Autonomous Lunar Orbit Determination using Star Occultation Measurements

A method has been developed for performing autonomous Lunar orbit determination based on measurements of the times at which stars set behind or rise above the Lunar limb. This system is being developed as a possible technology for use in the Lunar exploration initiative because it enables increased autonomy of operations near the Moon. The system consists of a specially modified star camera and an extended Kalman filter. The star camera keeps track of known stars in its field of view and reports the times when stars suddenly appear or disappear without crossing the edge of the field of view. These are times that the known line-of-sight vectors to the stars cross the Lunar limb, and this knowledge translates into position information. The Kalman filter uses a series of star occultation/rising times and an orbital dynamics model to estimate the spacecraft’s position and velocity. An observability analysis shows that this system is strongly observable, and a truth-model simulation has demonstrated an absolute position accuracy of 70 m per axis and an absolute velocity accuracy of 0.045 m/s per axis when using a Lunar topographic map with an RMS altitude accuracy of 100 m and a Lunar gravity model with an RMS accuracy of 1x10 m/s.