General Corporation

Systematics General Corporation

2922 Tdefitas

lltlrarf

Falls Ohurch. Uirqi~lia 221142

GPS/REFSAT

D e f i n i t i o n

Study

Report For

Low-Cost Terminals

November 1980

Prepared for

National Aeronautics and

Space

Admini

stv'rz t i

on Goddard

Space

f l i - " t Center

Earth

Obser*vation Sy. 2 : -

";-!i

sion User

Termilia1

& L o c a t i o n Sya

i.e,nr; Branch

Greenbelt, Mary1 and 20771

Prepared on

NASA/GSFC

Contract

NAS5-23433

by

SYSTEMATICS GENERAL CORPORATtON

2822 TELLSTAR Cf. FALLS CHURCH, VA. 12042

TABLE OF CONTENTS

SECT l ON

-

PAGE

Abs t racr 1

1 . NAVSTAR GPS System E GPS/REFSAT System--- 2

1.1 NAVSTAR GPS System--- 2

1.2 The GPS/REFSAT System---

4

1.2.1 I n i t i a l GPS S a t e l l i t e Selection--- . 8

1 . 2 . 2 Doppler and Delay A c q u i s i t i o n - - - 1.2.3 P o s i t i o n Fixing--- 8

8 1

.2.4

Format o f t h e REFSAT D a t a Message--- 10

2.1 Requi red Frequency Assignments-

... 1 4

2.2 S p a c e c r a f t Antenna F o o t p r i n g Coverage Requirement--- 2 . 3 RF L i n k Budget Computations--- 15

2 . 4 S p a c e c r a f t Anterlna System---.---

15

2.5

Transponder System--- 15

15 3. A l t e r n a t e Methods F o r Implementing a GPS/REFSAT System---

17

4.

GPS/REFSAT User T e r m i n a l Segment---.--- 2 2

4

1 2-Channel R e c e i v e r Requi rernen ts--- 22

4.2 D a t a

Transmitter---~---

22

4.3

S i m p l i f i e d B l o c k Diagram, GPS/REFSAT T e r m i n a l - - - 22

5.1 G e n e r a t i o n o f t h e REFSAT D a t a Message--- 25

5.2 REFSAT Reference C a r r i e r ,lnd Message Timing---

25

Appendix: Performance Speci f i c a t i o n f o r REFSAT/NAVSTAR C i v i 1

User

System---.-- A- 1

LlST O F TABLES

- ^-

TABLE

-

^PAGE

1 - 1 .

NAVSTAR GPS Signal

Charcteristics---an---

3 1-2. GPS Receiver Functions Comparison--- 2 2

LlST OF FIGURES

F

l GURE

-

PAGE

1 - 1 , GPS Satellite Ground

T r a c k s (20

degree elevation visibility

contour

shown

around Washington,

DC)---

5 1-1A. GPS Satellite Elevation Angles at

Washington, DC---

6

1-16.

Dilution-of-Precision (DOP) Values

at

Washington, DC--- 6

1-4. REFSAT vs. Conventional Receiver--- I 1

3- 1 , GPS/REFSAT Terminal (VHF

REFSAT

Signal

~i nk)

--- ¹⁸

3-2. GPS/REFSAT

System

Utilizing

the

ATS-3 Satellite--- 19 3 GPS/REFSAT System Using

a

Local Tower---

2

1

4-1. Simp1 i f

i e d Block

Diagram, GPS/REFSAT Terminal---

²

4

5-1.

Simplified Functional Block

Diagram,

REFSAT

Remote Control

~ t ~ t i ~ ~ - - - ~ - - - 26

ABSTRACT

The NAVSTAR G l o b a l P o s i t i o n i n g System (GPs) i s a s a t e l l i t e n a v i - g a t i o n system c u r r e n t Zy under development by t h e Department of Defense.

I t w i l l c o n s i s t o f 18 s a t e l l i t e s i n c i r c u l a r , 12-hour o r b i t s a t an a l t i t u d e o f 11,000 NM, i n c l i n e d

63"

t o t l i e e q u a t o r , GPS s a t e l l i t e s w i l l b r o a d c a s t pseudo-random n o i s e codes and ephemerides on two

L-band s i g n a l s t o u s e r s w o r l d w i d e , i n c l u d i n g m i l i t a r y and c i v i l u s e r s . T h i s r e p o r t d e s c r i b e s a concept t h a t u t i l i z e s a r e l a y transponder, l o c a t e d e i t h e r on a s a t e l l i t e i n g e o s t a ~ ~ o n a r y o r b i t o r on a l o c a l tower t o r e l a y a c q u i s i t i o n - a i d i n g data, ephemerides, e t c , from a ground-based remote c o n t r o l s t a t i o n t o a GPS c i v i l u s e r t e r m i n a l

l o c a t e d on a s h i p o r l a n d - t r a n s p q r t a t i o n v e h i c l e . Termed REFSAT ( ~ e f e r e n c e S a t e l l i t e ) , t h i s concept reduces s i g n i f i c a n t l y t h e c i r c u i t c o m p l e x i t y and c o s t o f u s e r t e r m i n a l s . A l a w - c o s t , REFSAT u s e r t e r m i n a l m i g h t r a n g e i n p r i c e f r o m $1,000 t o $2,000 per t e r m i n a l , i n l o t s o f 3,000.

T h i s r e p o r t d e f i n e s t h e v a r i o u s systems needed t o implement t h e REFSAT concept f o r l o w - c o s t , GPS c i v i l t e r m i n a l s . The GPS/REFSAT system c o m p a t i b l e w i t h t h e NAVSTAR GPS sys tern c o n s i s t s o f a space segment

(gees

t a t i o n a r y

r e l a y sa t e l l i t e )

,

^a ground t e r m i n a l segment ( c i v i 1 u s e r t e r m i n a Is), and t h e remote c o n t r o l s t a t i o n ( t h e c e n t r a l f a c i l i t y w h i c h p e r f o r m s o p e r a t i o n s common t o a1 ? users f o r r e l a y v i a t h e spacc segment), Each segmeat i s d e s c r i b e d i n d e t a i l .

A GPS/REFSAT system u t i l i z i n g a l o c a l tower f o r t h e r e l a y t r a n s p o n d e r i s a l s o d e s c r i b e d .

The

results

o f a s t u d y o f c i v i l u s e r r e q u i r e m e n t s i s p r e s e n t e d . D e t a i l e d s p e c i f i c a t i o n s f o r t h e GPj/REFSAT system and i t s i n d i v i d u a l segments a r e p r e s e n t e d i n t h e appendix.

1 , IAVSTAR GPS System & GPS/REFSAT System

The GPS/REFSAT Systemfi augments t h e p l a n n e d NAVSTAR G l o b a l P o s i t i o n i n g

(GPs)

System, a1 l o w i n g a s i g n i f i c a n t r e d u c t i o n i n t h e c o s t and conrplexi t y o f c i v i l u s e r t e r m i n a l s .

