Features

GPS works by timing signals transmitted by navigational satellites. Each satellite sends pulses at precisely known times so, by recording the time the signals are received on Earth, a handheld GPS receiver or in-car satellite navigation device can work out how far it is from some of the 24 satellites in orbit. Most of the satellites aren't visible from any particular location, but a GPS receiver needs to be able to get signals from at least four to get a fix. Knowing how far it is from one satellite is enough to place the receiver somewhere on the perimeter of a sphere centred on that satellite.

Similar information from a second satellite places the receiver on the perimeter of a second sphere, which means the actual location is one of the points where these two spheres intersects. This intersection, however, is a circle in space, so timing information from a third satellite is required to provide another sphere. The intersection between this and the circle is just two points.

Because one of these points is likely to be way out in space or, perhaps, in the middle of the Earth, these three satellites might seem enough to provide a location. The snag is that all this requires very accurate timing. The satellites have precise but very expensive atomic clocks onboard, but in-car sat-navs and mobile phones do not. What's more, because radio signals travel so quickly, even a timing inaccuracy of a few millionths of a second would put the location way out. Fortunately, a distance from a fourth satellite provides a solution. By using four satellites the timing error is cancelled out, allowing a position to be calculated to an accuracy of just a few metres.

This system applies to ordinary GPS, but many of the newer 3G mobile phones offer Assisted GPS (AGPS). Here, rather than having to perform all the work in the handset, the job of tracking satellites and working out a location is shared between the mobile phone and the network base stations.

One of the main areas in which the base stations provide assistance is in finding satellites. When a GPS receiver is first switched on, it can take a few minutes to search, build up a list of all the satellites that are visible from that location and download their orbital data. This is annoying to the user and it wastes valuable battery life. In an AGPS system, the network keeps track of the satellites and, whenever a handset needs to work out its location, it will provide it with information about the ones that ought to be visible.

Some implementations of AGPS also offer clever techniques to allow it to work indoors, which can be tricky with ordinary GPS.

	1 The NAVSTAR GPS system uses 24 satellites orbiting the Earth in six paths. To get a position, the user receives signals from four satellites.