Pamela S. Millar

The Geoscience Laser Altimeter System (GLAS) for the ICESat mission

Summary form only given.The Geoscience Laser Altimeter System is a next generation space lidar whose design combines a 10 cm precision surface measurements with a dual wavelength cloud and aerosol lidar in for long-term continuous use in orbit. GLAS is scheduled to be launched into a 598 km circular polar orbit in summer 2001 as part of NASAs ICESat mission. The orbit and mission parameters are summarized. The orbits 94 degree inclination was selected to optimize the crossing ground track patterns over Greenland and Antarctica and to enable data comparison with other NASA Earth Science instruments. GLAS will determine the orbit altitude, position and time from an on-board GPS receiver, and is designed to operate continuously for 3-5 years.

Two-color laser ranging to a cooperative airborne target

An experiment was performed during the summer of 1992 to measure the changes in the roundtrip time of flight of laser light due to horizontal gradients in the atmosphere. Dual laser frequencies were used to range from a fixed telescope ground system to an array of corner cubes mounted on an aircraft. The aircraft flew circular paths at slant ranges of 20 to 25 kilometers around the ground facility. To maximize the number of two color returns from the aircraft, both an acquisition and a closed loop tracking scheme were developed. A Global Positioning System (GPS) receiver placed onboard the aircraft relayed its position to the ground via a radio downlink; when this data was extrapolated in time, it gave telescope acquisition pointing angles to within 1 degree of truth. Closed loop tracking was then achieved by digitizing the image of a down-looking infra-red beacon which was viewed through a camera mounted on the telescope. The X-Y coordinates of the beacon in the camera field of view were sent to the telescope computer to continuously track the image. Stable tracking was achieved on each of the three experiment nights for over 90 minutes with tracking errors within the required 200 microradian limit. Enough two color waveform data was obtained to analyze the horizontal changes in the atmospheric delay.

New capabilities for Earth Science measurements with 6U CubeSat technologies

NASAs Earth Science Technology Office (ESTO) is responsible for the development of advanced technologies to address future Earth science measurement needs. In recent years, ESTO has flown various 1U - 3U CubeSats as a means to prove the applicability of information system and instrument subsystems in support for Earth science Decadal Survey mission concepts. In 2015 as part of the In-Space Validation of Earth Science Technologies (InVEST-15) solicitation ESTO awarded four new 6U CubeSat projects. While these projects are maturing specific new technologies, their capability and scope also have the potential to serve as platforms for stand-alone science observations. We describe the goals of the InVEST program and introduce the new technologies and science potential of the InVEST-15 selections.

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.