A number o f o p e r a t i o n s common t o a l l c i v i l users w i t h i n a p a r t i c u l a r g e o g r a p h i c a r e a may be p e r f o r m e d a t a c e n t r a l remote c o n t r o l s t a t i o n . I n f o r -

m a t i o n c o n t a i n i n g t h e r e s u l t o f t h e s e o p e r a t i o n s i s b r o a d c a s t a l o n g w i t h a p r e c i s i o n f r e q u e n c y r e f e r e n c e t o a l l c i v i l u s e r s t h r o u g h t h e REFSAT/GPS g e o s t a t ! o n a r y space seyment.

The f o l l o w i n g s e c t i o n s d e v e l o p t h e d e f i n i t i o n o f t h e GPS/REFSAT system, d e t a i l i n g i t s method o f o p e r a t i o n and r e l a t i o n t o t h e NAVSTAR G P S s y s t e m .

1 . 1 NAVSTAR GPS System

The NAVSTAR Global P o s i t i o n i n g System ( G P S ) o f f e r s a c c u r a t e three-dimensions 1 p o s i t i o n (and v c l o c i t y i f d e s i r e d ] i n f o r m a t !on t o u s e r s anywhere i n t h e w o r l d . A user p o s i t i o n f i x c o n s i s t s o f t h e f o l l o w i n g s t e p s :

Measuring t h e t r a n s i t t i m e o f RF s i g n a l s f r o m t h r e e GPS s a t e l l i t e s o f a t o t a l c o n s t e l l a t i o n o f 18 s a t e 1 1 i t e s and cornput i n g t h e r e s u l t a n t d i s t a n c e s . ( T r a n s i t t i m e i s d e t e r m i n e d u s i n g t h e speed o f

l

i g h t ) . Computing t h e c u r r e n t p o s i t i o n o f t h r e e GPS s a t e l l i t e s u s i n g t h e ephemeris d a t a t r a n s m i t t e d w i t h each s a t e l l i t e s i g n a l ,

S o l v i n g t h e r e s u l t a n t l l t r i a n g u l a t i o n l ' e q u a t i o n s t o d e t e r m i n e t h e pos i t i o n o f t h e user t e r m i n a l

.

I f u s e r t e r m i n a l s m a i n t a i n e d p r e c i s i o n c l o c k s s y n c h r o n i z e d w i t h GPS system t i m e , a p o s i t i o n f i x c o u l d be accomplished i n t h i s s i m p l e manner. The u s e r t e r m i n a l w o u l d then be a t t h e i n t e r s e c t i o n o f t h r e e spheres whose c e n t e r s were l o c a t e d a t t h e r e s p e c t i v e s a t e l l i t e s .

I n p r a c t i c e , t h e r e q u i r e m e n t f o r u s e r t e r m i n a l s t o c o n t a i n a p r e c i s i o n c l o c k i s e l i m i n a t e d by making range measurements t o f o u r s a t e l l i t e s . I n t h i s case, t h e n a v i g a t i o n e q u a t i o n s c o n t a i n f o u r unkqowns: t h e u s e r p o s i t i o n i n t h r e e dimensions and t h e e r r o r , o r f i x e d b i a s , i n t h e u s e r ' s i m p r e c i s e c l o c k .

The n a v i g a t i o n s i g n a l t r a n s m i t t e d f r o m each NAVSTAR G P S s a t e l l i t e c o n s i s t s o f two R F f r e q u e n c i e s , L1 and L2. As shown i n T a b l z 1-1, t h e L1 s i g n a l a t

1575.42 MHz i s modulated w i t h b o t h t h e P and C/A pseudo-random n o i s e (PRN) codes i n phase q u a d r a t u r e . The

12

s i g n a l a t 1227.6

MHz

i s m o d u l a t e d w i t h t h e P-code.

Both s i g n a l s a r e a l s o c o n t i n u o u s l y modulated w i t h t h e n a v i g a t i o n message d a t a - b i t s t r e a m a t

50

bps.

*

^{J. W.} S e n n o t t , A . K. Choudhury, R.E. T a y l o r , "The REFSAT Approach t o Low- Cost G P S Terminals", NASA Goddard Space F l i g h t Center, TM

79655,

A p r i l 1979.'

T a b l e 1 - 1 , NAVSTAR GPS S i g n a l C h a r a c t e r i s t i c s ,

L1 RF Frequency

= 1575.42

MHz

Conta i n s : PRN P-Code PRN C / A Code

50

BPS D a t a

Stream

L2 RF F r e q u e n c y = 1 2 2 7 . 6 0 YHz

Conta i ns PRN P-Code

SO

BPS Data Stream Long ( P ) Code

Frequency = 1 0 . 2 3 MHz

Epoch = 267 Days

(Reset each

7

d a y s ) Short ( c / P ) Code

Frequency

=

1 . 0 2 3 MHz

Epoch = 1

millisecond

The d a t a stream i n c l u d e s s r : t e l l j t e e p h e r ~ ~ e r i s i n f o r m a t i o n , t o a1 low a user t o compute t h e 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 needed f o r s o l u t i o n of t h e n a v i - g a t i o n e q u a t i o n , The PRN codes s e r v e two f u n c t i o n s :

1) S a t e l l i t e i d e n t i f i c a t i o n . The code p a t t e r n s a r e unique t o each s a t e l l i t e and a r e matched w i t h l i k e codes i n t h e u s e r r e c e i v e r and, 2 ) The measurement o f n a v i j a t i o n s i g n a l t r a n s i t time, by measuring t h e

phase s h i f t r e q u i r e d t o match t h e codes.

The u s e r t e r m t n a l s d i s c u s s e j i n t h i s document u t i l i z e t h e L l s i g n a l a t 1575,42 MHz and t h e s n o r t C/A code which r e p e a t s e v e r y m i 11 isecond. More s o p h i s t i c a t e d u s e r s may make use o f b o t h

L1

and

L2

s i g n a l s t o measure iono- s p h e r i c p r o p a g a t i o n d e l a y s o r t o u t i l i z e t h e l o r ~ g c r P-code f o r extreme a c c u r a c y ,

The b a s e l i n e c o n s t e l l a t i o n of I8 NAVSTAR s a t e l l i t e s w i l l be p l a c e d i n 12-hour o r b i t s p r o v i d i n g a t least f i v e s a t e l l f t e s a t

5

degrees o r more above t h e l o c a l h o r i z o n t o a w o r l d w i d e user.

F i g u r e 1-1 shows ground t r a c k s o f t h e f o u r Phase

1

NAVSTAR s a t e l l i t e s o p e r a t i o n a l a s o f 1 January 1980 ( o r b i t p o s i t i o n s 1 , 3,

5,

and 6 ) . A g i v e n s a t e l l i t e ground t r a c k r e p e a t s each 12 hours. A l s o shown i s t h e 20 degree v i s i b i l i t y c o n t o u r about Washington, DC. 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 t h e v i s i b i l i t y c o n t o u r w i l l appear 20 degrees o r more above t h e h o r i z o n .

