1. An apparatus for use with a plurality of GPS earth satellites in estimating one or more states of a platform, the platform having a reference point that represents the position of the platform and at least two receiving points corresponding to the locations of antennas for receiving satellite signals, the apparatus comprising: a processor for computing estimates of the errors in the platform states from inputs comprising (1) one or more measured phase double-differences obtained from signals transmitted by a plurality of satellites, (2) the estimated position of the reference point, (3) the estimated positions of the receiving points relative to the reference point, and (4) the positions of the satellites, a phase double-difference being defined as the difference in phase differences for signals received from two satellites, a phase difference being the difference in carrier phase of a satellite signal received at two receiving points on the platform.