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ST20-GP6

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19 GPS Performance

This chapter details the performance of a ST20-GP6 based GPS receiver.

Note that the performance is dependent on the quality of the user radio, antenna and software

used to track the signal and to calculate the resultant position.

19.1 Accuracy

19.1.1 Benign site

The accuracy performance of a GPS receiver is dependent on external factors, in particular the

deliberate degradation of the signal by the US DoD, known as Selective Availability (SA). This

results in an error speciﬁcation of 100m.

If signal errors are corrected by differential GPS, the ST20-GP6 can achieve better than 1m

accuracy with 1-second rate corrections. Note that the ST20-GP6 supports the RTCA-SC159

provided corrections with no additional hardware.

For surveying use, the resolution of the counters used in the phase/frequency tracking allows

resolution down to 1mm.

19.1.2 Under harsh conditions

Under harsh conditions, accuracy degrades due to:

•

noise on the weakened signal

•

reﬂected signals from buildings and cliffs

•

obstruction of satellites

The ST20-GP6 paysa2dB signal/noise ratio penalty by using 1-bit signal coding, there are then

no further losses in the signal processing hardware. The fast sampling rate, with both in-phase and

quadrature channels, results in the subsequent processing being 11 dB better, on a signal to noise

ratio, than earlier systems that sample at 2 MHz. Thus there is a 9 dB overall improvement.

Accuracy

Stand alone

with Selective Availability

without Selective Availability

< 100m

< 30m

Differential

< 1m

Surveying

< 1cm

Table 19.1 Accuracy performance

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