The Global Positioning System (GPS) was developed to provide highly precise position, velocity, and time information to users anywhere in the area of the Earth and at any time. The GPS, when fully operational, will consist of 18 satellites in six orbital planes. Any GPS user, by receiving and processing the radio signals from the satellite network can instantaneously determine navigation information to an accuracy of about 15 m in position and 0.1 m/s in velocity. The GPS is compared with other systems such as Loran-C, Omega, TACAN and Transit

Ananda, M.

English

NASA Technical Reports Server

5.8 $ NAVSTARIGLOBAL POSITIONING SYSTEM* 
M. Ananda, Ihe Aerospace Corporation 
3 0 
The Global Posit ioning System (GPS) has been developed t o  provice highly pre- 
cise pos i t ion ,  velocity,  and time information t o  use r s  anyuhere i n  t h e  neigh- 
borhood of  e a r t h  and a t  a n y  time. The GPS, when f u l l y  opera t iona l ,  w i l l  con- 
sist of 18 sa te l l i t es  i n  s i x  o r b i t a l  planes. These d a t e l l i t e s  w i l l  be a t  
about 20,000 km a l t i t u d e  with a 12-h period and t h e  o r b i t s  w i l l  be incl ined t o  
So. The s a t e l l i t e s  w i l l  t ransmit  L, ( 1 5 7 5 . 4 2 4 ~ z )  and L ( 1 2 2 7 . 6 4 ~ 2 )  sig- 
na ls .  Navigation information such a s  the  emphemeris o f  $he s a t e l l i t e s  and 
s a t e l l i t e  clock model parameters and the  system data  a r e  superimposed on these  
rad io  s ignqls .  Any GPS user, by receiving and processing t h e  rad io  s igna l s  
from the  cons t e l l a t ion  can instantaneously determine navigation information 
(pos i t i on  and ve loc i ty  parameters) t o  an accuracy of  about 15 m i n  pos i t ion  
and 0.1 m/s i n  veloci ty .  T. i s  rad io  navigation system is primari ly  developed 
f o r  u t i l i z a t i o n  by t h e  Department of  Defense. However, there exists a broad 
spectrum of c i v i l  u se r s  who would bene f i t  from t h i s  system. 
Ihe GPS system has th ree  separa te  segments. One is  the space segment, which 
is t'le s a t e l l i t e  i t s e l f ,  t h e n  what we c a l l  a cont ro l  segment, which i s  respon- 
s i b l e  f o r  providing t h e  information t o  t h e  s a t e l l i t e ,  atid t h e  t h i r d  is the  
user segment. The user segment includes individual  user MANPAC or automo- 
b i l e s ,  t rucks ,  s h i p s ,  a i r c r a f t s ,  and s a t e l l i t e s .  With regard t o  program h i s -  
t o r y ,  o r i g i n a l l y  t : -e  Navy was responsible  f o r  i n i t i a t i n g  a navigation sys tem,  
t h e  TRANSIT program-and it i s  s t i l l  operat ional  now. Il.ley have f i v e  sa t e l -  
l i t e s  and the  accuracy is reasonable.  The GPS mission program was a prel imi-  
nary study which t h e  Navy or iginated and 621-B was a progran t h e  Air Force 
i n i t i a t e d ;  t h e y  were subsequently combined and bocame t h e  Global Posit ioning 
System. During the  progran evolution there a r e  Lnree phases: ( 1 )  the  concept 
va l ida t ion  phase , ( 2 )  t h e  fu l l - sca le  engineering development and t e s t i n g  (pre-  
s e n t  phase),  and ( 3 )  f u l l  operation capabi l i ty .  In  Phase I de had four sa t e l -  
l i t e s  and two planes;  i n  Phase I1 we have 6 s a t e l l i t e s :  however, one of  the  
s a t e l l i t e s  i s  not  functioning properly because of t h e  on-board clock. It is 
not  giving the accuracy we need.  I n  Phase I11 we w i l l  eventual ly  have 18 sa t -  
e l l i t es  and t h i s  w i l l  be a d i f f e r e n t  s a t e l l i t e  than what we have now. 
Let's b r i e f l y  look a t  t h e  space segment. Once we have 18 s a t e l l i t e s ,  each 
s a t e l l i t e  w i l l  be i n  a 12-hour period a t  20,000 m a l t i t u d e .  (Figure 1 ) .  Cur- 
