L-Band DME Multipath Environment in the Microwave Landing System (MLS)

FAA-RD-82/19

Project Report ATC-116

L-Band DME Multipath Environment in the Microwave Landing System (MLS) Approach and Landing Region J. E. Evans

13 April 1982

Lincoln Laboratory MASSACHUSETTS INSTITUTE OF TECHNOLOGY LEXINGTON, MASSACHUSETTS

Prepared for the Federal Aviation Administration, Washington, D.C. 20591 This document is available to the public through the National Technical Information Service, Springfield, VA 22161

This document is disseminated under the sponsorship of the Department of Transportation in the interest of information exchange. The United States Government assumes no liability for its contents or use thereof.

ABSTRACT

The m u l t i p a t h environment i n t h e approach and l a n d i n g r e g i o n r e p r e s e n t s an

important

factor

in

the

o p t i m i z a t i o n and u l t i m a t e performance

of

the

b

“ 4

Microwave

Landing

(DME/P).

Various

System types

of

(MLS)

Precision

Distance

Measuring

m u l t i p a t h are a s s e s s e d i n t h e

Equipment

of

context

the

proposed DME/P implementation e r r o r c h a r a c t e r i s t i c s t o a s c e r t a i n t h e p r i n c i p a l challenges.

It

i s shown

(analytically

and e x p e r i m e n t a l l y ) t h a t

specular

r e f l e c t i o n s from b u i l d i n g s r e p r e s e n t a s i g n i f i c a n t c h a l l e n g e , p a r t i c u l a r l y a t low a l t i t u d e s

(e.g.,

c a t e g o r y I1 d e c i s i o n h e i g h t and below) where t e r r a i n

l o b i n g can cause t h e e f f e c t i v e m u l t i p a t h l e v e l s t o exceed t h e e f f e c t i v e d i r e c t signal level.

However, t h e t i m e

d e l a y d i s c r i m i n a t i o n c a p a b i l i t i e s of

the

proposed DME/P should e f f e c t i v e l y e l i m i n a t e t h e bulk of such m u l t i p a t h . Limited S-band

(3GHz) measurements of d i f f u s e r e f l e c t i o n s from nominally

f l a t t e r r a i n i n d i c a t e d very low l e v e l s .

iii

However,

s p e c u l a r r e f l e c t i o n s from

ACKNOWLEDGMENTS

S e v e r a l s e c t i o n s of Orr.

draw h e a v i l y on e a r l i e r

t h i s report

J. Yaeger-Charriere

I

coded t h e computer programs used t o a s s

l i k e l i h o o d of m u l t i p a t h as w e l l as g e n e r a t i n g much of

ck by R.S. is r e l a t i v e

the a i r p

t geometry

A. Vierstra w a s t h e p r i n c i p a l e n g i n e e r f o r t h e PDME m u l t i p a t t

measurement

system used i n the Quonset S t a t e A i r p o r t f i e l d tests summarizec

i n Chapter

d a t a base from maps provided by t h e FAA. p-.-.

V.

Crowder, and D.

J. B e r t r a m , W.

Hamilton a s s i s t e d i n t h o s e n

isurements.

office e

bled us t o

t h e Quonset State A i r p o r t manager's

The c o o p e r a t i o n of

e x e c u t e t h e measurement program i n a very s h o r t t i m e p e r i o d .

P.

I

!tt wa.s t h e

p r i n c i p a l e n g i n e e r f o r t h e major a i r p o r t tests summarized i n Chi

t e r V.

Gregory, J. K a l i l and T. Magnan a s s i s t e d i n t h e measurements whil

J. Yaeger-

C h a r r i e r e and K.

Roberts

reduced

the data.

C a t a l a n 0 assl

C.

A.

:ed i n t h e

s o f t w a r e development.

J.

A u s t i n was

the

principal

engineer

of

the

S-band

system used f o r t h e h i g h r e s o l u t i o n t i m e measurements a t L.G. presented

i n Chapter V I .

chani

1 sounding

Hans

)m a i r f i e l d

Murray made l o g i s t i c a l arrangeme :s f o r t h e

P.

experiments and reduced t h e d a t a . Concurrent s t u d i e s c a r r i e d o u t by a number of people i n con t h e AWOP WG/M reported here. Koshino

multipath

subgroup were

of

considerable

help

f

P a r t i c u l a r thanks a r e due t o Robert K e l l y (Ben

(N.E.C./Japan),

A.

Becker

(DFVLR/W.

Germany),

an

iction with t h e work x/USA), M.

T.

Gori

(Electronica/Italy). P

The r e p o r t was a b l y typed by C. Carter-Likas,

iv

D. Young and N.

TABLE OF CONTENTS Abstract

iii

Acknowledgments

iv

List of Illustrations

vii

I.

Introduction

1-1

11.

Principal DME/P Multipath Rejection Features

2-1

-9

A.

B. C. D.

E. F. G. H. 111.

IV

.

V.

3-1

A. B. C. D. E.

3-1

Reflections from Terrain in Front of Transponder Array Reflections from Terrain in the Approach Sector Shadowing by Overflying or Taxiing Aircraft Reflections from Parked or Taxiing Aircraft Reflections from Buildings

3-7 3-9

3-11 3-12

Simulation Studies of DME/P Multipath Effects

4-1

A. B. C.

4-1 4-14 4-33

Reflections from Aircraft and Surface Vehicles Effective M/D Levels Due to Building Reflections DME/P Multipath Scenarios

Experimental Studies by Lincoln Laboratory

B. C.

VII.

2-1 2-4 2-13 2-19 2-19 2-23 2-27 2-29

DME/P Multipath Sources and Characteristics

A.

VI.

Signal Waveform Receiver Pulse Processors Transponder Antenna Pattern Shaping Aircraft Antenna Pattern Shaping Motion Averaging Lateral Diversity Uplink/Downlink Error Combining Receiver Mismatched IF Filtering

ASTC Measurements of Direct Signal Lobing Summary Results of L-Band Airport Measurements Results of High Time Resolution S-Band Multipath Measurements at an Operational Airport

5-1 5-1 5-3 5-52

Likelihood of Encountering DME/P Reflections from Buildings on Final Approach

6-1

Conclusions and Recommendations for Further Study

7-1

A. B.

7-1 7-4

Conclusions Recommendations for Near Term Studies

R- 1

References Appendix A

A.l A.2 A.3

Derivation of DME Multipath Performance Formulas

Fixed Threshold Detection Real T i m e Threshold Detection Delay-and-Compare

Appendix B

Airport Maps Used to Determine Building Locations

A-1 A-2 A-4 A-8 B- 1

P

b

c

LIST OF ILLUSTRATIONS Figure

1-1

1-2 2- 1 2-2 2-3 2-4 2-5 2-6 2-7 2-8(a) 2-8( b ) 2-9 2-10 2-ll(a) 2-ll(b) 2-12 2-13 2-14 2-15 2-16( a ) 2-16( b ) 2-1 7 ( a ) 2-1 7 ( b )

3-1 3-2( a ) 3-2(b)

Page E f f e c t of a S i n g l e M u l t i p a t h Component on DME Pulse Constant Delay Contours o n A i r p o r t S u r f a c e Gaussian DME Waveform I l l u s t r a t i n g t h e D e f i n i t i o n of Risetime and Nominal Threshold C r o s s i n g Time DME/P cos/ cos2 Waveform Fixed T h r e s h o l d i n g E r r o r vs. M u l t i p a t h Delay Adaptive Thresholding E r r o r vs. M u l t i p a t h Delay DME Leading Edge Comparator C i r c u i t Delay-and-Compare E r r o r vs. M u l t i p a t h Delay Comparison of DAC E r r o r Characteristic With and Without I F F i 1t e r i n g Leading Edge Comparison of Various Proposed P u l s e Shapes f o r Air t o Ground (from [ 1 2 1 ) Leading Edge C h a r a c t e r i s t i c s of Various Proposed Ground t o Air P u l s e Shapes I n c l u d i n g I F F i l t e r E f f e c t (from [ 1 2 ] ) H a z e l t i n e AZ Antenna Azimuth P a t t e r n Showing C e n t e r l i n e Emphasis Measured H o r i z o n t a l P a t t e r n of Ground Antenna (from 1371) PALM Dipole Array P a t t e r n s of F i v e PALM Elements Averaging F a c t o r f o r Uniform and J i t t e r e d Spacing (TRSB) M u l t i p a t h A r r i v a l D i r e c t i o n s f o r Maximum Motion Averaging and F i r s t G r a t i n g l o b e Lateral D i v e r s i t y DME/P Transponder DME/P M u l t i p a t h E r r o r Reduction v i a Ground Averaging Lateral D i v e r s i t y (from I3211 Characteristic o f t h e CTOL/STOL System Receiver Filter C h a r a c t e r i s t i c of t h e VTOL System R e c e i v e r F i l t e r Experimental F i r s t Defined Transmitted P u l s e (from [ 4 3 1 ) Experimental F i r s t Defined Received P u l s e (from [431) M U M u l t i p a t h Phenomena S i g n a l P a t h s Considered i n "Naive" DME M u l t i p a t h Analysis Role of Ground R e f l e c t i o n s i n Determining DME M u l t i p a t h / D i r e c t Amplitude R a t i o

vii

1-4 1-5 2- 2 2-2 2-6 2-8 2-10 2-1 4 2-1 5 2-1 6 2-16 2-17 2-17 2-1 8 2-20 2-22 2-24 2-25 2-2 6 2-28 2-28 2-30 2-30 3-2 3-3

3-3

I

Figure

3 -3 3 -4

3-5

3-6 4-1 4-2 4-3 4-4 4-5 4-6 4-7 4-8 4-9 4-10 4-ll(a)

4-1 1( b ) 4-1 2 ( a) 4-12( b) 4-1 3 4-14 4-15

4-16

E, Expected S i t i n g C o n d i t i o n s f o r G l i d e S l o p e Systems M u l t i p a t h P r o p a g a t i o n Over Rough S u r f a c e s D i f f u s e M u l t i p a t h Level and S p a t i a l D i s t r i b u t i o n : ( a ) S p e c u l a r and D i f f u s e S c a t t e r i n g C o e f f i c i e n t vs. Roughness F a c t o r and ( b ) Angular E x t e n t of G l i s t e n i n g S u r f a c e f o r Rough S u r f a c e S c a t t e r i n g C o n f i g u r a t i o n Used t o Determine M u l t i p a t h Parame t e r s Due t o S c a t t e r i n g from B u i l d i n g Present-day L-band DME Waveform/Real T i m e Thresho l d i n g Receiver: S u r f a c e V e h i c l e R e f l e c t i o n s Present-day L-band M E Waveform/Real T i m e Thresho l d i n g Receiver: B747 R e f l e c t i o n s Present-day L-band DME Waveform/Real T i m e Thresho l d i n g Receiver: B747 R e f l e c t i o n s AWOP WG-M DME/P Strawman: " S u r f a c e Vehicle" Ref l e c t i o n s a t Threshold AWOP WG-M DME/P Strawman: " S u r f a c e V e h i c l e " Ref l e c t i o n s Near Touchdown AWOP WG-M DME/P Strawman: " S u r f a c e V e h i c l e " Ref l e c t i o n s Near Touchdown AWOP WG-M DME/P Strawman: B747 R e f l e c t i o n s a t Threshold AWOP WG-M DME/P Strawman: B747 R e f l e c t i o n s Near Touchdown AWOP WG-M DME/P Strawman: B747 R e f l e c t i o n s Near Touchdown AWOP WG-M DME/P Strawman: B747 R e f l e c t i o n s Near C a t I1 D e c i s i o n Height E f f e c t i v e L-band M/D Level from Large B u i l d i n g f o r R e c e i v e r Near Touchdown E f f e c t i v e L-band M/D R a t i o from Large B u i l d i n g f o r R e c e i v e r Near Touchdown E f f e c t i v e L-band M/D Level from Large B u i l d i n g f o r R e c e i v e r Near Touchdown E f f e c t i v e L-band M/D Level from Large B u i l d i n g f o r R e c e i v e r Near Touchdown E f f e c t i v e M/D L e v e l s from Large B u i l d i n g With Rec e i v e r a t Runway Threshold E f f e c t i v e M/D L e v e l s from Large B u i l d i n g With Rec e i v e r n e a r C a t I1 D e c i s i o n Height E f f e c t i v e M/D Levels from Large B u i l d i n g With Rec e i v e r Between C a t I and Cat I1 D e c i s i o n Height s E f f e c t i v e M/D L e v e l s from Large B u i l d i n g w i t h Rec e i v e r a t Cat I Decision Height

viii

-6 '-8

1-10

1-14 1-2

1-4 1-5 1-6 1

-a

1-9 1-10 r-11

1-1 2

1-13 1-16

1-17 1-18 1-1

9

1-20 1-21 s'

1-2 2 t

1-2 3

Figure

4-17 4-1 8 4-1 9 4-20 ( a ) 4-20( b) 4-2 1 4-22 4-23

4-24(a) 4-24(b) 4-25 4-26 4-27 4-28(a) 4-28( b) 4-28 ( C) 4-29(a) 4-29(b) 4-29(~) 4-30 4-3 1 ( a ) 4-3 1 ( b ) 4-32(a) 4-32( b) 4-32( c )

Page E f f e c t i v e M / D L e v e l s from Large B u i l d i n g w i t h Off Runway T e r r a i n 3 f e e t Below Runway Level When R e c e i v e r i s Near Touchdown E f f e c t i v e M/D Levels from Medium S i z e B u i l d i n g w i t h R e c e i v e r Near Touchdown E f f e c t i v e M/D Levels from Medium S i z e B u i l d i n g w i t h R e c e i v e r Near Touchdown E f f e c t i v e M/D Levels from Medium S i z e B u i l d i n g w i t h Receiver a t Threshold E f f e c t i v e M/D Levels from Medium S i z e B u i l d i n g w i t h R e c e i v e r a t Threshold E f f e c t i v e M/D Levels from Medium S i z e B u i l d i n g w i t h Receiver a t Cat I1 D e c i s i o n Height E f f e c t l v e M/D R a t i o from Medium S i z e B u i l d i n g w i t h R e c e i v e r a t C a t I D e c i s i o n Height E f f e c t i v e M/D R a t i o from Medium S i z e B u i l d i n g w i t h R e c e i v e r a t C a t T I D e c i s i o n Height and Off Runway T e r r a i n 6.0 Feet Below Runway Level Gain Contours f o r Boeing 727 Over 8, 9 ( a n t e n n a 2 ; g e a r down) (from [ 1 9 1 ) Boeing 727 Antenna P a t t e r n i n XY-plane ( a n t e n n a 2 ; g e a r down) (from [ 1 9 ] > A i r p o r t Map f o r WG-A Scenario 2 f o r L-band Carrier DME M u l t i p a t h Levels and R e l a t i v e T i m e Delays f o r AWOP S c e n a r i o 2 AWOP Scenario 3 Based on C r i s s e y Army A i r f i e l d i n San F r a n c i s c o DME M u l t i p a t h Levels and R e l a t i v e Time Delays f o r AWOP S c e n a r i o 3 E f f e c t i v e M/D R a t i o f o r B u i l d i n g # 1 i n AWOP S c e n a r i o 3 (from [ 4 2 ]) E f f e c t i v e M/D R a t i o f o r B u i l d i n g #7 i n AWOP S c e n a r i o 3 (from [ 4 2 1 ) Raw DME/P E r r o r f o r AWOP Scenario 3 CMN E r r o r F i l t e r Output f o r AWOP S c e n a r i o 3 PFE E r r o r F i l t e r Output f o r AWOP Scenario 3 P r e c i s i o n P u l s e CTOL Scenar€o E f f e c t i v e M/D R a t i o f o r B u i l d i n g #1 i n DME/P CTOL Scenario E f f e c t i v e M/D R a t i o f o r B u i l d i n g #2 Raw DME/P E r r o r s f o r DME/P CTOL S c e n a r i o CMN E r r o r F i l t e r Output f o r M E / P CTOL S c e n a r i o PFE E r r o r F i l t e r f o r DME/P CTOL S c e n a r i o

ix

4-24 4-26 4-27 4-28 4-29 4-30 4-31

4-32 4-34 4-35 4-36 4-37 4-38 4-40 4-41 4-42 4-43 4-44 4-45 4-46 4-48 4-49 4-50 4-5 1 4-52

'age --

Figure

5-1

,

5-2 5-3 5-4 5-5( a ) 5-5 ( b ) 5-5( c ) 5-6 5-7 5-8 5-9 5-10( a ) 5-10(b) 5-10( C ) 5-1 1 5-1 2 5-13 5-14( a ) 5-14(b) 5-1 S 5-16 5-17 5-18(a) 5-1 8 ( b) 5-18(~) 5-1 9 5-20

Measured S i g n a l L e v e l Along Hanscom Taxiway w i t h Both Antennas a t 5 Feet, Dipole P o i n t e d Down (from McGarty [ 2 2 ] ) DME /P Mu1t i pa t h Measur emen t System Example of D i g i t i z e d Waveform Washington N a t i o n a l A i r p o r t CME M u l t i p a t h Measurement S i t e Summary R e s u l t s f o r 3" Approach t o DCA Before Threshold Summary R e s u l t s f o r 3" Approach t o DCA Before Threshold Summary R e s u l t s f o r 3" Approach t o DCA Near Threshold Summary R e s u l t s f o r DCA Over Runway Computed M u l t i p a t h C h a r a c t e r i s t i c s f o r DCA 3" Approach S c e n a r i o A i r p o r t Geometry a t Wright P a t t e r s o n AFB WPAFB Waveforms Near Threshold Summary R e s u l t s f o r F l i g h t P r o f i l e 1 a t WPAFB Summary R e s u l t s f o r F l i g h t P r o f i l e 1 a t WPAFB Summary R e s u l t s f o r F l i g h t P r o f i l e 1 a t WPAFB Computed DME M u l t i p a t h Characteristics f o r WPAFB S c e n a r i o Lambert-St. Louis I n t e r n a t i o n a l , St. L o u i s , M i s s o u r i , Showing Azimuth R e f l e c t i o n P a t h s St. Louis Waveforms a t Threshold (50 f t . AGL) Data Summary f o r S t . L o u i s ' s Approach w i t h 50 f t . Threshold Height Data Summary f o r S t . L o u i s ' s Approach w i t h 50 f t . Threshold Height Computed M u l t i p a t h C h a r a c t e r i s t i c s f o r Lambert S t . Louis S c e n a r i o Philadelphia International Airport i n Vicinity of Runway 9R-27L P h i l a d e l p h i a Waveform a t and P a s t T h r e s h o l d Summary R e s u l t s f o r F l i g h t P r o f i l e 1 a t Philadelphia Summary R e s u l t s f o r F l i g h t P r o f i l e 1 a t Philadelphia Summary R e s u l t s f o r F l i g h t P r o f i l e 1 a t Philadelphia Computed M u l t i p a t h Characteristics f o r PHL M e asu r erne n t S c e n a r i o T u l s a S i t e 1 Measurement Geometry and R e f l e c t i o n

5-2 5-4 5-6 5-8 5-9 5-10 5-1 1 5-1 2 5-1 4 5-16 5-1 7 5-18 5-1 9 5-20 5-2 1 5-22 5-2 4 5-2 5 5-2 6 5-2 7 5-2 8 5-30 5-32 5-33 5-34 5-35 5-36

b Y S

X

1 "

Page

Figure 5-2 1 5-22 5-2 3 (a) 5-23(b) 5-24 5-25 5-26( a ) 5-26( b) 5-27 5-28 5-29 5-30 5-31 5-3 2 5-33

5-34 5-35 5-36 5-37 5-38 5-39 5-40 5-41 5-4 2 5-4 3 5-44 5-45 5-46 5-47 5-48 5-49 5-50 5-5 1

T e r r a i n Contour Along T u l s a Runway 17-35 Tulsa S i t e 1 Waveforms Over Runway on F l i g h t Profile 1 Tulsa S i t e 1 Data Summary f o r 25 f t . Threshold C r o s s i n g Height Tulsa S i t e 1 Data Summary f o r 25 f t . Threshold C r o s s i n g Height Computed M u l t i p a t h C h a r a c t e r i s t i c s f o r Simulation of T u l s a S i t e 1 Measurements Tulsa S i t e 2 Waveforms Near Threshold T u l s a S i t e 2 Data Summary f o r 50 f t . Threshold Crossing Height T u l s a S i t e 2 Data Summary f o r 50 f t . Threshold Crossing Height Computed DME M u l t i p a t h Characteristics f o r Simu l a t i o n of Tulsa DME Measurement S i t e 2 Quonset S t a t e A i r p o r t Measurement Geometry Measurement S t a t i o n %14 Data Comparison of Quonset S t a t e Measured and Simulated M/D Levels Measurement Van and Diesel Generator Closeup of Blade Antenna on H e l i c o p t e r High T i m e R e s o l u t i o n M u l t i p a t h Experiment Geometry a t L.G. Hanscom A i r p o r t (Bedford, MA) V i e w from Van S i t e f o r Runway 5-23 Measurements V i e w from Threshold of Runway 22 Received Envelope 150 f t . Above Ground Received Envelope a t Runway 22 Threshold Received Envelope 100 f t . Above Ground Received Envelope 1 5 f t . Above Runway V i e w from Van S i t e f o r Runway 11-29 Measurements V i e w from Threshold o f Runway 2 9 Hangars and Parked A i r c r a f t Near Runway 29 Threshold Hangars and Parked A i r c r a f t Near Runway 11-29 Midpoint S i t e 2 M u l t i p a t h a t 200 f t . A l t i t u d e S i t e 2 M u l t i p a t h a t 150 f t . A l t i t u d e S i t e 2 M u l t i p a t h a t 100 f t . A l t i t u d e S i t e 2 M u l t i p a t h a t Threshold S i t e 2 M u l t i p a t h a t 200 f t . A l t i t u d e S i t e 2 M u l t i p a t h a t 150 f t . A l t i t u d e S i t e 2 M u l t i p a t h a t 100 f t . A l t i t u d e S i t e 2 M u l t i p a t h a t Threshold

xi

5-37 5-38 5-39 5-40 5-42 5-43 5-44 5-4 5 5-46 5-48 5-49 5-50 5-54 5-55

5-56 5-58 5-60 5-6 1 5-62 5-63 5-64 5-65 5-66 5-67 5-68 5-70 5-7 1 5-72 5-73 5-74 5-75 5-76 5-77

