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1 ) A { L ~ Z i \ . i / b ~ h ~ L ~ L A L L L ~ L L T L J : i i t ' l - L e d 1 4 l'ii&!L i Z i t ~ ! l i d A ~ ~i JiiLvi k l i , ~ L L L A G ~ .~LL'UA~'L

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G j / j L 3111;

Systematics General Corpora tion

?!I22 J~ledar UIIII~I Falls mhur~h. Uirqillia 22ll42

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REFSA'T/GPS BREADBOARD

-

TEST TERMINAL

HARDWARE

DESIGN

REPORT

November 1 980

Prepared f o r

N a t i o n a l Aeronautics and Space Administration Goddard Space

Fl

i g h t Cen t p r

Earth Observation St --.IS D i v i s i o n User T e r m i n a l 6 Locat i o . .

,

,,IS Branch

Greenbelt, Maryland 20771

Prepared Under NASA/CSFC C o n t r a c t NAS5-26039

(3)

ABSTRACT

T b i s r e p o r t d e s c r i b e s t h e hardware d e s i g n and t h e test plan needed t o v e r i f y t h e REFSAT concept.

A t a g i v e n moment, all u s e r s o f t h e NAVSTAR G l o b a l P o s i t i o n i n g

S y s t e m ( G P S ) l o c a t e d w i t h i n a r a t h e r l a r g e g e o g r a p h i c a l a r e a a r e r e q u i r e d to p e r f o r m a n u m b e r o f i d e n t i c a l , common operations i n o r d e r t o a c c o m p l i s h a n a v i g a t i o n " f i x " ( a d d i t i o n a l " l o c a t i o n - d e p e n d e n t " operations a r e also requ i r e d )

.

Under t h e KEFSAT concept, t h e conlmon o p e r a t i o n s a r e p e r f o r m e d a t a c e n t r a l l o c a t i o n and results b r o a d c a s t v i a a g e o s t a t i o n a r y r e f e r e n c e satellite (REFSAT). T h i s concept a l l o w s a considerable r - e d u c t l o n i n t h e c o s t and c o m p l e x i t y o f individual c i v i l u s e r t e r m i n a l s .

The hardware design o f a

REFSAT/GPS

r e c e i v e r t e r m i n a l and the REFSAT t r a n s m i t t e r s i m u l a t o r i s described.

Since s i x o f t h e Phase I NAVSTAR GPS s a t e l l i t e s a r e now o p e r a t i o n a l

a

d e m o n s t r a t i o n test p l a n u t i l i z i n g t h e one-to-two hour p e r day v i e w i n g window i s a l s o d e s c r i b e d .

Equipment check-out p r e l i m i n a r y t e s t results a r e p r e s e n t e d .

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TABLE OF CONTENTS

SECTION

.

PAGE

1.0 REFSAT/GPS BREADBOARD-TEST T E R M I N A L HARWARE DESjGN

...

1

1.2 S y s tern D e s c r i p t i o n o f P r o t o t y p e REFSAT/GPS R e c e i v e r

... 5 ...

1.2.1 REFSAT/GPS R e c e i v e r N o i s e Budget 6

1.2.2 GPS and REFSAT S i g n a l RF L i n k Computations

...

8

1.3

C i r c u i t D e s c r i p t i o n o f P r o t o t y p e REFSAT/GPS

...

R e c e i v e r T e r m i n a l 10

1.3.1

REFSAT S i g n a l Channel

...

13

...

1.3.2 G P S S i g n a l Channel 13

1.3.3 R e c e i v e r Frequency S y n t h e s i z e r

... 14 ...

2.0 REFSAT TRANSMITTER SIMULATOR 16

...

3 . 0 REFSAT/GPS R E C E I V I N G T E R M 1 NAL TEST PLAN 18 3 . 1 T y p i c a l G P S S a t e l l i t e C o n s t e l l a t i o n V i e w i n g Window

....

20

...

3.2 GPS REFSAT T e s t B l o c k Diagram 27

...

3.2.1 REFSAT Word Generator D e s c r i p t i o n 29 3.2.2 Logging Mode Data H a n d l i n g D e s c r i p t i o n

...

31

3.2.3 Use o f Computer t o A i d T i m e Word G e n e r a t o r

...

S y n c h r o n i z a t i o n . 32

...

3.3

T e s t Procedure

3 3

4.0 CHECKOUT TEST PERFORMANCE

... 34

...

4.1 RF A m p l i f i c a t i o n and Frequency T r a n s l a t i o n Tests

34

...

4.2

REFSAT

S i g n a l C a r r i e r Recovery

35

...

4.3

REFSAT S i g n a l D a t a Demodulation.

35

...

4 . 4

D o p p l e r C o r r e c t i o n

35

4.5

C o r r e l a t i o n

... 35

4.6

Conversion of Analog C o r r e l a t i o n Data t o D i g i t a l

...

Form 36

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LIST

OF

FIGURES

F I G U R E

. P A G E

I .

The REFSAT Concept

...

2

...

2. P r o t o t y p e R E F S A t / G P S Receiver and Simulated R E F S A T

4 3 .

REFSAT/GPS Receiver Terminal

... 1 1

4 .

Receiving Antenna Elevation P a t t e r n

...

1 2

5 .

R e c e i v e r Frequency S y ~ c h e s i z e r

... 15

6 .

REFSAT T r a n s m i t t e r S l m u l a t o r B l o c k Diagram

... 17

7 .

GPS Satellite Ground Tracks (20 degree e l e v a t i o n

visibility c o n t o u r shown around Washington.

D.C.) ...

22

8 .

G P S Grocnd Tracks. 1600-2000 hrs GMT.

1

J a n u a r y

1979

(20 degree e l e v a t i o n visibility c o n t o u r s h o w n a r o u n d

Washington. D . C . )

...

22

...

9.

G P S Satellite E l e v a t i o n Angles at Washington.

D.C. 23

...

.

10 D l lution-of-Precision

(DOP)

Values at Washington.

D.C. 23

...

1 1 . GPS

Satellite E l e v a t i o n Angles at Washington.

D.C.

24

....

12

.

Di lut ion-of-Precision

(DOP)

Values at Washington.

D.C.

24

...

13

.

G P S REFSAT Test Block Diagram 28

...

14 .

M i c r o c o m p u t e r Interface to Test Equipment 30

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LIST

OF TABLES

TABLE

.

PAGE

1

.

R e c e i v e r Noise Budget

... 7

2

.

GPS Channel L i n k Computations

( 1 5 7 5 . 4 2

M H ~ )

...

8

3

.

REFSAT S i g n a l Channel L i n k Cornputat ions

(1556.40

MHz)

... 9

4 .

NAVS'TAR Phase I O r b i t D ~ t a a t F i r s t Ascending Node on

1 January

1979 ...

21

...

5 . GPS

Satellite Test Window at Washington.

D.C.

25

6 .

GPS Satellites I n

O r b i t

As O f June 19110

...

