Laser Reference Sensor Alignment Estimation Using Star Observations

Abstract

The laser reference sensor is the central instrument in the Ice, Cloud, and Land Elevation Satellite laser pointing knowledge system, simultaneously observing stars and the altimetry laser in a single instrument coordinate frame. The star measurements are sparse, with the intermittent tracking of individual stars because of the small field of view and no tracking in sunlight due to problems with scattered light. Attitude estimation for the laser reference sensor alone is challenging due to its limited stellar data output. There are also three commercial star trackers mounted on the science instrument and spacecraft bus. The time-varying alignments and attitudes of all four star trackers are tracked simultaneously using an alignment filter in order to determine the stability of the laser reference sensor and the effects of its sparse star observations on alignment and attitude knowledge. The filter is able to predict laser reference sensor star measurements with accuracies of approximately 0.5 to 1.5 arcseconds over time scales greater than the orbital period while tracking laser reference sensor alignment variations on the order of 40 arcseconds.