The GPS Navigation Message

Abstract

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.

Broadcast vs. precise GPS ephemerides: a historical perspective

“…UERE is dependent upon the receiver design and the environment in which a receiver is used. The original SPS GPS error budget allocated 6 metres to SISRE and 3.6 metres to UERE [Van Dierendonck 1980]. User Navigational Error is a measure of the total navigational error experienced by a user for defined equipment in a known environment.…”

GPS modeling for designing aerospace vehicle navigation systems

“…The GPS satellite ephemerides are obtained either in real-time from the GPS navigation message [15] or from satellite tracking data spanning a period of several days both before and after the time of interest. The accuracy of the ephemerides can be expressed as position and velocity standard deviations in height, long-track, cross-track (HLC) coordinates; Thble II presents the values used for this study [13].…”

Global Positioning System (GPS) Autonomous User System

“…The ENM ephemeris and clock parameters are updated by the estimated corrections every filter cycle so that X,-, is always zero in equation (5).…”

“…The navigation message is a multiparameter fit (15 parameters plus an epoch time for the ephemeris and 3 parameters plus an epoch time for the clock) to the predicted ephemeris and clock [5,7]. In order to maintain the required system accuracy, the fit period should be on the order of 4 to 6 h. The current fit period is 4 h, requiring new navigation messages to be radiated at intervals no longer than 4 h. The control segment nominally uploads new navigation messages at 8 h intervals so that the satellite-transmitted data processed by a user are nominally, at most, an 8 h prediction.…”