Abstract: A method and computer program for determining an error factor for a differential global positioning system is disclosed as well as a method of tracking a vehicle. In determining the error factor, estimated positional data is transmitted from a GPS via GPRS to a server. Since the GPS signals are being transmitted from a vehicle travelling along a known route, i.e. a road or rail track, the data can be matched to the route and a correction factor calculated. The error factor is then transmitted to differential GPS devices. For vehicle tracking a global positioning system sends, via GPRS, data a regular intervals relating only to its position.