Satellite attitude determination based on GPS carrier phase measurements

Özten, Evrim
With the developments in the GPS receiver technology, GPS receivers have been preferable equipments also for the space applications. In the early eighties, they have been used only for position, velocity and time determination. With the further improvements, it has been proven that they can provide not only the positioning information, but also attitude knowledge with the accuracy less than 1 degree. In this thesis, an estimation algorithm is developed in the MATLAB Simulink environment for the attitude determination using the GPS carrier phase measurements from four antennas. Because the attitude dynamics of a satellite is nonlinear, Extended Kalman Filter (EKF), a nonlinear estimator, is decided to use. The algorithm is implemented using three quaternion and three inertial angular velocity error states. For the attitude determination, four antennas in a square shape with 1 meter inter-antenna distance are assumed to be used. A simulation is conducted in order to evaluate the performance of the algorithm. The simulation includes a satellite in J2 perturbed orbit. The yaw stabilization of the satellite is maintained by using a momentum wheel with a spin axis perpendicular to the orbit plane. GPS pseudorange, pseudorange rate and differential carrier phase measurements are also modeled in the simulation environment. Because most of the measurement errors are quite small in the carrier phase measurement or they mostly cancel out after the single differencing process, only the multipath error and receiver noise is modeled in the simulation. An integrated GPS/Gyro case is also considered in this study. For the integrated GPS/Gyro case, the developed EKF is modified by adding gyro bias drift parameter to the state vector. In order to model the gyro measurements, a gyroscope error model is developed. The gyro measurements are generated by adding errors like bias, bias drift, scale factor and misalignment to the true angular rates. The algorithms developed for GPS only and GPS/Gyro integrated cases are tested with different GPS visibility conditions. After the simulation test, it is observed that the achieved attitude accuracy with the GPS only case is better than 0.4 degrees. Moreover, the integration with three axis gyro assembly improves the attitude performance especially for the cases with poor GPS satellite visibility.