The GPS/REFSAT System

The GPS/GEFSAT System i l l u s t r a t e d on F i g u r e 1-2 o f f e r s a s i g n i f i c a n t r e d u c t i o n i n b o t h t h e c o s t and c o m p l e x i t y o f u s e r ground t e r m i n a l s . A remote c o n t r o l s t a t i o n performs many o f t h e t a s k s r e q u i r e d o f a c o n v e n t i o n a l GPS u s e r t e r m i n a l , D a t a from t h e remote c o n t r o l s t a t i o n i s t h e n b r o a d c a s t over a w i d e geograph i ca l area 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 s a t e l 1 i ie (REFSAT) i n

t h e form o f an L-band " a i d i n g s i g n a l " . GPS/REFSAT u s e r t e r m i n a l s i n t h e s e r v i c e a r e a o f t h e remote ~ o n t r o l s t a t i o n use t h e L-band REFSAT s i g n a l t o a i d GPS

s a t e l l i t e s i g n a l a c q u i s i t i o n and t o s i m p l i f y computation o f t h e u s e r ' s p o s i t i o n . Before d i s c u s s i n g t h e

GPS/REFSAT

system I n d e t a i l , two o b s e r v a t i o n s f r o m F i g u r e s 1-1 and

I-lA/1-IR

are p e r t i n e n t . An e x a m i n a t i o n of Phase 1 GPS ground t r a c k s n o r t h of t h e 20' v i s i b l l i t y c o n t o u r shows t h a t t h e geometry o f t h e GPS s a t e l l i t e s r e l a t i v e t o a u s e r t e r m i n a l :

( 1 ) Changes r a p i d l y w i t h time. A p a r t i c u l a r group o f

4

s a t e l l i t e s remains v i s i b l e t o a u s e r t e r m i n a l , above t b s

20'

e l e v a t i o n angle, f o r 2 t o 3 hours. However, the 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 n

(HDoP) f o r t h e same f o u r s a t e 1 l ites i s

5

o r

less

f o r o n l y

2

hours.

( 2 ) Changes s l o w l y : 4 l t h u s e r g e o g r a p h i c a l l o c a t i o n . A t a g i v e n i n s t a n t i n t i m e , users anywhere i n t h e c c n t i n e n t a l U n i t e d S t a t e s c o u l d u t i l i z e t h e same 4 - s a t e l l i t e c o n s t e l l a t i o n .

20° Elevation Visibility Contour

Figure 1-1. GPS

Satellite

Ground Tracks ( 2 0 degree elevation

vis!billty

contour shown around Washington, G.c.)

80 Satellfte O r b I t 3 ¹ Posl tlon Number

70 ,

60 *

50 .

0 2 4 6 B 10 12 14 16 10 20 22 24

Haurs GMT. 1 January 79

F i g u r e 1 - 1 A , GPS

Satellite Elevation Angles

a t Wash,,

D.C,

F i g u r e 1-15.

Dilution-of-Precision

(DOP)

Values

a t Wash., D . C .

9 8

7 . 5 . 5

4 . 3 . 2 . 1 .

. . .

-

gcornetrlc d i l u t i o n o f prccislon

IW..-UCIIII..I...W--

p o s t t i o n d i l u t f o n o f precision

horizontal d i l r i t l o n o f precision

4

O ' ^'

4

^{' 6} - 8 ^' 10

12

^{14 16 1'8 ' 20 22 24}

Hours GHT. 1 January 79

Figure 1-2. The

GPS/REFSAT

System

The f i r s t s t e p r e q u i r e d t o p e r f o r m a GPS n a v f g a t i o n f i x i s t h a t o f s e l e c t i n g f r o m t h e v i s i b l e GPS s a t e l l i t e s t h o s e f o u r t h a t p r o v i d e t h e minimum h o r i z o n t a \ d l lution-of-precision (HDOP). T h i s i n i t i a l s t e p shown o n F i g u r e 1-3 i s i m p o r t a n t s i n c e the

magnltude

of t h e u s e r p o s i t l o n e r r o r s

I n t h e computed GPS n a v i g a t i o n f t x depend n o t o n l y upon t h e u s e r ' s r a n g i n g e r r o r s , b u t on t h e r e l a t i v e geometry o f t h e f o u r s e l e c t e d s a t e l l i t e s .

1.2.1 I n i t i a l G P S S a t e l l i t e S e l e c t i o n

I n g e n e r a l , a l l user t e r m i n a l s w i t h i n a l a r g e g e o g r a p h i c a l area would s e l e c t t h e same f o a r GPS s a t e l l i t e s i n o r d e r t o a c h i e v e minimum HDOP a t a p a r t i c u l a r t i m e . T h i s s e l e c t i d n r e q u i r e s t h e maintenance o f c u r r e n t G P S satel 1

l

t e almanac d a t e i n memory and t h c s o l u t i o n o f an I1HDOP1' a l g o r i t h m . The REFSAT s y s t e m performs t h i s t a s k a t a c e n t r a l l o c a t i o n and then b r o a d c a s t s

t h e r e s u l t s t o a l l users, relieving i n d i v t d u a l u s e r t e r m i n a l s of t h i s s o f t - ware and s t o r a g e f u n c t i o n .

1 . 2 . 2 Doppler and Delay A c q u i s i t i o n ^-,

..

For s i g n a l a c q u i s i t i o n

( 4

GPb s a t e l l i t e s ) , a c o n v e n t i o n a l u s e r t e r m i n a l may p e r f o r m a frequency a ~ d ccdc d e l a y search o r make use o f s t o r e d G P S almanac d a t a t o compute expected c n ? ; ' c ~ ' n i f t s f o r the f o u r s a t e l l i t e s i g n a l s . A p r e c i s i o n f r e q u e n c y r e f e r e n c e : s r e q u i r e d a t t h e u s e r t e r m i n a l t o reduce d o p p l e r a c q u i s i t i o n time.

I n t h e REFSAT System, a m a j o r p o r t i o n o f t h l s f u n c t i o n i s performed by t h e remote ? c o n t r o l s t a t i o n , A s shown on F i g u r e

1-3,

t h e REFSAT p r o v i d e s a

p r e c i 5 !on f r e q u e n c y r e f e r e n c e t o a1 1 users ( t h e REFSAT C a r r i e r Frequenctt S i g n a l )

.

I n a d d i t i o n , t h e data message c a r r i e d on t h e REFSAT s i g n a l p r o v i d e s I J d o p p l e r c o e f f i c i e n t 1 ' i n f o r m a t i o n a l l o w i n g a s i m p l e computation o f d o p p l e r o f f s e t s a t t h e user t e r m i n a l .

1.2.3 P o s i t i o n F i x i n g

Both c o n v e n t i o n a l GPS end GPS/REFSAT u s e r t e r m i n a l s make "pseudo-range'' measurements t o t h e

4

s e l e c t e d GPS s a t e l l i t e s , S o l u t i o n o f t h e n a v i g a t i o n e q u a t i o n r e q u i r e s a knowledge o f t h e p o s i t ion c o o r d i n a t e s o f t h e

4

s e l e c t e d G P S s a t e l l i t e s . These c o o r d i n a t e s must be computed from

G P S

ephemeris d a t a c o n t a i n e d i n the GPS s a t e l l i t e s i g n a l s . Dernodulatlan o f t h i s d a t a r e q u i r e s GPS c a r r i e r phase t r a c k i n g i n t h e c o n v e n t i o n a l GPS t e r m i n a l .