26

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0 REFSAT/GPS BREADBOARD-TEST TERMINAL HARDWARE D E S I G N

1.1 I n t r o d u c t i o n

T h i s s e c t i o n d e s c r i b e s t h e REFSAT/GPS r e c e i v e r hardware d e s i g n f o r an e x p e r i m e n t a l p r o t o t y p e G P S REFSAT-Aided R e c e i v e r . The p r o t o t y p e r e c e i v e r

i s t o be used t o v e r i f y t h e REFSAT c o n c e p t , making u s e o f t h e C l e a r / A c q u i - s i t i o n , C / A , code s i g n a l s from t h e Phase t G P S s a t e l l i t e s t o d e t e r m i n e a user t e r m i n a l ' s g e o g r a p h i c a l l o c a t i o n w i t h an e r r o r n o t exceeding 50 meters (rrns) ,

A REFSAT-aided r e c e i v e r makes use o f a r e f e r e n c e s i g n a l g e n e r a t e d by t h e REFSAT system t o a i d GPS s i g n a l a c q u i s i t i o n and t o reduce t h e c o m p l e x i t y o f user computations. The c o n c e p t i s i l l u s t r a t e d

In

F i g u r e 1

and c m p a r e d w i t h t h a t o f a c o n v e n t i o n a l GPS r e c e i v e r .

The REFSAl system would c o n s i s t o f an e a r t h s t a t i o n and geosynchronous r e l a y s a t e l l i t e s e r v i n g a p a r t i c u l a r g e o g r a p h i c a l area. A REFSAT Remote

C o n t r o l S t a t i o n ( R c S ) w o u l d p e r f o r m t h e f o l l o w i n g f u n c t i o n s normal l y performed by i n d i v i d u a ' c o n v e n t i o n a l

GfS.

r e c e i v e r s :

a S e l e c t s f o u r v i s i b l e GPS s a t e l l i t e s h a v i n g minimum h o r i z o n t a l - d i l u t i o n - o f - p r e c i s i o ~ , ( H D O P ) ;

A c q u i r e s t h e f o u r GPS s a t e l l i t e C/A s i g n a l s and demodulates t h e 50 bps d a t a channel t o o b t a i n GPS s a t e l l i t e ephemeris d a t a ; an o p t i o n i s t o o b t a i n ephemeris d a t a by Land Line, o r o t h e r means;

a Computes c u r r e n t GPS s a t e l l i t e p o s i t i o n c o o r d i n a t e s ; Computes c u r r e n t GPS s a t e l l i t e b o p p l e r c o e f f i c i e n t s .

I n a d d i t i o n t o GPS d o p p l e r c o e f f i c i e n t d a t a , t h e REFSAT R C S t r a n s m i t s a s t a b l e r e f e r e n c e c a r r i e r f r e q u e n c y , o f f s e t f r o m t h e G P S C/A s i g n a l s , For

r a p i d a c q u i s i t i o n .

The REFSAT s i g n a l t h u s c o n t a i n s a s t a b l e r e f e r e n c e c a r r i e r f r e q u e n c y . c u r r e n t GPS s a t e l l i t e p o s i t i o n c o o r d i n a t e s and d o p p l e r c o e f f i c i e n t d a t a which i s t r a n s m i t t e d t g t h e geosynchronous REFSAT s a t e l l i t e f o r b r o a d c a s t

t o GPS REFSAT Mobi i e T e r m i n a l s ( R M T S ) .

As shown i n F i g u r e

I ,

a REFSAT RMT p e r f o r m s t h e f o l l o w i n g f u n c t i o n s : A c q u i r e s t h e REFSAT r e f e r e n c e c a r r i e r frequency w h i c h i s s t a b l e w i t h i n

- +

10 Hz as a r e f e r e n c e f r e q u e n c y f o r GPS s i g n a l a c q u i s i t i o n ; m Demodulates REFSAT d o p p l e r c o e f f i c i e n t d a t a and computes d o p p l e r

c o r r e c t i o n s ; a c q u i r e s GPS s i g n a l s ;

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REMOTE COMTROL STATlOU [RTS)

I

4 COHYEEITIOHAL I

I GPS RECEIVER I

a g d u 1 1 1 r

I €*tit h ) r U TERHIMAL I

I 1 I

I I

1 I

I Wprlm GY I

I S I I C l l l t l P O ~ l t l O l 1

I Cmrdl~t~$ I

I I 1 I

User P c I l t l m I

I I I 1 I I 1 t 1 I I I I I I 1 t

Figure 1. The

REFSAT

Concept.

I Mnsun 4 I

I hbda I

I I

1 h I

I t I

I

I I

I tpwttm 1

I I I

I t

I I

I ' I

I I I I I t I I t 1 I 1

I

v

I I

(9)

Demodulates c u r r e n t G P S s a t e l l i t e p o s i t i o n c o o r d i n a t e d a t a f r o m t h e R E F S A T s i g n a l ;

Makes pseudo-range measurements t o f o u r s e l e c t e d GPS s a t e l l i t e s ; Solves G P S n a v i g a t i o n e q u a t i o n t o d e t e r m i n e u s e r p o s i t i o n ; an o p t i o n i s t o use a " r e p o r t - b a c k " t r a n s m i t t e r t o r e l a y pseudo- range measurements back t o t h e

R C S

f o r c o m p u t a t i o n o f u s e r p o s i t i o n .

For p r e l i m i n a r y t e s t s w i t h Phase 1 GPS s a t e l l i t e s , s e v e r a l o f t h e f u n c t i o n s shown i n F i g u r e 1 w i l l be s i m u l a t e d o r performed o f f - l i n e as f o l lows:

S i n c e t h e ATS-6 s a t e 1 1 i t e i s n o t a v a i \ a b l e t o t e s t t h e REFSAT c o n c e p t , a low-power ( i n the o r d e r o f

-120

dBW e . i . r a p . * ) t r a n s m i t t e r r a d i a t i n g w i t h i n a few f e e t o f t h e REFSAT-aided r e c e i v e r ' s antenna w i l l be used as a s i m u l a t e d REFSAT;

a S i n c e c u r r e n t GPS s a t e l l i t e ephemeris d a t a i s n o t a v a i l a b l e el t h e r i n zdvance o r i n r e a l time$+$:, t h e

REFSAT

r e f e r e n c e s i g n a l

used f o r e x p e r i m e n t a l p r o t o t y p e t e s t i n g w i l l c o n t a i n dummy G P S s a t e l l i t e p o s i t i o n c o o r d i n a t e d a t a messages. Measured pseudo- range v a l u e s o u t p u t f r o r r t h e r e c e i v e r w i l l be r e c o r d e d f o r l a t e r o f f - 1 i n e s o l u t i o n o f t h e GPS n a v i g a t i o n e q u a t i o n (when c o p i e s o f t h e d a i l y G P S u p l o a d d a t a c o n t a i n i n g t h e r e q u i r e d ephemeris d a t a have been r e c e i v e d by m a l l f r o m Vandenberg A F B ) .

F i g u r e 2 i l l u s t r a t e s t h e p r o t o t y p e REFSAT/GPS r e c e i v e r and t h e

REFSAT

s i m u l a t i o n t h a t w i 1 1 be used f o r p r e l i m i n a r y t e s t s .

The remainder o f t h i s document i s c o n t a i n e d i n f o u r sections:

a R E F s A T / G P S Breadboard-Test T e r m i n a l Hardware Design a REFSAT T r a n s m i t t e r S i m u l a t o r Hardware Design

a REFSAT/GPS R e c e i v e r T e r m i n a l T e s t P l a n Checkout Test Performance

; E f f e c t i v e I s o t r o p i c R a d i a t e d Power

**

The REFSAT-aided r e c e i v e r i s , by d e s i g n , i n c a p a b l e o f d i r e c t l y d e m o d u l a t i n g the G P S n a v i g a t i o n message (50 bps d a t a ) .

