THE GPS USERS CONTINUOUSLY receive navigation information from the GPS Space Vehicles in the form of data bits modulated on the received signals. This information, which is computed and controlled by the GPS Control Segment, includes the vehicle's time, its clock correction and ephemeris parameters, almanacs and health for all GPS Space Vehicles, and text messages. From this information the Users compute the Space Vehicle's precise position and clock offset and less precise positions and clock offsets of Space Vehicles yet to be acquired.The GPS Navigation Message design process included numerous trade studies which weighed various representations and algorithms against variables such as message size, accuracy, update frequency, User computational requirements, and legacy for the operational GPS. Other factors such as graceful degradation and future User requirements were also considered. Finally, upon selecting the appropriate design structure, the design was fine tuned to its final form and User algorithm implementation trade-offs were performed. The representation algorithms and User algorithms were jointly tested using a simulated Space Vehicle ephemeris trajectory and Space Vehicle clock. The results of these tests demonstrate that the User models represent the simulated ephemeris and clock to within 0.01 meters with precise parameters, and to within 0.1 meters with truncated parameters.

The concept validation phase (Phase I) of the Navstar GPS program will evaluate the performance of User receiving equipment and the Control Segment. The purpose of this paper is to describe how the Control Segment supports the User test program, and how Control Segment prediction errors in satellite ephemerides and atomic frequency standard behavior are propagated into User error. A summary of projected performance is also presented based on simulation results.

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