Laser Reference Sensor Alignment Tracking and Star Observations

Abstract

The laser reference sensor is the central instrument in the Ice, Cloud, and land Elevation Satellite 2 laser pointing knowledge system, simultaneously observing stars and the altimetry laser in a single instrument coordinate frame. The star observations are relatively sparse, with a predicted brightness cutoff near visual magnitude 5, and their density varies significantly across the sky. There are star gaps of up to approximately 200 s, and areas of the sky with 20 simultaneously observable stars. The star observations are augmented with observations from two spacecraft star trackers using an alignment filter. The filter tracks the motion of the laser reference sensor relative to the spacecraft. Monte Carlo simulation is used to characterize the effects of various magnitude cutoffs, types of alignment variations, and regions of the sky on alignment tracking and overall pointing knowledge performance. Multiple model adaptive estimation is used to map alignment process noise filter tuning with respect to the input parameters. The results include pointing knowledge uncertainty predictions over an input parameter space of three star brightness cutoffs, two types of alignment variations, and three alignment variation amplitudes. The results also map pointing uncertainty to regions of the sky and individual stars.