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GP5 Signal

1575.42 1!Rz R c q ~ 1 s I t i ~ n

REfSRl S l m l a l i o n Signal w/l.OZ> ABPS .5

1556.40 lUir PM C I A Code frat6ina

[-I20 dBY c.i.r.p.1

-1 ft.-

40.42 llilz GPS.

21.4 Wlz REF5RT Correcttan

17-4

REFSRT Signal

\556.4000 t!Nz Carrier.

1556.4010 tltiZ

6 21 HHz .. C I A Fn Code

1556.41172 MHz FSK Data

t

Reference O ~ c i l I a L ~ r

10 I W t ; lppm Domler

Uf rset

H-7

REFST Data Uard 25 Hz Steps

-

Reference

Frequency Osclllatnr Pseudo Range Data

-

Synthesizer 10 lMz

-

i 5 ~ 1 D - ~ ~

Rccefvtr

Figure

2.

Prototype REFSAT/GPS Receiver a n d Simulated REFSAT.

(11)

1.2 Sys tern Descr i p t I o n o f P r o t o t y p e REFSAT/GPS R e c e i v e r

T h i s s e c t i o n d e s c r i b e s the f u n c t i o n a l b l o c k d i a g r a m ( ~ i ~ u r e 2 ) f o r P r o t o t y p e REFSAT/GPS R e c e i v e r t o be used t o v e r i f y t h e REFSAT concept.

The f o l l o w i n q s e c t i o n s c o n t a i n c i r c u i r d e s c r i p t i o n s o f t h e r e c e i v e r .

The s i m u l a t e d REFSAT, a l s o shown on F i g u r e 2 i s d e s c r i b e d i n a l a t e r s e c t i o n , The r e c e i v e r processes:

S i g n a l s from f o u r GPS s a t s l l i t e s :

1575.42

MHz

.t 4

kHz maxlmum d o p p l e r , c o n t a i n i n g 1.023 MHz C/A code ( u n i q u e f o r each s a t e l 1 i t e )

a REFSAT r e f e r e n c e s i g n a l c o n t a i n i n g :

a s t a b l e r e f e r e n c e c a r r i e r frequency (+

- 5

p a r t s 10 10 ) a t 1556.40 MHz and,

t h e REFSAT d a t a word stream (128 bps FSK,

1556,4070

MHz and 1 556.4072 MHZ)

.

The r e c e i v e d slgnals a r e s e p a r a t e d f r o m o t h e r i n c i d e n t s i g n a l s by a 30 MHz wide bandpass f i l t e r (FL-1) b e f o r p r e a m p l i f l c a t l o n . The r e c e i v e r r e f e r e n c e osci 1 l a t e r ( T O MHz 1 p a r t i n 10

%

) and f r e q u e n c y s y n t h e s i z e r p r o v i d e t h e necessary r e c e i v e r l o c a l o s c i l l a t o r s i g n a l s a t

1535

MHz and 21.0 MHz.

F o l l o w i n g t h e f i r s t m i x e r

(M-I),

t h e GPS and REFSAT s i q n a l s a r e separated i n t o two channels. The r e s u l t a n t 1 s t I F f r e q u e n c i e s a r e 40.42 MHz -t

4

kHz maximum expected d o p p l e r s h i f t f o r t h e GPS channel and 21.4

MRZ

f o r t h e REFSAT channel. The f o l l ~ w i n g m i x e r s i n each c h a i n share a common 21 MHz l o c a l o s c i l l a t o r frequency, r e s u l t i n g i n 2nd

IF

f r e q u e n c i e s o f 19.42 MHz

2 4

kHz f o r t h e G P S channel and 0.40 MHz f o r the REFSAT channel.

The REFSAT s i g n a l i s a p p l i e d t o a PLL demodulator t o s e p a r a t e t h e 0.400 MHz r e f e r e n c e c a r r i e r f r o m t h e FSK d a t a s i g n a l (7.0 and 7.2 kHz).

The FSK d a t a s i g n a l i s passed t h r o u g h a demodulator and thence as a 128 bps s e r i a l b i t stream t o t h e r e c e i v e r m i c r o p r o c e s s o r .

The r e c e i v e r m i c r o p r o c e s s o r makes use o f t h e d o p p l e r c o e f f i c i e n t data f r o m t h e REFSAT data s t r e a m a l o n g w i t h an a p p r o x i m a t e knowledge o f r e c e i v e r p o s i t i o n t o compute d o p p l e r o f f s e t v a l u e s f o r each o f t h e f o u r GPS s a t e l - l i t e s i g n a l s t h a t must be a c q u i r e d . These d o p p l e r o f f s e t v a l u e s a r e

a p p l i e d t o a frequency s y n t h e s i z e r t o p r o d u c t a 19.020 MHz l o c a l o s c i l l a t o r s i g n a l w h i c h may be s h i f t e d

5

kHz i n 25 Hz s t e p s f o r d o p p l e r compensation.

T h i s I 1 d o p p l e r compensation" s i g n a l i s mixed w i t h t h e r e c o v e r e d REFSAT r e f e r e n c e (0.40 MHz) i n

M-7.

The r e s u l t a n t 13.42 MHz s i g n a l i s t h e n

modulated w i t h t h e a p p r o p r i a t e 1.023 Mbps C/A code i n

M-6

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

GPS

s a t e l l i r e .

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GPS s i g n a l c o r r e l a t l o n i s then accornpl ished i n

M-4

and

M-5

and associated c i r c u i t s under c o n t r o l o f t h e r e c e i v e r microprocessor, For a p a r t i c u l a r

GPS

satellite, i n p u t s t o t h e c o r r e l a t i o n mixers

M-4

and M-5 a r e :

a Four ( o r more) GPS s i g n a l s whose c e n t e r frequency f s 19.42 MHz

+ 4

kHz, and c o n t a l n l n g a frequency e r r o r p r e s e n t i n the r e c e i v e r

-

reference o s c i l l a t o r as much a s

1535

H:, and

"Doppler c o r r e c t ion" s i g n a l whose c a r r i e r frequency I s 19.420 !lHz

+

computed d o p p l e r c o r r e c t ion, and conta1 n i ng the same frequency e r r o r present i n the r e c e i v e r r e f e r e n c e o s c i l l a t o r ,

Thus, frequency e r r o r s due t o the r e c e i v e r r e f e r e n c e o s c ~ l l a t o r c a n c e l . The a f f e c t i v e ;requency accuracy o f the "doppler c o r r e c t t o n " s i g n a l

i s e s s e n t i a l l y t h a t o f t h e reference o s c i l l a t o r employed

In

the REFSAT t r a n s m i t t e r s i m u l a t o r

( 1 ~ 5 6 ~ 4

x

5 x

1 0 " ~ MHz = 0.8 H Z ) , l i m i t e d by the 25 Hz s y n t h e s i z e r r e s c l u t i o n . As a r e s u l t , GPS s l g n a l a c q u i s i t i o n can be a c c ~ m p l i s t ~ e d w i t h o u t a l e n s t h y frequency search.*

F o l l o w i n g GPS s i g n a l a c q t * ' - t i t i o n , t h e r e c e i v e r m i c r o p r o c e s ~ c ~ determines pseudo-range values t o each o f the f o u r G P S s a t e l 1 i tes by measuring t h e t i m e ofFset, ketwczrr the f o u r C/A code epochs from each o f f o u r G P S satellites, r c i s o u t p u t d a t a I s recorded f o r l a t e r o f f -

l i n e a n a l y s i s .