As shown on F i g u r e 1-3, t h i s f u n c t i o n i s performed by each c o n v e n t i o n a l

GPS

user t e r m i n a l , I n c o n t r a s t , t h e REFSAT system p e r f o r m s t h i s f u n c t i o n a t a c e n t r a l l o c a t i o n . C u r r e c t p o s i t i o n c o o r d i n a t e s f o r the f o u r s e l e c t e d GPS s a t e l l i t e s a r e t h e n broadcast t o

a l l

GPS/REFSAT u s e r s as p a r t o f t h e REFSAT s i g n a l . The u s e r may u t i l i z e t h i s i n f o r m a t i o n d i r e c t l y t o s o l v e t h e n a v i g a t i o n e q u a t i o n o r s i m p l y t r a n s m i t measured pseudo-range v a l u e s v i a a " r e p o r t back"

l i n k f o r p o s i t i o n computation a t a c e n t r a l l o c a t i o n .

I I I

I k q u i r e 4 GPS

Satellite Slgnr!s

I I

This concept a s s m e 5 rcccptlon o f 50 BPS data s1gnals fm 4 GPS satellftes acquired. b optlonal conccu; Is to use a 1 x 5 line frm CPS Raster Contml Stbtlon (RfS1 to PCS.

[see Flgure 1-21

*. In slternate concept 1s to carputt l(nr'5 location back a t R t 5 .

I

-

I GPS frttllltc

I I

I ---*--

I ^I 1 I

User Fosftlm 1

Figure

1-3.

T h e REFSAT Concept-

F i g u r e 1-3 and T a b l e 1-2 i l l u s t r a t e t h e manner i n which t h e above s t e p s a r e accomplished i n c o n v e n t i o n a l GPS and t h e proposed

GPS/REFSAT

systems.

F i g u r e

1-4

c o n t r a s t s the rece!ver f u n c t i o n s r e q u i r e d o f each system.

2 4

Format o f the REFSAT D a t a

M e s s a g c

F i g u r e 1-5 shows a proposed format f o r t h e REFSAT d a t a message b r o a d c a s t w i t h the REFAAT s i g n a l f o r G P S s a t e l l i t e s A, B, C and

0.

I n a d d i t i o n t o t h e

p r e c i s i o n REFSAT c a r r i e r w h i c h p r o v i d e s each GPS/REFSAT u s e r t e r m i n a l w i t h a p r e c i s e frequency r e f e r e n c e , the REFSAT s i g n a l c o n t a i n s a 128 b i t - p e r - s e c o n d FSK s i g n a l .

Each REFSAT d a t a frame i s t r a n s m i t r e d ''on t i m e t ' r e l a t i v e t o G P S system time. Each o f the

4

subf rames c o n t a i n s :

( 1 ) s y n c h r o n i z a t ion

(3

b y t e s , 1 4 - C i t s )

(2) c u r r e n t X, Y, and Z e a r t h - c e n t e r e d c o o r d f n a t e s f o r a s e l e c t e d G P S

s a t e l l i t e

(9

b y t e s ) .

( 3 )

c u r r e n t d o p p l a r c o e f f i c i e n t d a t a f o r c o m p u t a t i o n o f d o p p l e r g r a d i e n t (3 b y t e s )

.

(4)

^code s e l e c t ^{t o identify} t h e

selected

GPS s a t e l l i :e ( 1 b y t e ) .

GPS SIGNAL ^{( a )} CONVENTIONAL NAVSTAR GPS USER EQUIPMENT

POSITION

?

RF/lF

I

^SIGNAL

PROCESSING

STORAGE AND EPHEMERIS PREDICTION ALG.

(b) REFSAT-AUGMENT

L 3

GPS USER EQUlPMENT GPS SIGNAL

t

1-BAND

SYNTHESIZER PRECISION CODE COIIERENT

AND STABLE AND CARRIER

OSCl LLATO R TRACK LOOPS

DEMOD

-

1

'

REFSAT SIGNAL

MICRO- PROCESSOR

R F/I

F 1

F i g u r e

1-4.

REFSAT VS. Conventional Receiver.

I

-

SIGNAL POSIT ION

PROCESSING

T a b l e f - 2 .

GPS

Receiver functions Comparison.

REFSAT Sirnplifiwf ion Eliminate this software/

storage function VCXO

lo

I part in lo6 Major Function

Signal

Acquisition

'1

Conventional

GPS

Termina

t

Sub-Function

1.

Initial

Satellite

Sclcction

2.

Doppler Acquisition

Ijardwarc

1 part

10

8 synthcsizcr

(in oven)

Signal Tracking

Position Fixing

Software GPS

Almanac

Compute

satellites in

view Compute

range rate for scleclcd

satellites

PSK spread spcctrum

programable

synthesizer (in-phase

Pr

quadrature)

Delay-lock

loop

AFC

loop

Costas b o p

PSK

demodulate using above phase

reference 3. Delay Acquisition

4.

Fine Delay Track

5. Fine Doppter

Acquisition

6.

Carrier

Pliasc Track

7. TcIernctry Acquisition

8. Ephemeris

Update 9. Position Computation

Filter ernr signals

Advanccfrctard

commands

Filter error

signals

Advance/retard commands

-

Filter error signals

VCXO freq. step

commands Fillcr cror signals

VCXO pliasc step commands

Rcal-timc predictor

Pseudo

range to 1at.-long.

conversion

Reduce wdc generator precision

No

_punctual code,

cmploy

interpolation .

Not essential

for all

users

Eliminatc cnlircly

Simple

non-cohcrent FSK

Eliminate

entirely

Sat. A - Sat. B Sat. C Sat. D

I

^{Data I I D a t a I 1 Data I 1 D a t a I Data A I Data B I Seconds}

0 1 2 3

4 5

6

I

^I _I

I-

Subf rarne

1

^{1 Second}

-1 I

1 I

Code Select

I

1 I -I

Dopp

l

e r

I ^I

I

^Sync

I X~ I Y~ I =A

#Gradient 1 I

1

^{I I I 1 I}

-

MOTE: 1 Byte

- ⁸

^{B l t s}

Flgure

1-5. REFSAT

Data Format.

2.

R E F S A T

Space Segment

T h l s s e c t i o n examines t h e requirements o f the REFSAT space segment i n o r d e r t o form the performance s p e c i f i c a t i o n s l i s t e d i n Appendix

1 ,

The . g e o s t a t i o n a r y r e f e r e n c e s a t e l 1 i t e shown on F i g u r e 1-2

must

provide:

( 1 ) A space-to-earth down1 i n k broadcast o f t h e REFSAT r e f e r e n c e c a r r i e r , and t h e REFSAT d a t a message

(128

bps), r e l a y e d over an e a r t h - t o - s p a c e

up1

ink.

( 2 ) A

u s e r

t e r m i n a l - t o - s p a c e up1 i n k f o r those users r e q u i r i n g a report-back c a p a b i l i t y t o t h e REFSAT remote

control

s t a t i o n v i a s p a c e - t o - e a r t h

downlink,

2.1 Required Frequency Assignments

B e f o r e f i r m frequency assign~nents can be made, i t w i l l be necessary t o study t h e p o s s i b l e m u t u a l i n t e r f e r e n c e e f f e c t s between

REFSAT

and those users p r e s e n t l y a l l o c a t e d i n the frequency bands r h a t

m a y

be considered.

