GLAS is a laser altimeter designed to measure ice-sheet topography and associated temporal changes, as well as cloud and atmospheric properties.  In addition, operation of GLAS over land and water will provide along-track topography. For ice-sheet applications, the laser altimeter will measure height from the spacecraft to the ice sheet, with an intrinsic precision of better than 10 cm with a 70-m surface spot size.  The height measurement, coupled with knowledge of the radial orbit position, will provide the determination of topography.  Characteristics of the return pulse will be used to determine surface roughness.  Changes in ice-sheet thickness at a level of a few tens of cm (anticipated to occur on a subdecadal time scale) will provide information about ice-sheet mass balance and will support prediction analyses of cryospheric response to future climatic changes.  The ice-sheet mass balance and contribution to sea-level change will also be determined.  The accuracy of height determinations over land will be assessed using ground slope and roughness.  The height distribution will be digitized over a total dynamic range of several tens of m.

Along-track cloud and aerosol height distributions will be determined with a vertical resolution of 75 to 200 m and a horizontal resolution from 150 m for dense cloud to 50 km for aerosol structure and planetary boundary layer height.  Unambiguous measurements of cloud height and the vertical structure of thin clouds will support studies on the influence of clouds for radiation balance and climate feedbacks.  Polar clouds and haze will be detected and sampled with much greater reliability and accuracy than can be achieved by passive sensors.  Planetary boundary layer height will be directly and accurately measured for input into surface flux and air-sea and air-land interaction models.  Direct measurements of aerosol vertical profiles will contribute to understanding of aerosol-climate effects and aerosol transport.

The GLAS laser is a diode pumped, Q-switched Nd:YAG laser with energy levels of 75 mJ (1.064  µm) and 35 mJ (0.532  µm).  The pulse repetition rate is 40 pulses/sec, and the beam divergence is approximately 0.1 mrad.  The infrared pulse is used for surface altimetry, and the green pulse is used for atmosphere measurements.  The altimeter uses a 100-cm diameter telescope.

In the 120 days following launch, the ground track will repeat in 8 days to provide multiple overflights of ground verification/validation sites. The main mission will use a 183 day repeat track.

Last Update: January 1999