~~

I

Figure 5-52 5-53 5-54 6- 1 6-2

6 -3 6-4

6-5

6 -6

6-7

6-8

6-9 6-10 7-1 7 -2

7-3 7 -4 7 -5 A- 1

-Page . S i t e 2 M u l t i p a t h Near 125 f t . A l t i t u d e S i t e 2 M u l t i p a t h Near 125 f t . A l t i t u d e S i t e 2 M u l t i p a t h Near 125 f t . A l t i t u d e D e t e r m i n a t i o n of MLS M u l t i p a t h by Ray T r a c i n g T i m e Delay D i s t r i b u t i o n f o r DME/P S i t e d With Azimuth Array: (a-b) B u i l d i n g s Near Runway T h r e s h o l d and (c-d) B u i l d i n g s Near Runway S t o p End M u l t i p a t h Region D i s t r i b u t i o n f o r DME/P S i t e d With Azimuth Array: ( a ) B u i l d i n g s Near Thresho l d and ( b ) B u i l d i n g s Near Runway S t o p End D i s t r i b u t i o n of S c a l l o p i n g Rates f o r DME/P S i t e d With Azimuth Array: ( a ) B u i l d i n g s Near Thresho l d and ( b ) B u i l d i n g s Near Runway Stop End D i s t r i b u t i o n of R e l a t i v e Time Delay f o r DME/P S i t e d With Azimuth T r a n s m i t t e r B u i l d i n g : (a-b) B u i l d i n g s Near Runway T h r e s h o l d and (c-d) B u i l d i n g s Near Runway Stop End M u l t i p a t h Region When DME/P i s S i t e d With Azimuth T r a n s m i t t e r Building: ( a ) B u i l d i n g s Near Threshold and ( b ) B u i l d i n g s Near Runway S t o p End D i s t r i b u t i o n of S c a l l o p i n g F r e q u e n c i e s When DME/P i s S i t e d With Azimuth T r a n s m i t t e r B u i l d i n g : ( a ) B u i l d i n g s Near Threshold and ( b ) B u i l d i n g s Near Runway S t o p End M u l t i p a t h T i m e Delay D i s t r i b u t i o n s Wnen DME/P i s S i t e d With MLS E l e v a t i o n Antenna. Only B u i l d i n g s Near Runway Threshold M u l t i p a t h Region D i s t r i b u t i o n When DME/P i s S i t e d With E l e v a t i o n Array S c a l l o p i n g Frequency D i s t r i b u t i o n When DME/P i s S i t e d With MLS E l e v a t i o n T e r r a i n Along Road a t Camp Edwards, Mass. T e r r a i n P r o f i l e a t Camp Edwards, Mass. S i t e #2 (Gibbs Road) Received Power vs. E l e v a t i o n Angle a t Camp Edwards S i t e #2 Camp Edwards Measurement: Gibbs Road, L-band and C-band E l e v a t i o n A r r a y , EL a 2.5' DME M u l t i p a t h Bench T e s t Used i n UK Tests [ 3 8 ] E r r o r Curve f o r cos-cos2 P u l s e a n d DAC w i t h I F Filter

xii

5-78 5-7 9 5-83 6-2

6- 5

j-

8

i-9

5-10

j-11 i-12

5-1 3 7 -6 7 -7 7 -8

7 -9

7-10 1-10

Figure B- 1 B-2 B-3 B-4 B- 5 B-6 B- 7 B-8 B-9 B-10 B-11 B-12 B-13 B-14 B-15 B-16 B-17 B-18 B-19 B-20 B-2 1 B-22 B-23 B-24

Page Efinneapolis - S t . P a u l (Wold-Chamberlain F i e l d ) (MSP) John F. Kennedy I n t e r n a t i o n a l A i r p o r t (JFK) San F r a n c i s c o I n t e r n a t i o n a l A i r p o r t (SFO) O'Hare I n t e r n a t i o n a l A i r p o r t (ORD) M i a m i I n t e r n a t i o n a l Airport T u l s a I n t e r n a t i o n a l A i r p o r t (TUL) Los Angeles I n t e r n a t i o n a l A i r p o r t (LAX) P h i l a d e l p h i a I n t e r n a t i o n a l A i r p o r t (PHL) Melbourne A i r p o r t orly Airport (Paris) Heathrow A i r p o r t (London) Haneda A i r p o r t (Tokyo) Narita A i r p o r t (Tokyo) S a n t o s Dumont A i r p o r t ( B r a z i l ) Pulkovo A i r p o r t (Leningrad) Sheremetyevo A i r p o r t (Moscow) Vnukovo A i r p o r t (Moscow) Wright P a t t e r s o n AFB Hamburg, W. Germany A i r p o r t F r a n k f u r t ( b i n ) , W. Germany A i r p o r t Washington N a t i o n a l A i r p o r t (DCA) Gatwick A i r p o r t (London) Dorval A i r p o r t (Montreal) Sydney Ai r p o r t ( A u s t r a l i a )

xiii

B- 2 B- 3 B- 4 B- 5 B- 6 B- 7 B- 8 B- 9 B-lo B-10 B-11 B-12 B-13 B-14 B-15 B-16 B-17 8-18 B-19 B-20 B-2 1 B-22 B-23 B-24

A p p r o x i m a t e Conversions to M e t r i c Mrsrurrs

A p p r o r i m e r r Conrefaions f r o m Metric M e s r u ! s s

Simbd

LENGTH

-

.PM m c h s 1.a s q w I "Yd.

.qura males urns

X

6.5 0.09 0.1 2.6 0.4

M A S S (wei#ht)

MASS (weight)

P*

c

AREA

AREA

OI

QlllCIl

Ib

panas short ions

:-

, ,2

28 0.45 0.9

'b

Ibl

VOLUME

VOLUME rn,,l,l1l..,

0 03

15

Ill.,,

21

30

111as

5

l...LDOD.

iablrrgoma llud wos cup.

0.24

pints

0.47

w.ns gallor

0.95 3.8 0.03

cub*< 1-1 cub= r u d s

I

111as

' m

cubtc ratn, cubic m c a s

,'

I.c6 0.26 35

1.3

T E M P E R A T U R E (erect)

0.76

T E M P E R A T U R E (rract) C.lSl"S

't

il

P.

1.

INTRODUCTION

This

report

summarizes

a s t u d y of

t h e m u l t i p a t h environment

for the

Microwave Landing System (MLS) p r e c i s i o n D i s t a n c e Measuring Equipment (DME/P) subsystem. the

The s t u d y o b j e c t i v e s were t o determine a q u a n t i t a t i v e s t a t e m e n t of

anticipated

performance

environment

assessment.

as

In

a guide

t o DME/P

particular,

w e have

system o p t i m i z a t i o n and sought

to

consider

the

m u l t i p a t h environment f e a t u r e s which w i l l be of g r e a t e s t concern f o r t h e Lband DME/P implementations c u r r e n t l y under c o n s i d e r a t i o n by t h e I n t e r n a t i o n a l C i v i l A v i a t i o n O r g a n i z a t i o n (ICAO) A l l Weather O p e r a t i o n s Panel (AWOP). The p r e s e n t hope i s t h a t an L-band

DME which i s f u l l y compatible w i t h

c u r r e n t VOR/DME n a v i g a t i o n a n d / o r RNAV requirements can provide range guidance which i s adequate f o r a l l t h e MLS needs (e.g.,

RNAV t o MLS t r a n s i t i o n , complex

t e r m i n a l maneuvers f o r curved approach, f l a r e i n i t i a t i o n and t h e f l a r e maneuT y p i c a l DME/P requirements are shown i n Table 1-1.

v e r i t s e l f , etc.). There

is

at

this

t i m e a l i m i t e d L-band

t h e MLS phase

I1 program used

Performance e x t r a p o l a t i o n of

can be

A l l of t h e p r e c i s i o n DME's t e s t i n g i n t h e U.S.

d i r e c t l y a p p l i e d t o t h e DME/P. during

DME d a t a base which

fast

t h e s e C-band

rise t i m e p u l s e s

results

at

t o t h e L-band

C-band. DME/P

is

u n l i k e l y due t o t h e d i f f e r e n c e s i n p u l s e rise t i m e , c a r r i e r frequency and t h e u n r e p r e s e n t a t i v e r e f l e c t o r geometry of t h e m u l t i p a t h t e s t s i n t h e US MLS phase

11 assessment.

Subsequent L-band

Technical Analysis

Center

DME t e s t s conducted

( A t l a n t i c C i t y N.J.),

[ 2 3 , 3 0 ] a t t h e FAA

Crows Landing,

Calif.

and

Wallops I s l a n d , V a . used runways which have few i f any s i z a b l e scatterers. The L-band Braunschweig, W.

DME

testing

by

t h e F e d e r a l Republic of

Germany (FRG) a t

Germany a s s o c i a t e d w i t h t h e DLS p r o p o s a l [ 131 encountered a

c e r t a i n degree of

"indigenous" m u l t i p a t h from b u i l d i n g s , houses,

however, t h e a i r p o r t geometry w a s n o t t y p i c a l of normal a i r p o r t s .

etc.

[ 131,

The l i m i t e d

United Kingdom (UK) DME t r i a l s a t RAE Bedford u s i n g an L-band DME w i t h a f a s t

(100 n s e c ) risetime a p p a r e n t l y encountered only ground r e f l e c t i o n l o b i n g [ 141

*Minimum

Decision Altitude

1-1

TABLE

1-1

DME/P ACCURACY REOUIREMENTS

-maximum e r r o r ( 9 5 % p r o b a b i l i t y ) - c a l c u l a t i o n s b a s e d u p o n a 1 7 6 8 m' ( 5 8 0 0 f t ) DME t o r u n w a y t h r e s h o l d d i s t a n c e -NA f o r "non a p p l i c a b l e "

Function Segmented approach

Segmented approach

I I

Typical distance from t h e r e f e r e n c e d a t u m (NM)

-extended runway centerline

I

250m (800f t )

-

20

-at

40"

azimuth

3 7 5m (1200ft)

34m

85m (210ft)

- e x t e n d e d runway centerline

(50fE)

5 -at

40"

3 4ln ( 6 3f t )

127m

azimuth

(330ft)

1-middle

C a t TI Decision Height

Control Motion Error

Path Following

Error

40001 (1200ft)

0.57

marker

-CTOL

3 Om ( l o o t t)

0.3

i

N.4

NA

-STOL 15m (Soft)

Flare l n i t i a t i o n o v e r uneven t e r f a i n

-CTOL

I

0

-STOL

F l a r e Maneuver w i t h MLS F l a r e Antenna

___-

I

-

CTOL

-

STOL

Long F l a r e A l e r t

NA

-

3Om (1OOft)

1 RPI (100f t )

12m (40f t )

1201 (401't)

n

3Om (looft) 12m (40ft)

12m (40ft) 12m (40ft)

1

3 Om (looft)

NA

-

-

L.

Coordinate Translations and C o n v e r s i o n s

12m t o 30m (40ft) (100ft)

1-2

1210 (40f t )

i n much t h e same way t h a t i t

M u l t i p a t h a f f e c t s t h e DME signal-in-space

does t h e MLS a n g l e guidance s c a n n i n g beam s i g n a l , i.e., of

t h e r e c e i v e d p u l s e envelope.

by a d d i t i v e d i s t o r t i o n

An important d i s t i n c t i o n , however,

f o r DME, m u l t i p a t h r e t u r n s are one-sided

following:

i n time,

is t h e

t h a t i s , they

always a r r i v e l a t e w i t h r e s p e c t t o t h e d i r e c t component due t o t h e l o n g e r T h i s i s i n c o n t r a s t t o t h e s i t u a t i o n i n scanning beam i n

p a t h s they t r a v e r s e .

which m u l t i p a t h a r r i v a l t i m e i s a f u n c t i o n of s c a n d i r e c t i o n and t h e a n g u l a r t h e s c a t t e r e r , and i n f a c t , m u l t i p a t h which l e a d s t h e d i r e c t on

l o c a t i o n of the

scan w i l l

"TO"

trail

it

on

the

scan

"FRO"

and

vice

versa.

This

o b s e r v a t i o n h a s i m p o r t a n t i m p l i c a t i o n s on p r o c e s s o r implementation f o r b o t h i n t e r r o g a t o r and t r a n s p o n d e r . F i g u r e 1-1 i l l u s t r a t e s t h e e f f e c t of a s i n g l e m u l t i p a t h component on a t y p i c a l DME waveform.

The m u l t i p a t h s i g n a l i s a delayed

received d i r e c t s i g n a l s ( t ) ;

i n general,

w i l l d i f f e r from t h e d i r e c t .

I n terms of

r e p l i c a of

the

t h e m u l t i p a t h amplitude and phase T,

p , and

+,

the ( r e l a t i v e ) delay,

a m p l i t u d e , and phase, t h e complex envelope of t h e m u l t i p a t h can be w r i t t e n pej's(t

- TI.

respectively.

Figures 1-l(a)

The t o t a l r e c e i v e d envelope i s simply t h e magnitude of t h e sum

of t h e two complex waveforms. phase (+=Oo)

and ( b ) show t h e d i r e c t and m u l t i p a t h waves,

case.

T h i s i s i l l u s t r a t e d by F i g . 1 - l ( c ) f o r t h e i n -

One can n o t e t h a t t h e t r a i l i n g edge s u f f e r s f a r g r e a t e r

displacement t h a n t h e l e a d i n g edge f o r a f i x e d -6 dB t h r e s h o l d . F u r t h e r on, w e s h a l l see t h a t m u l t i p a t h d e l a y i s a key f a c t o r i n DME performance, and t h a t each DME implementation h a s a r e g i o n of what are c a l l e d

"critical delays,"

t h a t i s , v a l u e s of

range e r r o r i s l a r g e s t .

For some, t y p i c a l l y t h e slow risetime p u l s e s a s s o c i -

a t e d w i t h p r e s e n t day L-band o r d e r of

d i f f e r e n t i a l d e l a y f o r which t h e n e t

DME p r a c t i c e ,

t h e c r i t i c a l d e l a y s are on t h e

s e v e r a l hundred ns t o more t h a n one p s .

F i g u r e 1-2

illustrates

e l l i p t i c a l c o n t o u r s of c o n s t a n t d e l a y of t h i s o r d e r of magnitude on a n a i r p o r t p l a n view. end

of

a

threshold.

It i s assumed t h a t t h e DME t r a n s p o n d e r i s 1,000 f t behind t h e s t o p

10,000

ft

runway,

and

that

the

One can e a s i l y see t h a t t h e llis

-

receiving

aircraft

is

over

2ps t i m e delay contours extend

w e l l back i n t o p o r t i o n s of t h e a i r p o r t which could be occupied by b u i l d i n g s o r o t h e r s t r u c t u r e s ( t h e nominal 700 f t o b s t a c l e c l e a r a n c e l i n e i s s k e t c h e d f o r

1-3

a)

a 0

. I

s

a)

> C

w

( a ) direct

( b ) mu1t i p a t h

( c ) combined

Fig.1-1.

E f f e c t of a single m u l t i p a t h component on DME p

1-4

ilse.

see9

(117095-Sl-

-

-

delay in nanoseconds

A

700' clearance 1i n e

\

I5

L rc a-l V

C Y 0

i .C

n

Distance A1 ong Cen terl i ne ( ft )

Fig.1-2.

Constant d e l a y c o n t o u r s on a i r p o r t s u r f a c e .

1-5

11

reference).

Thus,

f o r such DMEs,

numerous p o t e n t i a l m u l t i p a t l

sourc.es can

e x i s t at an a i r p o r t . Other

DMEs

(fast

risetime

smaller c r i t i c a l d e l a y s , e.g., indicate

that

pulses

and

improved p r o c e s s o r ) have much

less t h a n 200 ns.

t h e s e DMEs are l a r g e l y

o b s t a c l e s on t h e a i r p o r t s u r f a c e .

The contour5 .:n F i g .

immune t o r e f l e c t i o n s f

1-2

)m permanent

They may be s u s c e p t i b l e t o r e i ect toris from

parked o r t a x i i n g a i r c r a f t and s e r v i c e v e h i c l e s which can be f o l d i n s i d e t h e obstacle clearance l i m i t s .

The e x t e n t of t h e s e s u s c e p t i b i l i t i e s

s discussed

i n S e c t i o n I V , where the c r i t i c a l / s e n s i t i v e area problem i s add e s s e d .

For

s one handy

of s u i t a b l y p r e s e n t day In a t a l l t o nnex 10 and 1s

(cat

111

app r op r i a t e ip "M"

s

(WGM)

than the

m occupancy

formarice i s p r o c e s s or rtain

which

s the three

iresho.lding,

.es so,me of ndependent if

multipath

and

1-6

.

motion

c

.-

Section

I11 c o n s i d e r s

the

DME m u l t i p a t h

environment

with

particular

emphasis on t h o s e t y p e s of m u l t i p a t h which are l i k e l y t o be of g r e a t e s t conc e r n f o r r e p r e s e n t a t i v e optimized waveform/processor d e s i g n s .

I t i s seen t h a t

l a t e r a l m u l t i p a t h from b u i l d i n g s or a i r c r a f t are of g r e a t e s t concern from t h e as

important

T e r r a i n r e f l e c t i o n s are s e e n t o a l s o be

r e l a t i v e time d e l a y s .

viewpoint of

a

contributing

factor

in

that

they

will

cause

substantial

d e c r e a s e s i n t h e d i r e c t s i g n a l s t r e n g t h and, may not cause as l a r g e a d e c r e a s e .-

i n the multipath s i g n a l levels. S e c t i o n I V c o n s i d e r s DME m u l t i p a t h s i m u l a t i o n s t u d i e s r e s u l t s t o d a t e . R e f l e c t i o n s from a i r c r a f t , t r u c k t y p e o b j e c t s , and b u i l d i n g s are examined by t h e approach used t o assess ElLS a n g l e guidance c r i t i c a l / s e n s i t i v e areas [8]. Next, b u i l d i n g r e f l e c t i o n s are examined i n t h e c o n t e x t of t h e m u l t i p a t h l e v e l s and s p a t i a l v a r i a t i o n as e x e m p l i f i e d i n two m u l t i p a t h s c e n a r i o s developed by It

AWOP.

is

shown

that

lateral

multipath

from

buildings

represents

a

s i g n i f i c a n t c h a l l e n g e t o s u c c e s s f u l DME/P o p e r a t i o n . S e c t i o n V examines

lateral multipath

in

t h e e x p e r i m e n t a l s t u d i e s t o d a t e r e g a r d i n g L-band

representative

geometries.

These i n c l u d e both work

d i r e c t l y aimed a t DME/P m u l t i p a t h and r e l a t e d work i n t h e c o n t e x t of a i r p o r t surveillance.

I t i s shown t h a t very high M/D l e v e l s (e.g.,

can be encountered a t low i n t e r r o g a t o r ( i . e . ,

i n excess of 0 dB)

a i r c r a f t ) h e i g h t s as a r e s u l t of

d i f f e r e n t i a l ground l o b i n g e f f e c t s . Another i m p o r t a n t various

issue is

characteristics

s c a l l o p i n g frequency).

(e.g.,

the

r e l a t i v e l i k e l i h o o d of

relative

time

delay,

multipath with

multipath

region,

To assess t h i s , a b u i l d i n g l o c a t i o n d a t a base d e r i v e d

from maps f o r some 24 U.S.

and f o r e i g n a i r p o r t s h a s been analyzed t o determine

e m p i r i c a l p r o b a b i l i t i e s of e n c o u n t e r i n g s p e c u l a r b u i l d i n g r e f l e c t i o n s w i t h a given c h a r a c t e r i s t i c value.

R e s u l t s of t h i s a n a l y s i s are p r e s e n t e d i n S e c t i o n

VI.

S e c t i o n V I 1 summarizes t h e r e s u l t s of t h e v a r i o u s s t u d i e s and p r e s e n t s a p r e l i m i n a r y q u a n t i t a t i v e asessment of

t h e expected m u l t i p a t h environment as

w e l l as i d e n t i f y i n g s e v e r a l i s s u e s r e q u i r i n g f u r t h e r s t u d y .

1-7

11. PRINCIPAL DME/P MULTIPATH REJECTION FEATURES

Our objective in this section is to examine the principal DME/P multipath rejection features with the objective of defining the characteristics of the

.

principal multipath threats.

*.

relationship exists between multipath level and delay and DME error behavior

.-

It is unfortunate that no simple rule-of-thumb

is strongly dependent

upon

the signal design as well

processing at both tkie transponder and interrogator. amplitude is fortunately somewhat simpler. In

Error

as the

Dependence on multipath order to convey some

understanding of what is involved, the following two sections examine common DME pulse shapes and pulse arrival time estimation techniques and some of their qualitative performance

characteristics.

Following that, several

additional multipath features are examined. Signal Waveform

A.

The signal waveform utilized for the DME/P can make a major impact on multipath performance to the extent that it permits one to make a distance measurement at reasonable signal-to-noise ratio (SNR) soon after the pulse has arrived. years.

A variety of pulse shapes have been proposed over the past two

We will consider here two of the most common proposals. 1.

Gaussian Pulse

Figure 2-1 illustrates the basic Gaussian pulse on a time scale measured in 10-90% risetime units.

We use the Gaussian pulse as the model for present

*The

Radio Technical Commission for Aeronautics RTCA 117 (SC-117) [15] used the relationship E

=

Special Committee No.

0.29 tr

where t, is the pulse risetime as a guideline for its DME design. This particular relationship would suggest significant multipath problems for many of the current DME pulse shape proposals. Fortunately, the physical/mathematical basis for applying this relationship to all pulse shape/processor combinations is highly suspect.

2- 1

I

I

I

jI I ~

i

I

I

!

~

I

I

Fig.2-1. Gaussian DME waveform illustrating the definition of risetime and nominal threshold crossing time.

1.50

1.35 1.20 1.05

1- I

I

-

117963-S

I

THE 5%

- 30% P A R T l A L RISE TIME

I II

I 1f

i

SHOULD BE ESSENTIALLY L I N E A R

-

3E05 nr

650

1300

1959

Fig.2-2.

2600 3250 TIME ( 1 , s )

3909

4550

DME/P cos-cos 2 waveform.

2-2

52DC

5850

5fOO

day DME waveform.

Thus, it is relevant to the use of a nonprecision DME

(DME/N) as a part of MLS as well as the DME based azimuth system (DAS) under study by the FRG [131. The ICAO standard pulse width (measured between the -6 points) is 3.5 ps, which corresponds to a 2.5

p s risetime [l].

dB envelope The 2.5 p s

risetime pulse meets the ICAO spectrum requirements with room to spare.

R.P.

Crow has calculated that the pulse will still meet the ICAO specifications if the risetime is decreased to 1.3

ps

[15].

For our purposes, the Gaussian

pulse will be described by the equation: 2

-8 ( t / tr 1

s(t) = e

(2-1)

where tr is the risetime and R = 1.423 so that the tr satisfies the ICAO 10%

-

90% definition. 2.

