Transfer orbit stage gyrocompass alignment algorithm in a twist and sway environment for Mars Observer Mission on commercial Titan

A robust numeric gyrocompass alignment algorithm, to meet the initial alignment requirement for the transfer orbit stage for the Mars Observer Mission has been developed. The algorithm contains a coarse alignment algorithm, a local level navigator which continuously propagates the attitude and generates the accumulated linear velocity error at 50 Hz, and a Kalman filter which corrects the attitude and accumulated linear velocity errors at 1 Hz. The gyrocompass algorithm performance over 7200 seconds of time simulation in west, north, and down axes is presented. The coarse alignment was performed for 30 seconds, and then the fine alignment was started with the coarse alignment matrix as the initial condition. From the simulation results, it was observed that the gyrocompass reduced the down alignment error to less than 0.5° (1800 arc secs) within minutes. After 20 minutes, the reduction rate in the down alignment error became small, and then it finally settled to a steady-state value. The west, north, and down axes error statistical information indicates that the numeric gyrocompass algorithm meets the present requirements with sufficient margins