Topex orbit determination using GPS tracking system

An extensive simulation study has been performed to analyze several aspects of Topex Precise Orbit Determination (POD) using the Global Positioning System (GPS). Since the error modeling is the key to such an experiment, extensive investigation has been performed based on real GPS data processing experience to create realistic measurement and dynamic error models for GPS. Double-differenced Topex-GPS phase measurements were evaluated in relation to improving the Earth geopotential model, which is the dominant error source for Topex POD. The effect of model errors on the Topex orbit and gravity recovery accuracies was evaluated. Long arc and short arc gravity solutions were compared to understand the effect of the model errors in using the long arc and the short arc solutions for the gravity recovery. Limiting factors for the long arc and the short arc gravity recovery were identified. The effect of ambiguity parameters on the gravity recovery was analyzed. Since the number of ambiguity parameters are quite large, an algorithm has been developed to handle the ambiguity parameters more efficiently. Subarc parameters and empirical parameters were tested to find an optimal set of parameters to be estimated for the gravity recovery. Several gravity models were obtained by combining Topex-GPS information with a nominal geopotential information. These combined gravity models were evaluated by the statistical comparison of the recovered geopotential model parameters and by the Topex orbit analysis. Significant improvement in the nominal geopotential model, especially for the low degree harmonics, was achieved. More than a factor of three improvement in Topex orbit accuracy was demonstrated using the recovered gravity model. Analysis associated with predicted Topex radial orbit errors using the solution covariance indicates error estimates for the orbit accuracy is realistic.