Cos/Cos2 Pulse

This waveform was designed in an attempt to find a suitable compromise between i)

a pulse shape usable at L-band whose spectrum adheres to ICAO Annex 10 and is thus compatible with present DME, and

ii)

a sharp leading edge suitable for low level thresholding and good multipath rejection.

The cos/cosz pulse adopted by WG-M[50] waveform for DME/P is shown in Fig. 2-2.

as the precision measurement

This pulse shape satisfies the basic

requirements above.

The leading edge has a much sharper rise than the

Gaussian-type pulse.

Its initial slope is such that the risetime would be

0.78 u s if it continued linearly.

2-3

B.

Receiver Pulse Processors

A l l t h e DME performance r e s u l t s found i n t h e e n s u i n g d i s c u s s k o n are based

on t h e assumption of one of

t h r e e canonical r e c e i v e r p r o c e s s i n i techniques;

f i x e d t h r e s h o l d , r e a l t i m e t h r e s h o l d , or delay-and-compare.

Each of t h e s e can

be used w i t h any of t h e p u l s e t y p e s , and t h e i r performance c h a r a j t e r i s L i . c s are more or less independent of t h e d e t a i l s of t h e p u l s e shape.

a d e s c r i p t i o n of each p r o c e s s o r i s given. t i o n of

I n Lhis s e c t i o n ,

T h i s i s supplemented bL a n explana-

i t € i m p o r t a n t performance c h a r a c t e r i s t i c s ; t h e e x p l a n aI t i o n is done

w i t h t h e a i d of a n a n a l y t i c a l formula which p r e d i c t s range e r r o r I s . m u l t i p a t h and p r o c e s s o r parameters.

l

A c o l l e c t i o n of such formulas i s d e r i v e d i n Appendix A.

I

These formulas

the

principal

r e s t r i c t i o n on t h e i r u s e i s t h a t they a p p l y f o r small-to-moderite

multipath

have

been

validated

against

computer

simulation

data;

l e v e l s . From among t h e s e , ' t h e r e s u l t s which a p p l y t o a Gaussian b u l s e w i l l be used as i l l u s t r a t i o n s . A l l t h e DME performance r e s u l t s found i n t h e e n s u i n g

on t h e assumption of one of t h r e e c a n o n i c a l r e c e i v e r

I processing

arle based techniques;

I

f i x e d t h r e s h o l d , r e a l t i m e t h r e s h o l d , or delay-and-compare.

of t h e s e can

are

be used w i t h any of t h e p u l s e t y p e s , and t h e i r performance more or less independent of t h e d e t a i l s of t h e p u l s e

a d e s c r i p t i o n of each p r o c e s s o r i s given. t i o n of

T h i s i s supplemented by a n explana-

i t s i m p o r t a n t performance c h a r a c t e r i s t i c s ; t h e e x p l a n a t i o n is done

w i t h t h e a i d of a n a n a l y t i c a l formula which p r e d i c t s range e r r o r k s . m u l t i p a t h and p r o c e s s o r parameters.

pulse.

The o b j e c t i v e i s t o d e f e a t m u l t i p a t h by d e t e c t i n g t h e F i l s e p r i o r t o

c o n t a m i n a t i o n by t h e d e l a y e d r e f l e c t i o n s . t i o n can be made,

I n general, t h e earli e r t h e detec-

t h e g r e a t e r t h e multipath suppression.

Oj

course,

the

e x t e n t t o which t h i s can be c a r r i e d out i s l i m i t e d by t h e receive r n o i s e l e v e l and t h e a v a i l a b l e s i g n a l power.

2-4

1.

Fixed Threshold Receiver

I n t h i s r e c e i v e r , t i m e of a r r i v a l is e s t i m a t e d by d e t e c t i n g t h e t i m e a t which t h e l e a d i n g edge of t h e f i r s t p u l s e c r o s s e s a t h r e s h o l d l e v e l a. i n t e r p r e t a t i o n of constant

fraction

" f i x e d " t h r e s h o l d is t h a t t h e t h r e s h o l d v o l t a g e remains a

of

performance r e s u l t s ,

"risetime" u n i t s .

The

the

peak

direct

signal

level

*.

For

some

of

the

i t i s convenient t o e x p r e s s t h e t h r e s h o l d i n terms of

The parameter, v,

e x p r e s s e s t h e t i m e e l a p s e d between t h e

nominal (no m u l t i p a t h ) t h r e s h o l d c r o s s i n g and t h e waveform peak as a f u n c t i o n

of

t h e risetime t r ( s e e F i g .

r a t i o s (M/D),

2-1).

For s m a l l m u l t i p a t h t o d i r e c t s i g n a l

t h e f o l l o w i n g e x p r e s s i o n is a n a c c u r a t e approximation t o t h e

s t a t i c a r r i v a l t i m e e s t i m a t i o n e r r o r f o r f i x e d t h r e s h o l d i n g on a Gaussian pulse:

-fl(*) E:

fix

c%-

Ptr

2f3v

e

r

(k+ 2v) r

(2-2)

COS$

where

-gv2

a = e p

= v o l t a g e MID r a t i o

$

= r e l a t i v e r f phase between d i r e c t and m u l t i p a t h s i g n a l s

'I

= r e l a t i v e t i m e d e l a y between d i r e c t and m u l t i p a t h s i g n a l s

An a p p r e c i a t i o n of t h e u t i l i t y of

t h i s formula can be gained by making

> 0)

some simple o b s e r v a t i o n s .

F i r s t of a l l , n o t e t h a t in-phase m u l t i p a t h ( c o s $

causes n e g a t i v e e r r o r s .

The m u l t i p a t h s i g n a l r e i n f o r c e s t h e d i r e c t s i g n a l ,

i n c r e a s i n g t h e envelope and c a u s i n g a l a t e t h r e s h o l d c r o s s i n g . out-of-phase

Similarly,

m u l t i p a t h w i l l r e s u l t i n an e a r l y c r o s s i n g .

*The

n o t i o n of a n a b s o l u t e l y f i x e d t h r e s h o l d is convenient f o r a n a l y t i c purposes, b u t does n o t n e c e s s a r i l y correspond t o what happens i n a real r e c e i v e r . The t h r e s h o l d could be a f i x e d v o l t a g e , i n which case t h e t h r e s h o l d c r o s s i n g p o i n t drops as t h e a i r c r a f t n e a r s t h e t r a n s p o n d e r , o r t h e t h r e s h o l d v o l t a g e could be range s c a l e d t o keep it a f i x e d number of dB below t h e nominal d i r e c t s i g n a l l e v e l .

2- 5

e -18

-20

-3e

-40

cLL

oc 0 CT oc w

-58

-€2

-73

-80

--------lee

Lr! 8

Fig.2-3.

= S i mu1 a t i on I

Theoret ical

I

I

--pGGG I

I

I

I

I

250

I

I

I

I I I I I I

soe

I

I

I

I

leeo Time Delay (nsec) 750

I

I

I

I

I

I

1258

F i x e d thresholding error vs. multipath delay

2-6

As

a

decreasing.

function

At

of

multipath

zero delay,

delay,

the

error

magnitude

t h e m u l t i p a t h a m p l i t u d e modulates

pulse, causing a l a r g e e r r o r s i n c e t h e threshold is fixed.

i s monotone the direct

A s t h e delay in-

creases, c o r r e s p o n d i n g l y less of t h e m u l t i p a t h p u l s e i n f l u e n c e s t h e l e a d i n g edge and smaller e r r o r s

result.

This behavior

i l l u s t r a t e s one f a v o r a b l e

a s p e c t of f i x e d t h r e s h o l d i n g , which is i n s e n s i t i v e t o l a t e m u l t i p a t h r e s u l t i n g from t h e decoupling of level.

t h r e s h o l d l e v e l and v a r i a t i o n s

i n received s i g n a l

Of c o u r s e , t h i s f e a t u r e comes a t t h e p r i c e of l a r g e e r r o r s f o r e a r l y

m u l t i p a t h and c o n s i d e r a b l e s e n s i t i v i t y t o s i g n a l l e v e l changes c h a r a c t e r i s t i c of

ground l o b i n g and a i r b o r n e antenna g a i n v a r i a t i o n s

*.

N e i t h e r of

these

t r a i t s i s i n h e r e n t i n e i t h e r of t h e o t h e r p r o c e s s o r s t o be considered. Formula (2-2)

c l e a r l y shows t h a t m u l t i p a t h performance i s improved by

u s i n g s h o r t risetimes and low t h r e s h o l d s

( l a r g e v a l u e s of

v).

The l a t t e r

o b s e r v a t i o n , i n t e r e s t i n g l y , i s o p p o s i t e t o what i s found f o r MLS a n g l e system m u l t i p a t h when a d w e l l g a t e p r o c e s s o r i s used.

There,

t h e problem i s t o

minimize t h e maximum d w e l l g a t e d i s p l a c e m e n t , which i s achieved by u s i n g a h i g h t h r e s h o l d value. F i g u r e 2-3 i l l u s t r a t e s t h e f i x e d t h r e s h o l d i n g b e h a v i o r i n a p l o t of e r r o r vs. m u l t i p a t h d e l a y .

A second c u r v e , which i s t h e comparable s i m u l a t i o n d a t a ,

i s p l o t t e d f o r comparison. -10 dB, t r = 1 . 3 psec, $I = 0'.

I n both cases, t h e m u l t i p a t h parameters are p = The t h r e s h o l d s e t t i n g i s -26 dB ( v = 1.27).

Real T i m e Threshold Receivers

T h i s p r o c e s s o r makes u s e of a t e c h n i q u e employed i n t h e MLS a n g l e processors.

The t h r e s h o l d v o l t a g e i s set a s p e c i f i e d number of dB below a r e f e r -

ence v a l u e r e l a t e d t o peak p u l s e amplitude. the present pulse,

The r e f e r e n c e could be t a k e n from

t h e p r e v i o u s p u l s e , o r i t could be a smoothed a v e r a g e of

s e v e r a l p a s t p u l s e s (motion a v e r a g i n g p r o p e r t i e s w i l l d i f f e r s l i g h t l y depend-

*For

example, L i n c o l n measurements [18-201 of L-band a i r b o r n e antenna g a i n f o r t y p i c a l DME l o c a t i o n s show v a r i a t i o n s of as much as 10 dB n e a r t h e forward d i r e c t i o n when t h e wheels are down [18-201.

2- 7

I ecI

GAUSSIAN PULSE t, = 1.3 psec

98

88

NO RECEIVER FILTERING 7e

68 c LL

sa 4e

30

I

\

= Simulation

--------e

I

e

I

I

I

I

258

:

!

I

[

I

Theoretical

# 1 1 1 1 1 1 1 1 1 1 1 1

see

758

1808

' 1 1 1 1

1250

Delay (nsec)

Fig.2-4.

Adaptive thresholding error vs. multipath del

-

2-8

i n g on t h e c h o i c e of r e f e r e n c e ) .

For t h e f o l l o w i n g , t h e t h r e s h o l d i s r e f e r r e d

t o t h e p r e s e n t p u l s e amplitude. A s t h e f o l l o w i n g formula i l l u s t r a t e s ,

adaptive thresholding induces a

somewhat d i f f e r e n t e r r o r behavior from t h a t s e e n p r e v i o u s l y :

'.

(2-4)

-

tr The major d i f f e r e n c e between t h i s and t h e p r e v i o u s formula i s t h e m u l t i p l i c a -

t i v e term i n T, i n d i c a t i n g t h a t as m u l t i p a t h d e l a y i n c r e a s e s from z e r o , e r r o r i n c r e a s e s from z e r o t o a maximum and s u b s e q u e n t l y d e c r e a s e s .

For example,

u s i n g a -20 dB t h r e s h o l d , t h e maximum e r r o r i s found t o occur a t a d e l a y e q u a l t o 35% of t h e risetime and t h e corresponding e r r o r i s roughly 0.28 pt,. b e h a v i o r can be e x p l a i n e d i n terms of

the processor operation.

m u l t i p a t h scales t h e l e a d i n g edge and peak i n p r o p o r t i o n , error.

For l o n g e r d e l a y s (e.g.,

Close-in

inducing l i t t l e

t h o s e i n t h e maximum e r r o r v i c i n i t y ) ,

peak i s d i s p l a c e d w h i l e t h e l e a d i n g edge i s r e l a t i v e l y c l e a n . i n c o r r e c t t h r e s h o l d s e t t i n g c a u s e s an e r r o r .

This

the

The r e s u l t i n g

As delay f u r t h e r increases, the

e r r o r does not f a l l o f f as f a s t as f o r t h e f i x e d t h r e s h o l d p r o c e s s o r due t o t h e r e s i d u a l e r r o r s i n peak amplitude. e x p o n e n t i a l terms i n ( 2 - 4 )

and (2-2).

T h i s can be s e e n by comparing t h e The dependence upon p and (b i s essen-

t i a l l y t h e same as f o r f i x e d t h r e s h o l d i n g , except now in-phase m u l t i p a t h t e n d s t o d e l a y t h e t h r e s h o l d c r o s s i n g and produce a p o s i t i v e e r r o r . F i g u r e 2-4 shows a n a l y t i c a l and s i m u l a t i o n r e s u l t s of e r r o r VS. d e l a y f o r The m u l t i p a t h and p r o c e s s o r parameters are t h e same

real t i m e t h r e s h o l d i n g .

as i n F i g . 2-3 ( f i x e d t h r e s h o l d i n g ) . 3.

Delay-and-Compare

R e c e i v e r s (DAC)

A block diagram of a delay-and-compare

p r o c e s s o r i s shown i n Fig.

2-5.

A r r i v a l t i m e i s d e t e c t e d by c o i n c i d e n c e of t h e l e a d i n g edges of t h e delayed and undelayed p u l s e s .

T h i s p r o c e s s o r i s self-AGC'd

with regard t o t h e thresh-

o l d s e t t i n g , s i n c e t h e comparator i n p u t s are s c a l e d r e p l i c a s .

2- 9

This f e a t u r e

I 117966-NI

*, Fig.2-5.

c

DME leading edge comparator circuit.

-.

-

2-10

provides a measure of l e v e l i n s e n s i t i v i t y similar t o a d a p t i v e t h r e s h o l d i n g . The combined choice of p r o c e s s o r g a i n ( G ) and comparator d e l a y ( T ~ )f i x e s t h e e f f e c t i v e t h r e s h o l d l e v e l by determining t h e p o i n t on t h e i n p u t waveform a t which t h e coincidence nominally occurs. yield equivalent thresholds.

Various combinations of G and Td

The g a i n r e q u i r e d t o a c h i e v e a t h r e s h o l d cross-

i n g v t r sec b e f o r e t h e waveform peak of a Gaussian p u l s e i s ( a s a f u n c t i o n of

Equation (2-6) below g i v e s Gaussian p u l s e s .

h e a r r i v a l t i m e e r r o r f o r a DAC p r o c e s s o r w i t h

It i s w r i t t e n i n a form which p l a c e s i n e v i d e n c e t h e h a l f -

width a t t h r e s h o l d parameter ( v ) used p r e v i o u s l y and t h e d e l a y Td; e x p l i c i t dependence on t h e g a i n has been suppressed through u s e of (2-5):

L

tr

2-11

I The above form a l l o w s d i r e c t comparison w i t h t h e p r e v i o u s e r r o k e x p r e s s i o n .

i s q u a l i t a t i v e l y i d e n t i c a l t o t h a t of a d a p t i v e l t h r e s h o l d i n g ,

The b e h a v i o r

d i f f e r i n g o n l y i n i t s d e t a i l e d dependence on t h e p r o c e s s i n g parameters.

The

e x p o n e n t i a l decay i s e s s e n t i a l l y t h e same as f o r f i x e d t h r e s h k l d i n g . , as i s I

more e a s i l y s e e n i n t h e f o l l o w i n g rewrite of (2-6):

I

! For

t o real t i m e

thresholding.

h a s a clear advantage over f i x e d t h r e s h o l d i n g

however, f o r

l a r g e multipath delays,

Delay-and-compare

small v a l u e s o f

both a r e s u p e r i o r

T.

A f u r t h e r advantage of DAC p r o c e s s i n g i s t h a t i t e f f e c t i v l y e l i m i n a t e s

a l l multipath

components

* delay .

comparator

whose

Thus,

by

delay

some s m a l l mu : i p l e

exceeds

d e c r e a s i n g 'd

at

fixed

of

the

equivaJently ,

gain

dropping t h e t h r e s h o l d ) , b e t t e r m u l t i p a t h immunity i s o b t a i n e d . A number of

carried

out

s t u d i e s of DAC p r o c e s s i n g f o r cos

[3,34,37,40-431.

-

cos2 pu: ;es have been ormula. f o r a

Appendix A d e r i v e s t h e e r r o r

c o s i n e l e a d i n g edge of d u r a t i o n D:

p s i n w T cos

where w =

n / 2 T and tc

T

+I

<

t

I

(2-8)

i s t h e DAC d e c i s i o n t i m e .

c

*A s i m i l a r s t a t e m e n t i s t r u e of f i x e d t h r e s h o l d i n g . The as umption of a Gaussian p u l s e d i s g u i s e s t h e s e f a c t s , s i n c e t h e l e a d i n g edge 'xtends i n f i n i t e l y i n t o the past. The r e s u l t s are easier t o see f o r a pu se t h a t rises from z e r o a m p l i t u d e , e.g., a cos-cos2 p u l s e .

2-12

r

Unfortunately,

eq.

(2-8)

is

not

appropriate

for

practical

implementations due t o t h e i n f l u e n c e of t h e r e c e i v e r IF f i l t e r . and 2-7

show DAC curves f o r Gaussian and cos

-

cos2 p u l s e s .

DME/P

F i g u r e s 2-6

2-7,

I n Fig.

we

see t h a t t h e r e g i o n of e r r o r s w i t h t h e I F considered i s approximately 50% g r e a t e r t h a n suggested by t h e p u l s e shape a l o n e .

However, t h e peak e r r o r is

Also n o t e t h a t t h e r e g i o n of n o t i c e a b l e e r r o r s l e n g t h e n s s l i g h t l y a t

reduced.

T h i s is due t o t h e I F f i l t e r o u t p u t s t a r t i n g o u t a t z e r o

h i g h M/D r a t i o s .

s l o p e as opposed t o a f i n i t e s l o p e (Fig. 2-8).

critical path delays f o r adaptive

The commentary concerning t h e most

t h r e s h o l d i n g a p p l i e s t o t h e p r e s e n t case as w e l l . vs.

The delay-and-compare

error

d e l a y curve peaks earlier t h a n t h e a d a p t i v e t h r e s h o l d i n g c u r v e , s o t h e

c r i t i c a l p a t h d i f f e r e n c e s l i e i n a somewhat smaller range. It is p o s s i b l e t o e x e r c i s e p a r t i a l c o n t r o l o v e r t h e DME m u l t i p a t h envi-

ronment by means o t h e r t h a n s i g n a l and r e c e i v e r t h r e s h o l d i n g c i r c u i t design. F i v e such means are now d i s c u s s e d :

antenna p a t t e r n s h a p i n g , motion averaging,

l a t e r a l d i v e r s i t y , uplink/downlink e r r o r a v e r a g i n g , and mismatched r e c e i v e r filtering.

C.

Transponder Antenna P a t t e r n Shaping

It is g e n e r a l l y assumed t h a t t h e DME t r a

near

the

coverage.

MLS The

azimuth pattern

ground of

antenna

the

will

and

w i l l be 1 c a t e d w i t h o r

ponde

ground-based

have

antenna

at

least

i s a key

the

same

factor

in

m a i n t a i n i n g s u f f i c i e n t l y high q u a l i t y range guidance throughout t h e coverage S i n c e t h e coverage is wide i n azimuth ( 4 0 ° ) , t h e azimuth p a t t e r n

volume.

cannot r o l l o f f a p p r e c i a b l y off c e n t e r l i n e without e x a g g e r a t i n g t h e problems of d i r e c t s i g n a l shadowing and m u l t i p a t h enhancement f o r a i r c r a f t on curved o r dog-leg

approaches.

However,

it

is c e r t a i n l y p o s s i b l e t o u s e a n azimuth

p a t t e r n w i t h c e n t e r l i n e emphasis such as t h a t shown i n F i g . proposed by H a z e l t i n e I l l ] .

A p a t t e r n such as Fig.

a r r a y phased t o y i e l d t h e d e s i r e d p a t t e r n . yield

a

degree

of

centerline

emphasis

2-9

which w a s

2-9 t y p i c a l l y r e q u i r e s a n

The u s e of a simple r e f l e c t o r can (Fig.

2-10).

The

range

accuracy

requirements f o r a n a i r c r a f t e x e c u t i n g a t e r m i n a l maneuver off c e n t e r l i n e are

2-13

GAUSSIAN PULSE

98

80

c

tr =

1.3 psec.

N O RECEIVER FILTERING

c LL

(L

0 (L

w ne

Delay (nsec)

Fig.2-6.

Delay-and-compare error vs. multipath delay

2-14

\

/

\

+= 0' Bw

1

I

I

I

10 0

&>-

do

/

Fig.2-7. Comparison of DAC error characteristic with and without IF filtering.

2-15

1.o

w

n

3

5

4a

5

PULSE TYPE

0.5

w

CHOPPED cos8

W

a

DATE Type A-1

cos28 0 1.o

0.5

0

RISETIME ( p s )

I

F i g . 2-8. (a) Leading edge comparison of v a r i o u s proposed p u l s e ! s h a p e s f o r a i r t o ground (from [ 1 2 ] ) and ( b ) l e a d i n g edge c h a r a c t e r i s t i c s of v a r i o u s proposed ground t o a i r p u l s e s h a p e s i n c l u d i n g I F f i l t e r e f f e c t (from [ 1 2 ] ) .

II

2-16

-

1117971-SI RKF

0

10

AN&€

20

w ~ r

I

1

30

10

W UW/Z&VtA&

I

'60

I

60 Cy ANI, O f M C S

1

I

70

a0 7COJl.i

Fig.2-9. Hazeltine AZ antenna azimuth pattern showing centerline emphasis.

ANGLE (deal e

Fig.2-10.

Measured horizontal pattern of ground antenna (from [ 3 7 ] ) .

2-17

I

I

c

Y

Fig.2-ll(a).

PALM dipole array.

2-18

lax

sufficiently

to

tolerate

a

decreased

signal

level.

The

reflection

m u l t i p a t h r i s k when o f f c e n t e r l i n e i s n o t i n c r e a s e d v e r y much by t h e u s e of c e n t e r l i n e emphasis s i n c e i t i s u n l i k e l y t h a t s i g n i f i c a n t scatterers w i l l l i e i n t h e emphasis region. I n e l e v a t i o n , t h e antenna p a t t e r n can be designed t o a m e l i o r a t e some of t h e r e f l e c t i o n m u l t i p a t h problems c i t e d earlier.