The

f o l l o w i n g d i s c u s s i o n serves the purpose o f demonstrating f e a s i b i l i t y and p o i n t i n g o u t

major requirements. The p r e c i s e f r e q u e n c i e s t h a t a r e discussed a r e not n e c e s s a r i l y those t h a t

w o u l d

be employed i n an o p e r a t i o n a l system.

REFSAT Space-towEarth Reference S i g n a l

The frequency band f o r the REFSAT s p a c e - t o - e a r t h r e f e r e n c e s i g n a l should be chosen t o minimize t h e s i z e , weight, c o m p l e x i t y , and c o s t of i n d i v i d u a l user t e r m i n a l s over those a l r e a d y r e q u i r e d t o r e c e i v e GPS s a t e l l i t e s i g n a l s a t

1575.420 MHz (a maximum dappl e r s h i f t o f

- ⁺⁴

^{kHz may be}

expected).

I n o r d e r t o best meet t h e above c o n s i d e r a t i o n s , t h e REFSAT s i g n a l frequency s h o u l d he w i t h i n a few p e r c e n t of the GPS s a t e l l i t e s i g n a l frequency so t h a t a common antenna and RF f r o n t - e n d may b~ used. For t h e purposes o f t h i s d e f -

i n i t i

on s t u d y ,

REFSAT down 1 i nk r e f e r e n c e c a r r i e r frequencies z t 1555 MHz and 1560 MHz w i 1 1 be assumed t o cover t h e c o n t i n e n t a l U n i t e d States (CONUS)

and

A l a s k a , r e s p e c t i v e l y .

User Rep, rt-Back, Earth-to-Space Frequency

Should a report-back f u n c t i o n be r e q u i r e d o f a p a r t i c u l a r user, a t r a n s m i t t e r and s u i t a b l e antenna must be added t o a b a s i c u s e r t e r m i n a l . The antenna should have s u f f i c i e n t beamwidth t o a v o i d t h e need for antenna p o i n t i n g . Although a

number o f frequency hands w o u l d be s u i t a b l e , t h i s d e f i n i t i o n s t u d y assumes t h a t t h e same u s e r antenna w o u l d be used f o r b o t h t r a n s m i s s i o n and r e c e p t i o n , p l a c i n g t h e d e s i r e d e a r t h - t o - s p a c e t r a n s m i t frequency n e a r 1600

MHz.

2.2

S p a c e c r a f t Antenna Footprint Coverage Requirement

U s i n g t h e REFSAT System, t h e c h o i c e o f a p a r t i c u l a r 4 - s a t e l l i t e G P S c o n s t e l l a t i o n ( f o r minimum HDoP) i s made a t t h e remote c o n t r o l s t a t i o n . As a r e s u l t , a l l GPS/REFSAT u s e r t e r m i n a l s i n a p a r t i c u l a r REFSAT coverage a r e a use the same 4-sate1 1 i t e GPS c o n s t e l l a t i o n . The maximum dimensions o f the REFSAT antenna f o o t p r i n t a r e t h e r e f o r e l i m i t e d .

A l t h o u g h a d d i t i o n a l s t u d y i s r e q u i r e d t o d e f i n e t h e maximum f e a s i b l e REFSAT coverage a r e a , p r e l i m i n a r y i n v e s t i g a t i o n i n d i c a t e s t h a t g e o g r a p h i c a l r e g i o n s a s l a r g e as the c o n t i n e n t a l U n i t e d S t a t e s

(CONUS)

a r e p r a c t i c a l . Separate beams c o v e r i n g CONUS and A l a s k a w i l l be assumed f o r t h i s document.

(See

s p e c i f i c a t i o n s i n Appendix 1 ) .

2.3 -

^RF ] . i n k Budget C t . n p u t a t i o n s

RF l i n k budget c o m p u t a t i o n s a r e g i v e n on T a b l e 9.1 o f t h e Appendix.

A nominal REFSAT 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

( E I R P )

o f about 40 d e c i b e l s above o n e - w a t t (dBW) and a u s e r t e r m i n a l r e p o r t - b a c k

ElRP

o f about

10

dBW would be

r e q u i r e d .

2.4 S p a c e c r a f t Antenna System

The REFSAT system p l a c e s

no

s p e c i a l d e s i g n c o n s t r a i n t s upon t h e s p a c e c r a f t antenna system. A CONUS-coverage antenna a r r a y s t r u c t u r e i s assumed f o r

b o t h t r a n s m i s s i o n and r e c e p t i o n .

2.5 Transponder System

The REFSAT r e f e r e n c e s i g n a l imposes v e r y modest r e q u i r e m e n t s on t h e space- c r a f t t r a n s p o n d e r i n t e r m s o f b a n d w i d t h (a few t e n s o f kHz) and o u t p u t power

( 1 2 ~ B W )

.

^{There a r e , however,}

^s

^{t r i ngent 1 1 mi t s} upon t h e f r e q u e n c y accuracy and s t a b i l i t y o f t h e REFSAT c a r r i e r as r e c e i v e d a t a u s e r t e r m i n a l .

I n o r d e r t o a v o i d t h e need t o p e r f o r m a f r e q u e n c y s e a r c h

to

a c q u i r e t h e s e l e c t e d GPS s a t e l l i t e s i g n a l s , i t i s d e s i r a b l e t h a t t h e REFSAT c a r r i e r be m a i n t a i n e d w i t h i n 50 Hz o f i t s nominal v a l u e

( 3

p a r t s i n l o 8 ) as a p r e - c i s i o n f r e q u e n c y r e f e r e n c e t o a i d GPS s i g n a l a c q u i s i t i o n a t t h e u s e r t e r m i n a l .

The carrier frequency o f

the

_REFSAT transponder,

as

seen by a user t e r m i n a l , must remain constant to

within 50

Hz by employing

a v e r y precise

nraster oscillator i n t h e spacecraft,

Doppler

s h i f t

due to

REFSAT satellite motion must

also be

h e l d

to

limits n o t exceeding

50

_Hz.

The

transponder used f o r user report-back signals

may

take

a

number o f

forms

depending upon whether

FDMA

o r

T D M A

techniques are

employed.

Considering t h e brevity and low data

rate

(e.g.

128 bps)

o f t h e report- back message t h a t would

be

r e q u i r e d o f any given user, a single

TDMA

channel o f

10 kHz

bandwidth should be adequate f o r CONUS coverage, A detailed l i s t of specifications i s g i v e n in the Appendix.

3.

A l t e r n a t e Methods F o r Implementing a GPS/REFSAT System

T h i s s e c t i o n d e s c r i b e s two a l t e r n a t e methods f o r i m p l e m e n t i n g a GPS/REFSAT system:

(1) Use of a VHF 1 i n k ( r a t h e r than L-band) f o r t h e REFSAT r e f e r e n c e s i g n a l ,

( 2 ) Usc o f a l o c a l t o w e r ( r a t h e r t h a n a g e o s t a t i o n a r y s a t e l

l

i t e ) t o b r o a d c a s t t h e REFSAT s i g n a l over a l i m f t e d g e o g r a p h i c a l a r e a ,

3.1

Use o f a VHF L i n k f o r t h e REFSAT S i g n a l

The R E F S A T concept p r e v i o u s 1 i l l u s t r a t e d on F i g u r e 1-2 assumed a REFSAT s i g n a 1 frequency ( 1

555

MHz

!

