| Two nonlinear state estimation techniques, the Sliding Mode Observer and the Extended Kalman Filter, are compared in terms of their ability to provide accurate relative position and velocity estimates for a formation flying mission about the Earth/Moon - Sun L2 libration point. The observers are individually tested on the NASA Constellation X simulation model. Constellation X is a proposed x-ray telescope mission, where formation flying spacecraft was considered as a possible mission scenario. A follower spacecraft is controlled to maintain a fixed distance (50 meters) from a leader spacecraft to with 1 millimeter accuracy.; The state estimates propagated by each observer were of sufficient accuracy to maintain the required separation distance to within mission design requirements. For these particular formations of the Extended Kalman Filter and the Sliding Mode Observer, the Extended Kalman Filter is shown to be less sensitive to measurement noise levels, and the Sliding Mode Observer is shown to be less sensitive to input disturbances. There is no overall significant difference in sensitivity to parametric uncertainties between observers. |