S p e c u l a r r e f l e c t i o n s from

f l a t o r t i l t e d t e r r a i n w i l l u s u a l l y i n t e r c e p t t h e t r a n s p o n d e r at n e g a t i v e o r low ( r e l a t i v e t o t h e g l i d e s l o p e ) e l e v a t i o n a n g l e s .

Each of t h e p r o c e s s o r s

d i s c u s s e d i n t h e p r e v i o u s s e c t i o n , e s p e c i a l l y f i x e d t h r e s h o l d , has a c e r t a i n s u s c e p t i b i l i t y t o such m u l t i p a t h and would b e n e f i t from low e l e v a t i o n c u t o f f i n the elevation pattern. rolloff

at

the horizon w a s

The d e s i g n of L-band the object

of

p a t t e r n s t o y i e l d a maximal

considerable

s t u d y i n t h e PALM

program [9]; Fig. 2-11 shows r e p r e s e n t a t i v e PALM p a t t e r n s .

D.

A i r c r a f t Antenna P a t t e r n Shaping

Onboard

the aircraft,

wide

azimuth coverage i s r e q u i r e d f o r guidance

d u r i n g complex p r e f i n a l maneuvers, s o t h a t l i t t l e m u l t i p a t h s u p p r e s s i o n can be a c h i e v e d by a i r b o r n e antenna azimuth shaping i f a s i n g l e a i r b o r n e antenna i s used f o r DME.

S i m i l a r l y , e x c e p t f o r t h e cases of t i l t e d t e r r a i n o r t i l t e d

scatters

as

(such

a r r i v a l angles at cannot

be

aircraft

fuselages)

the aircraft

below

the

aircraft,

the

multipath

are n o t widely d i s p e r s e d i n e l e v a t i o n and

s i g n i f i c a n t l y reduced by

the elevation pattern

of

the aircraft

antenna.

E.

Motion Averaging

Like t h e MLS a n g l e f u n c t i o n s , benefit

from motion a v e r a g i n g .

t h e DME i s a multi-scan

system and can

Since t h e i n t e r r o g a t i o n rate

( 4 0 Hz) i s

t y p i c a l l y e i g h t times t h e MLS d a t a r a t e ( 5 Hz), e i g h t r e p l i e s can be averaged

p e r o u t p u t range r e a d i n g .

The a v e r a g i n g improvement v a r i e s w i t h s c a l l o p i n g

frequency i n a manner dependent upon t h e s i n g l e - s c a n phase f u n c t i o n .

e r r o r vs r e l a t i v e r f

A l l t h e e r r o r formulas p r e s e n t e d above show t h e e r r o r i s

p r o p o r t i o n a l t o cos 4 , where Q i s t h e d i f f e r e n t i a l phase a n g l e between d i r e c t

2-19

,

0

-5 -1

m -10 73

v

Z a (3

W

> ta

-15 -20 -25

-J W -30

K

-35 -40 - 12

-8

0 4 ELEVATION ANGLE k k g ) -4

8

' I I

Fig.2-ll(b).

Patterns of five PALM elements.

2-20

12

and

multipath.

Differential phase

advances by

an amount w,T

between

interrogations separated by T sec when the scalloping frequency is w s rad/sec, and the consequent reduction in error obtained by averaging M consecutive scans is given by the averaging factor [81

sin A(ws)

=

&ST 2

M sin

2

Because the phase behavior of DME error is not truly sinusoidal, it is more conservative to assume that the actual error improvement is the maximum of (29) and

l/dE (the improvement to be expected

from averaging independent

errors).

This function is plotted vs. fs(ws/2n)

in Fig. 2-12 for M=8 and an

interrogation rate of 40 Hz.

The grating lobes (points of poor motion

averaging) are at multiples of 40 Hz, and the points of maximum improvement are at multiples of 5 Hz which are not harmonics of 40 Hz.

In Section IV, the

effects of this type of motion averaging upon DME/P sensitivity to aircraft and vehicle reflections are studied. An informative way to look at motion averaging results is from relating the scalloping frequency to the multipath geometry.

For an aircraft on

centerline approach, multipath arriving at conical angle p (relative to the

A/C heading) has scalloping frequency [ 91 V

fs

where v

=-

x

=

(1

-

cos R )

A/C speed and X

(2-10) =

wavelength.

If this expression is substituted

into (2-91, the averaging factor can be graphed in a polar plot showing averaging improvement as a function of arrival angle.

The scalloping fre-

quency formula inverts to give -1 p = cos (1

2Xfs

-*

- . V

XfS - 7)

,

(2-1 1)

small p

2-21

iI I I

~

I

i

I I

-

I

1 .oo

111j95Gq

= Uniform spacing = J i t t e r e d spacing

I

---__-__

-.-

= Improvement from

averaging 8 random

0.75

I

c, S Q)

5>

c

0

L

n E

'\

0.50

'

cn

\

I

c

.r

VI I8

L

\

I

\

1

\

Q)

>

I

U

./ I

0.35

I

I - 1 I

0.25

0 0

20

40

60

i 8o

100

I

Scal 1 oping Frequency (Hz)

Fig.2-12.

Averaging f a c t o r f o r uniform and j i t t e r e d spacing (TRSB)l

.

.-

2-22

I

The a n g l e s a t which g r e a t e s t a v e r a g i n g improvement can be expected are t h o s e where f s = n/MT, n L-band.

Figure

grating lobe,

on

The f i r s t g r a t i n g l o b e a n g l e i s 36.9'

m u l t i p l e of M.

2-13

shows

these angular directions.

the scalloping persistance

Outside

at

the f i r s t

[81 may be t o o s h o r t f o r c o h e r e n t

Denoting by Y t h e d i s t a n c e normal t o c e n t e r l i n e t o t h e s p e c u l a r

averaging. point

f

the

scatterer,

motion

averaging

becomes

at

ineffective

ranges

(measured from t h e c e n t e r l i n e p r o j e c t i o n of t h e s p e c u l a r p o i n t ) 2vT x

R > > Y

A t L-band,

(2-12)

2vT/X = 3.16 f o r o u r example.

Further

improvement

can

be

( j i t t e r e d ) i n t e r r o g a t i o n sequence. Phase I1 EL-1

obtained

using

a

non-unif ormly

spaced

T h i s p r i n c i p l e w a s demonstrated i n t h e MLS

tests, and f o r t h e ICAO submission a more e l a b o r a t e j i t t e r

sequence w a s d e v i s e d which has b e t t e r a v e r a g i n g improvement t h a n t h e Phase I1 sequence.

F i g u r e 2-12 shows t h e a v e r a g i n g f a c t o r f o r one phase of t h e l a t t e r

j i t t e r sequence, a g a i n f o r 8 p u l s e s .

Although j i t t e r e d p u l s e sequences have

n o t been proposed f o r t h e MLS DME, i t can be s e e n by comparing t h e two c u r v e s i n Fig. 2-12 t h a t j i t t e r can s u b s t a n t i a l l y reduce t h e e r r o r i n t h e v i c i n i t y of t h e g r a t i n g l o b e s ( m u l t i p l e s of 1 / T ,

e.g.,

m u l t i p l e s of 40 Hz f o r t h e cases

cited).

F.

Lateral D i v e r s i t y

Another means of o b t a i n i n g a phase change between t h e d i r e c t and r e f l e c t e d s i g n a l s i s t o u t i l i z e s p a t i a l d i v e r s i t y a t t h e ground s t a t i o n . i l l u s t r a t e s the principle.

C.

F i g u r e 2-14

Two modes of o p e r a t i o n have been s u g g e s t e d [32]:

(a)

simultaneous a v e r a g i n g of t h e r e c e i v e d i n t e r r o g a t i o n by N a n t e n n a s and r e c e i v e r s , whereby N p u l s e r e c e p t i o n times s are averaged t o y i e l d a s t a r t i n g p o i n t f o r t h e t r a n s p o n d e r d e l a y c i r c u i t , and

(b)

s e q u e n t i a l r e c e p t i o n / t r a n s m i s s i o n , where t h e o r d i n a r y DMF, t r a n s p o n d e r i s connected t o a n a r r a y of N a n t e n n a s by a s i n g l e p o l e N throw switch. I n t h i s case, t h e a v e r a g i n g

2-23

First I.-Band Grating Lobe

'\

'.

L-band

D i r e c t Path

- i!

V

Fig. 2-13. Multipath arrival directions for maximum motion averaging and first grating lobe.

i

c

2-24

N Antennas

.

(ilalNl AVERAGING Receiver - Multip[e

Time Marks ti Aver a g in g

N Antennas

Switch

n

AIRIBRNE

AVERAGING

Transponder

Fig.2-14.

Switching

Antennas are roil d om L y

Lateral diversity DME/P transponder.

2-25

DME, Ci rcula r Array Pulse : cos2’COS 2 # tr= 1.5 ha

p$ I

Trigger: DAC, 25 %/SOX A Antenna: 23 Elements (Exarnplt:) Diameter 3.5 A I

WD: ‘t

I

0.5

: 0.4

P

HT-Phasc! at rcfcrencc p o i n t :

‘y

= 0

or

~

y = 180’

Fig. 2-15. DME/P multipath error reduction via ground aveiaging lateral diversity (from [ 3 2 ] ) .

2-26

I

over phase processing.

changes

is

obtained

by

the

airborne

data

Method ( a ) has t h e advantage of reducing t h e downlink e r r o r s i g n i f i c a n t l y on each i n t e r r o g a t i o n , as shown i n Fig.

.

2-15.

reduce t h e r e s i d u a l e r r o r y e t f u r t h e r .

Moreover, motion a v e r a g i n g may

However, t h e c o s t i s h i g h , and u p l i n k

Method ( b ) has a much smaller c o s t impact and does

e r r o r s are n o t a f f e c t e d .

reduce u p l i n k e r r o r s as w e l l .

However, i t :

(1)

does not y i e l d as s i g n i f i c a n t a n e r r o r r e d u c t i o n on s i n g l e i n t e r r o g a t i o n and

(2)

a c t s i n p a r a l l e l w i t h motion averaging.

One s i g n i f i c a n t d i f f i c u l t y with both l a t e r a l d i v e r s i t y modes (and, motion a v e r a g i n g ) i s t h a t t h e f r a c t i o n a l e r r o r r e d u c t i o n d e c r e a s e s w i t h i n c r e a s e d M/D l e v e l (p), since the bias e r r o r typically is proportional t o p 2 Consequent-

.

ly, i n s i t u a t i o n s where improvement i s most needed, t h e amount of improvement decreases.

G.

Uplink/Downlink E r r o r Combining

The a i r c r a f t i n t e r r o g a t i o n of a ground t r a n s p o n d e r and t h e ground t r a n s ponder r e p l y are a t d i f f e r e n t carrier f r e q u e n c i e s which t y p i c a l l y d i f f e r by 6 3

MHz.

Consequently, t h e r e l a t i v e phase,

Q

,

of t h e m u l t i p a t h s i g n a l on t h e

u p l i n k w i l l d i f f e r from t h a t on t h e downlink by

AQ

where

4

=

271 (Af

Af =

tr =

T

+

fs

t

>

uplink/down l i n k channel frequency o f f s e t ( t y p i c a l l y 6 3 MHz). t i m e i n t e r v a l between t r a n s m i t t i n g t h e i n t e r r o g a t i o n and r e c e i v i n g a r e p l y .

.+

= 50 ns

time

2 x a i r c r a f t t o ground p r o p a g a t i o n

~,

2-27

(2-13)

I F CENTRAL FREQUENCY

-2

-3

0

-1

+1

+2

+3 RELATIVE

FREQUENCY

(a)

'(UHZ) I

I

..

-40

.. -50

I

I

I

.. AMPLITUDE

1

(dB)

I

I F CENTRAL

ji

FREQI. ENCY -3

-.

-2

-1

+1

0

+2

+3

!

REqATIVE FREQUENCY (~MHZ )

I

II

-

20

Fig.2-16. (a) Characteristic of the CTOL/STOL system receiver Eilter and (b) characteristic of the VTOL system receiver filter.

2-28

.

The

error

r e d u c t i o n due t o

situations. the

M/D

the

t r term i s n e g l i b l e

fs

i n virtually all

Consequently, we may f o c u s o u r a t t e n t i o n on t h e

ratio

is

such

that

e r r o r s are

the

proportional

AfT term.

If

,

the

to

cos 4

r e s u l t i n g d i s t a n c e e r r o r w i l l be changed by a f a c t o r B (T) =

.

27rAf-r

COS

(2-14)

o v e r t h e one way e r r o r a t a frequency midway between t h e u p l i n k and downlink T h i s uplink/downlink e r r o r f a c t o r i s p e r i o d i c i n T a t i n t e r v a l s

frequencies. of

16 ns.

l/Af =

multiples

of

Since p r a c t i c a l multipath delays

t h i s period,

are g e n e r a l l y l a r g e

i t i s r e a s o n a b l e t o assume t h a t uplink/downlink

e r r o r combining w i l l on t h e a v e r a g e d e c r e a s e t h e rms range e r r o r by

4 2 over

t h a t f o r t h e one way p u l s e r e c e p t i o n .

H.

Receiver Mismatched I F F i l t e r i n g

The problem of p u l s e a r r i v a l t i m e e s t i m a t i o n i n Gaussian n o i s e a l o n e h a s been a s t a n d a r d r a d a r problem f o r y e a r s

[33,35].

The r e c e i v e d s i g n a l is

passed through a matched f i l t e r and t h e range i s e s t i m a t e d from t h e c e n t r o i d of t h e peak f i l t e r o u t p u t . However, t h i s procedure i s

not o p t i m a l

i n t h e presence of m u l t i p a t h .

s t a n d a r d approach t o DME m u l t i p a t h m i t i g a t i o n h a s been t o use one of

The the

l e a d i n g edge t h r e s h o l d i n g methods t o g e t h e r w i t h a f i l t e r which i s more o r less matched t o t h e l e a d i n g edge of t h e p u l s e , e.g.

,a

B u t t e r w o r t h I F f i l t e r whose

bandwidth i s approximately t h e r e c i p r o c a l of t h e l e a d i n g edge e q u i v a l e n t rise time

.

R e c e n t l y , i t has been suggested by M. Gori of I t a l y [43] t h a t y e t f u r t h e r improvement i n DME m u l t i p a t h performance can be achieved by u s i n g a r e c e i v e r f i l t e r which i s s i g n i f i c a n t l y mismatched w i t h respect t o t h e r e c e i v e d p u l s e waveform.

F i g u r e 2-16

shows r e p r e s e n t a t i v e mismatched f i l t e r s s u g g e s t e d f o r

CTOL and V/STOL a p p l i c a t i o n s .

The f i l t e r emphasis a t f r e q u e n c i e s o f f c e n t e r

frequency i s i n t e n d e d t o y i e l d a f a s t e r p u l s e risetime a t t h e f i l t e r o u t p u t and hence improved m u l t i p a t h r e j e c t i o n . and

output

waveforms

for

a cos-cosL

2-29

F i g u r e 2-17

2 ns

shows t h e I F f i l t e r i n p u t

risetime p u l s e w i t h mismatched

I

Fig.2-17. Experimental f i r s t d e f i n e d p u l s e s (from [ 4 3 ] ) (a) t r a n s m i t t e d and ( b ) r e c e i v e d .

filter.

The rise t i m e of t h e p u l s e a t t h e mismatched o u t p u t f i l t e r i s f a s t e r

t h a n t h a t f o r a matched f i l t e r . On t h e o t h e r hand,

T h i s y i e l d s improved m u l t i p a t h performance.

t h e mismatch does a d v e r s e l y e f f e c t performance a g a i n s t

r e c e i v e r n o i s e a n d / o r a d j a c e n t channel i n t e r f e r e n c e . One d i f f i c u l t y a s s o c i a t e d w i t h t h e u s e of a mismatched f i l t e r i s t h e need

..

t o u t i l i z e i t on a l l a i r b o r n e DME/P r e c e i v e r s i f i t i s f e l t t h a t t h e improved performance on t h e i n t e r r o g a t o r r e p l y w i l l be r e q u i r e d a t some a i r p o r t runway. 4

By

contrast,

some

of

the

other

multipath

reduction

techniques

such

as

c e n t e r l i n e emphasis need o n l y be i n c o r p o r a t e d i n t h e ground equipment f o r "d if f i cu 1t " s i t e s

.

2-31

I

,

DME/P MULTIPATH SOURCES AND CHARACTERISTICS

111.

F i g u r e 3-1

shows t h e p r i n c i p a l m u l t i p a t h s o u r c e s c o n s i d e r e d t o d a t e i n

Our o b j e c t i v e i n t h i s c h a p t e r i s t o a s c e r t a i n which s o u r c e s are

MLS s t u d i e s .

of g r e a t e s t concern f o r DME/P o p e r a t i o n and t o bound t h e expected m u l t i p a t h effects. The two p r i n c i p a l parameters of concern h e r e are t h e l e v e l ( p ) and t i m e d e l a y (T).

The t i m e d e l a y is s t r i c t l y a f u n c t i o n of scatterer l o c a t i o n , and

can t h u s be c h a r a c t e r i z e d g e o m e t r i c a l l y by e l l i p s e s of c o n s t a n t d e l a y such as were shown i n F i g . 1-2.

The peak r e l a t i v e m u l t i p a t h l e v e l is more complicated

s i n c e i t r e p r e s e n t s t h e product of s e v e r a l f a c t o r s :

(3-1)

Req P R P r P sec 'div

= 'size

where Psize

-

size factor

=

e q u i v a l e n t F r e s n e l r e f l e c t i o n c o e f f i c i e n t which t a k e s i n t o account t h e s u r f a c e d i e l e c t r i c and c o n d u c t i v i t y

PR

=

distance factor distance)

P r

=

s u r f a c e roughness f a c t o r ( a l s o , mode loss f a c t o r i n case of p e r i o d i c a l l y corrugated s u r f a c e s )

P sec

=

f a c t o r corresponding t o v a r i o u s secondary p a t h s i n v o l v i n g t e r r a i n r e f l e c t i o n s as i l l u s t r a t e d i n F i g . 3-2.

div

=

d i v e r g e n c e f a c t o r f o r curved s u r f a c e s

Req

S i n c e t h e v a r i o u s f a c t o r s i n (3-1)

(=

direct

path

distance/multipath path

are a s t r o n g f u n c t i o n of scatterer t y p e ,

one must c o n s i d e r each p a r t i c u l a r s c a t t e r e r shown i n Fig. 3-1 s e p a r a t e l y .

A.

R e f l e c t i o n s from T e r r a i n i n F r o n t of Transponder Array

The p r i n c i p a l e f f e c t of

these reflections

i s t o change t h e e f f e c t i v e

d i r e c t s i g n a l l e v e l s i n c e t h e m u l t i p a t h d e l a y t u r n s o u t t o be s o small t h a t t h e two p u l s e s completely o v e r l a p .

The e f f e c t i v e d i r e c t s i g n a l i s g i v e n by:

3 -1

3- 2

a W

I

rr! h W

0

a

cn clr

m

1118029-51

Race I ve r

Tranrmi t t e r

Measured "DIRECT" s i q n a l level depends on paths a and ag Measured "MULTIPATH" s i g n a l l w e l depends on paths b. bg, c . c g

(a) Signal paths considered in "naive" DME multipath analysis Fig.3-2. and (b) role of ground reflections in determining DME multipath/direct amplitude ratio.

3- 3

I

I

I

jkr

1

I

(i3-2) I

i I

where rl i s t h e d i r e c t p a t h l e n g t h , r 2 t h e ground r e f l e c t e d p a t h l e n g t h ; k , t h e wavenumber e q u a l s 2n/X, where X i s t h e wavelength;

is the effective pg t h e r f phase change due t o r e f l e c t i o n and

terrain reflection coefficient, G(Bi,

Ei)

t h e antenna ( v o l t a g e ) g a i n a t azimuth B i and e l e v a t i o n Ei.

-.I

I

Using t h e s t a n d a r d approximations we can show t h a t t h e r e c e i v e d pbwer i s : N

2

0

Pr =

+ Po

2 (471 r l >

+

2POCOS

(

2rA r x + +)I

(3-3

1

I

where p o i s t h e e f f e c t i v e r e f l e c t i o n c o e f f i c i e n t [pG(02,@2)/G(Bl,@l)] and i s t h e range d i f f e r e n c e ( r 2 - r l ) .

It i s e a s i l y shown t h a t

!ir

I I

A r =

CT

=

22 z 1 2

~

I

1:

I I

1

where z2 i s t h e t r a n s m i t t e r h e i g h t and z1 t h e r e c e i v e r h e i g h t .

The term Po is

t h e power of t h e t r a n s m i t t e r .

The term i n t h e b r a c k e t s i s u s u a l l y tekned t h e

p a t t e r n propagation f a c t o r

When z1 and z2

.



.

N

m

+ F-

.

m

x

@

.

m W

4-37

>

> >

m

m

u

>

v-\ sr

cp ?

R

CI

; (0

t" 0

SAN FRANCISCO BAY

M L S Elevation Site

1-

200

Fig.4-27.

0

200

-- +--.x=--j

400

603 Scale in Feet

800

AWOP s c e n a r i o 3 based o n C r i s s e y Army A i r f i e l d i n San F r a n c i s c o .

1000

m u l t i p a t h w i t h r e l a t i v e d e l a y s g r e a t e r t h a n 400 n s e c , we see t h a t s c e n a r i o 2 would n o t be a s t r i n g e n t t e s t of t h e optimized d e s i g n . S c e n a r i o 3 shown i n Fig.

.

Base near San F r a n c i s c o .

w a s based on t h e geometry a t C r i s s e y Army

4-27

T h i s STOL s c e n a r i o has h i g h l e v e l b u i l d i n g m u l t i p a t h

w i t h s h o r t r e l a t i v e d e l a y s ( s e e Fig.

as w e l l as low s c a l l o p i n g rates.

4-28)

T h i s combination of m u l t i p a t h c h a r a c t e r i s t i c s i s such t h a t m u l t i p a t h e r r o r s can be expected f o r even a n optimized DME/P design. F i g u r e 4-29

shows t h e computed e r r o r s computed by DFVLR[42] f o r t h i s

s c e n a r i o w i t h t h e f o l l o w i n g DME/P system implementation:

-

c o s 2 w i t h 10% - 90% risetime of 800 ns

pulse:

cos

t h r e s h o l d i n g:

delay-and-compare delay

ground antenna:

4 f t . v e r t i c a l aperture with a pattern rolloff of approximately 3 dB/deg a t t h e h o r i z o n [ 9 ]

height:

6 feet.

w i t h g a i n of 2 and 100 ns

A comparison of t h e e r r o r trace w i t h t h e m u l t i p a t h d i a g n o s t i c p l o t shows t h a t

t h e peak e r r o r o c c u r s where t h e B 1 m u l t i p a t h is l a r g e s t (M/D = 0 dB, T = 110 nsec).

