In early 1985, an improved ground Operational Control System (OCS) will maintain the navigation service. Primary among the OCS improvements over past GPS navigation systems is a global network of ground antennas (to upload satellite navigation data) and tracking/monitor stations. In addition, a refined Kalman filter will continuously estimate the Global Positioning System (GPS) satellite ephemerides and clock phases and frequencies. Although the user-GPS interface remains unaltered, the accuracy of the GPS navigation service is expected to improve by a factor of three.Using real and simulated GPS pseudo range radiometric tracking data, The Aerospace Corporation has completed a detailed error analysis which shows that the satellite clock noise contributes more than 90 percent of the total satellite-touser pseudo range error. If the number of OCS uploads is increased to three per day (as planned), then the accuracy of the navigation service is also expected to improve by nearly a factor of three because the clock-noise contribution to the range error increases linearly with time. Also, this analysis shows the consequence of satellite ephermeris uncertainties in the GPS navigation application.

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