^be low t h e GPS down 1 i n k frequency

(1575.42

M H Z ) , i n o r d e r t o m i n i m i z e GPS/REFSAT C i v i 1 User T e r m i n a l corn- p l e x i t y .

The narrowband ( 1 0 kHz) R E F S A T s i g n a l may, o f c o u r s e , be b r o a d c a s t a t any d e s i r e d frequency p r o v i d e d t h a t t h e u s e r t e r m i n a l i s equipped w i t h an antenna and r e c e i v e r channel t o t r a n s l a t e t h e r e c e i v e d R E F S A T s i g n a l t o t h e t e r m i n a l I F frequency (21.4

MHz).

A b l o c k diagram f o r a G P S / R E F S A T c i v i 1 user t e r m i n a l accommodating a VHF R E F S A T b r o a d c a s t s i g n a l i s shown o n F i g u r e 3-1. Except f o r t h e s e p a r a t e V H F and L-band antenna and R F f r o n t - e n d components, t h e t e r m i n a l b l o c k diagram f u n c t i o n s a r e i d e n t i c a l t o t h o s e d e s c r i b e d f o r an L-band R E F S A T system d e s c r i b e d i n S e c t i o n

4

o f t h i s document.

S i n c e the REFSAT s i g n a l i s used as

the

u s e r t e r m i n a l p r e c i s i o n frequency r e f e r e n c e , t h e f i r s t LO s i g n a l s ( t o M 1 and

M 6

on F i g u r e 3-1)

must be s u f f i c i e n t l y s t a b l e i n f r e q u e n c y t o m a i n t a i n t h e p r e c i s e 50-Hz, f r e - quency d i f f e r e n c e r e q u i r e d between t h e GPS and

REFSAT

c h a n n e l s . The recovered r e f e r e n c e c a r r i e r from t h e R E F S A T s l g n a l i s c o r r e c t e d f o r GPS s a t e l l i t e d o p p l e r ( i n M4) and used wi t h t h e l o c a 1 l y - g e n e r a t e d PRN codes

(M5)

as t h e l o c a l

r e f e r e n c e for c o r r e l a t i o n w i t h r e c e i v e d GPS s a t e l l i t e s i g n a l s .

Demodulated d a t a f r o m t h e R E F S A T s i g n a l p r o v i d e s t h e t e r m i n a l w i t h : (1) I d e n t i f i c a t i o n o f t h e

4

GPS s a t e l l i t e s w h i c h c u r r e n t l y p r o v i d e

minimum h o r i z o n t a l d i l u t i o n - o f - p c s c i s i o n

,

( H D O P ) ,

( 2 ) D o p p l e r c o e f f i c i e n t d a t a ( a l l o w s c o m p u t a t i o n o f d o p p l e r s h i f t s f o r a c q u i s i t i o n ) ,

( 3 )

P o s i t i o n c o o r d i n a t e s f o r the

4

s e l e c t e d GPS s a t e l l i t e s .

An tcnnas

I

VHF L-Band.

Correlator PRH Code

AcqufsitIon

-

t o r r e l a t i o n and

Reference Trackf ng

co Corrected

for Doppler

Oopp l e r

I Demod

REFSAT Channel H 3 ^Doppler

Correctlon

Doppler Carff sAents

11

-

_-

Satellite I d e n t l f l c a t l o n

4 b b - Code

S e l t c t l o n Master

OscFllator L ~ t c l l l t e P o r l t l o n

i C o s d ' i n a t e s Posg t i o n ^{C I ,}

Cmputatlon

Hicroprocessor liser Posltlon

I

Opt lonal Report

Back Xmtr

-

Pseudo Ranges ^-

Figure 3-1. GPSfREFSAT

Terminal

(VHF

REFSAT

Signal ink),

ATS-3 Satellite VHF Band

0

..'

#

,'

^/

1575 Wz

(!; Signal) _I ^I

I

.' '-

0 #'

0 0 f .

4 / /

/ /

#

.'

'

/

.-8and VHF Band

v v

I Pos l t Ion

I 1

,,-,---

---

¹

Figure 3-2.

GPS/REFSAT

System

Utilizing

the ATS-3 Satellite.

An o l t e r n a t l v e Implementat i o n o f a GPS/REFSAT system i s shown i n F i g u r e

3-2

u s i n g the g e o s t a t i o n a r y ATS-3 s p a c e c r a f t as the REFSAT t r a n s - ponder

.

The REFSAT transponder broadcasts t h e REFSAT s i g n a l over the geo- g r a p h i c a l coverage area. i n d i v i d u a l

GPS/REFSAT

user t e r m i n a l s may then make use o f t h e REFSAT s i g n a l t o a c q u i r e t h e f o u r GPS s a t e l l i t e s i g n a l s which c u r r e n t l y p r o v i d e t h e mlnimurn v a l u e o f h o r l z o n t a l - d i l u t i o n c o f -

p r e c i s i o n , and measure pseudo-range values.

Once t h e user termina l has measured pseudo-range values t o t h e

4

s e l e c t e d GPS s a t e l l i t e s , s e v e r a l o p t i o n s a r e a v a i l a b l e :

(1) The user may make use o f t h e 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 d a t a contained i n t h e

REFSAT

s i g n a l t o compute t h e p o s i t i o n of t h e user terminal, and,

( a ) Make use o f pos i t ion informat i o n f o r n a v i g a t i o n , o r o t h e r purposes o r ,

(b)

Report-back computed p o s i t Ion v i a a user report-back t r a n s m i t t e r and t h e REFSAT transponder t o a Report-back Receiving Terminal o r ,

( 2 ) The user may s i m p l y r e p o r t - b a c k measured pseudo-range, t i m i n g , and user I D data. Computation o f user p o s i t i o n may then he

accomplished a t t h e Report-back Receiving Terminal, r e l i e v i n g the u s e r t e r m i n a l o f t h i s t a s k .

3.2 Use o f a Local Tower f o r REFSAT S i g n a l Breadcast

There a r e

many

a p p l i c a t i o n s where a s p e c i a l i z e d group o f users occupy a l i m i t e d geographical area t h a t may be economically served by p l a c i n g t h e REFSAT transponder on a l o c a l tower r a t h e r than on a g e o s t a t i o n a r y s a t e l l i t e .

F i g u r e 3-3 i l l u s t r a t e s t h e a p p l i c a t i o n o f t h e REFSAT concept t o

mari-

time n a v i g a t i o n and c o n t r o l i n a congested waterway, A REFSAT transponder placed on an

800 f t .

h i g h tower, f o r example, would p r o v i d e coverage over a t l e a s t 40 m i l e r a d i u s . P r e c i s i o n t r a c k i n g and c o n t r o l would then be a v a i l a b l e f o r a l l vessels e q u i p p e d w i t h a user t e r m i n a l such a s t h a t shown p r e v i o u s l y on F i g u r e 3 - 1 .

Figure 3-3. EPS/REFSAT System

Using

a Local Tower.

Local T o m r

Harltime Vess

- - - . - - . - - -

-

^{I - - - t - -}

'*

^I Report-Back REFSAT R m t *

1 VHF or UHF I ^{Recc'ving control} s t a t i o n

I L-Band Band

1

^{Termlnal 1}

I I YCssel Tracking 6 Control

I

I

^{I Center}

I ^{r' L}

REFSAT I

I Report-Back

I

I

^{Terminal Transml t t t r} I I

I I

L

I

^I

I

^I

1 I

I Shlp PosltIon 1

1 I

GPS/REFSAT U s e r Terminal Segment

T h i s s e c t i o n d e s c r i b e s t h e r e q u i r e m e n t s o f t h e GPSi'REFSAT

user

t e r m i n a l segment o f t h e REFSAT system. A l i s t of s p e c i f i c a t l o r i s g i v e n i n

S e c t l o n 8 i n t h e Appendix.

4.1

2-Channel Receiver Requirement2

Under t h e REFSAT concept d e s c r i b e d i n S e c t i o n 1, t h e u s e r t e r m i n a l r e c e i v e r must process b o t h GPS s a t e l

l l

t e s l g n a l s

(1575.420 MHz +

a maxlmum

4

kHz d o p p l e r

s h i f t ) and the REFSAT r e f e r e n c e s i g n a l . F o r t h e most ~ o n o m i c a l r e c e i v e r design, the REFSAT c a r r i e r should be w l t h i n a few p e r c e n t o f t h e GPS s i g n a l frequency t o a1 low a common u s e r antenna and RF f r o n t end t o be used.

I n o r d e r t o s a t i s f y t h e r n a j o r i t y o f pro.j+!~.ted u s e r r e q u i r e m e n t s (see S e c t i o n

6 ) ,

a maximum t i m e - t o - f i r s t - f i x o f a b o u t i 00 seconds s h o u l d be a1 lowed

(REFSAT s i g n a l a c q u i s i t i o n

+

acquisition o f

4

s e l e c t e d GPS s a t e l l i t e s i g n a l s

+

o u t p u t o f u s e r p o s i t i o n o r pseudo-ranse v a l u e s ) . A maximum p o s i t i o n update time o f 10 seconds f o l l o w i n g f n l t i a l a c q u i s i t i o n w i l l meet t h e needs of most p o t e n t la 1 u s e r s .

A s u r v e y o f p o t e n t i a l u s e r r e q u i r e m e n t s i n d i c a t e s t h a t a h o r i z o n t a l p o s i t i o ~ i a c c u r a c y o f about 100 meters would meet a l l b u t t h e most s t r i n g e n t u s e r needs. The C/A code (1.023 ~ b p s ) i s c a p a b l e o f m e e t i n g

t h i

s accuracy r e q u i rernent

.

4.2 Data T r a n s m i t t e r

Since a s i g n i f i c a n t p o r t i o n o f t h e p o t e n t i a l c l v i l u s e r market i d e n t i f i e d i n S e c t i o n 6 c o n s i s t s o f " l o c a t i o n " o r " s ~ r v e i l l a n c e ' ~ t y p e users, a d a t a t r a n s - m i t t e r f o r " r e p o r t - b a c k " o f u s e r p o s i t i o n may be r e q u i r e d as an o p t i o n . The

r e p o r t - b a c k message may c o n s i s t o f u s e r p o s i t i o n c o o r d i n a t e s computed a t t h e u s e r t e r m i n a l o r measured pseudo-range v a l u e s t o a l l o w l a t e r c o m p u t a t i o n o f user p o s i t i o n a t a c e n t r a l l o c a t i o n .

S p e c i f i c a t i o n s f o r t h e r e p o r t - b a c k rnessage a r e g i v e n i n S e c t i o n

8.5

i n t h e Appendix,

The c h o i c e o f t h e p a r t i c u l a r frequency band t o be employed f o r t h e u s e r r e p o r t - b a c k l i n k r e q u i r e s f u r t h e r study (see S e c t i o n 2 ) , b u t s h o u l d be near t h e

GPS

s i g n a l frequency t o a1 low use o f a s i n g l e antenna f o r

user

t e r m i n a l t r a n s - m i s s i o n and r e c e p t i o n . A t r a n s m i t t e r power l e v e l i n the o r d e r o f 10 w a t t s would be needed (see Appendix f o r l ink budget cornputat ions).

4.3

Simp1 i f i e d B l o c k Diagram, GPS/REFSAT User T e r m i n a l

Figure 4-1 i l l u s t r a t e s a s i m p l i f i e d GPS/REFSAT u s e r t e r m i n a l b l o c k diagram c a p a b l e o f m e e t i n g t h e r e q u i r e m e n t s d i s c u s s e d above. The GPS s a t -

e l l i t e and REFSAT reference s i g n a l s a r e sufficiently

close

i n frequency t o share a common RF f r o n t - e n d and f i r s t

mixer

{MI). Separate i n t e r m e d i a t e - frequency channels a r e then employed. Should a second frequency conversion be d e s i r a b l e , the l o c a l o s c i l l a t o r frequencies a p p i i e d t o t h e two channels must be i d e n t i c a l o r d e r i v e d f r o m t h e same source i n o r d e r t o m a i n t a l n t h e p r e c f se frequency d l f f e r e n c e

between

t h e REFSAT r e f e r e n c e c a r r i e r and the G P S sate1 l i t e c a r r i e r frequencies (nore M2 and

M3).

F o l l o w i n g the REFSAT channel on the b l o c k diagram, the REFSAT c a r r i e r i s recovered f o r l a t e r use i n t h e c o r r e l a t o r . The REFSAT d a t a message i s demodulated and a p p l i e d t o t h e micro-processor,

The micro-processor makes use o f t h e satellite l d e n t i f i c a t l o n p o r t i o n of the REFSAT d a t a message t o s e l e c t t h e

4

PRN codes corresponding t o t h e

4

G P S

sate' 1 i t e s c u r r e n t l y v i s i b l e w i t h minimum h o r i z o n t a l d i l u t l o n - o f - p r e c i s i o n (HDoP).

The micro-processor 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 p o r t i o n of the REFSAT d a t a message a l o n g w i t h estimated u s e r p o s i t i o n ( w i t h i n 150 Lm) t o compute doppler c o r r e c t

{on.

The dopp t e r c o r r e c t ion, when compared t o the recovered

REFSAT r e f e r e n c e c a r r i e r , f n s c r e s a c o r r e c t e d c o r r e l a t l o n reference t h a t i s w i t h i n 50 t o 100 Hz of the GPS channel s i g n a l . GPS s i g n a l a c q u i s i t i o n may

then

be r a p i d l y accomplished w i t h o u t the need t o p e r f o r m a frequency search and w i t h o u t employing a p r e c i s i o n o s c i l l a t o r w i t h i n the user t e r m i n a l . A

long-term frequency s t a b ! 1 i t y o f 1 p a r t 106 i s adequate instead o f 1 p a r t

l o a .

The d o p p l e r - c o r r e c t i o n s i g n a l from the micro-processor i s a p p l i e d t o a low-f requency 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 )

.

