The calculation of position by GPS is based on the principle of range triangulation. The principle of triangulation relies on the intersection of distances from known points. In order to compute the unknown position of the receiver it needs to know the distance to the satellites as well as the position of the satellites. As the receiver computes the distance travelled based upon the time delay between the transmission and reception of a signal, the clocks on the satellite and receiver have to be synchronised. Not only do the satellite and receiver have to be synchronised but all satellites have to be synchronised with each other in order to allow the simultaneous calculation of multiple distances needed to compute a position. The quality of time keeping devices onboard a satellite is generally far superior to that of a receiver and consequently time is considered a variable and solved for. the major sources of error involved with GPS include:
- Ionospheric Propagation Error -The ionosphere consists of gases that have ionised from solar radiation. It extends from approximately 50 to 1000 kilometres above the Earth’s surface. The ionised particles cause diffraction of GPS signals which causes a delay. The result is referred to as the ionospheric delay and it is influenced by the solar magnetic activity. It is also influenced by the time of day, being larger during the day, and by the elevation angle of the satellite relative to the receiver, the signals have to propagate farther through the ionosphere as the angle from zenith increases. The ionosphere is non-homogenous and the delay can vary over short periods, affecting the ability of the ionospheric correction values that are broadcast with satellite positions and times.
- Tropospheric Propagation Errors - The troposphere is the lower part of the earth’s atmosphere that is comprised of dry gases and water vapour which lengthen the propagation path due to local changes in the refractive index. This is referred to as the tropospheric delay which varies according to temperature, pressure and humidity at all points along the propagation path through the troposphere. The troposphere can be modelled well enough to suitably reduce its effect on the propagation path.
- Satellite Errors - Satellite errors occur from the clock and positional errors that are inevitable in these systems. Although the satellite clock is synchronised with the other satellites the ability to do this perfectly is limited, resultantly the satellite clocks are allowed a small range of drift and corrections are applied to bring all the satellites to a common time known as GPS time which is used to correct the transmitted clock time. The satellite orbit also has errors present within it. These errors in the transmitted position of the satellite also cause small errors in the range calculations that use this flawed information. Satellite errors result in smaller errors in the calculated range in comparison to the other errors.
- Multipath Errors - Multipath errors result from reflection of the GPS signals that create secondary propagation paths producing an error at the receiver due to the longer propagation time of the reflected GPS signal. These errors can be divided into two categories, stationary and dynamic, depending on the motion of the receiver. Stationary multipath errors slowly change with the motion of the satellite due to reflection path changes and consequent changes in propagation length. Dynamic receivers can be subject to numerous different multipath errors in a short period of time from passing different reflective surfaces that can propagate GPS signals. The short window of time that may occur for each reflective surface to influence the receiver causes unpredictable errors.