It must be stated at the outset that the present author has a technical knowledge of Navstar GPS based only on fragments of the literature open to the public, and no knowledge whatever about the plans for its military use. An attempt will be made to discuss, from first principles, the consequences to both civil and friendly military users of the availability of a global, satellite-based navaid that makes possible the determination of position in three dimensions to a high accuracy. For civil users, confined to the ‘coarse acquisition’ mode, there is a 95 per cent probability that the horizontal position error will not exceed about 20 metres. For military users holding the key to a more sophisticated mode of operation, a significantly greater accuracy is available. The GPS also determines velocity, and data can be updated every second or so. It will be assumed that the navaid covers all the Earth's surface of interest, down to ground level. The cost of the satellite system is assumed to be high.