Equation (A-35)

p r e d i c t s a maximum e r r o r of

p o i n t ; t h e e r r o r trace i n d i c a t e s a 50 f o o t e r r o r .

about 40 f e e t a t t h i s

Motion a v e r a g i n g h e r e w a s

e f f e c t i v e only i n t h e v i c i n i t y of runway t h r e s h o l d . F i g u r e 4-30

shows a DME/P

m u l t i p a t h s c e n a r i o which w a s developed by

t r a n s l a t i n g t h e l o c a t i o n s of s e v e r a l b u i l d i n g s a t Heathrow A i r p o r t down t h e runway and i n c r e a s i n g t h e b u i l d i n g h e i g h t s locations

and

heights

assumed

for

this

*.

Table 4-1

scenario.

e f f e c t i v e M/D r a t i o and t i m e d e l a y s w h i l e Fig. 4-32 by DFVLR[42].

*

shows t h e b u i l d i n g

Figure

4-31

shows t h e

shows t h e e r r o r s computed

The e r r o r s h e r e are low due t o t h e r e l a t i v e l y long t i m e d e l a y s .

DME/P m u l t i p a t h l e v e l s from t h e s e a c t u a l b u i l d i n g s w i l l be q u i t e low due t o t h e i r l o c a t i o n and low h e i g h t s .

4-40

i

m 0

cd

$4

-4

rn

5U

7

c

(YD

E

11 b

4-41

I I

I

1

I

I

4-42

I

I

1

I

,

I *

I

I I

I

I

I

I

4-43

0 E I

0 0

:I

W 0

* 0

0

OnE ERROR

4-44

til)

0

F

t

L

. I

1

w

? w In a0 P

I 1

8

I

I

B DtlE

I

I 0

ERROR

4-45

I

(ll)

I

I

I

I

I

P

0

I

E

a I

0 0 Y

x

I

?

A

i+ 3 I cE

4-46

b

1

TARLE 4-1 R U I L D I N C : AND AIRCRAFT LOCATIONS FOR PRECISION PULSE CTOL SCENARIO

Building

C e n t e r of S p e c u l a r Region x Coordinate ( f t )

5

R u 1. rl i ng Lo c a t 1 on s

12000

83

1 4 600

100

15100

5n

16100

5n

19000

(963 9,836)

( 9 8 0 ? , 83 6 )

A i r c r a f t Locations

Nose Location 6 5

Height ( f t)

(5500,492) (6400,410)

Orientation Paral le1 t o Runwav Paral 1 e l t o Runwav

O r i g i n o f Azimuth T r a n s m i t t c r i s a t (n,n,5) DME i s c o l o c a t e d w i t h Azimuth T r a n s m l t t e r a n d t s a t (O,n,30)

DME Height ( P h a s e C e n t r e ) i s 70 f r .

A i r c r a f t "Touchdown" i s a t 11,000 f t . F l i g h t Path: L i n e a r from f 2 1 0 0 0 , n , 4 1 0 ) t o (I?ROC),n,5n) L i n e a r from (12800,n,50) t o ( l l n 0 0 , 0 , P ) F l a t Terrain: (e/eo = 3)

Building S u r f a c e R e f l e c t i o n C o e f f i c i e n t a s i n AlJOP GIG-A (-1 dB f o r cases shown)

4-47

4n

B

I - 4

8,

LL

I

-9

I

I

I

0

C

I

L

L?

0 IMi

'

4- 48

1

WU

'

I

jfl

I

I

i

9

.

r

a

4

c

9

C

o

4-49

1

c

0

u ..

(d $4

t i 4U I

*

0

0

i

n W 0

DnE ERROR

4-50

I

I I

I

I

I

I

I

I

I

53 0 0 Y I

x

.

I

I

I

I

I

I

!

4-51

P

n

v

I

m

N

U

I

8

onE

8

I I

I I

I I

I I

I

0

ERROR

4-52

tn)

8

8

.

t’

It should be emphasized t h a t both of t h e s e s c e n a r i o s were contrived t o be

q u i t e s t r e s s f u l t o DME/P.

However, the f a c t that f a i r l y good performance was

obtained i n both c a s e s with the "nominal" DME/P design i s encouraging.

4

4-53

5

a

V.

EXPERIMENTAL STUDIES BY LINCOLN LABORATORY The analysis in Chapter I11 and simulation studies of Chapter IV indicate

that specular reflections from buildings are an important challenge to successful DME/P operation, and, that there is some uncertainty regarding the reflection levels from irregular terrain and/or small scatterers at the airport.

* Laboratory

In this which

preceding chapters.

section, we

summarize experimental work

at Lincoln

substantiates the principal factors identified in the The first of the factors concerns the lobing in the

direct level as the interrogator nears the threshold.

Quantitative L-band

data related to this issue was obtained in the context of airport surface traffic control (ASTC) studies and is reported in Section A.

Section B

describes the results of building reflection measurements made at some six US airports

to

(partially)

validate

the computer simulations.

Section C

discusses the results of S-band ( 3 GHz) high time resolution measurements at L. G. Hanscom airfield to assess the reflection levels from a nominally flat airport runway environment while section D considers multipath from irregular terrain.

A.

ASTC Measurements of Direct Signal Lobing

The classical flat earth propagation model presented in Chapter I11 has been used for the power budget calculation in the bulk of the PDME proposals (e.g.,

[ 3 , 12, 1 4 1 ) to date.

undulations which

Nevertheless, most airport runways have various

potentially

could

yield

a

received

interrogator height which is rather different from the

signal level vs. 2

h R-

4

dependence

indicated in Chapter 111. As an outgrowth of studies at Lincoln Laboratory on

* It

had originally been hoped that a more substantive quantitative multipath data base would be available given the wide spread use of DME. However, it appears that the current DME has not been used for precision approach and landing guidance, and (as noted earlier) the DME/P tests to date have not been conducted at sites with substantial building multipath. Appendix B discusses related experimental work by others.

5-1

6

-11

ANTENNA UP

-10 n

m E -151 .

E

n -301 W

-451

-50 -55

-

-60

-

1

-65 I 100

1

200

I

I

400

'I

1

600

I

r-l

1

I

1000

2000

r

, 4000

1

I

l

l

,

10.000

RANGE (ft) Fig.5-1. Measured s i g n a l l e v e l a l o n g Hanscom taxiway w i t h b o t h a n t e n n a s a t 5 f e e t , d i p o l e p o i n t e d down (from McGarty [ 2 2 ] ) .

5- 2

the

feasibility

of

achieving

airport

surface

traffic

control with

an

ATCRBS/DABS based system, measurements of the received signal power were made along several taxiways at the L.G.

Hanscom Ahport (Bedford, MA) at various

transmitter-receiver geometries [ 2 2 ] . such measurement.

Figure 5-1

shows the results of one

We see that the measured levels at two different heights

fit the Rm4 law quite well over a distance range of several octaves. Measurements at various receiver heights for fixed ranges showed similar good agreement. B.

Summary Results of L-Band Airport Measurements

In this section, we summarize the principal results of measurements accomplished at a number of US airports to quantify the L-band multipath environment.

The program objectives were:

1)

measurement of the principal multipath parameters (amplitude and time delay) with realistic aircraft/ ground site locations at runways which had the major DME/P multipath sources (large buildings) identified in previous analytical (simulation) studies.

2)

determination of whether significant DME/P multipath sources exist which had not been considered to date in the DME/P studies to date, and

3)

comparison of the measured results with computer simulation results obtained with simplified airport models (such as have been used for DME/P system design to date).

The measurements placed particular emphasis on the final approach region including the flare and rollout regions since these areas correspond to the most

stringent DME/P accuracy requirements and, have not been utilized

operationally with the current L band DME. Measurements

were

made

at

five

major

operational

US

airports

(Philadelphia, Washington National, Wright Patterson AFB, St. Louis, and Tulsa) as well as a preliminary test at Quonset Point, RI.

5- 3

Since these

11 18646-S

I

Reply pulse width

- 100 nsec

Received amolitude digitized (8bits) every 50 nsec

Aircraft range also recorded Fig.5-2.

DME/P multipath measurement system.

c

5-4

measurements are described in depth in a companion report [ 4 6 1 , the discussion here attempts to provide an example of the results obtained at each airport and then summarizes the remaining data. A highly mobile equipment was desired which could measure the multipath parameters of greatest interest. narrow (100

nsec

-

200

This was accomplished by transmitting a

nsec wide) L-band

pulse from an aircraft and

(digitally) recording the received signal envelope at a ground antenna as a function of time as shown in Figs. 5-2 and 5-3.

By examination of the

digitized envelope, it was then possible to determine the pertinent multipath characteristics (amplitude and time delay relative to the direct signal level) on a given signal reception.

The aircraft transmitted signals at a 10 Hz

rate, corresponding to approximately 18 feet of aircraft displacement between successive measurements. multipath

environment

This relatively dense spatial sampling of the

allowed

us

to

use

correlation

between

adjacent

measurements to reject erroneous data due to cochannel interference and/or low signal to noise ratio (SNR). Aircraft

range information was obtained by having the narrow pulse

transmission times controlled by a standard Air Traffic Control Radar Beacon (ATCRBS) transponder which was being interrogated by the ground measurement system.

In this way, the delay time between the interrogation and received

ATCRBS reply yielded the aircraft range.

The flight profiles were such (e.g.,

centerline approaches using an ILS localizer to furnish vertical position) that knowledge of the range generally would permit one to determine the aircraft position. Each (digitized) received waveform was examined to locate discrete pulses according

to

criteria based

on pulse width

and

magnitude.

Co channe1

interference due to asynchronous replies from other ATCRBS transponders was rejected by measurement of the pulse width.

The reduced pulse parameter data

were then displayed in plots of multipath level and time delay versus distance along the flight path.

By considering the nature of adjacent multipath

environment estimates and repeatability of the phenomena between successive

5-5

5-6

8

? ?

3 9 N

In

? t

00

t

w

? ? 8

v

?

&!

% a !

y

? S e>

N

?8 00

N

? v

N Q

si N

2

?

?

r(

N

? m

? t

0 8

V

-

't

(nominally) identical runs, it was possible to identify questionable data which required hand analysis of thewaveforms. 1.

Washington National Airport (DCA)

Figure 5-4 shows the airport geometry at DCA. 3"

Measurements were made on

and 6" approaches to runway 15 since the runway is to be used for STOL

operations with a small community MLS.

The principal multipath identified in

the preliminary airport survey was the row of hangars (hangars 9-12) the runway from the measurement van.

The hangar fronts were largely smooth

metal with a height of 20 m ( 6 0 feet). Figures 5-5

and 5-6

across

*

show results of analyzing

the received waveforms

*The

principal focus for the automated analysis was specular reflections which are manifested by large pulses which are well separated from the direct signal as shown in Fig. 5-3. The criteria used to identify the pulses were: 2.

the peak amplitude should correspond to an SNR of at least +10 dB or, the minimum M/D ratio of concern (typically -20 dB)

3.

the pulse width between -6 dB points should lie in the interval (W -50 psec, W +lo0 psec) where W = expected pulse width in psec. (W = 15011s for the narrowest pulses used)

The first pulse encountered in the digitized time interval which meets the above criteria was assumed to be the direct signal. The peak level of the pulse is taken to be the direct signal amplitude and the direct signal amplitude and the point midway between the first leading and trailing edge digitized amplitudes which are at least 6 dB down from the peak level is taken to be the centroid. If no pulses meeting the above criteria were encountered in the digitized waveform, an "M" is placed on the M/D summary plot at the -25 dB M/D level and no symbol is placed in the corresponding time delay (T) plot. If only a "direct" pulse is encountered, an "X" is plotted at -20 dB on the M/D plot with no corresponding symbol on the T plot. Any additional pulses meeting criteria (1) and ( 2 ) are assumed to be multipath. Their peak amplitude and centroid are computed as for the direct signal. The displayed M/D ratio represents the ratio of peak amplitudes while (con' t )

.

5- 7

5-8

UJ

*

a

c

n.

80.

X

rs-

xrx

0.

x

X

!2

P

X

-I0.

-n.

m

-n.

x

Fig.5-5(a).

Summary r e s u l t s f o r 3" approach t o DCA b e f o r e t h r e s h o l d .

5- 9

I L

i

V

I

l-*ln sa.

I

?

Fig.5-5(b).

Summary results f o r 3' approach t o DCA b e f o r e t h r e s h t i l d .

5- 10

t a.

--

L

xx

r

*nun

4

,

U'

n u n

I-

IC n-n

/&

I

I

*In-

K" I . .

i

i

n

V

U

Fig.5-5(c).

Summary r e s u l t s f o r 3' approach t o DCA n e a r t h r e s h o l d .

5- 11

x

I

a

x

I X

x x

mm

I

Y

I

Fig.5-6.

X I

I X X

x1x

XXSXX

x

X

x

X

I x

I X *

xxxux x x l x x x X I * x a x I

x

xIOIxx

I

x

I

x x

Summary r e s u l t s for DCA over runway.

5- 1 2

.XI

. I

u I

( s u c h as shown i n Fig.

5-3) t o determine t h e r e l a t i v e m u l t i p a t h l e v e l s and

F i g u r e 5-7

t i m e delays.

shows a r e p r e s e n t a t i v e s i m u l a t i o n which took i n t o

account t h e n o n v e r t i c a l i t y of t h e hangar w a l l above t h e doors. The measured and s i m u l a t i o n l e v e l s i n c r e a s e r a p i d l y n e a r 1.5 nmi from t h r e s h o l d ; however, t h e measured M/D r a t i o v a l u e s range from -10 dB t o 0 dB, whereas t h e s i m u l a t i o n l e v e l s a r e c l o s e r t o -27

dB.

s h a r p l y and t h e n i n c r e a s e t o n e a r 0 dB near t h r e s h o l d .

Both l e v e l s d e c r e a s e Sizable oscillations

i n t h e M/D l e v e l s are e v i d e n t on e i t h e r s i d e of t h r e s h o l d ( j o y s t i c k range =

0.83 nmi).

T h i s r e f l e c t s t h e i n f l u e n c e of m u l t i p a t h from d i f f e r e n t b u i l d i n g s

as w e l l as o s c i l l a t i o n s i n t h e m u l t i p a t h l e v e l from i n d i v i d u a l scatterers as w i l l be d i s c u s s e d i n t h e s e c t i o n on s i m u l a t i o n r e s u l t s .

For t h e most p a r t ,

t h e m u l t i p a t h d e l a y s i n t h i s r e g i o n are t i g h t l y grouped i n t h e 700 n s e c

-

1100

nsec r e g i o n p r e d i c t e d by r a y t r a c i n g c o n s i d e r a t i o n s . The f a i r l y h i g h l e v e l (-8 dB t o 0 dB M/D r a t i o ) 800 ns d e l a y m u l t i p a t h measured near 3.0 nmi from t h r e s h o l d cannot be e x p l a i n e d by t h e simple a i r p o r t model.

The a i r c r a f t x-y l o c a t i o n h e r e i s a t t h e edge of t h e s p e c u l a r r e g i o n

f o r t h e North Hangar complex, but t h e e l e v a t i o n a n g l e of t h e a i r c r a f t i s f a r i n e x c e s s of t h e a n g l e subtended by t h e lower l e v e l b u i l d i n g s (e.g.,

general

a v i a t i o n t e r m i n a l and North Terminal complex) which are s o u t h of hangar 8. Thus, i f t h e hangar walls and doors were v e r t i c a l , l a r g e s p e c u l a r r e f l e c t i o n s should n o t have been encountered i n t h i s region.

Another p o s s i b i l i t y i s t h a t

t h i s m u l t i p a t h a r o s e from t h e h i l l s i d e between t h e p u b l i c p a r k i n g area and Thomas

Avenue

and/or

the

Washington

Metro

Station

which

borders

Smith

t h e r e l a t i v e t i m e d e l a y i s computed as t h e t i m e between t h e r e s p e c t i v e p u l s e c e n t r o i d s . The f i r s t m u l t i p a t h s i g n a l encountered a f t e r t h e d i r e c t s i g n a l i s denoted by an "X" i n t h e M/D l e v e l and T p l o t s . Succeeding m u l t i p a t h s i g n a l s , i f any, are denoted by t h e l e t t e r s Y , Z , A , and B , r e s p e c t i v e l y , on both plots. The r e p l i e s of o t h e r ATCRBS t r a n s p o n d e r s t o o t h e r ATCRBS i n t e r r o g a t o r s may l i e w i t h i n t h e d a t a r e c o r d i n g i n t e r v a l . These " f r u i t " p u l s e s w e r e r e a d i l y Measurements i d e n t i f i e d s i n c e t h e i r p u l s e width i s approximately 450 nsec. w i t h f r u i t p r e s e n t a r e f l a g g e d i n t h e summary p l o t s w i t h an "F" a t a n M/D l e v e l of -30 dB as a warning t h a t t h e d a t a on t h a t i n d i v i d u a l r e p l y may have been c o r r u p t e d by t h e f r u i t .

5-13

@4/24/81 168e1833 DCA LL DCIE 6T 3eFT REIS. 3DC 25FT 1 . C X = B l + = B 7 V = B 5

OVER O = B 4

2 9 B Q

-8.8

-16.8

-24.

e

-32.8

-48.9 1

I

I

1

I

I

I

I

gee. e

669.8

Runway threshold I

We.8

1.4

2.05

Joy stick range ( M i ) 2.71 3.4 4.0

8.8

Fig.5-7.

'I

Computed multipath characteristics for DCA 3" approach scenario.

I. i

at.

Boulevard, s i n c e t h e 800 n s e c d e l a y i s s l i g h t l y g r e a t e r t h a n t h a t a s s o c i a t e d w i t h t h e hangar complex a t t h i s range. I n g e n e r a l , t h e m u l t i p a t h r e g i o n s near t h e t h r e s h o l d of DCA runway 15-33 c o r r e l a t e d f a i r l y w e l l w i t h t h e s p e c u l a r r e g i o n s a s s o c i a t e d w i t h a row of The t i m e d e l a y s of t h e m u l t i p a t h n e a r t h r e s h o l d

hangars b o r d e r i n g t h e runway.

agreed q u i t e w e l l q u a n t i t a t i v e l y w i t h t h e p r e d i c t i o n s u s i n g a simple a i r p o r t model, 1

but t h e e x p e r i m e n t a l M/D

l a r g e r t h a n were p r e d i c t e d .

l e v e l s were i n s e v e r a l cases s u b s t a n t i a l l y

Also, s t r o n g m u l t i p a t h w a s encountered a t l o n g e r

ranges on t h e approach (e.g.,

3

-

5 nmi from t h r e s h o l d ) which could not be

e x p l a i n e d by r e f l e c t i o n s from v e r t i c a l w a l l s of t h e hangars which border t h e runway.

2.

Wright P a t t e r s o n A i r Force Base (WPAFB)

F i g u r e 5-8 shows t h e a i r p o r t geometry a t WPAFB runway 5-23.

This a i r p o r t

had been shown t o have high l e v e l m u l t i p a t h i n t h e f l a r e and r o l l o u t r e g i o n i n

e a r l i e r tests a t

C-band

The p r i n c i p a l m u l t i p a t h t h r e a t h e r e i s hangar

[27].

206 which i s over 25 m h i g h and some 133 m long. The runway i s approximately level

for

the f i r s t

1500 m (4500 f t )

and t h e n s l o p e s upward

toward

the

threshold. F i g u r e s 5-9

and 5-10

show r e p r e s e n t a t i v e r e c e i v e d waveforms and summary

r e s u l t s a t WPAFB on a 3" approach t o runway 23.

Only fragmentary r e s u l t s were

o b t a i n e d i n t h e r e g i o n p a s t t h r e s h o l d due t o low SNR.

T h i s low SNR a r o s e from

extreme ground l o b i n g due t o t h e runway l e n g t h and contour. F i g u r e 5-11

shows t h e corresponding s i m u l a t i o n r e s u l t s .

p r e d i c t s low l e v e l (-12

dB M/D)

The s i m u l a t i o n

r e f l e c t i o n s from b u i l d i n g 152 a t t h r e s h o l d

w i t h a d e l a y of approximately 1.6 u s and h i g h l e v e l (-3 dB M/D) r e f l e c t i o n s from hangar 206 w i t h a t i m e d e l a y of 2 us. delays

correlate

reasonably

well

with

the

The m u l t i p a t h r e g i o n s and t i m e field

measurements,

p r e d i c t e d M / D l e v e l s a r e , i n some cases, c o n s i d e r a b l y lower (e.g., t h a n t h e measured v a l u e s .

10

but

-

the

15 dB)

T h i s d i f f e r e n c e could a r i s e from s e v e r a l f a c t o r s :

5-15

Taxiwav Test Region

<

11 18654-S

Measurement site

--

I-

I

--

' I*

-

-.

Y I C - w ~ = m r r s l e

Fig.5-8.

-1..C-*--*-..L

Airport geometry at Wright Patterson AFB.

5-16

--

I

H o r i z o n t a l scale t i c k s every 0.5 p s e c

V e r t i c a l s c a l e t i c k s every ll dB URIWT MTTERSON

DHEBlW 1

.

IGWT P6tTERSON

DHE212V

11 18655-S

r

I

a,-

a

1

u

.rl

I

I

l

I

1

f

1

IVRMCES

1

DHEB12U

I I W PATTERSON

m t

I

I

1

I

2.7

I

0 4

1

!iWR-S

1

1

I

I

1

2.6

time I

-

I -

Fig.5-9.

-

WPAFB waveforms n e a r t h r e s h o l d .

5-17

I

I

I

11 18656-Si

s

4

x

x

X

.. c

Fig.5-lO(a).

Summary results for flight profile 1 at W A F

5-18

I

a

I

I I

Fig.5-lO(b).

Summary results for flight profile 1 at W A F B .

5-19

I*-

.

B

t

1 L

X

j I...

x

x

u

X

X

a

x

V Y

I....

x x x

x

X

X

K

X

x x

x

v X

x

I

x

I

i

I .H

x

Fig.5-lO(c).

Summary r e s u l t s f o r f l i g h t p r o f i l e 1 a t WP 7B.

5-20

-

c

5-21

5-22

.

r

1. the terrain contour along the runway and building reflection paths was assumed to be flat in the simulation. This may have understated the amount of differential direct signal lobing due to the ground since the off runway terrain is lower than the along runway terrain, and ,

2. the staggering of the doors on building 206 was ignored. In studies of C band reflection behavior along this runway, it was found that the reflected signal levels could oscillate very rapidly in the specular region due to reinforcement and cancellation of signals from adjacent doors [27]. In general, the measured data at WPAFB correlated reasonably well with the multipath regions and time delays expected from ray tracing and computer simulations.

