Bob Schutz (Team Leader) reviewed the science objectives that GLAS is designed to address. He reviewed the general aspects of the instrument and mission design and how GLAS will meet the objectives for the cryosphere, atmosphere and land applications. The link to the IPCC recommendations on the cryosphere were discussed.
The GLAS data products were reviewed and discussed and the DAACs where each of the data products will reside were presented. The overall context of the data products within EOSDIS was summarized and it was noted that the data would be readily available to the community. There would be no period of exclusive access of the data, although the calibration/verification assessment of the instrument in the first 90-120 days may delay the rapid flow of data during this interval.
The Laser Altimetry Project Scientist, Jay Zwally (GSFC), reviewed the evolution of GLAS into the current concept as a free-flyer with a three-year design, but five-year goal. He described the current study of new concepts that was recently begun under the direction of John Oberright and Dan Mark, both from GSFC. Zwally also summarized the January meeting of the Cryosphere Working Group and he discussed aspects of the new IPCC Report.
Instrument and Spacecraft Status
Bert Johnson (GSFC) and Rob Afzal (GSFC), on behalf of the instrument team, summarized the status of on-going instrument development activities. They described the receiver breadboard being developed by X. Sung at Johns Hopkins University and the updated version of the laser altimeter simulator software, which has been enhanced to handle surface roughness and variability in reflectivity.
Significant progress has been made in the laser transmitter development. A laser transmitter breadboard has been developed to evaluate the beam quality and operation over the design lifetime. With the GLAS requirement of operation at 40 Hz, a design goal of 3.15 billion shots per laser has been adopted. This design goal provides for laser operation for approximately 2.5 years, thus an instrument design with three lasers would achieve the 3-year goal with redundancy.
A quarter scale breadboard laser oscillator/amplifier assembly has been constructed that exhibits good Gaussian beam quality. The oscillator beam produces a 4 nsec pulse width with 10% jitter and 2 mJ power followed by amplifiers to increase the pulse energy. About one week after the team meeting, an experiment on the diode pump arrays achieved well over 4 billion shots with a 17% power reduction using an accelerated pulse rate of 200 Hz, well within the specifications. This milestone demonstrated the viability of the design goals.
Additional tests on optics survivability have been initiated using an accelerated test with a 500 Hz laser system. This system, known as AGES, achieved 3.1 billion shots about one week after the GLAS Team meeting and has exhibited excellent performance.
Dan Mark (GSFC) described the plans and initial results for the GLAS study. Preliminary results from the study team, which is examining all aspects of the mission, has concluded that the current small satellite design could be launched in the year 2000 with an overall cost savings. A final report of the study is due in the May-June time period.
Cryospheric Science Applications
Ken Jezek and Ingrid Zabel (Ohio State) reviewed potential GLAS cryospheric applications. They noted that the frequencies used by radar altimeters, such as TOPEX or ERS-1, tend to penetrate the snow layer to depths that are dependent on factors such as snow wetness, whereas laser altimetry will measure the snow surface elevation. The combination of laser and radar altimetry could provide an estimate of snow depth in regions such as the percolation zone, where the radar measurement is dominated by the return signal from the most recent annual ice layer and the laser return is from the snow surface.
Terry Wilson (Ohio State) noted GLAS may contribute to understanding the rifting processes in the Transantarctic Mountains, especially in regions where the surface structures are below the ice surface. Studies with radar echo soundings suggest that a high correlation exists between surface elevation and bedrock topography.
Ian Whillans (Ohio State) reviewed the various contributions to the characterization of the ice surface. He noted that sastrugi topography grows with time after a blizzard. He discussed the scale of surface topography in terms of the vertical and horizontal size of the features as well as their temporal stability.
Kees van der Veen (Ohio State) discussed the problem of separating long-term changes in the surface from variations at shorter time scales. He noted the need for a long-term data set, but also noted the importance of initiating the collection of essential measurements.
Steve Forman (Ohio State) noted that GLAS may contribute to measurements of coastal erosion in the Arctic. Submerging areas erode quickly because of permafrost changes.
Airborne and Spaceborne Activities
Bob Thomas (NASA HQ) reviewed the NASA program in polar research. He made the observation that when GLAS is launched, the data collected in the airborne laser altimeter will provide data sets spanning 10 years over particular regions of the ice sheet. The combination of aircraft data and GLAS will provide an early assessment of mass balance in those regions.
Jack Bufton (GSFC) discussed the status of the laser experiment he will fly on the Shuttle, STS-72. This experiment will provide data for analysis of the performance of the laser altimeter over a variety of mid-latitude surface topographies (land, vegetation, water). The laser footprint of 100 m is similar to the GLAS footprint of 70 m.
The generation of a digital elevation model (DEM) using a scanning airborne laser altimeter (AOL) was discussed by Bea Csatho (Ohio State). The DEMs have high resolution of a few tens of centimeters and were created from parallel AOL swaths measured along a selected ERS-1 ground track.
Tony Schenk (Ohio State) described a possible procedure for calibration of airborne laser altimetry using photogrammetry. The proposed technique may be applicable to GLAS calibration as well.
James Choe (University of Texas) described his analysis of 1993 and 1994 airborne laser altimetry over Greenland along an ERS-l track. He described the technique he uses to correct for the troposphere delay and noted that comparison of laser data along nearly coincident tracks requires information on the cross-track slope, such as the DEM.
Charlie Vaughn (NASA Wallops) discussed the problem of geolocating the laser footprint of the airborne altimeter. The contribution to the vertical component from roll and pitch biases and the bias determination was summarized. Jim Spinhirne (GSFC) described an analysis using the ISCCP data base to examine the seasonal variation of clouds in the polar regions. Initial estimates of the coverage of ice sheet altimetry returns can be given, but much better knowledge of the distribution of cloud optical thickness is needed. The establishment of a few ground-based lidar systems in the polar regions would enable a useful characterization of the cloud factor in GLAS measurements
GLAS Orbit, Algorithm Theoretical Basis Document
Bob Schutz (University of Texas) reviewed the constraints and considerations that led to the following recommended GLAS orbit paramters: 94 degree inclination, 705 km altitude, 182-day repeat cycle, frozen orbit. The reasons for recommending a retrograde orbit (94 degree) vs. a prograde orbit (86 degree) that provides comparable coverage were reviewed. The 4-degree region at the pole where no data can be collected by a nadir-viewing altimeter at this inclination was discussed within the context of the tradeoffs of science and analysis techniques. The possibility of reducing the size of the "hole" by changing to a more nearly polar orbit was examined in detail after the meeting. In early April, the Team reiterated the 94 degree inclination based on considerations associated with the characteristics of altimeter crossovers over the Greenland and Antarctic ice sheets.
The GLAS data products were reviewed and a discussion of the algorithm theoretical basis documents for the data products was conducted. The GLAS Science Management Plan addressed the individual team tasks related to the ATBD aspects. Working group meetings for the preparation of the GLAS ATBDs are planned.
The meeting concluded with the GLAS Team thanking the Byrd Polar Research Center for their kind hospitality. The next regular meeting will be in the September/October time period.