1.2.1

REFSAT/GPS

Receiver Noise Budget

T a b l e

1

shows receiver n o i s e budget camputations. Components from

Figure 1 a r e l i s t e d , i n o r d e r , from t h e antenna through the 1 s t I F a r n p l i f l e r along w i t h the e q u i v a l e n t n o i s e temperature o f each referenced t o the

antenna t e r m i n a l s .

Since the present r e c e i v e r design does n o t i n c l u d e an image f i l t e r preceeding the 1st m i x e r , t h e additional "image" cioise has a l s o been computed. The a d d i t i o n o f an image f i l t e r would reduce t h e e q u i v a l e n t

system temperature from

1434

t o 1017 K e l v i n . The r e s u l t a n t

1.5

dB r e d u c t i o n does n o t j u s t i f y the a d d i t i o n a l expdnse o f such a f i l t e r .

Note t h a t t h e n o i s e budget g i v e n i n Table 1 I n c l u d e s t h e e f f e c t o f 30 f e e t o f RG-214 t r a n s m i s s i o n 1 i n e between t h e antenna and the receiver.

Should t h i s t r a n s m i s s i o n l i n e be removed, o r the 1 s t mixer, LNA-I, and the f i l t e r FL-1 be remotely l o c a t e d a t t h e antenna, t h e e q u i v a l e n t

-

A The four r e q u i r e d GPS s i g n a l s a r e acqiured, and then sampled, s e q u e n t i a l l y .

(13)

TABLE I

RECEIVER NOlSE BUDGET

N o i s e Loss

Factor Factor C -wputat ion

Noise Temperature

Referred to Antenna

Antenna

- -

170

K

30' RG-214

-

2.00 290 (2.00-1 )

( 3

di! loss)

Filter,

FL-1 -

1.23 290(1.23-ljx2.~3

(0.9 d B loss)

LNA-1 (30 dB 1.58 1 /lo00 290(1.58-ijx? - 2 3 ~ 2 . 3 3

g a i n , 2 d B

NF)

M i x e r ,

M-1 (6

d B 5.01

3.98

2 9 0 ( 5 . 0 1 - 1 ) x ~ 1 / 5 0 0 ~ ) ~ 1 . 2 3 x 2 . 0 0

-

- 3

K

loss,

7

dB NF)

Amp-1 (28 dB g a i n ,

3.31

5.2 dB NF)

1017 #

Since no image filter is used: Noise a t image frequency from LNA-1 414 K N o i s e a t image frequency from M-1

3

K

E q u i v a l e n t system nolse temperature 1434

K

(14)

r e c e i v e r n o i s e t e m p e r a t u r e would be reduced t o 653 K. Use o f an image f i l t e r c o u l d reduce t h i s v a l u e t o

445

K.

1.2.2 G P S and REFSA'T S i g n a l RF L I n k Computations

GPS s i g n a l RF l i n k cornputatlons a r e g i v e n i n T a b l e 2. The GPS

s i g n a l power a t t h e r e c e i v i n g antenna o u t p u t i s expect3d t o be -158.5 dBW.

For t h e 1434 # r e c e i v e r n o i s e t e m p e r a t u r e shown on T a b l e 1, t h e

r e s u l t i n g G P S c a r r i e r - t o - n o i s o d e n s i t y r a t i o i s expected t o be

38.5

dB-Hz f o r a 5.5 dB C / N i n t h e nominal 2 k t z p o s t c o r r e l a t i o n bandwidth. T h i s v a l u e may be increased t o 41.9 dB-H2 f o r a 8.9 dB C / N r a t i o s h o u l d t h e e f f e c t o f t h e antenna t r a n s m i s s i o n l i n e be e l i m i n a t e d ,

REFSAT s i g n a l l i n k c o m p u t a t i o n s a r e g i v e n c n T a b l e 3. The R E F S A T c a r r i e r s i g n a l power a t the r e c e i v i n g antenna o u t p u t i s e x p e c t e d t o be

-146

dBW. F o r t h e r e c e i v e r n o i s e t e m p e r a t u r e shown on T s b l e 1, t h e r e s u l t i n g REFSAT c a r r i e r - t o - n o i s e d e n s i t y i s expected t o be 5 1 , O dB-Hz, c o r r e s p o n d i n g

to

an Eb/N, v a l u e o f 20.0 dB.

TABLE 2

GPS CHANNEL

t

INK COMPUTATIONS

(1575.42

M,!z)

R a d i a t e d Power ( s a t e 1 1 i t e ) T r a n s m i t t i n g Antenna G a i n Free Space Loss

Atmospheric Loss P o l a r i z a t i o n Loss R e c e i v i n g Antenna Gain

I

R e s u l t a n t power a t r e c e i v i n g a n ~ e n n a o u t p u t

-158.5

d0W I

E q u i v a l e n t r e c e i v i n g system n n i s e t e m p e r a t u r e 31.6 dBK

I

(1434

K

f r o m

Table

1 )

I

Bol tzmann's c o n s t a n t ( J / K ) -228.6 dB(J/K)

I

R e s u l t a n t c a r r i e r - t o - n o i s e d e n s i t y r a t i o , C/No

38.5

dB-Hz

Requi r e d

C

f o r s p e c i f i e d ~ e r f o r k a n c e .

35.2

dB-Hz

Excess M a r g i n 3.3 dB

I

(15)

TABLE 3

REFSAT S

l

GNAL CHANNEL

L

l NK COMPUTATIONS ( 1

556.40

MHZ)

E q u i v a l e n t I s o t r c p i c R a d i a t e d Power - 1 2 0 . 0 dBW

F r e e Space Loss (1 f o o t ) - 2 6 . 0 dB

P o l a r i z a t i o n Loss - 3 . 0 dB

R e c e i v i n g Antenna Gain 3 . 0 d 5 i

-

R e s u l t a n t c a r r i e r power a t r e c e i v i n g antenna - 1 4 6 . 0 dBW o u t p u t

E q u i v a l e n t r e c e i v i n g system n o i s e (1434 K 3 1 . 6 dBK from T a b l e 1 )

Bol tzmann's constant -228.6 dB (J/K)

R e s u l t a n t c a r r i e r - t o - n o i s e d e n s i t y r a t i o

51

- 0 dB-Hz

C a r r ier-to-FSK sideband r a t i o - 1 0 . 0 dB

- - --

R e s u l t a n t d a t a s i g n a l - t o - n o i s e d e n s i t y r a t i o , C/No 4 1 . 0 dB-Hz

D a t a r a t e (128 b / s ) 2 1 . 0 dB

Resvl t a n t

Eb/No

r a t i o E s t i m a t e d b i t e r r o r r a t e

20.0 dB l e s s t h a n 10-9

(16)

1.3 C i r c u i t D e s c r i p t i o n o f P r o t o t y p e REFSAT/GPS R e c e i v e r T e r m i n a l

T h i s s e c t i o n d e s c r i b e s a breadboard REFSAT/GPS R e c e i v e r T e r m i n a l shown i n F i g u r e 3 .

