The Limits of Direct Satellite Tracking with GPS

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Abstract
Recent advances in high-precision differential GPS-based satellite tracking can be applied to the more conventional GPS-based direct tracking of low-earth satellites. To properly evaluate the limiting accuracy of direct GPS-based tracking (i.e., with no differential corrections provided from simultaneous ground observations), it is necessary to account for the correlations between the a priori errors in GPS states, Y-bias (unmodeled forces on the GPS satellites), and GPS solar pressure parameters. These can be obtained only by careful analysis of the process by which the a priori GPS orbits have been determined. The analysis indicates that submeter real-time accuracy can be readily achieved for a user above 700 km altitude, even when the user solution is based on a GPS ephemeris that is more than 12 h old. In the presence of Selective Availability, a user without access to the Precise Positioning Service will require a near-real-time correction to achieve meter-level accuracy.

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