Curt H. Davis
Electrical & Computer Engineering
University of Missouri-Columbia/Kansas City
I. Introduction
Volume changes in the polar ice sheets are directly related to global sea levels. During the past century, the sea level, as recorded from tide-gauge data around the world, has risen by 10 to 20 cm. Although both thermal expansion of the oceans and ice-sheet melting contribute to sea-level rise, no more than 25% of this increase can be attributed to thermal expansion. It is not currently known with any degree of certainty whether or not the Greenland and Antarctic ice sheets are growing or shrinking. The uncertainty in the mass balance of the ice sheets is around ± 30% which translates to a ± 5 cm/yr change in average ice thickness (for both ice sheets). Thus it is necessary to develop a baseline set of mass-balance measurements for the ice sheets. This is extremely important for assessing the current state of the polar ice sheets and for predicting future changes in the ice sheets. Accurate measurements of ice-sheet growth/shrinkage would quantify one important source of sea-level change and thus enable a better understanding of the effects of greenhouse gases on global climate change.
II. Satellite Radar Altimetry
Over the last two decades spaceborne radar altimeters have provided unprecedented coverage of the Earth's surface. Satellite radar altimeters were developed primarily for the study of oceanic physics and the Seasat, Geosat, Topex/Posedion, and ERS-1 satellites have produced datasets of the mean sea-level, wave height, and surface windspeed over a large percentage of the world's oceans. Even though radar altimeters were developed primarily for oceanographic applications, the orbits of Seasat and Geosat extended to latitudes of ± 72.1°, covering major portions of the Greenland and Antarctic ice sheets. Currently the ERS-1 satellite radar altimeter is providing elevation measurements up to ± 82°. Datasets provided by these satellites have been used to produce surface elevation maps of large portions of Greenland and Antarctica.
Scientists have used the 10 year time interval between the operation of the Seasat and Geosat altimeters to estimate the mass balance of the southern portion (south of 72° N) of the Greenland ice sheet. However, these results have some major limitations because of the inherent design limitations of the radar altimeter. First, a typical radar altimeter has an antenna beamwidth of 1-2° corresponding to a footprint diameter on the Earth's surface on the order of 10 - 20 km, depending upon the orbital altitude of the altimeter. Thus, the radar altimeter does not obtain an elevation measurement at a distinct location, rather it is a spatial average of the surface elevation over the altimeter's entire footprint. This introduces significant errors (tens of meters) to the elevation measurement when the surface slopes are large (> 1°). Second, the radar altimeter depends on multiple pulse averaging because of low signal-to-noise ratios (SNR) for individual pulses. Thus, the averaging of the received pulses (typically 100) along the satellite track also represents another form of spatial averaging. Finally, the areas of the ice sheets that are the most sensitive to climatic variations are near the margin where increases in the melting and drainage rates will have the most dramatic effect on surface elevation. The tracking circuitry on-board the radar altimeters was designed for the slowly varying surface elevations over the oceans. When the abrupt transition in elevation from ocean to ice-sheet margin occurred, the radar altimeter frequently lost track and the majority of data in this critical region was lost.
III. Laser Altimetry Requirements
Because of the critical link between the ice sheets and global sea levels and the inherent limitations of radar altimeters for measuring ice-sheet elevations, the development of a spaceborne laser altimeter system specifically dedicated for ice-sheet observations is currently being pursued. Studies conducted in the 1980's identified the key scientific requirements for a spaceborne laser altimeter to adequately measure the topography of the polar ice sheets. These requirements include: a 70 m footprint diameter, a ± 10 cm range precision, a sampling rate of 20 Hz (corresponding to a ground sample spacing of 350 m), a high SNR for each individual pulse (no averaging), a highly accurate knowledge of the laser pointing with respect to the altimeter nadir angle, and 10 million elevation measurements over 3.5 months to perform a complete survey of the ice sheets. These requirements would enable a second ice-sheet survey to detect elevation changes of ± 15 cm over a time interval of only 1 year. This level of precision is only attained by satellite radar altimeter systems for periods of a decade or longer, and then only in the flatter interior portions of the ice sheets.
IV. Geoscience Laser Altimeter System (GLAS)
The GLAS instrument is currently being developed by NASA under the Earth Observing System (EOS) program. The primary purpose of GLAS is to determine the mass balance of the polar ice sheets and their contributions to global sea level change. This would be the first spaceborne altimeter (laser or radar) whose primary purpose is the study of the ice sheets. Consequently, the design parameters of the system have been tailored to meet the requirements outlined in the previous section. The divergence of the laser beam is 100 mrad producing a 70 m surface footprint from an orbital altitude of 700 km. Four diode pumped, conductively cooled Nd:YAG lasers operating at 1064 nm with an output energy of 100 mJ and a pulsewidth of 4 nsec will produce individual pulses with high SNR and a range precision (after tracking) of 10 cm. Four lasers are included for mission redundancy to produce continuous 40 Hz operation (175 m ground sample spacing) and a design lifetime of 5 years or 6 billion laser shots. To achieve the stringent laser pointing requirements required for ice-sheet measurements, zenith pointing star camera couplers will be used to resolve the laser pointing to an accuracy greater than 1 mrad. GLAS will provide 96% aerial coverage of the Greenland and Antarctic ice sheets and the ice-sheet mapping will be repeated every 6 months so that mass balance estimates can be produced. The GLAS system has been approved for development and is scheduled for a launch in July, 2002.
In addition to its primary mission, the GLAS instrument also has several important secondary benefits. When not located over the ice sheets the laser altimeters will provide useful information over land or ocean surfaces, including reflectivity, surface roughness, vegetation heights, and topography. Also, GLAS will use four lidars operating at 532 nm with an output energy of 50 mJ for atmospheric profiling from the ground to 40 km. These lasers will be used to measure the cloud heights and the vertical structure of clouds and aerosols in the atmosphere. The atmospheric measurements will provide important information about cloud distributions needed for Earth radiation budget analyses that could not otherwise be obtained unambiguously from passive optical techniques. Finally, the development of the laser technology, the star camera pointing, and the precision orbit determination methods required for GLAS will have direct benefits to future geoscientific satellites as well as contributing to the technology base of industry. For example, the laser technology can be applied to future commercial areas such as satellite communications and ranging.
V. Conclusion
Laser altimetry is a new technology that represents the next generation of spaceborne altimeter systems. This technology can overcome many of the limitations of previous spaceborne radar altimeter systems for monitoring the surface elevation of the polar ice sheets. Advantages of the laser technology include small footprint size, individual pulse processing of surface elevations, and dense ground sample spacing. This technology offers the best hope for accurately assessing the current state of the mass balance of the polar ice sheets. This knowledge is critically important for detecting global climate change signals and the contribution of the ice sheets to global sea level change.
FURTHER READING
Zwally, H.J., R.H. Thomas, and R.A. Bindschadler (1981), "Ice-sheet dynamics by satellite laser altimetry," NASA Technical Memorandum 82128, Washington, DC 20546.
Curran, R.J. et al. (1987), "LASA-Lidar Atmospheric Sounder and Altimeter," EOS Instrument Panel Report-Vol. IId, NASA, Washington, DC 20546.
World Wide Web: http://gdglas.gsfc.nasa.gov/lam/lammiss.html
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