Guides

How the GPS works

The GPS is owned and operated by the US Department of Defense, but is available for general use around the world (the US military refers to it as NAVSTAR GPS – Navigation Signal Timing and Ranging Global Positioning System). All you require is a suitable receiver. Briefly, here's how it works:

• 24 GPS satellites (21 active and three spares) orbit around the Earth at approximately 20,000km above the Earth. The satellites are well-spaced so that four satellites will be above the horizon from any point on Earth.
• Each satellite has a computer and an atomic clock. With an understanding of its own orbit and the clock, it continually broadcasts its changing position and time.
• On the ground, a GPS receiver "triangulates" its own position by calculating the distance from three or more satellites. This gives it a geographic position (latitude and longitude) with an accuracy usually better than 10m (for most receivers). The accuracy is determined by the number of satellites (more is better), their positions across the sky, and whether there is any interference to the satellite signals (from tall buildings and trees).
• Many receivers are also equipped with a display screen (often now in colour) and can show the user's current position on a map.
• If the GPS unit can receive a signal from four or more satellites, it can also figure out the altitude as well as the geographic position.

Many GPS units (those used in vehicles, for instance) can also calculate your speed and direction when moving, and even estimate your time of arrival at a specified destination.