Design of a robust nonlinear attitude estimation algorithm for Space Rider mission An important part of almost every space mission post-launch analysis is the at- titude sensors performance assessment. Launch vibrations, orbit insertion and a harsh operating environment could degrade the sensors data reliability. Even mounting errors during vehicle assembly and integration could occur, mining the mission success. Generally, spacecraft attitude sensors require extensive in-flight calibration during their operational life to ensure pointing requirements are satis- fied. Continuous on-board calibration also provides a means for fault prediction and detection trough parameters tracking. Star trackers are among the most accurate instruments to estimate a spacecraft orientation in space, achieving accuracies to the arc-second range in the boresight pointing direction.