GPS - the Global Positioning System - is a network of satellites launched for use by the US Military. It is now being exploited world-wide, and the calculator-sized GPS receivers are getting smaller and cheaper. These receivers pin-point your position accurately, anywhere in the world. They are already available for less than £200, and are appearing on the high street in Tandy Stores, Maplin Catalogues, and other shops soon.

There are no hidden costs, beyond the cost of the receiver, and the US Government have promised to give 10 years notice of any change in policy in provision of the service to an accuracy of typically 100 metres. More expensive `differential' receivers can provide much greater accuracy of better than 10 metres, although this may soon be unnecessary as the Americans are expected to stop feeding in the 'deliberate error' which has limited accuracy to 100 metres. Note: SA was switched off in 2000, so all GPS are now accurate to nearer 5 metres or better.

Some GPS receivers are now designed for use with a PC computer. They do not need buttons or a display. They simply consist of a matchbox sized plastic box with two leads: one that plugs into the car's cigar lighter socket for power, and the other that plugs into the computer.

Many of these GPS receivers have what is known as an "NMEA" interface - RS232 data as ASCII text, that can be plugged into the COM port of a notebook computer and used in a moving car. The GPS data provides position as latitude and longitude, and velocity as a course and speed. The GPSS software uses this data to provide car navigation facilities.