Unfortunately, the severe reflection environment (terrain lobing

and/or building

reflections) was

such that

only

fragmentary data were

available in the flare region where the highest M/D levels were anticipated. The measured data available in that region suggest that the actual M/D levels were comparable to and, in many cases in excess of, the simulation results using a simple airport model. 3.

St. Louis (Lambert) International (STL)

Figure 5-12 shows the airport geometry for measurements at STL. principal multipath sources were

The

the McDonnell Douglas aircraft factory

buildings (labeled M-D in Fig. 5-12) and the terminal building/hangar complex to the south of runway 12R. high.

These structures were typically 20m (60 feet)

The runway sloped downward from the measurement site to the runway

threshold. Figures c

5-13

and

5-14

show

representative waveforms

measurement results for a 3" glideslope approach to runway 12R. v

and

summary

Figure 5-15

shows the corresponding simulation results for a simple airport model.

We see

that the multipath from the M-D building 42 with a level of approximately -8

I -

dB is expected near threshold with a time delay of approximately 1.4

ps

to

5-23

I\

I

X

m d (u

W S CI

t v)

5-24

I

*

3

?

2

-1..

L t

x

I : I

x

x

S ~ O O ~ RJDW S ~ C CeucsimIB LWIS

,..

m~srm.

m w i u - muam

Fig.5-14(a). D a t a summary f o r S t . L o u i s ' s approach w i t h 50 f t . threshold height.

5- 25

11 186624

.

I I

x

=* I

I

I?

" I

t

I

t

..""

.I 4

Fig.5-14(b). Data summary f o r S t . Louis's approach w i t h threshold height.

0 ft.

3

5-26

t

9 (P

? ?!

n

8

6

a

. . ?

8

8

8 f

? Z

1

n I

t u 1

5-27

8

N

8

8

w

8 8

N

8

m

8 8

!!!

8

8 8 0

9

8

/

I

c/

3

i /

5-28

I 15

.

2.0

T h i s l e v e l a g r e e s r e a s o n a b l y w e l l w i t h t h e measured r e s u l t s i n Fig.

ps.

S i m i l a r l e v e l s / t i m e d e l a y s are p r e d i c t e d from t h e o t h e r M-D

5-14.

i n a 1200 f o o t r e g i o n (0.2

buildings

nmi) s t a r t i n g 1200 f e e t (0.2 nmi) a f t e r t h r e s h o l d

and, i n f a c t , t h i s a p p e a r s t o be t h e c a s e a l t h o u g h s e v e r a l v e r y h i g h l e v e l M/D e x p e r i m e n t a l p o i n t s occur which are n o t s u g g e s t e d by t h e s i m u l a t i o n model. I

These

undoubtedly

building

surfaces

a r i s e from t h e which was

not

complicated considered

fine

in

structure

of

the

M-D

developing t h e simulation

a i r p o r t model. t

The e x p e r i m e n t a l d a t a m u l t i p a t h a f t e r t h r e s h o l d has d e l a y s comparable t o t h o s e p r e d i c t e d f o r t h e t e r m i n a l b u i l d i n g east concourse; however, t h e l e v e l s and s p a t i a l d u r a t i o n are s i g n i f i c a n t l y less t h a n s u g g e s t e d by t h e s i m u l a t i o n T h i s d r a m a t i c d i f f e r e n c e arises because t h e l o a d i n g g a t e s and parked

result. aircraft

block

most

of

the

multipath

from

e x p e r i m e n t a l s h o r t d u r a t i o n m u l t i p a t h a t 1.0

the

building

surface.

The

nmi j o y s t i c k range c o r r e l a t e s

w i t h t h e r e g i o n p r e d i c t e d f o r t h e TWA hangar m u l t i p a t h . I n g e n e r a l , t h e m u l t i p a t h r e g i o n s a t S t . Louis i n t h e approach and f l a r e regions correlated f a i r l y w e l l with the specular regions associated with the l a r g e buildings

which f a c e

the

runway.

The M/D

l e v e l s and t i m e d e l a y s

p r e d i c t e d u s i n g t h e MLS p r o p a g a t i o n model and a v e r y s i m p l e a i r p o r t model agree

fairly well

for

t h e M-D

buildings

modeled,

a l t h o u g h some i s o l a t e d

measurements s u g g e s t e d M/D l e v e l s much h i g h e r t h a n p r e d i c t e d . The

measured

s u b s t a n t i a l l y lower

M/D

levels

for

the

t h a n s u g g e s t e d by

terminal

concourse

t h e s i m p l e a i r p o r t model.

wing

were

The low

t e r m i n a l concourse l e v e l s a r e a t t r i b u t e d t o blockage of t h e r e f l e c t i o n p a t h s by t h e parked a i r c r a f t and jetways.

,

S i m i l a r phenomena were noted i n C-band

m u l t i p a t h measurements a t Logan A i r p o r t 181.

4.

P h i l a d e l p h i a I n t e r n a t i o n a l A i r p o r t (PHL)

\

F i g u r e 5-16

shows t h e a i r p o r t geometry a t PHL.

The p r i n c i p a l m u l t i p a t h

s o u r c e s h e r e were t h e v a r i o u s hangars t o t h e n o r t h of t h e runway.

5-29

The runway

HILfiDELPHIA

HILADELPHIA

DRE204B

DnE204B

Probable f r u i t

'

I I

-piEiq 7

I

Y

i

I

I

1

1

I

I

1.7

IYRAHCES

1

WRANCES

1

1.7

1

1

i I

'

I jILADELPHIfi

.(ILADELPHI CI

DPIE284B

DMEZ04B

1

I

1

1

>YRAmES

1

1

1

1

1.5

time

Fig.5-17.

P h i l a d e l p h i a waveform a t and p a s t t h r e s h o l d .

5- 30

1

1

sloped downward approximately 12 feet from the measurement site to the runway midpoint and was flat thereafter. Figures 5-17

i

and 5-18 show representative signal waveforms and summary

results for a 3" approach to runway 27L. Most of the measurements were missed at or just after threshold due to low SNR. This low SNR probably arose from the runway contour causing excessive lobing and/or excess signal losses due to mismatch in the receiver cables.

c

Figure 5-19 shows the computed multipath characteristics using a simple airport model.

The predicted M/D level of -8

correlates reasonably well with Fig. 5-18. no

dB for the UA cargo unit

It should be noted, however, that

multipath within -20 dB of the direct signal was detected on any of the

other approaches.

The predicted peak M/D levels of -18 dB and -28 dB for the

M / E A cargo building and cargo unit # l are not inconsistent with Fig. 5-18, although here again the experimental data show large variations which are not suggested by the computer simulation. In general, the Philadelphia measured results correlated reasonably well with the predictions from ray tracing analysis and computer simulations using a

simple airport

model.

The measured

M/D

ratios

and

T

values were

quantitatively in reasonable agreement on the approaches with adequate SNR; however, in most cases, the SNR was so low as to cause significant problems in data interpretation. 5.

Tulsa Internationl Airport (TUL)

Figure 5-20 shows the airport geometry at TUL where 3" approaches were *

conducted to both ends of runway 17L-35R.

A distinctive feature of the TUL

environment was the sizable hump (see Fig. 5-21) which is approximately 1000 m (3300 ft.) from the threshold of runway 35R.

The principal multipath

threat here were the American Airlines hangar (approximately 30 m high) and the McDonnell Douglas aircraft factory (approximately 20 m high). Figures 5-22 and 5-23 show representative waveforms and summary results 7

5-31

n

.

1.. X

x

x

.

-I..

-1.

S10OTMED J O I SllCl D W C C # I I I 1I

.wnm

I I ~ ~ ~ ~ c .

Fig.5-18(a).

rwimm-Im i n

nrssio((- 3s

DnirTiIr. a r n 4 n

Summary r e s u l t s f o r f l i g h t p r o f i l e 1 a t Philade: p h i a .

5-32

eo.

11 18667-SI IO.

x

x

X

0.

e:

x X

!i 2

x

-10.

x

nx

x

x

X

xx

x

x

'x

rn Ir I

-a*. x n

n

m

)((XIOD(R

x

x

X

X

.

P

R

nnn

n n

m

-

x x

m m nm

m nn

FFFFF P R

F

a

0

-30.

F

FFFF

F

F F

l i F

F

FF F

RFFFF

x *o(

F

F

F

R

x

x

X X M X

x m m x

n~

x

m x x x x x

x

Fig.5-18(b).

Summary r e s u l t s f o r f l i g h t p r o f i l e 1 a t P h i l a d e l p h i a .

5-33

X X

X X

X

xx

X

t

X X

n X

X

X

X

Fig.5-18(c).

Summary r e s u l t s f o r f l i g h t p r o f i l e 1 at P h i l a d e : p h i a .

5-34

1118669-SI

Fig.5-19.

Computed multipath characteristics for PHL measurement scenario.

Re

I 1 18670-SI Fig.5-20.

Tulsa site 1 measurement geometry and reflection

5-36

5-37

k v)

a H

7

rl

LOG AMPLITUDE (TICKS EVERY 11 dB)

. *

Fig.5-23(a).

T u l s a s i t e 1 d a t a summary f o r 25 f t . t h r e s h o l d c r o s s i n g h e i g h t .

5-39

X

X V

I

Fig.5-23(b).

T u l s a s i t e 1 d a t a summary f o r 2 5 f t . t h r e s h o l d c r o s s i n g h e i g h t .

5-40

f o r a 3" approach t o runway 35R.

Much of t h e d a t a i n t h e expected m u l t i p a t h

r e g i o n a f t e r t h r e s h o l d were missed due t o a combination of s i g n a l a t t e n u a t i o n by t h e runway hump and/or s u p p r e s s i o n of t h e ATCRBS t r a n s p o n d e r by h i g h l e v e l m u l t i p a t h from t h e M-D F i g u r e 5-24 with

a

50-foot

hangar ( w i t h a t i m e d e l a y of approximately 2

shows t h e s i m u l a t i o n r e s u l t s f o r a n approach t o runway 35R threshold

crossing height

W e see t h a t low l e v e l (

runway.

LIS).

> -5 dB

and a 25-foot

height

) multipath with a

T

along t h e

of 700 -1100 ns

i s a n t i c i p a t e d i n a r e g i o n approximately 2000 f e e t p r i o r t o t h e t h r e s h o l d from t h e McDonnell Douglas f a c t o r y b u i l d i n g .

This c o r r e l a t e s reasonably w e l l with

-15 dB m u l t i p a t h a t 2.0 nmi j o y s t i c k range i n Fig. 5-23. High l e v e l m u l t i p a t h i s expected i n t h e f l a r e r e g i o n (approximately 800 f e e t p a s t t h r e s h o l d t o 2000 f e e t p a s t t h r e s h o l d ) from b o t h t h e AA hangar (800 n s d e l a y ) and McDonnell Douglas f a c t o r y b u i l d i n g (1000 t o 3000 ns d e l a y s ) . These m u l t i p a t h l e v e l s and d e l a y v a l u e s do c o r r e l a t e w i t h t h e few d a t a p o i n t s t h a t were o b t a i n e d i n t h i s r e g i o n . F i g u r e s 5-25

and 5-26

show r e p r e s e n t a t i v e waveforms and summary r e s u l t s

f o r a 3" approach t o runway 17L.

The v e r y h i g h l e v e l m u l t i p a t h (+5 dB t o as

h i g h as +15 dB M/D r a t i o s ) a t t h r e s h o l d w i t h r e l a t i v e t i m e d e l a y s i n t h e 400

-

600 ns range c o r r e l a t e s q u i t e w e l l w i t h t h e expected t i m e d e l a y s and m u l t i p a t h r e g i o n f o r t h e AA hangar.

F a r t h e r down t h e runway, h i g h t o very h i g h l e v e l

m u l t i p a t h i s encountered w i t h a v a r i e t y of m u l t i p a t h d e l a y s c o r r e s p o n d i n g t o r e f l e c t i o n s from s e v e r a l of t h e b u i l d i n g s . F i g u r e 5-27 with

a

50-foot

runway.

shows t h e s i m u l a t i o n r e s u l t s f o r a n approach t o runway 1 7 L threshold

crossing height

and a 25-foot

High l e v e l (> 0 dB) m u l t i p a t h w i t h a

T

of 550

-

height

along t h e

650 ns i s p r e d i c t e d

i n a 600 f o o t r e g i o n approximately 1000 f e e t p r i o r t o t h r e s h o l d ( c o r r e s p o n d i n g t o a j o y s t i c k range of approximately 2 nmi).

T h i s p r e d i c t i o n of m u l t i p a t h

r e g i o n and d e l a y c o r r e l a t e s q u i t e w e l l w i t h t h e measured r e s u l t s i n Fig. 5-26; however, t h e peak measured M/D l e v e l s are c o n s i d e r a b l y h i g h e r ( 6 dB t o 1 2 dB) than

the

simulation r e s u l t s .

This

discrepancy

contour f e a t u r e s not considered i n t h e simulation. -r

5-41

probably r e f l e c t s

terrain

044W81 14141112 llffsll H E IIEAS. SITE 1 t=W FT. t - n x a n + - c v a n 2

0-B4

'1118675-sl

2-Bl

-8.0

-24.

e

-32.8

-49.8

r 3soo.9

.

I

I

1

I

\ Runway Threshold

~z7O.A

-

Y

3 im.o Sm.0

.

Fig.5-24.

Y

Computed multipath characteristics for simulation of Tulsa site 1 measurements.

time t i c k s every 500 nsec TULS4 SIT€ 8

*JOYRANCES 1

nsn SITE

I

I

I

2.2

ii

wLsn SITE e

DREBIQE

I

1

-JOVRANCES . 1

M216E

1

I

JLSA SITE i!

DIIE216E

1

I

1

1

1

2.2

DflEBlCE

1

I

wRA)(cEs

1

1

1

e.2

time

Fig.5-25.

T u l s a s i t e 2 waveforms n e a r t h r e s h o l d .

5-43

.

,

Fig.5-26(a).

T u l s a s i t e 2 d a t a summary f o r 50 f t . t h r e s h o l d crossling h e i g h t . -.

,-.

5-44

n.

I*.

S

i "X

ec

E

s

I

*.

-I*.

-n.

-am.

I-

" I* x I

sa

XI.

x

I X

"

I-

" X

X

x

x

W

x x

X""

Y

Y

"

X

"

2

asn.

Fig.5-26(b).

T u l s a s i t e 2 d a t a summary f o r 50 f t . t h r e s h o l d c r o s s i n g h e i g h t .

5-45

5-46

1

!.

m k

0 GI

a

FrE

-4

.

In general, International

t h e measured m u l t i p a t h r e g i o n s

agreed

simple ray t r a c i n g . predicted

quite w e l l

with

The measured M/D

and t i m e

the predicted

delays a t Tulsa

c h a r a c t e r i s t i c s using

l e v e l s agreed r e a s o n a b l y w e l l w i t h t h e

l e v e l s a t one s i t e ( a l t h o u g h a d e t a i l e d comparison i n t h e f l a r e

r e g i o n w a s n o t p o s s i b l e due t o t h e many missed measurements), w h i l e a t t h e other,

t h e observed M/D

than those predicted.

.

l e v e l s were c o n s i d e r a b l y l a r g e r (e.g., The l a r g e d i f f e r e n c e s

6 t o 1 2 dB)

are f e l t t o a r i s e from t h e

( s i z a b l e ) d i f f e r e n c e s i n t e r r a i n c o n t o u r f e a t u r e s a l o n g runway c e n t e r l i n e and along t h e path

to t h e b u i l d i n g s which were n o t

considered i n t h e simple

a i r p o r t model.

6.

Quonset S t a t e A i r p o r t , Rhode I s l a n d

F i g u r e 5-28 shows t h e measurement geometry f o r van t e s t s a t Quonset S t a t e a i r p o r t which i s a former m i l i t a r y f i e l d now being used f o r g e n e r a l a v i a t i o n aircraft.

Four s i z a b l e h a n g a r s ( a p p r o x i m a t e l y 20 m h i g h ) were expected t o be

the principal threats.

T h i s a i r p o r t i s very f l a t and much of t h e s u r f a c e i s

paved s o t h a t t h e ground l o b i n g h e r e should a g r e e f a i r l y w e l l w i t h t h e c l a s s i c models. The measurements h e r e were accomplished b e f o r e t h e d i g i t a l system was available.

Thus, t h e M/D r a t i o s determined by a n a l y s i s of s l o p e photographs

t a k e n a t s e l e c t e d p o i n t s i n s i d e and o u t s i d e t h e expected m u l t i p a t h region. F i g u r e 5-29 shows r e p r e s e n t a t i v e scope photographs w h i l e Fig. 5-30 shows t h a t t h e measured M/D l e v e l s agreed f a i r l y w e l l w i t h t h o s e p r e d i c t e d u s i n g a s i m p l e a i r p o r t model. I n g e n e r a l , a t Quonset S t a t e :

.

1.

The peak l e v e l s of M/D as measured a g r e e d w i t h t h e model p r e d i c t i o n s , and

2.

t h e v a r i a t i o n of peak M/D l e v e l s w i t h t r a n s m i t t e r height was a l s o successfully predicted.

5-4 7

//

QUONSET STATE AlRPORT QUONSET. R.I.

Fig.5-28.

d

\'\

quonset State A i r p o r t measurement geometry.

11 18681 -SI

M/D

=

4.3 d B

A

=

420 p sec.

M/D

=

0.6 d B

XMTR HT

Fig.5-29.

Measurement s t a t i o n 1/14 d a t a .

5-49

=

28 ft.

;

!

I

I

I I

. a,

a

R

M .rl

m

0

0

w

U VI

(d

U

a

3a

d

(d

a

a

I

rl

\

U

n -

\

5-50

.

The agreement here was quite good considering that a very crude building model used and that blockage by intervening aircraft was ignored.

7.

Summary of L-Band Results

All of the L-band measurement program objectives were achieved although in some cases [especially, WPAFB, PHL, and Tulsa site # 1 ] the experimental t

data in the flare/rollout region was of poor quality due to low signal to noise ratio.

The spatial region and time deal of specular multipath generally

correlated well with expectations based on simple ray tracing for these cases in which adequate airport maps were available. Washington National (DCA),

no significant (MID

With the exception of ratio

multipath was encountered which was not predicted.

>

-10 dB) specular

In the case of DCA, there

is some question as to whether the multipath encountered at 2-3

nmi from

threshold arose from the identified buildings as opposed to other airport features. The absence of significant specular multipath

*

from other than readily

identified structures at aircraft altitudes above 100 feet is viewed as particularly important for the initial implementation of MLS since the vast majority of the installations will provide category 1/11 service only. When the aircraft antenna was at low altitudes (e.g.,

10-20 feet) over

runways and/or taxiways, a variety of multipath signals were encountered which generally correlated with the principal identified structures.

On the other

hand, the large number of potential multipath sources in this region precluded a detailed quantitative analysis f o r each of the various sites. The airport models used for DME/P analyses to date have typically made a number of simplifying assumptions such as:

*The

-.

possible existence of numerous low level (e.g., reflections in this region is discussed below.

5-51

diffuse)

specular

1.

buildings are represented by rectangular plates with a coefficient

single flat vertical constant reflection

I !

2.

the terrain is assumed to be flat both along and off the runway centerline, and

3.

blockage of reflection paths by intervening objects is ignored.

~

,

The physical features of actual airports differ considerably from each of these assumptions, but arguments can be advanced to support either higher or lower levels than predicted by the simplified models.

Thus, we sought to

determine to what extent simplified airport models could predict the measured data.

The quantitative predictions of the simple airport models generally

agreed with the experimental data, although in some cases (especially near threshold at WPAFB, DCA, and Tulsa), the measured M/D values were conbiderably higher than predictions.

We attribute the WPAFB and Tulsa high levels to

terrain contour features.

In this context, it should be noted that 41 of the 6

airports had runway contours which differed considerably from the nomina.lly flat model used for DME/P power budget computations. Although the M/D levels encountered at several of these airports were above the -3 to -6 dB levels assumed in some DME/P "worst case" analyses (see, e.g., [ 3 ] ) , the relative time delay were in all cases considerably larger than the 300 ns value which is the upper limit of the "sensitive region" for the proposed DAC designs.

Thus, building reflection multipath at these,airports

should not pose a threat to the DME/P precision mode.

On the other hand,, a

conventional DME ( e . g . , slow rise time Gaussian pulse and -6 dB RTT receiver) would clearly have multipath problems at several of these sites.

I C.

Results of High Time Resolution S-Band Multipath Measurements I

at an Operational Airport

The multipath measurements summarized in the preceding section'utilieed I

pulses with relatively large widths (e.g.,

I

100

- 200

ns).

Consequently, it I

5-52

was not possible to resolve multipath returns whose time delay relative to the direct signal fell within the 0 the precision mode of DME/P.

-

200 ns range which is of greatest concern to

It had been suggested that there may be diffuse

multipath from small scatterers near the runway which could (by virtue of its ?

short relative time delays) significantly degrade the performance of DME/P

W I I

To quantify the magnitude of such diffuse multipath, a set of measurements were carried out using a wideband (100 MHz bandwidth) channel probing system developed at M . 1 - T ,

Lincoln Laboratory.

Frequency allocation

constraints necessitated operation at S-band (3 GHz).

However, in view of the

general

and L-bands in earlier

similarity between terrain reflections at C-

measurement programs [17, 271, it is anticipated that the S-band results are applicable to L-band.

1.

Measurement System

The high time resolution multipath measurement ground van is shown in Fig. 5-31.

S-band (3.0 GHz) signals are transmitted from a helicopter through

a blade antenna (Fig. 5-32)

*

to the receiving horn antenna

(27"

beamwidth).

The IF signal is correlated with an internal replica of the transmitted waveform and the correlator output envelope displayed on an oscilloscope.

For

the experiments described here, the oscilloscope display was recorded on a standard (Sony) TV video recorder for subsequent playback.

At the same time,

a call out of helicopter position and other data was placed on the audio track so

that the measured multipath characteristics could be correlated with

airport features. The transmitted waveform is obtained by modulating a CW carrier with a biphase pseudonoise waveform.

The pseudonoise waveform is generated by a 10

stage, maximal length shift register generator with 1023 code elements and a

*The

horn antenna is at the top of the ladder on the measurement van side in Fig. 5-32.

5-53

5- 54

5-55

5-56

I I ~

bit rate which can be as high as 200 MHz.

The correlation peak width is

approximately 10 ns at the 200 MHz bit rate and the peak to sidelobe ratio is approximately 30 dB ( = 10 log 1023). The correlator produces an output for each component of the received signal which is in phase with the waveform generated at the receiver.

