Robust spacecraft attitude determination using global positioning system receivers
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This dissertation presents the development of a new algorithm for processing GPS signals to compute attitude solutions. This new algorithm utilizes an Extended Kalman Filter (EKF) with a quaternion state to combine signal to noise ratio (SNR) and differential carrier phase measurements. Attitude solutions can be obtained from these two measurement sources if multiple antennas are utilized which have non-aligned boresight vectors. The algorithm achieves improved integer resolution and accuracy for the carrier phase approach, and is able to perform on on-orbit antenna gain pattern calibration to aid the SNR approach. The developed algorithm is tested using both hardware in the loop space simulations and actual rooftop data. These tests are used to adjust the filter parameters and algorithm logic to achieve good performance. Testing is undertaken that demonstrates the accuracy and speed of the integer resolution process. Comparisons between rooftop and simulation results demonstrate that simulations accurately represent anticipated orbit conditions. These comparisons further show that the non-aligned antenna boresight vectors introduce little or no errors to the double difference carrier phase measurements. A trade study is conducted to assess the impact of the SNR measurements on the overall solution accuracy once the more accurate carrier phase measurements become available. The final version of the algorithm demonstrates solution accuracies of less than 0.5 degrees RMS in all three angles of rotation during rooftop tests, and accuracies on the order of 0.1 RMS in multipath-free orbit simulations. The developed and tested algorithm is then ported from its original code into a flight ready version available in NASA GEONS software. The versatility of the basic algorithm design is explored by creating a new system that incorporates magnetometer data with SNR and carrier phase measurements. The addition of the magnetometer measurements is shown to improve the integer ambiguity estimation accuracy. Single antenna experiments demonstrate how magnetometer measurements and single antenna boresight estimates provide attitude solutions that are accurate enough to resolve integer ambiguities on systems with aligned antennas. In conclusion, some possible future tests are described that may provide better results by utilizing hardware more appropriate to the developed algorithm.