, The REFSAT/GPS R e c e i v e r uses a r e f e r e n c e s i g n a l g e n e r a t e d by t h e REFSAT system t o a i d G P S s i g n a l a c q u i s i t i o n 2nd t o reduce t h e c o m p l e x i t y o f user cornputat i o n s .

B r i e f l y , t h e dual-channel REFSAT-aided r e c e i v e r p e r f o r m s t h e f o l l o w i n g f u n c t i o n s :

1 ) Acqu i r e s t h e REFSAT r e f e r e n c e s igna 1 c o n t a i n i ng:

a ) A s t a b l e c a r r i e r f r e q u e n c y s i g n a l w h i c h serves as t h e r e c e i v e r p r e c i s i o n frequency r e f e r e n c e .

b) GPS d o p p l e r c o c f f i c i e n t d a t a (128 bps FSK)

2 ) U t i l i z e s t h e s t a b l e REFSAT c a r r i e r s i g n a l and d o p p l e r d a t a f o r a c q u i s i t i o n o f

4

GPS s a t e l l i t e s i g n a l s .

The d u a l - c h a n n e l , L-band REFSAT-aided r e c e i v e r shown i n F i g u r e 3

r e c e i v e s b o t h t h e 3EFSAT r e f e r e n c e s i g P t > l (1556.4011 MHz) and GPS NAVSTAR s a t e l l i t e e m i s s i o n s (1575.420 MHz C/A s i g n a l ) . The CHU A s s o c i a t e s

antenna p r o v i d e s e s s e n t i a l 1 y hemi s p h e r i c a l coverage ( 3 dBi norni n a l g a i n f o r e l e v a t i o n a n g l c s o f 30 degrees o r g r e a t e r above t h e h o r i z o n as shown on F i g u r e

4 ) .

For i n s t a l l a t i o n s r e q u i r i n g a remote antenna l o c a t i o n , t h e p r e s e l e c t o r f i l t e r , preamp1 i f i e r , and f i r s t m i x e r may be p l a c e d c l o s e t o t h e antenna, s i g n i f i c a n t l y r e d u c i n g t r a n s m i s s i o n c a b l e l o s s e s .

S i g n a l s f r o m t h e antenna a r e passed t h r o u g h a 4-pole c a v i t y p r e s e l e c t o r f i 1 t e r (1565 MHz fo, 30 MHz BW a t -3 dB p o i n t s ) , r e d u c i n g e f f e c t s o f o u t - of-band i n t e r f e r e n c e . A l o w - n o i s e preamp] i f i e r ( 2 dB NF, 30 dB g a i n )

i s p r o v i d e d ahead o f the f i r s t m i x e r , where a 1535 MHz local oscillator

s i g n 3 1 r e s u l t s i n 1 s t I F f r e q u e n c i e s o f 40.42 MHz f o r GPS s i g n a l s and 21.4 MHz f o r t h e REFSAT r e f e r e n c e s i g n a l . Tandem p o s t amp1 i f i e r s (28 dB g a i n each) p r o v i d e a d d i t i o n a l g a i n b e f o r e t h e two channels a r e s e p a r a t e d by a power d i v i d e r and bandpass f i l t e r s , FL-2 and FL-3.

(17)

HCMI L u T C M ~ L

c r u r~ .atat A G C ~ I F A M *

-.

I

I I f I . rn+Cb,HI U*1'7 u. +

1 . L - I

u.3 POW ALPS I ~ u r M-• v c THV Ed u n - 4 0

c l 5 V 1 J H , l > l

Twv.)O rL.f 4 I P I MO.IW

I f

r

.

I

C G C

- t l l V

F 4 1 W Fato fr"',fH

- . . -

rc M P r \ . C V C 0 FIUE

TUWC -7 -

I Y * l $ I A M P T l u l HO

L P 4 l . t VCC

t

- -

Y + !

IF 1 1 0 1

A M P F-CO(Imnl t l C l U4r .+I

FL.3 4~ mtl

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P L L ' I

M 8 * € 161

Z J 4 UUZ

=

C G C . q = +I

- t ILV u ~ i - 4 1 ~r t a t s M O 11 LV O.F.~T

--

I I

F i g u r e

3 .

Dua 1 Channel REFSAT Breadboard-Tes t Receiver.

I I

~ O ] ; D O U T FRll?.tl$

4

I

a

f

(18)

t 1

7

L % I , ] I

to C M l U Y l L t I

3 r A - r COMVE.I

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--

bl

~ Y T E P V A L T I M E R

U P r t s l P

L w

0

-

.- - -

-

. . . -- - - . - .

-

T I U I Y ~ - . - . . . . . - -. .

-

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1

Toup

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+ I I Y A R l

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-

I b

L O ~ O

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cnou ur

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TO V L L - I

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.

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T R A N S C C L I G I S T E * 9 Realsrcu

(19)

CHU ASSOCIATES

'

Horizon

ANTENNA:m-3124 51u 3 P A m R N CU'L H:

FREOUENCY: ?rjV5+ 4 U HZ A;LrML17H P M R AT-' ELNATION

PLOHED IN; ZENlTI-4 PLANE AT&' REUTIVE W M U T H

VCLTAG- POWER-

.

POIARUt?DN:

- E , RHCA

DATE: 5-15-79

5,-

CHC-

F i g u r e

4. Receiving Antenna Elevation Pattern.

(20)

1.3.1 REFSAT S i g n a l Channel

A f t e r 1 s t I F p o s t a m p l i f i c a t i o n , t h e REFSAT s l g n a l i s s e p a r a t e d f r o m t h e

G P S

s i g n a l s by bandpass f i l t e r i n g (21.4

MHz

f,, 30 kHz BW c r y s t a l f i 1 t e r

FL-3).

A f t e r f u r t h e r ampl l f i c a t io n , t h e REFSAT s l g n a l i s c o n v e r t e d t o a 2nd I F o f 400

kHz

u s i n g balanced m i x e r M-3 and a 21 MHz LO s i g n a l . The s i g n a l

Is

f u r t h e r ampl l f i e d and f i l t e r e d b e f o r e c a r r i e r r e c o v e r y and d e m o d u l a t l o n i n t h e p h a s e - l o c k e d l o o p , PLL-I, A t t h i s p o i n t , t h e REFSAT s l g n a l c o n s i s t s o f a c a r r i e r a t 400 kHz and d a t a c o n t a i n e d i n two FSK s i d e tones a t 407.0 kHz and 407.2 kHz. The s i g n a l i s s p l i t i n t o two p a t h s ( i n - p h a s e and q u a d r a t u r e ) t o p r o v l d e t h e needed i n p u t s t o t h e phase-locked l o o p demodulator, PLL-1.

The AM o u t p u t o f

PLL-1

c o n t a i n s REFSAT d a t a i n t h e f o r m o f FSK tones a t 7.0

kHz

and 7.2 kHz. T h i s s i g n a l i s f i l t e r e d and a p p l i e d t o PLL-2 f o r d e m o d u l a t i o n appearing as a s t a n d a r d TTL l e v e l s e r i a l b i t s t r e a m a t

"FSK o u t " f r o m PLL-2. The b i t stream Is s u p p l i e d as an o u t p u t t o t h e r n i c r o p r o c e s s ~ r fo r G P S d o p p l e r c o e f f i c i e n t d a t a .