Thus,

by varying the relative delay between the receiver and transmitter waveforms, received signals with relative time delays differing by more than 10 ns can be resolved as long as the relative amplitudes do not differ by more than (approximately) 25 dB. The matched filter output envelope is linearly proportional to input amplitude, thus it was difficult to quantify multipath with M/D levels of less than -20 dB when the direct signal peak was also displayed. Consequently each flight profile was repeated with a variety of known receiver gains so as to display lower level multipath.

However, the minimum discernable M/D level i s

approximately -27 dB due to the -30 dB pseudonoise sequence signal sidelobes. Figure 5-33 shows the measurement locations for the measurements on two runways at L. G. Hanscom airport (Bedford, MA).

Hanscom airport is a former

military airport which is now actively used by general aviation aircraft (including jets) as well as some large military transport aircraft.

The

measurements were made at locations corresponding to key points on a 3" cat I, I1 decision height and threshold) since the receiving

approach (e.g.,

system required a nearly stationary helicopter position to avoid Doppler artifacts. The receiving antenna heights were chosen to be approximately the same number of wavelengths above the ground as would be a DME/P antenna at the same height.

This choice i s appropriate because the effective direct signal level

near the threshold is:

D = [ 1 - p e3

4n(ht/d> (hr/X 1 Dfs

where Dfs

=

free space direct signal level at range d

5-57

5-58

v)

4J

E

c aJ

aJ

m

3

!4

E

al

(d

m CJ

h

I vl (d

3

b

3

c

k 0 aJ

w

U

m

*d

C ?

rp

E 0

k

u4 3 aJ

3

v i

u. m

oc

I vl

*d

h

ht

=

transmitter (i.e.., aircraft) height

hr/X = receiver (i.e., wavelengths p =

ground antenna) height in

ground reflection coefficient (approximately 1.0 for the elevation angles in these experiments).

Thus, by keeping hr/X constant, one has roughly the same degree of ground lobing at S-band as would be the case at L-band. The first set of measurements on runway 5-23 1981.

Figure 5-34

were conducted 30 July

shows the view from measurement van site while Fig. 5-35

shows the runway as seen from the 23 threshold.

The van to threshold distance

was approximately 5880 feet and the receiving antenna height approximately 7 feet.

The environment near the runway is seen to be quite flat and devoid of

sizable objects near the runway.

However, there is a sizable (30m high) hill

with trees and a variety of buildings aproximately 1000 feet to the side of the runway threshold. Figure 5-36

shows the nominal envelope and, the envelope with 10 db

additional gain ( s o as to show low level multipath) at 150 feet altitude. Figures 5-37

and 5-38

show corresponding results at 100 feet altitude and

threshold, respectively.

In all cases, the multipath levels in the 0-400 ns

relative time delay region are at least 25 dB below the direct signal level. Figure 5-39 shows the output envelope at an altitude of 15 feet when the helicopter was at the intersection of runways 5-23 discernable multipath with relative delays in the 0

-

and 11-29.

Again, no

400 ns region is evident

in the correlator output envelope. Figure 5-40 shows runway 11-29 from van site 2 while Fig. 5-41 shows the view from the 29 threshold. scatterers

border

the

In this case, a greater number of potential

runway.

These

include

transmitter building (left hand side of Fig. 5-40) (left hand side of Fig. 5 - 4 1 ) ,

the

ILS glideslope and

as well as a small hill

trees and shrubs near the threshold as well as

a number of hangars and parked aircraft approximately 1000 feet off the runway.

Figures 5-42 and 5-43 show the hangar and parked aircraft area to the

5-59

Y

5- 6 0

.

Fig.5-36.

Received envelope 150 ft. above ground.

5-61

1

I I I

a

Fig.5-37.

Received e n v e l o p e a t ' r u n w a y 22 t h r e s h o l d . I

5-62

.

Fig.5-38.

Received envelope 100 f t . above ground.

5-63

Fig.5-39.

Received envelope 1 5 f t . above runway.

5-64

I

.

.

5-65

7

!4

v)

h

m 3

C

?

c la

E

0

3 al

5- 66

V

5-67

5-68

s o u t h of runway 11-29. F i g u r e s 5-44

t o 5-47

show t h e r e c e i v e d envelopes a t t h e f o u r p r i n c i p a l

measurement p o i n t s f o r a n approach t o runway 29 w i t h t h e d i r e c t s i g n a l peak d i s p l a y e d (upper photo) and, w i t h t h e r e c e i v e r g a i n i n c r e a s e d by 10 dB t o show

*

low l e v e l m u l t i p a t h (lower p h o t o ) .

For t h e s e measurements, t h e r e c e i v i n g h o r n

w a s aimed down t h e runway w i t h phase c e n t e r h e i g h t of approximately 10 f e e t above t h e nominal runway l e v e l . On a second series of approaches, t h e horn was d i r e c t e d t o a p o i n t midway

.c

between t h e runway and t h e l a r g e hangar s o t h a t l o n g e r d e l a y m u l t i p a t h from t h e hangar/parked a i r c r a f t complex would be d i s p l a y e d .

F i g u r e s 5-48

t o 5-51

show t h e r e c e i v e d envelopes a t t h e f o u r r e c e i v e r l o c a t i o n s corresponding t o 5-44

Figs.

t o 5-47.

For both r e c e i v i n g a n t e n n a o r i e n t a t i o n s , l i t t l e o r no

m u l t i p a t h was observed w i t h s h o r t d e l a y s (T

<

300 n s ) .

However, when t h e h e l i c o p t e r w a s between measurement p o i n t s 2 and 3 , s p e c u l a r r e f l e c t i o n s were encountered from t h e l a r g e hangar which b o r d e r s t h e runway

as w e l l

representative

as

several adjacent

correlator

envelopes

hangars. in

this

F i g u r e s 5-52 region.

The

t o 5-54 high

show level

( o c c a s i o n a l l y g r e a t e r t h a n +O dB M/D r a t i o ) m u l t i p a t h w i t h a r e l a t i v e d e l a y of a p p r o x i m a t e l y 800 n s c o r r e l a t e s w i t h t h e l a r g e hangar t o t h e s o u t h of runway midpoint.

the

T h i s S-band b u i l d i n g m u l t i p a t h l e v e l i s c o n s i s t e n t w i t h t h e

e a r l i e r L-band a n a l y s i s and measurements.

5-69

/&/ /nHd

MIL-

07-3

Fig.5-44.

Site 2 multipath at 200 ft. altitude.

I

5-70 I

s

(sAnAb9

1

/ZuN sot~ . 9 &/&& -

PfK-

dlii Fig.5-45.

3

0 7 -31-

Site 2 multipath at 150 ft. altitude.

5-71

*'

t

csa4un4atA3 &kCt

Fig.5-46.

S i t e 2 m u l t i p a t h a t 100 f t . a l t i t u d e .

5-72

Fig.5-47.

Site 2 multipath at threshold.

5-73

I

T

0 7- > I - r/

+

/7 u / v , ME--/

diA9Ct

2q5'

Fig.5-48.

Site 2 multipath at 200 ft. altitude.

5-74

A6L

,

T

d i d Fig.5-49.

Site 2 multipath at 150 ft. altitude.

5-75

lW'R6L

Fig.5-50.

Site 2 multipath at 100 ft. altitude.

5-76

0 7 - v - v7 /LcMq

'''k3

0 7- 3/

mo

lyr

501

Fig.5-51.

Site 2 multipath at threshold.

5-77

P

3

Fig.5-52.

Site 2 multipath near 125 ft. altitude.

5-78

7-

.DiRtcT

Fig.5-53.

II -

HULT'riQR7-H

6 1 -j/*a' RUN- 3 floc/-/ /m-.q

Site 2 multipath near 125 ft. altitude.

5-79

I

----

~

-

-

.

c

Fig.5-54.

Site 2 multipath near 125 ft. altitude.

5-80 I

VI.

LIKELIHOOD OF ENCOUNTERING DME/P REFLECTIONS FROM BUILDINGS ON FINAL APPROACH We have seen from the preceding sections that specular reflections from

buildings represent a major challenge to successful PDME operation.

In this

section, we consider how likely it is that one would encounter such multipath in the final approach and landing region based on maps of some 24 airports from a number of countries.

In particular, we have focused on

(1)

distribution of time delays

(2)

specular reflection regions, and

(3)

the distribution of scalloping frequencies

*

as representing computable

relevant parameters for system design/analysis.

The method used to obtain these distributions was as follows: 1. the

specular reflection region is obtained by assuming classical geometric optics reflection applies and then determining the points (xn,xf) where the reflections from the ends of the walls pass through the (extended) runway centerline as shown in Fig. 6-1.

2. the

vertical regions of the reflections is considered by comparing the receiver elevation angle at the points xn, xf with the elevation angle subtended by the corresponding building walls. If the building elevation angle is at least as large as the aircraft elevation angle at either point, it is assumed that a specular reflection occurs.

Prevention of building wall illumination by objects not on the airport maps is

*Another

key parameter - relative multipath amplitude - is not readily computable unless we were to consider many airport environment details (e.g., building surface composition, terrain contours, blockage by intervening obstacles) as well as implementation dependent factors (e.g., transponder and interrogator antenna characteristics).

6-1

.-

n/ 1

1

TRAWSYITTLR

w:

YULTIPATW REGION ON CURVED APPROACH

Fig.6-1.

'L

Determination of MLS multipath b y r a y tracing.

6-2

not considered nor i s blockage of t h e r e f l e c t e d r a y s by o b j e c t s .

Similarly,

p e r i o d i c s u r f a c e c o r r u g a t i o n s are ignored s i n c e t h e a s s o c i a t e d d/X v a l u e s are

less t h a n u n i t y a t L-band. Table

6-1

shows

the

airports

considered

in

the

data

base.

The

corresponding a i r p o r t maps are a v a i l a b l e i n Appendix B. S e v e r a l MLS PDME s i t e s were considered:

The

(1)

c o s i t e d with t h e azimuth a r r a y on t h e extended runway c e n t e r l i n e 1000 f e e t beyond t h e s t o p end of t h e runway.

(2)

s i t e d f 2 0 0 f e e t (66 m e t e r s ) t o e i t h e r s i d e of t h e azimuth s i t e ( e . g . , as with t h e MLS azimuth t r a n s m i t t e r b u i l d i n g ) so as t o permit a h i g h e r phase c e n t e r h e i g h t .

(3)

s i t e d w i t h t h e e l e v a t i o n antenna some 820 f e e t from runway t h r e s h o l d and f400 f e e t o f f t h e runway c e n t e r l i n e .

building

data

base

had

been

originally

accumulated

for

azimuth

c l e a r a n c e and o u t of coverage f u n c t i o n s t u d i e s i n which case o n l y b u i l d i n g s w i t h i n 6000 f e e t of c u r r e n t assessment,

t h e azimuth s i t e were considered. many o t h e r b u i l d i n g s are a l s o of

However, concern.

for the

To permit

t h e e a r l i e r d a t a b a s e , t h e b u i l d i n g s were a s s e s s e d i n two

maximum u s e of groups :

a.

t h o s e n e a r t h e runway s t o p end

b.

t h o s e n e a r t h e runway t h r e s h o l d

The v a r i o u s s t a t i s t i c s f o r t h e v a r i o u s DME/P sites are shown i n F i g s . 6-2 to

6-10.

The

probabilities

shown are

obtained

o c c u r r e n c e s of a g i v e n m u l t i p a t h parameter (e.g.,

T,

r e g i o n v a l u e ) and d i v i d i n g by t h e number of runways.

by

summing up

the

f s o r specular reflection Thus,

1. a s i n g l e b u i l d i n g w i l l y i e l d T , f s and r e g i o n c o n t r i b u t i o n s t o t h e a b s c i s s a a t a number of v a l u e s of t h e o r d i n a t e i n each case, and

6-3

all

TABLE 6-1 AIRPORTS CONSIDERED I N DATA BASE FOR MULTIPATH S T A T I S T I C S

Airport

Runway A p p r o a c h e s

8 JFK 8 LOS ANGELES MIAMI 6 MINNEAPOLIS 6 12 O'HARE PHILADELPHIA 6 TULSA 6 SAN FRANCISCO 8 HEATHROW 6 MELBOURNE 4 4 ORLY 4 SANTOS DUMONT 4 OLD TOKYO 2 NEW TOKYO LENINGRAD 4 (Pulkova) MOSCOW 2 (Sheremetyevo) 4 ( V n u k o v 0) WASHINGTON (National) 6 4 FRANKFURT HAMBURG 6 4 SYDNEY 2 GATWICK MONTREAL 2 WRIGHT PATTERSON 2 ( Ohio)

TOTAL

122

Runway Approaches With B l d g s .

5 8 6

Actual Heights

A c t lJi31 Surface- Known YES YES YES YES YES YES YES YES YES NO NO NO NO NO

2

YES YES YES YES YES YES YES YES YES NO NO YES YES YES

0

NO

NO

0

NO

0

NO

NO NO

2 4 6 2 2 2

YES

YES YES NO YES

2

YES

6 10 6 3 4 5

4 2

4 4

, I.'

YES

,

YES YES YES YES NO NO YES

92

C

PERCENTAGE RUNWAYS

PERCENTAGE RUNWAYS

PERCENTAGE RUNWAYS

PERCENTAGE RUNWAYS

,

Y

lT.1

s u

E5 n

I...

7.5

1

DISTANCE FROM APPROACH END (IT)

10:19:57

06/23/81

(0)

a7.s

as..

11.5

e..

2.5

s.

25".

sno.

=e..

in...

125ee.

15.n.

n5.e.

me..

a15w.

I

DISTANCE FROM APPROACH END (!TI

06/23/81

10:29:44

(b)

F i g . 6-3. M u l t i p a t h r e g i o n d i s t r i b u t i o n f o r DME/P s i t e d w i t h azimuth a r r a y : ( a ) b u i l d i n g s n e a r t h r e s h o l d and (b) b u i l d i n g s near runway s t o p end.

6- 6

e.0

cn

s.e

4.e

i 1.e

2.e

1.e

e.e

MEASURED IN H Z MULTIPATH SCALLOPING RATES

1020r20

06/23/81

( 0 ) e. 14 l I w o R T Y I " ' " " " ' " ' XD.

sa

co

RUNWVS "

"

"

"

"

"

MEASURED IN H Z MULTIPATH SCALLOPING RATES L'

"

"

"

"

10 0

an..

"

06/23/81

lhB.04

(b)

D i s t r i b u t i o n of s c a l l o p i n g r a t e s f o r DME/P s i t e d w i t h azimuth a r r a y : Fig.6-4. ( a ) b u i l d i n g s n e a r t h r e s h o l d and (b) b u i l d i n g s n e a r runway s t o p end.

6-7

5

PERCENTAGE RUNWAYS

PERCENTAGE RUNWAYS

i

,

,

,

,

,

,

,

,

. . . . . PERCENTAGE RUNWAYS

; I , .. , .

r :

PERCENTAGE RUNWAYS

i

DISTANCE FROM APPROACH END (IT)

06/23/81

142651

(a)

Fig.6-6. M u l t i p a t h r e g i o n when DME/P i s s i t e d w i t h azimuth transmitter building: ( a ) b u i l d i n g s n e a r t h r e s h o l d and (b) b u i l d i n g s n e a r runway s t o p end.

6-9

4s.

4.-

1.5.

i 3.Y

1.5.

1.n

,.sa

c l.Y

b.U

*.a

MEASURED IN H Z MULTI PATH SCALLOPING RATES

06/23/81

1012S10

( 0 )

3..

1..

MEASURED IN H Z MULTI PATH SCALLOPING RATES

06/23/81

10135:Ol P

(b)

Fig.6-7. D i s t r i b u t i o n of s c a l l o p i n g f r e q u e n c i e s when DME/P i s s i t e d r i t h azimuth t r a n s m i t t e r b u i l d i n g : (a) b u i l d i n g s n e a r t h r e s h o l d and (b) b u i l d i n g s n e a r runway s t o p end.

6- 10

t

25.0.'

2z.s 2e.o

17.5

.

'

.

,

.

'

*

.

"

'

.

I

.

.

4,

. .

MULTIPATH TIME DELAY (NSEC)

06/22/81

15:38:49

(01

MULTI PATH TIME DELAY (NSEC)

06122181

l5:36: I3

(b)

Fig.6-8. Multipath time delay distributions when DME/P is sited with MLS elevation antenna. Only buildings near runway threshold.

6-11

3

cumaan.

35.0

4 30.0

X.0

2o.r

IS..

10.0

C..

..

m. ...os

7s.o.

lono.

ILW..

1w..

I z u .

DISTANCE FROM APPROACH END (FT)

zoeeo.

-.

06/22/81

15M30

Fig.6-9. M u l t i p a t h r e g i o n d i s t r i b u t i o n when DME/P i s s i t e d with elevation array.

6-12

r

V

e.

W.

7s.

I...

MEASURED IN HZ MULTIPATH SCALLOPING RATES

1Y.

1TI.

06/22/81

an.

E%51

S c a l l o p i n g f r e q u e n c y d i s t r i b u t i o n when DME/P i s s i t e d Fig.6-10. w i t h MLS e l e v a t i o n .

6-13

2.

Consequently,

a s i n g l e runway w i l l y i e l d graphs f o r each b u i l d i n g .

contributions

to

the

t h e area under each of t h e p r o b a b i l i t y c u r v e s i s not e q u a l t o

t h e p r o b a b i l i t y of e n c o u n t e r i n g some DME/P m u l t i p a t h a t a g i v e n runwa:y.

We

see

that

the

likelihood

b u i l d i n g s w i t h d e l a y s less

of

DME/P

multipath

from

the

data

base

-. V

t h a n 200 ns i s q u i t e low f o r t h e DME:/P s i t e s

l o c a t e d n e a r t h e azimuth s i t e .

It might be thought t h a t DME/P cos:tted w i t h

t h e e l e v a t i o n system would have a low i n c i d e n c e of m u l t i p a t h due t o ithe h i g h e l e v a t i o n a n g l e of t h e a i r c r a f t and t h e ( r e l a t i v e l y ) s h o r t DME/P t o < a i r c r a f t distance.

However t h e c o n s i d e r a b l e o f f s e t from c e n t e r l i n e p l a c e s t h e DME/P

n e a r t o s i z a b l e b u i l d i n g s and t h u s i n c r e a s e s t h e p r o b a b i l i t y of s h o r t d u r a t i o n multipath.

.-.

VII. CONCLUSIONS AND RECOMMENDATIONS FOR FURTHER STUDY A.

Conclusions

In this report, we have reviewed the knowledge to date on the DME/P multipath in the landing region with the aim of identifying the principal challenges to successful operation.

This analysis necessarily considered the

major DME/P multipath rejection mechanisms (e.g.,

pulse shape, processing

techniques, and antenna design) in order to assess the likely impact of a given threat as well as reviewing the available relevant L-band propagation data. A number of studies and experimental measurements related t o the DME/P

multipath environment have been carried out over the past two years.

Table

7-1 summarizes the principal multipath challenges and the current status of knowledge regarding them. The results indicate that reflections from buildings which have been increased in relative level by specular reflections from the terrain could represent the major challenge to DME/P operation in the landing region if an appropriate

pulse

shape

and

signal

particular,

simulations suggest that

processors the

are

not

relative M/D

utilized. ratios

In

could

be

considerably in excess of 0 dB, whereas the levels assumed in the bulk of the DME/P proposals to date (e.g.,

[2,

3 , 121) typically have been substantially

less (e.g., 6 dB) than 0 dB. However, the time delay discrimination available by the use of a sharper rise time pulse and improved signal processing (e.g., the building multipath.

DAC) exclude the bulk of

In Chapter 11, we saw that the error using a

representative DME/P receiver (3.5 MHz bandwidth IF filter with a delay and compare processor) with the proposed pulse shape is essentially immune to multipath with time delays greater than 300 ns. Based on the results of Chapter VI, it can be concluded that the vast majority (at least 95%) of runway ends should not contain any high level building reflection multipath with a time delay of less than 300 ns. the precision mode of DME/P can successfully withstand lower level (e.g.,

7-1

Since

TABLE 7-1

Challenge

Specular b u i l d i n g reflection

Satisfactory Theory Yes

PRINCIPAL DME/P WLTIPATH CHALLENGES

S i mu 1a t i o n Studies Yes DFVLR [ 4 2 ]

Experimental Measurements

CO mmen t s

Yes L i n c o l n Lab.

[46]

Tech Univ Uraunschwelg [331 Specular a i r c r a f t reflections

L e v e l s c a n n o t b e bounded by a s i n g l e number (e.g., -6dB M/D); and, a i r c r a f t a n t e n n a p a t t e r n s may create e v e n highe r l e v e l s . However, r e l a t i v e t i m e d e l a y s are n e a r l y always g r e a t e r t h a n 300 n s

Yes

Ye s DFVLR [ 4 2 ] L i n c o l n Lab.*

Only a t C-band

L-band l e v e l s s h o u l d b e l o w e r t h a n C-band. S i m u l a t i o n s sugg e s t m i n i m a l e f f e c t s o n DME/P precision pulse

Ye s

Ye s

Yes

Direct s i g n a l l o b i n g

No

Ye s L i n c o l n Lab [ 2 7 , 4 4 ] Tech Univ Braunschweig [33,471

May b e problem a t a i r p o r t s w i t h rough, b a r e t e r r a i n i n a p p r o a c h zone

No

Yes Tech Univ Braunschweig [331 L i n c o l n Lab.

Measured l e v e l s i n S-band t e s t s a t Hanscom a i r f i e l d ( B e d f o r d , MA: s u g g e s t v e r y low l e v e l s i n nominal ( f l a t ) e n v i r o n m e n t s

S p e c u l a r g r o u n d reE l e c t i o n s from: Elat t e r r a i n

rough t e r r a i n a i t h w e l l defined Eacets

3iffuse specular reflections

Pa r t i a l l y

No

*

q h i s report

i

-6 dB M/D ratio) multipath with delays between 0 ns and 300 ns, multipath from

the smaller objects (e.g.

,

aircraft, trucks, ASR radars) which often are

inside the 300 ns time delay contours should not offer a significant challenge to DME/P. The peak building reflections multipath levels which may occur with delays less than 300 ns are difficult to bound due to the strong sensitivity to terrain contour features and aircraft antenna patterns. i

In many cases, the

M/D level should be less than the -3 dB level which can be readily tolerated

by the current DME/P proposal. However, as was shown in the measurements at U.S. generated by the AWOP WG-M

airports and scenarios

multipath subgroup [ 4 2 ] , building reflections

levels approaching and exceeding the direct signal level can arise in some cases.

Several possible options exist for improving the performance of the

"nominal" DME/P system to cope with these cases on an as needed basis: a. Use of a "centerline emphasis" azimuth pattern on the ground antenna. This will probably necessitate a lower phase center-height, but the increased gain along centerline should offset much of the height/gain loss.

b.

