How does GPS work?

The 32 GPS satellites orbit Earth transmitting signals continuously. The radio signals are sent at several different L-band frequencies with civilian and military purposes. These signals are modulated by a PRN code sequence and a data signal. The PRN codes allow to separate signals sent by different satellites (GPS is a CDMA type system). In addition the PRN codes are used as a mean to measure the signal propagation time. The GPS signal is made in such a manner that at its reception it is possible nearly instantly read out this signal transmission time.

The satellites move on accurately determined orbits, allowing us the possibility to determine their position at any given moment. The satellite orbit information is called ephemeris. Each GPS satellite is broadcasting its ephemeris data.

On Earth the receiver compares the received PRN sequence with its own local sequence, and determines how long it took for the GPS signal to travel from the satellite to the receiver. The travel time then is converted to the distance from the receiver to the satellite - the value known under pseudorange name.

For position computation the GPS receiver positioning algorithm needs at least four pseudoranges and the positions of the satellites. The satellite positions are computed from ephemeris information and GPS signal transmission time information. In this position computation process also the receiver speed vectors and precise GPS time are obtained.

The ordinary GPS clock quality is equivalent to your clock quality in a watch. This clock is actively involved in pseudorange measurements. A GPS receiver has a very poor clock comparing to the atomic clocks in GPS satellites and the GPS control segment. Such clock error would easily accumulate up to about 300000km distance measurement errors per day. But a careful design of the receiver and the positioning algorithms allow a normal GPS receiver operation even with such quality clocks. The receiver clock error is estimated and monitored by the positioning algorithms. Therefore the GPS receiver can provide time synchronization down to 10ns precision.

This is accurate but very simplified information on how the GPS system works. There are many problems, error sources, integrity, availability and other issues that are not covered here. A lot more issues arise when GPS is used to measure position with accuracy of centimeters or millimeters. All this broad spectrum of information is given at our courses and lectures for academia and industry. Please feel free to contact us.

For more introductory information on the subject you can visit:

Danish GPS Center, Fredrik Bajers Vej 7C, DK-9220 Aalborg Ø, Denmark, Tel +45 9940 8362, E-mail: gpscenter(at)gps.aau.dk