The GPS System
GPS is an acronym for Global Positioning System. In its simplest form, the Global Positioning System allows a user to identify his position in the World down to an accuracy of just a few metres. In addition, the GPS service can provide altitude information and, by using simple maths, the velocity, speed and bearing of the receiver can be calculated.

The GPS system is based on a network of satellites that are in orbit around the Earth. The satellites have been built up over the years by the United States Department of Defence - many of them have been manufactured by Rockwell. There is now a full GPS service available and this is based around 24 satellites or SVs (Space Vehicles). The satellites orbit the Earth at an altitude of 20,183 Km and pass overhead once every 12 hours. Each satellite has a different orbit so that at any one time there are a handful of satellites ‘visible’ from any position on the globe. (The orbits are arranged so that between 5 and 11 SVs will always be visible 5 degrees above the horizon from anywhere on the globe.) The satellites are constantly transmitting information and by decoding these signals a position can be derived.

There are two qualities of service available from GPS; one for military use and one for civilian use. These services are called PPS (Precision Positioning Service) and SPS (Standard Positioning Service) respectively.

The PPS system is able to locate position down to an accuracy of a couple of metres but is reserved for military applications and as such is cryptographically coded to prevent unauthorised use. The SPS service is essentially similar to the PPS service but has an accuracy of about 5-10 metres.

Where GPS Will Not Work
GPS is a satellite tracking system, and the antenna needs a reasonable view of the sky. Therefore, it is not reasonable to expect GPS navigation to be performed while underground, in buildings, or in tunnels unless Dead Reckoning techniques are applied.

But virtually every other outdoor location will work, whatever the altitude (less than about 20 km!), latitude or longitude and at speeds exceeding 500 metres a second (about MACH 2!).