In order to determine the distance the receiver is from the satellite, the receiver basically calculates the time it takes for the signal to travel an x amount of distance at the speed of light (approximately 300,000km/s). Which is the familiar velocity times time problem. Three satellite positions can get you a position in three dimensional space (x,y,z), however due to the nature of the signal transmitted by the GPS satellites the precision may be degraded due to timing offsets. The satellite transmits a coded signal at a specific timing interval using a very accurate clock to time the transmissions (atomic clock). The receiver transmits the same signal at the same timing interval, however the receiver's clock is much less accurate (quartz clock). To compensate for this signal timing offset a fourth satellite is used to calculate a fourth variable, time (x,y,z,t). Even atomic clocks cannot be perfectly syncronized, however the use of four imperfect measurements does help to eliminate the timing offsets.