The recovered REFSAT c d r r i e r appears a t p i n

5

o f PtL-1 and i s used as t h e s t a b l e frequency r e f e r e n c e needed f o r GPS s i g n a l a c q u i s i t i o n . Note t h a t a t t h e antenna, t h e REFSAT c a r r i e r f r e q u e n c y i s p r e c i s e l y 19.02 MHz below the G P S s a t e l l i t e c a r r i e r frequency ( n e g l e c t i n g d o p p l e r ) . S i n c e b o t h the REFSAT and GPS s i g n a l channels share common LO s i g n a l s f r o m t h e r e c e i v e r i s master o s c i l l a t o r , t h i s p r e c i s e f r e q u e n c y d i f f e r e n c e i s m a i n t a i n e d independent o f i n s t a b i l i t i e s a s s o c i a t e d w i t h t h e r e c e i v e r master o s c i 1 l a t o r .

The 400 kHz r e c o v e r e d REFSAT c a r r i e r a t p i n

5

o f PLL-I i s a p p l i e d t o m i x e r

M-7

a l o n g w i t h a d i g i t a l l y - c o n t r o l l e d d o p p l e r o f f s e t s i g n a l f r o m t h e f r e q u e n c y s y n t h e s i z e r . The d i g i t a l l y - c o n t r o l l e d s i g n a l has a c e n t e r frequency o f 19.020 MHz and may be v a r i e d

2 5

kHz i n 25 Hz s t e p s by t h e m i c r o p r o c e s s o r f o r d o p p l e r compensation. The o u t p u t o f m i x e r

M-7

(19.420 MWZ) Is i d e n t i c a l i n f r e q u e n c y t o t h a t o f GPS s i g n a l channel 2nd I F , n e g l e c t i n g d o p p l e r , T h i s o u t p u t i s f i l t e r e d i n

FL-4

( 1 9 . 4 2 0 MHz fo, 390 kHz BW) t o reduce unwanted m i x i n g components. T h i s r e f e r e n c e c a r r l e r i s modulated i n m i x e r M - 6 w i t h a r e p l i c a o f t h e GPS C/A code g e n e r a t e d by t h e m i c r o p r o c e s s o r and phase s h i f t e d i n t h e h y b r i d t o produce in-phase "I"

and q u a d r a t u r e "Q" channel r e f e r e n c e s i g n a l s f o r C/A code c o r r e l a t i o n ,

1.3.2 GPS S i g n a l Channel

R e t u r n i n g t o the 1 s t I F post a m p l i f i e r s , t h e GPS s i g n a l s a r e s e p a r a t e d f r o m t h e REFSAT s i g n a l b y bandpass f i l t e r i n g (6% BW FL-2)

.

A f t e r amp1 i f i - c a t t o n

(28

dB), t h e GPS channel i s c o n v e r t e d t o a 19.42 MHz 2nd I F i n m i x e r M-2 ( s h a r i n g a 21 MHz l o c a l o s c i l l a t o r s i g n a l w i t h t h e REFSAT c h a n n e l ) .

(21)

T h l s 2nd I F s i g n a l i s t h e n passed t h r o u g h an AGC amplifier t o produce c o n s t a n t amp1 f tude s i g n a l s f a r code c o r r e l a t l o n . The o u t p u t o f t h e AGC a m p l i f i e r i s d i v i d e d i n t o two channels f o r c o r r e l a t i o n w l t h the two q u a d r a t u r e r e f e r e n c e

s

i g n a l s d e r i v e d f r o m t h e REFSAT channel (mixers M-4 and

M-5).

The r e s u l t a n t I and Q channels a r e then p r o c e s s e d t d e n l l c a l

l y

t h r o d g h 2

kHz

BW a c t i v e f i l t e r s , I n t e g r . ~ t e - a n d - d u m p c i r c u i t s , and f a s t AfD c o n v e r t o r s

( 4

p s ) , The t i m i n g f o r t h e integrate-and-dump

and

A/D c f r c u f t s i s synchronized end c o n t r o l l e d f r o m t h e m i c r o p r o c e s s o r v i a t h e

8253

programmable i n t e r v a l t i m e r a t a 1 - m i l l i s e c o n d (ms)

r a r e .

The I n t e g r a t e and s t a r t - c o n v e r s i o n commands a r e t r i g g e r e d by programmed delays i n c o u n t e r s w i t h i n t h e

8253.

The e n d - o f - c o n v e r s f o n p u l s e from the I channel ADT-EBHB t r i g g e r s t h e

8253

t o r e s e t t h e I n t z g r a t Ion-and- dump c i r c u i t s , s e t t i n g t h e code loop t o i n t e g r a t e over a n o t h e r code word f o r s e q u e n t i a l c o r r e l a t i o n w i t h

4

GPS s a t e l l i t e s i g n a l s .

1.3.3

R e c e i v e r Frequency S y n t h e s i z e r

The r e c e i v e r f r e q u e n c y s y n t h e s i z e r shown on F i g c r e

5

g e n e r a t e s t h r e e local o s c i l l a t o r s i g n a l s :

a 1 s t LO, 1535.00

MHz

r 2nd

LO,

21.00 MHz

a 3 r d LO and d o p p l e r compensation, 19.020 MHz

.t- 5 kHz

i n 25 Hz steps,

A t

l

s i g n a l s a r e d e r i v e d f r o m t h e r e c e i v e r master o s c i l l a t o r (10 MHz X O ,

I

ppm) u s i n g d i g i t a l d i v i d e r s , phase-locked mu1 t i p 1 i e r s , and balanced mixers. The L-band 1 s t LO s i g n a l makes use o f a s t e p - r e c o v e r y dfode

( S R D )

.

D o p p l e r compensation i s p r o v i d e d by v a r y i n g t h e f r e q u e n c y o f a 20 kHz VCO -t-

5

kHz I n 25 Hz s t e p s upon command o f the p p r o c e s s o r v i a t h e D/A c o n v e r t e r shown on F i g u r e

5,

(22)

.

b x 3

1 ]!Hz

,

21 21 MHz ,d

t10

.

2 L.O. Signal

t

1535Rlz

I

7

1 MHz 3% 1 MHz 1 0z3 1 MHz

.

1 5

0.5 MHz t 5 KHz I n

15 MHz

q 1 2 1 q q - I

I

IC--

I I

0.520 MHz

i 5 KHz

Flgure 5 .

Recefver Frequency Synthesizer.

lSt 1.0. Signal 130

=

x 118

+20

A

0.5 MHz

.

- Doppler Control Signal

f r o m p Processor L4 D/A

-

SRD

c. I

120

I k

1

x 7 -3.5HHz 4.020 MHz -

+_5 KHz

(23)

2.0 REFSAT TRANSMITTER SIMULATOR

The REFSAT/GPS R e c e i v e r makes use o f t h e REFSAT r e f e r e n c e s i g n a l t o . a i d acquisition o f GPS s a t e l l i t e e m i s s i o n s and t o reduce c o m p u t a t i o n s a t

t h e u s e r r e c e i v e r . The REFSAT r e f e r e n c e s l g n a l c o n s i s t s o f : a ) a s t a b l e c a r r i e r a t a f r e q u e n c y o f 19.02 MHz b e l o w t h a t

o f GPS s a r e l 1 i t e e m i s s i o n s

(1556.40

MHz f o r

REFSAT)

and, b ) a 128 bps d a t a s t r e a m (FSK, 1556,4070 MHz and 1556,4072

MHz)

c o n t a i n i n g GPS d o p p l e r c o e f f i s i e n t and o t h e r d a t a .

