Abstract
The Ice, Cloud, and land Elevation Satellite (ICESat) mission used laser altimetry measurements to determine changes in elevations of glaciers and ice sheets, as well as sea ice thickness distribution. These measurements have provided important information on the response of the cryopshere (Earth's frozen surfaces) to changes in atmosphere and ocean condition. ICESat operated from 2003 to 2009 and provided repeat altimetry measurements not only to the cryosphere scientific community but also to the ocean, terrestrial and atmospheric scientific communities. The conclusive assessment of significant ongoing rapid changes in the Earth's ice cover, in part supported by ICESat observations, has strengthened the need for sustained, high accuracy, repeat observations similar to what was provided by the ICESat mission. Following recommendations from the National Research Council for an ICESat follow-on mission, the ICESat-2 mission is now under development for planned launch in 2018. The primary scientific aims of the ICESat-2 mission are to continue measurements of sea ice freeboard and ice sheet elevation to determine their changes at scales from outlet glaciers to the entire ice sheet, and from 10s of meters to the entire polar oceans for sea ice freeboard. ICESat carried a single beam profiling laser altimeter that produced ~70m diameter footprints on the surface of the Earth at ~150m along-track intervals. In contrast, ICESat-2 will operate with three pairs of beams, each pair separated by about 3km cross-track with a pair spacing of 90m. Each of the beams will have a nominal 17m diameter footprint with an along-track sampling interval of 0.7m. The differences in the ICESat-2 measurement concept are a result of overcoming some limitations associated with the approach used in the ICESat mission. The beam pair configuration of ICESat-2 allows for the determination of local cross-track slope, a significant factor in measuring elevation change for the outlet glaciers surrounding the Greenland and Antarctica coasts. The multiple beam pairs also provide improved spatial coverage. The dense spatial sampling eliminates along-track measurement gaps, and the small footprint diameter is especially useful for sea surface height measurements in the often narrow leads needed for sea ice freeboard and ice thickness retrievals. The ICESat-2 instrumentation concept uses a low energy 532nm (green) laser in conjunction with single-photon sensitive detectors to measure range. Combining ICESat-2 data with altimetry data collected since the start of the ICESat mission in 2003, such as Operation IceBridge and ESA's CryoSat-2, will yield a 15+ year record of changes in ice sheet elevation and sea ice thickness. ICESat-2 will also provide information of mountain glacier and ice cap elevations changes, land and vegetation heights, inland water elevations, sea surface heights, and cloud layering and optical thickness.
Highlights
68 69 ICESat was the first spaceborne laser altimetry mission for Earth science and was in operation from 2003 – 2009 [Schutz et al, 2005]
The dense spatial sampling eliminates along-track measurement gaps, and the small footprint diameter is especially useful for sea surface height measurements in the often narrow leads needed for sea ice freeboard and ice thickness retrievals
Because of laser lifetime issues, 71 ICESat’s collection strategy was changed from continual operation to 30 day 72 campaign periods two to three times each year. Despite this campaign mode operation, it was a very successful mission that enabled estimates of the overall mass change of the Greenland and Antarctic ice sheets, as well as the regional changes that illuminate the underlying processes [Pritchard et al, 2009; Zwally et al, 2011 and 2015; Sørensen et al, 2011; Sasgen et al, Csatho et al, 2014, Khan et al, 2014]. 78 79 One of the key findings of ICESat was that some outlet glaciers around the margins of these ice sheets are losing more mass quicker than expected [e.g., Pritchard et al.,2009; Zwally et al, 2011]
Summary
68 69 ICESat was the first spaceborne laser altimetry mission for Earth science and was in operation from 2003 – 2009 [Schutz et al, 2005]. 309 e) ICESat-2 shall provide monthly surface elevation products to enable, when sea surface height references (leads) are available and under clear sky conditions, the determination of sea-ice freeboard to an uncertainty of less than or equal to 3 cm along 25-km segments for the Arctic and Southern Oceans; the track spacing should be less than or equal to 35 km at 70 degrees latitude on a monthly basis. 499 A pair-spacing requirement of 90 m is based on a sampling analysis of airborne laser-altimetry data collected with the Airborne Topographic Mapper (ATM) over Russell Glacier, in Southwest Greenland, which spans a wide range of surface roughnesses (Figure 4, top) In this analysis, the collection of point elevation measurements was sampled using different potential beam spacings and random repeat-track geometries, and the RMS error calculated in the resulting surfacechange measurements. For high reflectivity targets, such as ice sheets, the weak beams returns a sufficient number of laser photons to enable elevation measurements
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