Global Positioning System (GPS)

The GPS system has been designed to be as nearly accurate as possible. However, there are still errors. Added together, these errors can cause a deviation of +/- 50 -100 meters from the actual GPS receiver position. Most significant errors are discussed below:

1) Atmospheric Conditions

The ionosphere and troposphere both refract the GPS signals. This causes the speed of the GPS signal in the ionosphere and troposphere to be different from the speed of the GPS signal in space. Therefore, the distance calculated from “Signal Speed x Time” will be different for the portion of the GPS signal path that passes through the ionosphere and troposphere and for the portion that passes through space.

2) Ephemeris Errors/Clock Drift/Measurement Noise

As mentioned earlier, GPS signals contain information about ephemeris (orbital position) errors, and about the rate of clock drift for the broadcasting satellite. The data concerning ephemeris errors may not exactly model the true satellite motion or the exact rate of clock drift. Distortion of the signal by measurement noise can further increase positional error. The disparity in ephemeris data can introduce 1-5 meters of positional error, clock drift disparity can introduce 0-1.5 meters of positional error and measurement noise can introduce 0-10 meters of positional error.

3) Selective Availability

Ephemeris errors should not be confused with Selective Availability (SA), which is the intentional alteration of the time and ephemeris signal by the Department of Defense. SA can introduce 0-70 meters of positional error. Fortunately, positional errors caused by SA can be removed by differential correction.

4) Multi-path

A GPS signal bouncing off a reflective surface prior to reaching the GPS receiver antenna is referred to as multi-path. Because it is difficult to completely correct multi-path error, even in high precision GPS units, multi-path error is a serious concern to the GPS user.

GPS Error Budget is list as below: