There now exists radar altimeter measurements, from 1978 through the present, of the Greenland and Antarctic Ice sheets, the spatial and temporal coverage is summarized in this table. The NASA Pathfinder program is endeavoring to reference data from all missions into a common datum using a common set of atmospheric, instrument, and earth dynamic corrections where possible. This will give scientists a time-series of consistent data over 20 years to investigate seasonal and interannual variations of the ice-sheet surface balance, which is caused by variations in snowfall and ice melting ( Zwally et al. 1989, and Lingle et al. 1994 ). In addition, precise measurements of elevation changes can be used to estimate total mass balance of the ice sheets, which had not been obtainable to sufficient accuracy with other techniques. The crossover data gives elevations for different tracks at the same location, are best used for ice sheet change studies.
The multi-mission data can also be combined to give dense topographic coverage crucial for a variety of glaciological studies, from numerical modeling of the ice dynamics to monitoring of key glaciological features: ice drainage basins, ice-shelf grounding lines, ice divides, subglacial lakes, ice shelf fronts, and ice rises. Topographic grids (Level 4) using multi-mission data are crucial to delineating the key glaciological features of the ice sheets mentioned above. For more detailed elevation information the georeferenced databases (Level 3) that contain the full-rate along track data can be utilized.
The ice elevation data set (Level 2 - Ice Data Record) consists of the full rate altimetry with location, time, surface height and all pertinent corrections. It is mainly used by scientists who are trying to improve the algorithms to calculate the corrections or who require knowledge of what effect the corrections have on the data. Most researchers do not require this much detail and would be better served by the georeference data bases which contain the precise surface height after all corrections have been applied at the full data rate.
The altimeter waveform return gives a measure of the surface roughness on scales of sub-meter to tens of kilometers and indicates if surface penetration and/or volume scattering is present. These results are used to investigate regional, interdecadal and interannual changes in the ice sheet and sea ice surface characteristics (Yi and Bentley 1994, Davis and Zwally 1993). The returns are also used to identify sea ice boundaries and extent. These are available on the Level 1 - Waveform Data Records.