Siting the ground station so as to mitigate the multipath (e.g., atop the hump of a runway). If the DME/P transponder and MLS azimuth are not colocated, there is a possibility of ambiguous aircraft locations when the aircraft is close to the MLS azimuth (e.g., as during rollout or missed approach). The operational impact of such ambiguities (e.g., excluding the use of DME/P information within a certain minimum range) would have to be traded off against the improvement in DME/P multipath performance on a case by case basis.

c.

Improved signal processing techniques at the transponder (e.g., "mismatched" IF filter), and

d.

Lateral diversity transponder antennas.

7- 3

For the AWOP WG-M scenario with excessive control motion errors, the use of a I

centerline emphasis transponder antenna would probably have reduced the errors to well within the current limits. In summary, the experimental data base to date with precision DME systems (Crows Landing, California, and Wallops Island, Va) together with special. DME multipath environment measurements at some seven airports (four of which were major civilian airports) suggest that the DME/P multipath performance and environment are sufficiently well understood to develop SARPS at this lime. For the AWOP WG-M proposed system [ 4 9 ] there is a high degree of confidence

that the "nominal" system should provide the desired performance at the vast majority of runways (e.g.,

over 9 5 % ) ; and there are several additional

features which could be used to improve performance at those runways where the desired performance may not be met with the "nominal" system due to the :Local multipath environment. Near term (e.g.,

within the next year) experimental measurements at

additional airports should not substantially change the above conclusions regarding the multipath environment and the multipath performance of DME/P. If additional multipath performance data are needed for SARPS refinement, this might be accomplished by additional (limited) simulations.

These simulations

could involve additional scenarios and/or the inclusion of additional llME/P system possibilities for multipath rejection in the existing scenarios. When significant operational experience has been obtained with DME/P, the FAA and AWOP

should review the data base to determine whether additional

multipath measurements and/or SARPS guidance material may be warranted. are,

of

course,

some

second

order

investigated in the next few years. B.

issues

which

could

be

There

profitably

These are summarized below.

Recommendations for Near Term Studies 1.

Irregular Terrain Reflections

One uncertainty in the DME/P multipath environment is the nature of

7-4

reflections from rough and/or irregular terrain such as encountered in hilly o r mountainous regions.

Several of the U.S.

located in mountainous regions (e.g., suggested

[141

that

three

interim MLS installations are

Aspen, Colorado) and it has been

dimensional aircraft

particularly important in such regions.

position information is

Limited L-band measurements were

conducted by the FRG at Salzburg, Austria [ 1 5 ] ,

but the pulse widths used

( 2 p s ) were too large to resolve the multipath of greatest concern to DME/P.

Long delay (211s to 20 ps) diffuse multipath was observed as well as some discrete specular multipath. L-band measurements by Lincoln Laboratory using an aperture sampling technique have shown that high level specular reflections can arise from irregular terrain which is not heavily vegetated [ 8 , 4 4 ] .

Figure 7-1 shows one

such site at Camp Edwards, MA, while Fig. 7-2 shows the terrain profile. Figures 7-3 and 7-4 show the measured angular power spectrum as a function of elevation angle from the antenna phase center for two receiver angles. Multiple specular reflections from the terrain occurred in both cases as well as at several other sites. However, the geometries used in the Lincoln measurements had a ground antenna much closer to the rough terrain than would be the case with the normal DME/P siting.

Greater ground antenna to surface distances should

reduce the number of terrain reflections (since the range of elevation angles to the ground antenna is much smaller) and may reduce the M/D levels (since the Fresnel zone size will be larger).

Experimental measurements with more

realistic geometries would be useful. If the DME/P performance was substandard due to irregular terrain reflections, siting the transponder nearer the elevation antenna and utilizing a sharp cutoff elevation pattern on the transponder antenna appears to be the most attractive option for improving system performance. e

Due to the small

differences between the direct and ground reflection signals in terms of time delay, doppler shift and azimuth, the options suggested above for building reflections will not be useful against ground reflections.

7- 5

VI VI

E

EL

rd U

DISTANCE FROM ANTENNA (ft)

Fig.7-2.

T e r r a i n p r o f i l e a t Camp' Edwards, Mass. s i t e #2 (Gibbs Road).

i

7-7

I $

0

-10

-20

-30

-40

ELEVATION ANGLE (DEG)

Fig.7-3.

Received power vs. elevation angle at Camp Edwards site #Z.

7-8

.

b

.

F i e l d Measured

Simulation Predicted (ground . e f l e c t i o n c a l c u l a t i o d f o c u s i n g ground)

C-band I f

I

I I

i : :

-...-...-...-...-...-..-

,..-...-I..-...

I..

...

. . 1

4..

.. ..

7..

#.*

0..

.e

ELEVATION ANGLE (DEG) Fig.7-4. array,

Camp Edwards measurement:

= 2.5O.

Gibbs Road, L-band and C-band e l e v a t i o n

e.

-4

s.l

t

I

I

7-10

m

03

Y

al

rn u rn u

a al

5

rn al

(I]

u u

8

w

c,

2.

Bench Tests of Proposed Receivers

Bench tests of the proposed DME/P pulse shape and receivers in the

of

the

computer

simulations as well as confirming the analytical studies to date.

Whereas

presence

multipath

will

be

invaluable

for

validating

with the MLS angle systems, the development of such a simulation is a nontrivial undertaking, the situation is much simpler for a PDME.

Figure 7-5

shows the realization used in UK bench tests of a phase coded waveform [ 3 8 ] .

It is suggested that errors be determined at a range of time delays for M/D ratios of -10 dB, -6 dB, -3 dB, +3 dB and +6 dB (levels in the ranges -1 dB to +1 dB are not recommended as the results in the antiphase condition

will be very sensitive to precise level adjustment). rf phases should be explored for each

(T, p )

A full range of relative

combination.

Additionally, some

measurements should be made at low SNR to ascertain whether multipath and front end noise effects can be root sum squared. 3.

Investigation of STOL/VTOL Environments

The airport data set used to generate the empirical relative likelihood results in Chapter VI was heavily weighted toward CTOL operations.

However,

STOL/VTOL airports will be an important initial application of MLS (since ILS cannot be used in such cases).

Thus, examination of representative STOL/VTOL

airports geometries would be helpful in determining the appropriate DME/P hardware features.

7-11

'.

c

J

c

I

REFERENCES 1. International Standards and Recommended Practices, Aeronautical Telecommunications, Annex 10, ICAO, 3rd ed. (July 1972).

2. H. Ecklundt, "Technical Proposal for the PDME (FRG Proposal')," Presentation at the FAA/DMFT Experts Meeting (Dec. 12, 1979). 3. R. Kelly and E. LaBerge, "Guidance Accuracy Considerations for the MicroI

,

.

wave Landing System L-Band Precision DMF,;' 1980).

Jour. of Navigation (May

4. C. Hirsch, "L-Band DME for the MLS," FAA ContrPct WI-71-3086-1, Final Report, (Feb. 1972). I '

5. C. Hirsch, "Experimentation for the use of L-Band DME with MLS," Final Report (April 1974). 6.C. Burrows, et al., "Microwave Landing System Requirements for STOL Operations ," AIM-Paper 79-997, 6th Aircraft Design, Flight Test and Operation Meeting, Los Angeles (12-14 Aug. 1974). 7. "Final Report of Analytic Study of MLS Operations with L-Band DME," Bendix Flight Systems Division, Peterboro, NJ (Sept. 1975).

8. J. Evans, T. Burchsted, R. Orr, J. Capon, D. Shnidman and R. Sussman, "MLS Multipath Studies," Lincoln Laboratory, M.I.T. Project Report ATC-63, Volumes I and I1 (1976), FAA-RD-76-3, DDC AD-A023040/9 and A025108/2. 9. J.E. Evans, "Synthesis of Equiripple Sector Antenna Patterns," IEEE Trans. Antennas Propag. AP-24,pp. 347-353 (May 1976). 10. P. Beckmann and A. Spizzichino, The Scattering of Electromagnetic Waves from Rough Surfaces (Pergamon Press, The Macmillan Co., New York, 1963).

11. "Refined Microwave Landing System (MLS) Development Program Plan, Feasibility Demonstration Phase 11," Report No. 11009, Vol. I1 Part 3 Section 30, Hazeltine Corp., Greenlawn, NY (15 May 1974). 12. S . Nakamura, "Some Considerations for the DME/M," ICAO AWOP Background Information Paper WGM/3-BPI/2 (March 1980). 13. "The DME-Based Landing System," Federal Republic of Germany, Vol. 2 (Sept. 1975).

14. M. Whitney, "DME(M) Trial Results," ICAO AWOP WG-S Background Information Paper AWOP-WG-S/BIP-6 (Sept. 1979).

R- 1

15. R. Crow, Private Communications to R.S. Orr (May 1976).

16. "A New Guidance System for Approach and Landing," Document No. DO-148, Radio Technical Commission for Aeronautics, SC-117 (18 Dec. 1970). 17. J. Capon, "Multipath Parameter Computations for the MLS Simulation! Computer Program," Lincoln Laboratory, M.I.T. ATC Project Report A9C-68 (8 April 19761, FAA-RD-76-55, DDC AD-A024350/1.

18. K.J. Keeping and J.C. Sureau, "Scale Model Measurements of Aircraft L-Band Beacon Antenna," Lincoln Laboratory, M.I.T. Project Report ATC-47 (4 March 1975), FAA-RD-75-23.

19. G.J. Schlieckert, "An Analysis of L-Band Beacon Antenna Patterns," Lincoln Laboratory, M.I.T. Project Report ATC-37 (15 January 19751, FAA-RDI-74-144. 20. D.W. Mayweather, "Model Aircraft L-Band Beacon Antenna Pattern Gain! Maps," Lincoln Laboratory, M.I.T. Project Report ATC-44 (16 May 19751, FAA-RD-7575. 21. R. Butler (Rappoteur), "Report of Multipath Subgroup of WG-A Held Near Boston, Mass., USA, October 11-15, 1976." L.WP/3 in the report of AWOP Working Group A, Seventh Meeting, London (Nov 1-12, 1976). 22. T. McGarty, "Measurements of Beacon Propagation and Multipath on t h e Airport Surface," Lincoln Laboratory, M.I.T. ATC Working Paper 42WP-5038 (22 Nov. 1974). 23. H. Postel, "Precision L-Band DME Tests," FAA Technical Center Report No. FAA-CT-80-25, (August 1980), FAA-RD-80-74. 24. "Flight Trials of TRSB/Interscan Equipment at Sydney International Airport, Australia," Paper AWO/78-WP/88 presented by Australia at ICA.0AWO Division Meeting (April 1978). 25. "Tracked Flight Trials of Doppler MLS at Manchester Airport," Paper AWO/78-WP/123 presented by United Kingdom at ICAO AWO Division Meeting (April 1978). 26. A. Lopez, "Scanning-Beam Microwave Landinn System - MultiDath Errors and Antenna Design Philosophy, IEEE Trans. lniennas Propagi AP-25, pp. 290296 (May 1977). '@

27. J. Evans, D. Sun, S. Dolinar, and D. Shnidman, "MLS Multipath Studies, Phase 3 Final Report, Volume I: Overview and Propagation Model Validation/Refinement Studies," Lincoln Laboratory, M.I.T. Project Report ATC-88 (25 April 19791, FAA-RD-79-21, DTIC AD-A08782712. 28. "Siting Criteria for Instrument Landing System," Dept. of Transportation, Federal Aviation Administration (FAA Order 7650.16A).

R- 2

Y

29. D. Shnidman, "Airport Survey for MLS Multipath Issues," Lincoln Laboratory, M.I.T. Project Report ATC-58, (Dec. 1975), FAA-RD-75-195. 30. P. Cornwell and P. Greene, "Fine Resolution Forward Scatter Measurements, IEEE Int'l Conf Radar-77, London, England (Oct 1977). I

I I '

.

31. "Time Reference Scanning Beam Microwave Landing System," U.S. Proposal to ICAO (Dec. 1975). 32. G. Blaschke, "Reduction of Errors Caused by Echo-Signals," Presentation to the FAA/BMFT Experts Meeting (Dec 12, 1979).

' i

33. P. Form, K. Westphal, and W. Schroer, "Progress in Measurement and Simulation of Multipath," Report of the Sonderforschungs-Bereich 58 "Flugfuhrung" of the Tech Univ. Braunschweig (Sept. 1981). 34. T. Tapsell (presenter), "A New Approach to Optimisation of the Essential Characteristics of DME/M to meet the Operation Requirements," AWOP WGM Background Information Paper to be presented in Rio de Janeiro (Sept. 1980). 35. P. Woodward, Probability and Information Theory, With Applications to Radar (Pergamon Press, London, 1964). 36. D. Barton and H. Ward, Handbook of Radar Measurement Jersey, 1969).

(Prentice Hall, New

37. J. Edwards (presenter), "DME/P Flight Test Data Results," AWOP WGM Background Information Paper AWOP/WG M.5/BIP-2 (Sept. 1981). 38. M. Whitney (presenter), "An alternative ranging system in L-Band," AWOP Working Group A Background Information Paper, L. BIP/1, (Nov. 1976). 39. J. Evans, S . Dolinar, D. Shnidman, and R. Burchsted, "MLS Multipath Studies, Phase 3 Final Report Volume 11: Development and Validation of Model for MLS Techniques," Lincoln Laboratory, M.I.T. Project Report ATC-88 Vol. I1 (Feb. 1980), FAA-RD-79-21, DTIC AD-A088001/3.

40.

S. Nakamura (presenter), "Proposal for Optimized Pulse Shapes and Related System Design," AWOP Paper AWOP/WGM.4/BIP (Mar. 1981).

41. J. Hetyei, "Functioning of the DAC Trigger and P-DME Time Reference," AWOP WG-M Paper AWOP/WGM.5/WG-13 (Sept. 1981). 42. W. Kabiersch, "PDME Simulation Program Description and Discussion of Results", DFVLR Inst. Flugfuhring Draft Report presented by T. Bohr as AWOP WG-M Paper AWOP/WGM.5/BIP-3 (Sept. 1981). 4 3 . M. Gori and M. Carnevale, "The Italian Proposal for the DME-M System," AWOP WG-M Paper,Dec. 1981.

R- 3

44. D.

Sun, "Experimental Measurements of Low Angle Ground Reflection Characteristics at L-Band and C-Band for Irregular Terrain," Lincoln Laboratory, M.I.T. Project Report ATC-107 (29 Dec 198l), DOT/FAA/RXl-81/65.

45. J. Evans and S . Dolinar, "MLS Multipath Studies Phase 3 Final Report volume 111: Comparative Assessment Results," Lincoln Laboratory, M.I.T. Project Report ATC 88, Volume I11 (8 June 198l), FAA-RD-79-21.

46. J. Evans, and P. Swett, "Results of L-Band Multipath Measurements at Operational United States (US) Airports in Support of the Microwave! Landing System (MLS) Precision Distance Measuring Equipment (DME/P),"' Lincoln Laboratory, M.I.T. Project Report ATC-109 (23 July 1981) , DOT/'FAA/RD81/63. 47. P. Form and R. Springer, "Field Tests for Multipath Propagation Meaisurements in Mountainous Sites," ICAO AWOP/WP.101 (April 1978).

48. R. Ullrich, "Error Response of a Trapezoid Pulse DME Technique 1)i.sturbed by Multipath Effects and Noise," DFVLR report (draft) (May 1981). 49. S . Nakamura (presenter), "Long Range Echo Problem of Existing :DME and TACAN," AWOP WG-M paper AWOP/WGM.5/BIP-6 (Sept. 1981) (drawn from an article in the NEC Research and Development Journal No. 39 of Oct 1975). 50. D. Care1 (rapporteur), "Report of the Fifth Meeting of AWOP Working Group M," Neuilly, France (Sept. 1981).

R- 4

APPENDIX A DERIVATION OF DME MULTIPATH PERFORMANCE FORMULAS

A l l m u l t i p a t h e r r o r formulas used i n t h i s r e p o r t are d e r i v e d ,y a common N

method.

The procedure i s t o f i r s t determine t h e nominal a r r i v a l t i m e ( t ) ,

which i s t h e r e f e r e n c e

t i m e a t which t h e DME p r o c e s s o r would e m i t a range

marker i n t h e absence of m u l t i p a t h ( i . e . ,

t h e t i m e of t h r e s h o l d c r o s s i n g o r

The second s t e p i s t o approximate t h e time

envelope c o i n c i d e n c e ) .

(t)

at

which t h e marker o c c u r s i n t h e p r e s e n c e of a s i n g l e m u l t i p a t h component having s p e c i f i e d parameters.

The d i f f e r e n c e

i s the s i n g l e scan e r r o r .

Motion a v e r a g i n g i s accounted f o r by a v e r a g i n g sev-

e r a l v a l u e s of t h e e r r o r i n which t h e d i f f e r e n t i a l phase has been incremented i n accordance w i t h a p a r t i c u l a r s c a l l o p i n g frequency and p u l s e spacing. h

I n most c a s e s t h e p r o c e s s o r e q u a t i o n cannot be s o l v e d a n a l y t i c a l l y f o r t i n t h e p r e s e n c e of a r b i t r a r y m u l t i p a t h ; even i n some of those cases f o r which i t can be, t h e s o l u t i o n i s n e e d l e s s l y obscure.

Thus i t i s g e n e r a l l y n e c e s s a r y

t o make some assumption about t h e m u l t i p a t h s i g n a l i n o r d e r t o complete t h e calculation. amplitude ( p )

I n the

f o l l o w i n g i t i s assumed t h a t t h e r e l a t i v e m u l t i p a t h

i s s m a l l enough t h a t t h e sum envelope ( d i r e c t

+

m u l t i p a t h ) can

be approximated by a McLaurin expansion i n p t r u n c a t e d a t t h e l i n e a r term:

e(t) =

A- 1

Although t h i s r e s t r i c t i o n on t h e s i z e of

p i s t h e major a n a l y t i c assumption,

o t h e r s p e c i a l i z e d a p p r o x i m a t i o n s may a r i s e from t i m e t o t i m e i n t h e c o u r s e o f the error derivation.

Each r e c e i v e r t y p e i s t r e a t e d s e p a r a t e l y below, and t h e

d e r i v a t i o n i l l u s t r a t e d w i t h t h e Gaussian p u l s e r e s u l t .

Formulas a r e a l s o g i v e n

f o r t h e c o s / c o s 2 and t r a p e z o i d a l p u l s e s . A.l

4

Fixed T h r e s h o l d D e t e c t i o n The d i r e c t p u l s e s a r e a l l normalized t o have u n i t y peak a m p l i t u d e , e , g . ,

t h e Gaussian pul.se i s t

e

-B(T) r

2 (A. 3)

The t h r e s h o l d a (O

' 0

sin

-1 < ~ < s i na d -

-1

a (A. 3 5 )

The r e q u i r e d comparator g a i n i s I

(A. 3 6 )

G =

'. l

Trapezoidal P u l s e : The a n a l y s i s f o l l o w s t h e p a t t e r n e s t a b l i s h e d i n t h e c o s / c o s

b

in

A-9

2

case, r e s u l t i n g

70

4

60

50

c

In

40

C

c

a

sa W

30

20

10

0

TIME DELAY DAC (ns)

Fig.A-1.

Error c u r v e for cos-cos2 p u l s e and DAC w i t h I F f i l t e r .

t .4

.

A-10

i

p r cos $

G

1

Cos

-

; ‘ G a t

-

‘d (A. 37)

[1

=

-$I-’

(A. 38)

C o s 2 PULSE W I T H I F FILTERING

The bandwidth of t y p i c a l I F f i l t e r s is n e c e s s a r i l y r e s t r i c t e d t o reduce r e c e i v e r n o i s e and i n t e r f e r e n c e from a d j a c e n t channels.

This causes t h e i n p u t

p u l s e t o t h e DAC t o rise more slowly than t h e c o s i n e p u l s e l e a d i n g edge. Simulations of

t h e DAC e r r o r c h a r a c t e r i s t i c s u g g e s t t h a t t h e e r r o r can be

approximated by :

8DC

= p f ( r ) cos Q

(A. 39)

~

I

r

where f(r) i s shown i n F i g . A-1.

A-11

.

APPENDIX B AIRPORT MAPS USED T O DETERMINE BUILDING LOCATIONS

Figures B-1 to B-24 show the airport maps used to determine building locations and orientations.

B- 1

B- 2

W

1 I-

II

O

E

i?

u

I

v)

m .d

a

0

rl

dr

i d

c c

e

8

B- 3

B- 4

-=

0

n

0

P

I .

B- 5

0

E u

v

a

k 0

1

B- 6

0

a k

c*

..

. 4.

c

t

O X

L

t-

B- 7

-I

4

E

a W

c

U H

B- 8

. .

J

v

0

k

u

a k

z! d rd

k

al

u d

ffl

H

al

9)

d M

ffl

4

0 +l

v L

i

B- 9

h

0

SCALE ( f t )

Fig.B-9.

Melbourne A i r p o r t .

-

0

2000

4000

i

t 17.330'X 150'

6000

SCALE ( f t )

Fig.B-10.

Orly A i r p o r t ( P a r i s ) .

B-10

I.'

. !

c

I,

tr

RUNWAY NO. 1

13,200'X 300'

10L+

HEATHROW CENTRAL

td I

r

P

NO. 5

13,200'X 300'

10R+

0

SCALE (ft) Fig.B-11.

Heathrow Airport (London).

i

,

i

.-

B-12

,

B-13

I

'I

.. 8

!

i

.I

I

!

!

'I

-

-

I :'

I ,

B- 14

rc

.n$ ' .. a

3

'

. . ..

. .... .

d

a

mumm

0-

111

O O O O O O O O O O O O O O O O O O O O O O O O O O O / O Q O O O O O O O O O O O 0 0 0 0 0 0 0 0 0 0 0 0 0 c . 0 0 0 0 0 0 0 0 0 0 0 0 ~ 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0 0

~nioiu wn

0

-I

0

0

0

4-

0

0

0

oDo

0

0

0

0

0

w

PYtlU

rmu IUIJlllIm

smii

-+ nua

A

r

MU&V af W Wi.Yo8

Fig.B-15.

Pulkovo Airport (Leningrad).

d

! I

I !

I I I I I I I i

1

I i

1 . 1

1I .o t I.

I:

I

B-16

i

i f t

n

I.1

h

0 a,,

1 ;

Fc

a, 0,

WY

..c

c

Y..'

-0

R

L

B-17

Taxiwav Test Region Measuranent site

Fig.B-18.

Wright Patterson AFB.

B-18

Fig.B-19.

Hamburg, W. Germany A i r p o r t .

B-19

H d

B-21

n

4 n

W

P

1

I

B-22

.f

B-24