Introduction to GPS

The global positioning system is a satellite-based navigation system consisting of a network of 24 orbiting satellites that are orbiting in space eleven thousand miles from Earth. The satellites are constantly moving, making two complete orbits around the Earth every 24 hours.
The first GPS satellite was launched by the US Military in February 1978 and civilian use began in the early 1990's. Since then receivers have decreased rapidly in price and now as little as £100.
Each satellite transmits a message containing three pieces of information, the satellite number, it's position in space and the time at which the message was sent. The GPS receiver reads the message and saves the information.
The GPS receiver can compare the time at which a signal was transmitted by a satellite with the time it was received by the GPS receiver. This allows it to determine how far away that particular satellite is.
With distance measurements from four satellites, the position from on the ground can be calculated.
With a minimum of three or more satellites, the GPS receiver can determine a latitude/longitude position. This can be converted into the UK National Grid.
With four or more satellites, a GPS receiver can determine a 3D position which includes latitude, longitude, and altitude. By continuously updating your position, a GPS receiver can also provide data on your speed and direction of travel.

(Source: The Virtual Field Course c/o Department of Geography University of Leicester)

A tour of the menu pages on the Garmin™ GPS 12XL