The orbits of navigation satellites have to be determined very precisely. The Transit broadcast (predicted) ephemeris, which is computed by the US Navy Astronautics Group, has an estimated orbital positional accuracy of the order of 25 m in each direction. By contrast, the precise (post-mission) ephemeris, which is determined by the US Defense Mapping Agency, from tracking data collected by the global TRANET network, reaches accuracies of the order of 10 m. These orbital precisions affect the navigation and (static) positioning accuracies which can be achieved by users of the system. The same is true of the GPS system which will become fully operational some time during 1988—89. However, unlike Transit, GPS will allow quasi-instantaneous absolute positioning (i.e. real-time navigation) as well as very high relative positioning accuracies. The latter will be obtained by using special operational and processing techniques (e.g. ‘differential GPS’ and ‘GPS interferometry’).