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What accuracy can i expect?
The Standard Positioning Service (SPS) available to civilian users
should give 20 metre horizontal accuracy, however it is normally
degraded to 100 metres (95% of the time) due to Selective
Availability (SA). (That is, the reported position will be within
100 metres of the true position 95% of the time.) The vertical
accuracy is about 1.5 times worse than horizontal, due to
satellite geometry. (Satellites are more likely to be near the
horizon, than directly overhead.)
Trimble Navigation, in their booklet "GPS - A guide to the next
utility" give the following error budget for commercial navigation
receivers:
Satellite clock error 2 ft.
Ephemeris error 2 ft.
Receiver errors 4 ft.
Atmospheric/ionospheric 12 ft.
Selective Availability 25 ft.
Total (root-sum-square) 15 - 30 ft depending on SA
The predicted acuracy is calculated by multiplying the above
figure by the PDOP (Position Dilution of Precision) which
typically will range from 4 to 6. This gives accuracies of 60 -
100 ft (30 m) without SA, up to 350 ft (100 m) with SA.
The accuracy can be improved by averaging readings over some time.
When taking readings for this purpose, there is apparently no
point in taking the readings more often than every 15 min, or so.
One user reports the following results:
Averaging for 15 - 20 hours: 10 metre accuracy
24 hours 5 metres
48 hours 3 metres
The error values are given as "2*sigma" values - for those (like
myself) who don't do statistics, this means that the readings
should be distributed as follows (for a 2*sigma of 100 metres):
32% of values are worse than sigma (50 metres)
5% of values are worse than 2*sigma (100 metres)
1% of values are worse than 2.6*sigma (130 metres)
0.3% of values are worse than 3*sigma (150 metres)
0.006% of values are worse than 4*sigma (200 metres)
0.00006% of values are worse than 5*sigma (250 metres)
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