trer,!ely good even though t h e  design spec i f i ca t ions  show only 2: lO-'3. We have 
two  L-band frequency s i g n a l s ,  L-1 and L-2, a t  1 .2  CHz and 1.5 CHz,  and we a l so  
have an L-3 b u t  t h a t ' s  not a user oriented band, i t  is used  f o r  a spec i f i c  
purpose. We may use t h a t  L-3 which is a cross- l ink capab i l i t y  from one s a t e l -  
l i t e  t o  another s a t e l l i t e .  
rent clock accuracy i s  about l:lO"3 t o  5:lO' n . Clock performance i s  ex- 
Now t h e  concept o f  ge t t i ng  navigation information from t h e  CPS s a t e l l i t e s .  
The s a t e l l i t e s  w i l l  provide s igna l s  fran which you can,  i f  you have a receiv- 
e r ,  ge t  range in fomat ion  and simultaneously i f  you can g e t  f u l l  rsnge infor-  
mation properly d i s t r ibu ted  i n  the s k y ,  you can determine your own posi t ion 
and the b i a s  i n  your clock. I f  your clock i s  not a s  precise  as the  s a t e l l i t e  
c lock ,  you can determine instantaneously our precis ion p l u s  t he  timing b ias .  
'Edited o r a l  presentat ion.  
2 02 
https://ntrs.nasa.gov/search.jsp?R=19820020843 2020-03-21T08:00:06+00:00Z
If you have the range r a t e  infomation i n  addition t o  the range infomation 
you can also determine the velocity i f  you are  a user. The accuracy we are 
currently broadcasting (Figure 2) we would get  is 15 meters of spherical d i s -  
t r i b u t e d  error for a l l  these types of users, whether a HANPAC, an automobile, 
s h i p ,  a i r c r a f t  or Shut t le .  men you deviate from 800-1,000 km a l t i t u d e ,  l i k e  
Landsat, your accuracy w i l l  b e  improved because of some of the e r rors  a re  as- 
sociated with atmospheric modeling. 
I n  comparison of navigation sys t ems  (Figure 31 ,  the CPS is  a navigation sys tem 
purely based upon radianetric measurements. There are  a quite a large nunber 
of navigation systems i n  existence. Currently, LORAN-C, OMEGA, INS, TACAN, 
TRANSIT-these are  a l l  radio based navigation systems. Ihe TRANSIT is probab- 
l y  closer to  CPS because it has a global capability: however, the accuracy d e -  
grades i n  between orb i t s .  CPS provides the sane accuracy globally everywhere 
anytime. That's the key point. Trt other systems the e r rors  are  f a i r l y  large 
and obviously there is a l o t  of l o s t  infomation you will not recei..-e, CPS is 
a very simple concept and versa t i le ,  and obviously, a v e r y  expensive sys tem 
also. For user equipment ,  as I said e a r l i e r ,  you need t o  have four s a t e l l i t e s  
simultaneously visible.  Simultaneous i n  the sense that  you don't need t o  have 
a four-channel receiver i n  order t o  do it;  you can do i t  sequentially. How- 
ever, the four s a t e l l i t e s  have t o  b e  available t o  the user and the constella- 
tion is designed t o  be able t o  achieve a t  l e a s t  four s a t e l l i t e s  a t  any given 
time, anywhere near the earth.  
The performance of the user system depends upon several t h i n g s  (Figure 4). 
The vehicle enviroriment -- of concern i n  particular t o  defense related prob- 
lems and constellation geanetry - is  very  geometry dependent and dependent on 
the dynanics of t h e  vehicle and the way t h e  ionosphere and troposphere are 
modeled. The ionosphere can >e monitored bet ter  i f  you have a two-channel re- 
ceiver because you have two frequencies going i n  and separated i n  frequencies 
so we can cal ibrate  real  time. For a s i n g l e  receiver, we have provision i n  
t h e  Ravigation message which w i l l  provide some k i n d  of table  for modeling t h e  
ionospheric effects .  It may not be very accurate, b u t  suf f ic ien t  for  t h a t  
type of user. 
3eviewing t h e  e r o r  budget (Figure 21, t rue navigation error is a product o f  
