Calibration technique for high-resolution laser pointing determination system

Summary form only given. The Geoscience Laser Altimeter System (GLAS) is a next generation space lidar whose design combines a 10-cm precision surface measurement with a dual wavelength cloud and aerosol lidar for long-term continuous use in orbit. The primary mission for GLAS is to measure the seasonal changes in the heights of the Greenland and Antarctic ice sheets. Since the accuracy of 10 cm in range measurements needs to be achieved even over surfaces with /spl sim/3 deg slope, the pointing angle of the ranging laser has to be known to within /spl sim/7.3 urad. For this purpose GLAS uses a stellar reference system (SRS) to measure the pointing angle of each laser shot relative to inertial space. To achieve this accuracy, the SRS itself needs to be calibrated and tested with an instrument capable of considerably better accuracy and stability. To meet these requirements the following technique has been developed. A Newtonian telescope with a point source in its focal plane is used as a source of collimated light (i.e. artificial star generator). It has FOV of /spl plusmn/0.5/spl deg/ and uses three corrective lenses to: (a) achieve diffraction-limited performance across entire FOV; (b) reduce distortions to <0.01% across entire FOV; (c) make the telescope telecentric; (d) minimize sensitivity to lateral misalignment. In the focal plane of the star generator there is an array of pinholes. The array, when used with the corrected star generator telescope, serves as an absolute angle reference. The star generator is calibrated using interferometric techniques, which allows for absolute angle determination. It is then used to calibrate the reference measurement telescope used for independent monitoring of the SRS measurements.