A report describes a series of computational models conceived as an improvement over prior models for determining effects of solar-radiation pressure on orbits of Global Positioning System (GPS) satellites. These models are based on fitting coefficients of Fourier functions of Sun-spacecraft-Earth angles to observed spacecraft orbital motions. Construction of a model in this series involves the following steps:
1. Form 10-day "truth" orbit arcs from precise daily GPS orbit data gathered during more than four years.
2. Construct a model of the solar-radiation pressure and estimate modelparameter values that make a leastsquares best fit of the model-predicted trajectory to each of the "truth" 10-day orbit arcs.
3. Using a least-squares procedure and utilizing the full covariance information from each 10-day fit, combine the estimates from all satellite arcs into a single set of model parameters for the two GPS constellations of the satellites now or soon to be placed in service.
4. Evaluate the model thus derived by means of orbit-data-fit and orbit-prediction tests. In evaluations performed thus far, these models have been found to offer accuracies significantly greater than those of the prior models.