what we c a l l  CEODOFF, tha t  is, geometry d e l u s i o n  of precision. If you r n u l t i -  
F l y  t h e  CECWFF times the user equivalent range error component you get the 
navigation e r ror .  The GEODOFF data varies from one place t o  another place and 
the best CEODOFF is 2 or 2.2. Sa i f  the s a t e l l i t e  dependent range error is 
mul t ip l ie !d  by 2.2, it would give you around 15-meter accuracy, which is the 
t o t a l  of the variou:. error  sources. The s a t e l i t e  clock i s  m i n u s  13 over a 
24-hour period and we update once i n  10-hours. Each clock w i l l  have a differ-  
ent elapsed time because the u ate time w i l l  be different  for  each s a t e l l i t e .  
h single s a t e l l i t e  w i l l  be v pd s i b l e  once or twice a day from a given ground 
control s ta t ion.  Another interest ing element i s  tha t  i f  you use CIPS i n  a rel-  
a t ive mode for a re la t ive  navigation, a l l  these errors tend t o  cancel out. So 
the only error you w i l l  be l e f t  w i t h  i s  a user related error of approximtitely 
2 meters. Most of the c iv i l ian  t y p e s  of purposes can be used in a r e l s t i v e  
mode depending on uhat your particular application is. For that  reason, we 
have what we c a l l  for security purposes se:ecti#e ava i lab i l i ty ,  so we cannot 
guarantee a n y  user the real  accuracy the system would provide. However, t h e  
degraded accuracy would be available t o  everyone. We don't  know what the de- 
graded accuracy is or whether i t  w i l l  be opted: those decisions are t o  be made 
2 03 
at a much higher level ,  but we have the a b i l i t y  to introduce whatever error we 
want. However, that will not affect  a user using a relative mode. 
6 
USERS NAVIGATE IN .RELATIVE 
COOROINATES USING TH! ' N O R M  
THREE S AM S ATELU TES 
EfiRORS THAT D I S A P f U R  SIGMA . IONOSPHERIC 6.9 
a 0 TROPOSPHERIC 6.0 
! EPHEMERIS PREDlCTlON 7.5 
0 CLOCK STABILITY 8.1 
3.6 - 0 SV PERTURBATIONS 
RSS l4.83 
0 REMAINING hRORS' 
THREE 
SIGMA 
WITH * 
6.9 
6.0 
353.0 
353.0 
3.6 
500 
r
0 RECEIVER NOISE 4.5 4.5 
0 OTHER - 1.5 1.5 0 MULTIPATH * 3.6 3.6 
RSS;, 6.0 6,O 
Figure 1. Re la t ive  Navigat ion 
I RANGE ERROR COMPONENTS I 
STABILITY 
SPACE 
PERTURW? ON 
1.2 MULTIPATH , 
l r l l l l l r  
4 3 2 1 0  
USER USE8 
1 2 
*Degraded clock and epheme ri s 
ERROR CORRtLAItON 
IN MUlf l -Rk€IVER 
>ATE11 I T E  DEPEND€ NI 
RANGE ERROR LSORQ 
CORRELATED ACROSS 
RECEIVERS 
24-HOUR k U S t R  EOUIVALENI RANGE ERROR WERE) METfRS 
Figure 2. User Equivalent  Range Error Budget 
205 
Systam 
(Nota 1) 
(Nota 1) 
Std INS 
(Note 2) 
TACAN 
(Note 1) 
Tramit 
(Not.. 31 
NOTES: 
Im lm t Opwatlon 
arlsl 
180 (CEP) No velocity 
data 
-tz- 2,200 KEPI No vebcity 
1,M)o max 
after 1st (RMS per axld 
hour K E P I  
100 KEPI No veloclty 
0.0 after 2 hr 
--- 
data 
200 (CEP) No veloclty 
date 
- 
. Federal Radio Navivt ion Pla 
'. ENAC-77-IV. Chractalr t ic  f 
August 1979 
us. coast, 
Continental 
us., selected 
Overseas areas 
Near global 
(BOK comw) 
Worldwida 
- 
Line of sight 
(pnont air 
routas) 
Worldwb 
Cornnunti 
Operational worldwldr with PI-hour 
all-weather cowrrgl. 
0 p . r r i i C d  wlth l ~ l l l o d  c o v r m .  
Limited by skywwa Interferoncr. 
Currently aperatlonal 4 t h  localized 
coverage. Systam h subject to multl- 
path errors. 
Operatlonal worldwide with 24.hour 
alhmather cowrego. Degraded per- 
formance in polar areas. 
Posltion rccureccy h *add malnly 
beciure of ulmuth uncrrtrlnty whlch 
Is  typically on thr or& of f 1 .tJ 
dG(rW. 
The Interval between podtlon flxm Is 
about 80 minutes. :or UI ' slow 
moving vehicles 
I 
, July 1680 
r a Moderata f.ccurrcy Inertial Navigation Systmm, 
I 3. Journal of the lnit ltute of Wavipetion, Volume 27, No. 2, Summer 1979 1 
Figbce 3. Navigat ion System Comparison 
Figure 4 .  UE S e t  Performance 
206 


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