# GPS Errors

Most Significant Factors Affecting GPS Accuracy

Ionospheric Propagation Delay (Atmospheric)

One of the most significant errors in the pseudo-range measurements results from the passage of the satellite signal through the Earth’s ionosphere, which varies depending on the time of day, solar activity and a range of other factors. Ionospheric delays can be predicted and an average correction applied to the GPS position, although there will still be some minor error introduced by this phenomenon.

Satellite Orbital Variations (Ephemeris)

Although the satellite orbits are extremely stable and predictable, some perturbations do exist. These are caused by gravitational effects of the Earth and Moon and the pressure of solar radiation.

Satellite Clock Error

Timing errors due to inaccuracies in both the satellite and receiver clocks, as well as relativity effects, can result in position errors of up to two metres.

Dilution of Precision

Geometric Dilution of Precision (GDOP) is an effect that degrades the accuracy of a position fix, due to the number and relative geometry of satellites in view at the time of calculation. The value given is the factor by which the system range errors are multiplied to give a total system error.

Position Dilution of Precision (PDOP) is a subset of GDOP that effect latitude, longitude and altitude. Many GPS receivers are able to provide an estimate of PDOP.

Multipath

An error in the pseudo-range measurement results from the reflection and refraction of the satellite signal by objects and ground near the receiver. This is known as Multipath error. Ghosting of television pictures is an example of Multipath effect.

As GNSS is a three-dimensional navigation system, the errors do not all lie along a line and therefore should not be added algebraically. Total system range error is calculated by the root-sum-square method, where the total is the square root of the sum of the squares of the individual errors

Other GPS Errors