I t was o r i g i n a l l y p l a n n e d t o t r a n s m i t t h e REFSAT r e f e r e n c e s i g n a l t h r o u g h ttte ATS-6 s a t e 1 1 f r e , S i n c e ATS-6 i s no l o n g e r a v a i l a b t e , i t has been necessary t o p r o v i d e a s e p a r a t e t r a n s m i t t e r t o s i m u l a t e t h e REFSAT 4ransmissIon. T h i s s e c t i o n p r o v i d e s a more d e t a i l e d description o f t h e c i r c u i t shown i n F i g u r e 2 t o p e r f o r m t h i s f u n c t i o n .

A block diagram o f t h e REFSAT t r a n s m i t t e r s i m u l a t o r i s shown on F i g u r e 6. A t e m p e r a t u r e c o n t r o l f e d c r y s t a l o s c i 1 l a t o r ( T C X O )

,

s t a b l e t o

5

p a r t s

In

101O, i s t h e r e f e r e n c e f r o m w h i c h the REFSAT r e f e r e n c e c a r r i e r i s d e r i v e d i n o r d e r t o a c h i e v e t h e r e q u i r e d frequency s t a b i l i t y

(+

-

10 HZ)

.

The s e r i a l REFSAT d a t a stream (128 bps) modulates a v o l t a g e c o n t r o l l e d o s c i 1 l a t o r ( V C O ) , p r o d u c i n g FSK s i d e t o n e s i g n a l s a t 7.0 and 7.2 kHz. The FSK s l d e t o n e s and a

455

kHz c a r r i e r s i g n a l , d e r i v e d f r o m t h e 10 MHz master o s c i l l a t o r , a r e a p p l i e d t o an AM m o d u l a t o r . A bandpass f i l t e r r e j e c t s the

lower sideband, y i e l d i n g :

a A r e f e r e n c e c a r r i e r a t 455

kHz

and,

FSK

s i d e t o n e s a t 462.0 and 462.2 kHz.

(FSK

s i d e t o n e s a r e 10 dB below the r e f e r e n c e c a r r i e r . )

The composite REFSAT s i g n a l i s t h e n t r a n s l a t e d t o 21.4 MHz, 121.4 MHz, and f i n a l 1 y t o 1556.4 MHz ( c a r r i e r f r e q u e n c y ) f o r t r a n s m i s s i o n .

(24)

NOTE: Frequencies s h m in HHz

Figure

6 .

REFSAT

Transmitter Simulator Block Diagram.

(25)

3.0 REFSAT/GPS RECEIVING TERMlN4L TEST PLAN

T h i s t e s t p l a n d e f i n e s t h e equipment c o n n e c t i o n s and p r o c e d u r e s npcessary t o p e r f o r m l a b o r a t o r y and s a t e l l i t e p r o o f - o f - c o n c e p t t e s t s o f the REFSAT/GPS sys tem i n c l u d l n g as t e s t o b j e c t i v e s :

V e r i f i c a t i o n o f t h e REFSAT c o n c e p t ,

Measurement o f t h e t e r m i n a l ' s g e o g r a p h i c a l l o c a t i o n w i t h an e r r o r o f n o t more t h a n 50 m e t e r s (rms) f o r s i g n a l s

(C/A code) f r o m t h e Phase I GPS s a t e 1 1

I

t e s .

As shown on F i g u r e 1 ( S e c t i o n I o f t h i s dqcument), t h e REFSAT-aided user r e c e i v e r :

e A c q u i r e s t h e REFSAT r e f e r e n c e c a r r i e r ( w h i c h s e r v e s as a s t a b l e r e c e i v e r r e f e r e n c e f o r GPS s i g n a l a c q u i s i t i o n ) ,

Demodulates c u r r e n t d o p p l e r c o e f f i c i e n t d a t a and computes doppler c o r r e c t i o n s f o r r a p i d G P S s i g n a l a c q u i s i t i o n , Demodulates c u r r e n t GPS s a t e l l i t e p o s i t i o n s o o r d i n a t e d a t a f o r d i r e c t i n s e r t i o n i n t o GPS n a v i g a t i o n e q u a t i o n s o l u t i o n ; a Makes use o f REFSAT c a r r i e r r e f e r e n c e and computed d o p p l e r

c o r r e c t ions t o a c q u i r e s e l e c t e d GPS C/A s i g n a l s ; a Measures pseudo-ranges t o

4

s e l e c t e d GPS s a t e l 1 i t e s ;

Solves GPS n a v i g a t i o n e q u a t i o n t o d e t e r m i n e user p o s i t i o n , S i n c e t h e ATS-6 s a t e l l i t e w i l l n o t be a v a i l a b l e t o t e s t t h e REFSAT concept as o r i g i n a l l y p l a n n e d , a low-power t r a n s m i t t e r r a d i a t i n g i n the immediate v i c i n i t y of t h e REFSAT-aided r e c e i v e r ' s antenna w i l l be used.

S i n c e c u r r e n t GPS s a t e l l i t e ephemeris d a t a i s n o t r e a d i l y a v a i l a b l e e i t h e r i n advance o r i n r e a l t i m e s , t h e REFSAT r e f s r e n c e s i g n a l used f o r t e s t i n g w i l l c o n t a i n duminy G P S s a t e l l i t e p o s i t i o n c o o r d i n a t e d a t a , Measured pseudo-range v a l u e s f r o m t h e r e c e i v e r w i l l be r e c o r d e d f o r

l a t e r o f f - l i n e s o l u t i o n o f t h e GPS n a v i g a t i o n e q u a t i o n when c c ~ i e s of d a i l y GPS u p l o a d d a t a c o n t a i n i n g t h e r e q u i r e d ephemeris d a t a a r e a v a i l - a b l e s e v e r a l days l a t e r f r o m the Vandeberg A i r F o r c e Base.

-- --

+ The REFSAT-aided r e c e i v e r i s , by d e s i g n , i n c a p a b l e o f d e m o d u l a t i n g t h e GPS n a v i g a t i o n message.

(26)

The remainder o f thls

section

i s

devoted to detailed deflnitlons

o f the

GPS satellite constellation viewing IJwindowa1,

the

experimental

test c o n n e c t i o n s , and procedures

for conducting

the necessary tests and d a t a

reduct ion.

(27)

3 . 1

T y p i c a l

G P S

S a t e l l i t e C o n s t e l l a t i o n V i e w i n g Window

NAVSTAR Phase I nominal o r b i t i n f o r m a t I o n f o r t h e

4

GPS s a t e l l

I

t e s o p e r a t i o n a l a t t h e b e g i n n i n g o f 1980 I s l i s t e d on T a b l e

4 ; (GFS

s a t e l l i t e o r b 1 t p o s i t i o n s

1 ,

2, 3,

4, 5 ,

and

6

a r e c u r r e n t l y o c c u p l e d by o p e r a t i o n a l s a t e l ?

l

t e s )

.

