Interferometric GPS/Micro-Mechanical Gyro Attitude Determination System: A Study Into the Integration Issues

摘要

The near future will see a proliferation of small low cost communication and science satellites with modest (0.1-0.5 deg) pointing requirements which will use attitude determination Systems (ADSs) of low power, weight, size and cost. This thesis is a study into the integration issues of such a system, that uses micro-mechanical inertial sensors and an interferometric GPS (IGPS) attitude determining receiver that are tightly coupled using 'model based' estimation techniques. The integration issues analyzed include: the effect on performance of limited IGPS use, the effect on performance of various gyro qualities and model accuracies, the inherent robustness to failure of the system and a power consumption analysis for a minimum energy IGPS receiver. It was found that attitude performance varied less than 0.05 degrees (RSS) when the interval between GPS phase measurements was varied from once every second to once every 30 seconds. This justifies the development of a minimum energy receiver to take advantage of the power savings of less frequent GPS measurements. During times of low dynamic model accuracy, the IGPS/MM Gyro ADS was bound by the steady state attitude error of the IGPS which averages a 1 sigma RSS error of about 0.28 degrees within the GPS update range mentioned above. Attitude error also depends on the gyro quality, and various projected system accuracies are given using several gyro error models. Real micro-mechanical gyro data from a prototype gyro was successfully integrated with real GPS phase measurements using an Extended Kalman filter. Failure scenarios and power expenditures were also analyzed using steady state linear covariance analysis as well as a space simulation which uses orbital parameters of a telecommunication satellite constellation. Total power expenditure of 200 mWatts can be achieved for this system and still fulfill mission requirements.