
When fewer than three of the satellites of a satellite global positioning system (GPS) such as NAVSTAR are visible to a user (15), the user cannot obtain independent positional information from the system. Since a satellite (11) in such a system is moving with respect to a user (15), its signals are received with a Doppler offset from their normal centre frequency and the frequency offset due to the satellite motion alone is calculable for a user at an approximate location from a given satellite...











