(:3a5A-L~;- 1 u b u
r
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{ S f = ;
tcma tic; ~ ; c : r r e r d ~
C u r l . . ) 43HC t i i ~ J / ~ l k uU 1 L L L L 1 7 i i Ilricl ik
G j / j L 3111;
Systematics General Corpora tion
?!I22 J~ledar UIIII~I Falls mhur~h. Uirqillia 22ll42
REFSA'T/GPS BREADBOARD
-
TEST TERMINALHARDWARE
DESIGN
REPORTNovember 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 rEarth 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 BranchGreenbelt, Maryland 20771
Prepared Under NASA/CSFC C o n t r a c t NAS5-26039
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 .
TABLE OF CONTENTS
SECTION
.
PAGE1.0 REFSAT/GPS BREADBOARD-TEST T E R M I N A L HARWARE DESjGN
...
11.2 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
...
81.3
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
...
313.2.3 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 Procedure3 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 Recovery35
...
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 n35
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
LIST
OFFIGURES
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 25 .
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 nvisibility c o n t o u r shown around Washington.
D.C.) ...
228 .
G P S Grocnd Tracks. 1600-2000 hrs GMT.1
J a n u a r y1979
(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 dWashington. 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 30LIST
OF TABLESTABLE
.
PAGE1
.
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 ~ )...
83
.
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 on1 January
1979 ...
21...
5 . GPS
Satellite Test Window at Washington.D.C.
256 .
GPS Satellites I nO r b i t
As O f June 19110...
260 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 1and 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 rc 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 ;
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
REFSATConcept.
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 IDemodulates 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 lused 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 ) .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
-
ReferenceFrequency Osclllatnr Pseudo Range Data
-
Synthesizer 10 lMz-
i 5 ~ 1 D - ~ ~Rccefvtr
Figure
2.
Prototype REFSAT/GPS Receiver a n d Simulated REFSAT.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 t1535
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 -t4
kHz maximum expected d o p p l e r s h i f t f o r t h e GPS channel and 21.4MRZ
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 2ndIF
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 nmodulated 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 rGPS
s a t e l l i r e .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
andM-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 rGPS
satellite, i n p u t s t o t h e c o r r e l a t i o n mixersM-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 s1535
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
x5 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 BudgetT a b l e
1
shows receiver n o i s e budget camputations. Components fromFigure 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 t1.5
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 .
TABLE I
RECEIVER NOlSE BUDGET
N o i s e Loss
Factor Factor C -wputat ion
Noise Temperature
Referred to Antenna
Antenna
- -
170K
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 5.013.98
2 9 0 ( 5 . 0 1 - 1 ) x ~ 1 / 5 0 0 ~ ) ~ 1 . 2 3 x 2 . 0 0-
- 3K
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
KE q u i v a l e n t system nolse temperature 1434
K
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 gto
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 IE 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 mTable
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-HzRequi 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-HzExcess M a r g i n 3.3 dB
I
TABLE 3
REFSAT S
l
GNAL CHANNELL
l NK COMPUTATIONS ( 1556.40
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-HzC 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 e20.0 dB l e s s t h a n 10-9
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.
HCMI L u T C M ~ L
c r u r~ .atat A G C ~ I F A M *
-.
II 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
.
IC 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
I I
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
af
t 1
7
L % I , ] I
to C M l U Y l L t I
3 r A - r COMVE.I
C
~ O C Y A N U C C 1
--
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 ~ - . - . . . . . - -. .
-
C14
C a
-. . .- ... . Ltmn x
1
ToupU E $64 * I I Y cup -15v -sv
V U 7 1 0 1
-.-- t t M ¶ B \ I 0 P I C . U I I I E I 4UTpUT
. A
* L V b S * * P l C * l a r l l t I l D M V
* NfCTEUEblS
-
I T O R I ET
014 c a w I
-
*TJ
W . rulr sMtt ILLINKS
N&TU"*L ~ ' e = * L Q
k
+ I I Y A R l
-
+---
--
. C I e l ¶ I-
I b
L O ~ O
<
cnou urL
TO V L L - I
vco r ~ u t rur 6
rtu 1
TTL
.
U I C H . M O L D.
T R A N S C C L I G I S T E * 9 RealsrcuCHU 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.
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 (21.4MHz
f,, 30 kHz BW c r y s t a l f i 1 t e rFL-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 400kHz
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 lIs
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 7.0kHz
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 rM-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 d2 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 rM-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 ) .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 andM-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 ll 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-dumpand
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 e8253
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 e8253.
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 e8253
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 h4
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 rThe 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 2ndLO,
21.00 MHza 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 e5,
.
b x 31 ]!Hz
,
21 21 MHz ,dt10
.
2 L.O. Signalt
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 Signalf r o m p Processor L4 D/A
-
SRD
c. I
120
I k
1
x 7 -3.5HHz 4.020 MHz -+_5 KHz
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 rREFSAT)
and, b ) a 128 bps d a t a s t r e a m (FSK, 1556,4070 MHz and 1556,4072MHz)
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 o5
p a r t sIn
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 thelower 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 .
NOTE: Frequencies s h m in HHz
Figure
6 .
REFSATTransmitter Simulator Block Diagram.
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.
The remainder o f thls
section
i sdevoted to detailed deflnitlons
o f theGPS satellite constellation viewing IJwindowa1,
theexperimental
test c o n n e c t i o n s , and proceduresfor conducting
the necessary tests and d a t areduct ion.
3 . 1
T y p i c a lG 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 WindowNAVSTAR 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 lI
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 e4 ; (GFS
s a t e l l i t e o r b 1 t p o s i t i o n s1 ,
2, 3,4, 5 ,
and6
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 t4
hours and44.5
minures on t h e 6MT r i m e s c a l e , Some43,078
seconds l a t e r (about 11 h o u r s58
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 be4
m i n u t e s ,3.4
seconds e a r l i e r eachfol
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 e4
GPS a r b ! t positions l i s t e d i n T a b l e4.
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 groundtrack
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 l4
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 f4
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 f5,
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 about5
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 12a 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 ) lmodulo 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 s55
m i n u t e s and56.6
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.TABLE 4
NAVSTAR PHASE I ORBIT DATA
AT FIRST ASCENDING
NODE
ON 1 JANUARYI979
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 NAVSTARSATELLITE
#) ,1
ARGUMENT
OF
PER
l GEEdeg
.
343
- -
97
--
ECCENTRICITY
0.0038 0 0.0048 0
F
l RSTASCEND 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
56
INCLINATION
deg
.
63.25
6 3
63.18
63
l i t e
O r b i ttion 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,
GPSGround 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.)
a0 , 3
70
.
1
60 '
Satelllte O r b i t
50
.
Posi tim10
[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 24Hours GMT, 1 January 7 9
F i g u r e
10. Dilution-of-Precision (DOP) Values at Wash., D.C.
F i g u r e
11,
GPSSatellite 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 tWash., D . C .
~ > 2 0 ° e l e v w i rrdou*
*
>1OD eltv wfndow*
1