Timekeeping in future NASA missions

The GPS has provided a model for precise time dissemination in the environment of the Earth. In the future, principles analogous to those now used in the GPS will need to be applied to space missions beyond the Earth. This paper presents some of the theoretical and technical issues that will be important for time synchronization among atomic clocks on Earth, on the surface of Mars, and on satellites in orbit about Mars. These issues include relativistic time transformations, environmental effects, and hardware factors. Potential applications are solar system very long baseline interferometry (VLBI), navigation, and refined tests of general relativity. The paper also considers issues regarding the formation of a distributed time scale using clocks on Earth and Mars, with an eventual extension of the time scale throughout the solar system.