st20-gp1 STMicroelectronics, st20-gp1 Datasheet - Page 100

• Manufacturer Part Number: st20-gp1
• Description: Gps Processor
• Manufacturer: STMicroelectronics

ST20-GP1
18 GPS Performance
This chapter details the performance of a ST20-GP1 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.
18.1 Accuracy
18.1.1 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-GP1 can achieve better than 1m
accuracy with 1-second rate corrections. Note that the ST20-GP1 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.
18.1.2 Under harsh conditions
Under harsh conditions, accuracy degrades due to:
The ST20-GP1 pays a 2 dB 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.
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noise on the weakened signal
reflected signals from buildings and cliffs
obstruction of satellites
Stand alone
Differential
Surveying
with Selective Availability
without Selective Availability
Table 18.1 Accuracy performance
Accuracy
< 100m
< 30m
< 1m
< 1cm