Accuracies (single GPS receiver)

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Positional accuracy with a single receiver, to civilian users approximately equals 5m to 10m, 95% of the time, and the height accuracy is generally 15m to 20m 95% of the time.

Until May 2000, the accuracy was a lot worse (100 m, 95%) before the US DoD turned off Selective Availability (SA). SA was the deliberate degradation of the GPS signal to limit its real-time accuracy to civilian users. The US DoD now have other ways of doing this if necessary (for example, in time of conflict).

Military users have access to a more accurate coded signal from the satellites. Positional accuracy = ? (but significantly better than civilian users).

The positional accuracy is affected by GPS satellite orbit errors, the atmosphere and receiver clock errors.

To give better accuracy the known errors must be accounted for.

Like the satellites, the receiver uses a clock to generate the codes used in measuring the ranges. This clock is not as accurate as the one in the satellite; if it was, GPS receivers would be unaffordable! The receiver clock is therefore assumed to contain an error. Fortunately receiver clock errors can easily be computed if distances to 4 or more satellites are measured.

The GPS satellite orbit errors and errors introduced into the signal travel time due to it travelling through the atmosphere, cannot be computed by a single receiver in real time.