Inertially referenced laser pointing determination system

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Abstract
Summary form only given.In a high precision space based altimetry system such as GLAS, accurate knowledge of the laser beam's pointing angle is critical. For instance, over a 3/spl deg/ surface slope pointing knowledge to 1.5 arcsec is the largest error source (23 cm) in achieving the GLAS accuracy requirement. The GLAS design incorporates a stellar reference system (SRS) to relate the laser beam pointing angle to the star field to an accuracy of /spl sim/7.3 urad. The stellar reference system combines an attitude determination system (ADS) operating from 4 to 10 Hz coupled to a 40 Hz laser reference system (LRS) to perform this task. The simplest approach for measuring the pointing of the GLAS laser beam with respect to the star field is to couple the laser directly into a star camera. Unfortunately this approach is not feasible with current star camera technology. An approach using two separate sensors to measure the pointing of the sampled laser beam at 40 Hz has been built. The overall approach for the stellar reference system is shown. The ADS measures the pointing of the instrument platform with respect to the star field while the LRS samples the laser beam at 10 Hz and measures its alignment with respect to the components of the ADS. The laser profiling array measures the spatial profile of the laser beam at 40 Hz. In the SRS approach a small fraction of the GLAS laser beam is folded into the laser reference sensor's FOV with two lateral transfer retroreflectors.

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