3.3.2 The GPS Navigation Message:

SATELLITE CLOCK ERROR DATA


The GPS clocks are free-running and their rate is monitored against GPS Time (GPST). GPST is itself kept synchronised to UTC as defined by the U.S. Naval Observatory (after taking into account the leap second offsets). The clock behaviour so determined is used to occasionally reset the satellite clock so that it is kept within 1 millisecond of GPST, and is also made available to all GPS users via clock error coefficients in a polynomial form:

= a0 + a1 (t - toc) + a2 (t - toc)2 (3.3-1)

where:

a0 is the clock bias term,
a1 is the clock drift term,
a2 is the clock drift-rate,
t is the satellite clock time (seconds in the GPS week), and
toc is the reference epoch for the coefficients (seconds in the GPS week).

 

The clock bias, drift and drift-rate are explicitly determined in the same procedure as the determination of the satellite ephemeris. What is therefore available to users is really a prediction of the clock behaviour some time into the future. As the random deviations of even cesium and rubidium oscillators are not predictable, such deterministic models of satellite clock error (that is, non-synchronisation with GPST) are accurate to about 20 nanoseconds, or approximately six metres in equivalent range.

Typical values for these coefficients are:

SV PRN number a0(sec) a1(sec/day) a2(sec/day2)
1

-73.208

-0.196

0.0

2

-58.831

-0.157

0.0

3*

-21.739

-1.306

0.0

4

3.579

0.131

0.0

5

14.151

0.196

0.0

7

42.269

0.206

0.0

12*

18.436

-8.418

0.0

14

3.454

0.020

0.0

15

45.545

0.216

0.0

16

-71.942

0.029

0.0

17

-38.796

-0.079

0.0

18

-3.796

-0.010

0.0

19

132.122

0.216

0.0

20

36.064

0.018

0.0

21

-14.313

-0.059

0.0

22

77.125

0.304

0.0

23

2.402

0.029

0.0


* Block I satellites

Note, under the policy of Selective Availability is implemented on all Block II series satellites, the satellite clock error data are intentionally falsified and hence the residual satellite clock error is of the order of several dekametres (corresponding to few hundred nanoseconds accuracy in the signal time delay).

 

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© Chris Rizos, SNAP-UNSW, 1999