The r e s u l t i n g doppler c o r r e c t i o n i s added t o t h e recovered REFSAT c a r r i e r {mixer

~ 4 ) .

The c o r r e c t e d c o r r e l a t i o n reference i s then modulat2d w i t h t h e a p p r o p r i a t e PRN code (mixer

M5)

and a p p l i e d t o t h e c o r r e l a t o r . The micro-processor m o n i t o r s t h e c o r r e l a t o r output w h i l e performing a code d e l a y search u n t i l G P S s i g n a l a c q u i s i t i o n has been ach i eved

.

A f t e r s i g n a l a c q u i s i t i o n

(4

GPS sate1 1 i t e s ) , t h e micro-processor a d j u s t s both the d o p p l e r c o r r e c t i o n and PRN code d e l a y ( s e q u e n t i a l l y f o r

4

G P S sate1 1 i t e s ) t o minimize c o r r e l a t i o n e r r o r s . The r e l a t i v e code phases when computed u s i n g t h e speed c f l i g h t , c o n s t i t u t e t h e r e q u i r e d pseudo-range measured values,

Various o p t i o n s a r e a v a i l a b l e f o r the user t e r m i n a l design:

( 1 ) .Pseudo-range v a l ues may be transmi t t e d v i a the r e p o r t - b a c k t r a n s m i t t e r , e l i m i n a t i n g t h e need f o r the p o s i t i o n computation f u n c t f o n i n t h e micro-processor o r ,

( 2 ) Computed user pos i t i o n l a t i tude/long i t u d e c o o r d i n a t e s may be t r a n s m i t t e d v i a t h e report-back t r a n s m i t t e r o r ,

( 3 )

User p o s i t i o n c o o r d i n a t e s may be d i s p l a y e d o r o t h e r w i s e used d i r e c t l y by t h e user t e r m i n a l when a report-back t r a n s m i t t e r i s n o t r e q u i r e d . D e t a i l e d s p e c i f i c a t i o n s a r e g i v e n i n S e c t i o n 6 . 0 i n the Appendix.

Ffgure

4-1.

Simp?if!ed

Block

DIagrarn, GPS/REFSAT

Hobfle Terminal.

A Lwa H i

_-

GPs Channel HZ

I ^ns

PRH Code

-

Correlatar Code

A c q u l s l t l o n

, ,

^Preselect _{F l l t e r}

t

_{Cmpu t a t ton}

-

Hicroprocesror

4 User Position

/

Optional Report Back X m t r o r

Pseudo Ranges

.

^and

Correlation _Tracking

-

Reference

c1 Corrected

for Doppler

I&,

^{Doppl e r ~-}

REFSAT Channel "3 Doppler

Correction Doppler CoefFlcIcnts

I

1 s t LO 2nd LO Satellite I d e n t I f l c a t l o n Code _-L

5 e l e c Z l m S a t e t f I t c Pa~ltlon

-

Coord I na to P o l l t t o n CI

5 .

GPS/REFSAT Remote C o n t r o l S t a t i o n

The G P S / R E F S A T remote c o n t r o l s t a t i o n ( R c S ) as p a r t o f t h e REFSAT system i s i l l u s t r a t e d t n F i g u r e 1 - 2 . The remote c o n t r o l s t a t i o n performs t h r e e p r i m a r y f u n c t i o n s :

( 1 Generat i o n and t r a n s m i s s i o n o f t h e REFSAT r e f e r e n c e c a r r i e r and REFSAT data message f o r r e l a y broadcast by a g e o s t a t i o n a r y reference s a t e l l i t e ,

(2) Generation and t r a n s m i s s i o n o f a p o l l i n g message t o c o n t r o l those u s e r t e r m i n a l s h a v i n g a report-back c a p a b i l i t y and

( 3 )

Recept i o n and process i n g o f report-back messages, F i g u r e 5-1 i l l u s t r a t e s these f u n c t i o n s .

5.1 Generation of the R E F S A T Data Message

The REFSAT d a t a message p r o v l d e s I n f o r m a t i o n t o a l l users t o a i d i n r a p i d a c q u i s i t i o n of the

4

^GPS satellites v i s i b l e w i t h 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 n (HDoP). I n o r d e r t o accomplish t h i s f u n c t i o n , the remote c o n t r o l s t a t i o n must have access t o t h e weekly navigation message upload data f o r the e n t i r e c o n s t e l l a t i o n o f GPS s a t e l l i t e s .

The remote c o n t r o l s t a t i o n s e l e c t s those

4

GPS sate1 1 i t e s which a r e

v i s i b l e and p r o v i d e the minimum

H O O P

over t h e REFSAT

user

area. The s e l e c t i o n should be updated a t 15-minute i n t e r v a l s .

I n a d d i t i o n , t h e remote c o n t r o l s t a t i o n computes d o p p l e r c o e f f i c i e n t s f o r the f o u r s e l e c t e d GPS s a t e l l i t e s t o

allow

computation o f GPS s i g n a l doppler s h i f t s by the i n d i v i d u a l user t e r m i n a l s . T h i s p o r t i o n o f the REFSAT data message should be updated a t 2-minute i n t e r v a l s .

For n a v i g a t i o n , and o t h e r asers w i s h i n g t o r e p o r t t h e i r p o s i t i o n s , t h e remote c o n t r o l s t a t i o n computes p o s i t i o n c o o r d i n a t e s f o r t h e f o u r s e l e c t e d GPS s a t e l l i t e s f o r i n c l u s i o n i n t h e REFSAT data message. A 4-second update i n t e r v a l i s a v a i l a b l e and p r o v i d e d i n the R E F S A T data message (see F i g u r e 5-1).

5.2

REFSAT Reference C a r r i e r and Message Timing

The REFSAT reference c a r r i e r p r o v i d e s each user t e r m i n a l w i t h a p r e c i s i o n frequency r e f e r e n c e , a l l o w i n g r a p i d GPS s a t e l l i t e s i g n a l a c q u i s i t i o n w i t h o u t

the

n e c e s s i t y t o perform a frequency search. The r e f e r e n c e c a r r i e r must be m a i n t a i n e d w i t h i n about 50 Hz o f I t s nominal v a l u e as r e c e i v e d a t a user t e r m i na 1 .

GPS Navlgat Ion Message Upload fran SAnSO

Select 4 CPS Satcllftcs for

Hin. HDOP

Compute Doppler Compute Posit Ion f o r 4 Selected Coordinates For

Satellites Selected Satel l i tes (4-5ec. Update)

TO REFSAT Relay Satel 11 te

b.

REFSAT Reference C a r r i e r (* 50 Hz

a t User Terminal)

i

_{Transmi Earth} ^REFSAT

d

S t a t i o n ttIng Reference Clock

(GPS Time f 200 us)

Figure

5-1.

Simp1 if ied Funct Ional

Block

Diagram, REFSAT Remote Control S t a t i o n .

In

o r d e r t o a l l o w user t e r m i n a l s t o p r o p e r l y a s s o c i a t e measured pseudo- range v a l u e s w i t h the corresponding 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 s , i t i s necessary t h a t t h e REFSAT d a t a message be synchronized w i t h GPS system t l m e t o ari accuracy o f about 200 microseconds,