1. A method of determining the relative dilution of precision (RDOP) of a GPS measurement comprising: receiving signals from a plurality of GPS satellites at a first, fixed, known location; receiving the signals from the plurality of GPS satellites at a second, fixed locations; measuring the time of receipt of the signals; determining the RDOP of a measurement of the locations of the second locations with respect to the first location in accordance with the relative changes in satellite positions during the receipt of the signals.