6.2.2 Measurement Biases and Errors

SATELLITE CLOCK UNCERTAINTIES

 


The GPS satellite clock bias, drift and drift-rate are explicitly determined in the same procedure as the estimation of the satellite ephemeris. The behaviour of each GPS satellite clock is monitored with respect to GPS Time, as maintained by an ensemble of atomic clocks at the GPS Master Control Station in Colorado Springs. The offset, drift and drift-rate of the satellite clocks are available to all GPS users as clock error coefficients broadcast in the Navigation Message (section 3.3.2).

What is available to users is actually a prediction of the clock behaviour for some time into the future (24 hours or more ahead). As the random deviations of even cesium and rubidium oscillators are not predictable (section1.3.2), such deterministic models of satellite clock error are accurate to about 20 nanoseconds, or six metres in equivalent range, depending upon the time since the last Navigation Message update. Selective Availability is a further artificial dithering of the satellite clocks causing several dekametres error in the range (or phase-range equivalent).


SATELLITE CLOCK BIAS

MAGNITUDE:

The residual satellite clock error (after correcting for the broadcast error model) cannot be neglected for any GPS surveying application as its magnitude is at the several dekametre level (under SA conditions). Clock error and corresponding range error:

1 nanosecond ==> 0.3m
1 microsecond ==> 300m
 
OPTIONS:
Construct a range-like observable from which the satellite clock error has been eliminated -- DIFFERENCE between-receivers.
Model the satellite clock error as a "random process" -- ESTIMATE a constantly changing parameter.

In the context of GPS phase data processing, the former involves observation differencing (section 6.3.2), while the latter requires explicit estimation of the clock error on an epoch-by-epoch basis (section 1.4.4). In general, the latter option usually requires the clock error to be modelled as "white noise". (This is in fact not a model in the stochastic sense, as no attempt is made to relate the clock error from epoch to epoch.)


STRATEGY:

Requires that two or more GPS receivers simultaneously track the same satellite. Both options are very effective at COMPLETELY eliminating the effect of the satellite clock bias.

 

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© Chris Rizos, SNAP-UNSW, 1999