The o r b ; t parameters a r e referenced t o m i d n i g h t

(0

hours) Greenwich Mean Time (GMT) on 1 January 1979. On t h a t d a t e , t h e ground t r a c k o f NAVSTAR

#5

occupying o r b i t p o s i t i o n # 1 would f i r s r c r o s s t h e e q u a t o r i n t h e S o u t h - t o - N o r t h d i r e c t i o n a t a l o n g i t u d e o f 47.0 degrees a t

4

hours and

44.5

minures on t h e 6MT r i m e s c a l e , Some

43,078

seconds l a t e r (about 11 h o u r s

58

minutes) i t w i I 1 a g a i n c r o s s t h e e q u a t o r i n t h e S o u t h - t o - N o r t h d i r e c t i o n , t h i s t i m e a t a l o n g i t u d e o f 227.0 degrees. The t i v e o f succeeding a s c e n d i n g node c r o s s i n g s w i 1 1 be

4

m i n u t e s ,

3.4

seconds e a r l i e r each

fol

l o w i n g day:k.

F i g u r e

7

shows t h e computed ground t r a c k s f o r t h o s e

4

GPS a r b ! t positions l i s t e d i n T a b l e

4.

The 20 degree e l e v a t i o n v i s i b i l i t y c o n t o u r about t h e t e s t s i t e (washington, D . C . ) i s a l s o shown. A s a t e l l i t e whose ground t r a c k i s N o r t h o f the v i s i b i l i t y c o n t o u r will appear a t an e l e v a t i o n o f 20 degrees o r g r e a t e r when viewed f r o m t h e t e s t s i t e ,

F i g u r e

8

shows t h o s e p o r t l o n s o f t h e GPS ground t r a c k s between 1600 and 2'200 h o u r s GMT ( t h e GPS c o n s t e l l a t io n v i e w i n g window f r o m t h e t e s t s i t e ) . The d i r e c t i o n o f ground

track

t r a v e l i s n o t e d and e a c h t r a c k i s marked a t 1 hour I n t e r v a l s .

F i g u r e 9 shows a p l o t o f t h e individual sat ell it^ e l ~ v a t i o n a n g l e s as viewed from t h e t e s t s i t e as a f b n c t i o n o f t i m e o f day. F i g u r e 10 shows carnpdted d i l u t ! o n - o f - p r e c i s i o n v a l u e s . The t y p e s o f d i l u t i o n - o f - p r e c i s i o n va 1 ues shown a r e geornet r i c (GDOP)

,

t h r e e d i m e n s i o n a l pos i t i o n

(PDOP) ,

and two d i m e n s i o n a l h o r i z o n t a l (HDOP). T h i s d a t a i s r e p e a t e d on an expanded s c a l e on F i g u r e s 1 1 arid 12. Ti-re v i e w i n g window when a l l

4

s a t e l 1 i t e s a r s v i s i b l e above t h e h o r i z o n a t t h e t e s t s i t e extends f r o m a b o u t 1600 t o 1950 h o u r s ( 1 January 1979 r e f e r e n c e ) d a i l y . From F i g u r e 12, the d i l u t i o n - o f - p r e c i s i o n v a l u e s reach a minimum o f

4

t o 6 w i t h i n t h e t e s t window, r i s i n g r a p i d l y a f t e r 1800 h o u r s . ( f o r example, f o r a DOP o f

5,

e r r o r i n computed u s e r p o s i t i o n a r e e x p e c t e d t o be about

5

t i m e s t h e e r r o r i n measured pseudo- range values.) The u s e a b l e t e s t window w i 1 l t h e r e f o r e e x t e n d f r o m about 1630 hours t h r o u g h 1830 h o u r s d a i l y ( 1 January 1979 r e f e r e n c e ) .

N o t e t h a t t h e GMT time s c a l e v a l u e s g i v e n on F i g u r e s

8

t h r o u g h 12

a p p l y t o 1 January 1979. The t i m e , GMT' a t w h i c h a p a r t i c u l a r event o c c u r s a t a l a t e r d a t e I s g i v e n i n d e c i m a l hours b y :

GMT1 [GMT

-

0.0676 ( t i m e s t h e day o f y e a r a f t e r 1 J a n u s r y 1 9 7 9 ) l

modulo 24 ( 1 )

*

The nominal ground t r a c e o f each s a t e l l i t e i s f i x e d and t n e r e f o r e r e p e a t s e v e r y 23 h o u r s

55

m i n u t e s and

56.6

seconds. T h i s d e p a r t u r e from a 24 h r . p e r i o d i s p r i m a r i l y due t o t h e d i f f e r e n c e s between a s o l a r and a s i d e r e a l day.

(28)

TABLE 4

NAVSTAR PHASE I ORBIT DATA

AT FIRST ASCENDING

NODE

ON 1 JANUARY

I979

NODAL PER 1

OD

s

43,078.3

43,078.3

4 3 , 0 7 8 - 3 43,078.3

Note: Parameters above a r e nominal. Nodal p e r i o d i s c o n t r o l l e d t o L 2 s and l o n g i t u d e o f ascending node i s c o n t r o l l e d t o 2 deg. Hence t h e t i m e o f an ascending node may vary k 8 minutes.

(Above d a t a from memo

-

J.E. Farr, P.S. Jorgensen, 8 August 1978; Modified June 1980 NAVSTAR

SATELLITE

#) ,

1

ARGUMENT

OF

PER

l GEE

deg

.

343

- -

97

--

ECCENTRICITY

0.0038 0 0.0048 0

F

l RST

ASCEND l NG NODE TIME (GMT)

4h44rn30s 2h00m59s 1 h54m07s Oh27mlSs F l RST

ASCEND l NG NODE LONG ITUDE

deg

.

' 47.0 89.5

332.0 353.0 NAVSTAR

SATELLITE

#

NAV

5

NAV

4

NAV 6

NAV 3

GPS ORB I T POSITIOV

#

1

3

5

6

INCLINATION

deg

.

63.25

6 3

63.18

63

(29)

l i t e

O r b i t

tion f: 5

S a t e Pos

Figure 7, GPS S a t e l l i t e Ground Tracks (20 deg. e l e v a t i o n v t s i b i l i t y contour shown around Wash., D.C. )

1 - Hour Intervals

Satelli

P o s i t

Figure 8,

GPS

Ground Tracks, 1600-2000 hrs

GMT,

1 Jan.

1979

(20

deg.

elevation v i s i b i 7 i t y contour shown around

Wash., D.C.)

(30)

a0 , 3

70

.

1

60 '

Satelllte O r b i t

50

.

Posi tim

10

[1 2 4 6 8 10 12 14 16 18 20 22 24

Hours GMT, 1 January 79

Figure 9, GPS Satellite E l e v a t i o n Angles at Wash., D.C.

'

2 4 6 b 10 12 14 16 18 20 22 24

Hours GMT, 1 January 7 9

F i g u r e

10. Dilution-of-Precision (DOP) Values at Wash., D.C.

(31)

F i g u r e

11,

GPS

Satellite Elevation Angles a t Wash,, D.C.

0

16 17 I8 19 20

Hours GMT, 1 January 79

Figure

12.

Dilution-of-Precision

(DOP)

Values

a t

Wash., D . C .

~ > 2 0 ° e l e v w i rrdou*

*

>1OD eltv wfndow

*

1

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