Sea Technology

AUG 2012

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of ice. Unlike lidar, radar can see through snow. The lidar de- termined the distance between the aircraft and the top of the snow, and the radar altimeter determined the distance from the aircraft to the top of the ice. The lidar and radar measurements of the freeboard were periodically re-referenced to sea level by measuring distance to the water over openings in the ice. The data collected could not be observed on board the Twin Otter during flight and were analyzed during post-mission analysis. Thus, the distance from the water surface to the top of the freeboard could be calculated. By determining the depth of the snow and ice above the sea surface, and assuming the den- sity of the snow and ice, a model using the principle of isostasy was used to determine the equilibrium between the seawater, atmosphere and ice to determine the ice thickness below the waterline, or ice draft. Satellites Satellites have been playing a major role in determining sea ice thickness. The ICESat (Ice, Cloud and land Elevation Satel- lite) mission that ran from 2003 to 2010 was part of NASA's Earth Observing System (EOS) used to study and monitor global ice cover. It helped determine freeboard and measure ice sheet mass balance, cloud and aerosol heights, land topog- raphy and vegetation characteristics. ICESat carried the Geo- science Laser Altimeter System, a lidar instrument that took measurements along a ground track every eight days. NASA intends to launch ICESat-2 in 2014. Until then, NASA's Operation IceBridge airborne research, a five-year pro- gram now in its third year, will fill the gap. NASA's 2012 Ice- Bridge flights were conducted with a P-3 four-engine turbo- prop airborne laboratory. The IceBridge missions carry NASA's Airborne Topographic Mapper, a high-precision, airborne scanning laser altimeter; the Digital Mapping System, a nadir-viewing digital camera; and the University of Kansas's ultrawideband frequency-mod- ulated continuous-wave snow radar. IceBridge missions are also studying the Greenland ice sheet, glaciers in Alaska and the Antarctic ice sheet. The new ICESat-2 satellite mission is being planned by the NASA Goddard Space Flight Center and is expected to launch in 2016 in a near-circular, near-polar orbit. Carrying the Ad- vanced Topographic Laser Altimeter System lidar, ICESat-2 will continue to measure changes in polar ice cover, as well as bio- mass and carbon in vegetation. In the meantime, space-based observations are also being conducted by the European Space Agency CryoSat-2 satellite, an international collaboration program launched in 2010. CryoSat-2 uses a synthetic aperture interferometric radar al- timeter to derive sea ice thickness via freeboard measurements. Some segments of the satellite's data are processed at NOAA's Laboratory for Satellite Altimetry to estimate wind speed and wave height to aid forecasters, and to supply data for the lab's Sea Ice Research Group. Validating Data The data from ICESat, CryoSat-2 and IceBridge flights have been difficult to validate. Snow moisture can affect the pene- tration of radar and cause errors in snow depth estimates. Wet, slushy snow on top of the ice is particularly problematic. OSIL Salinity Measurement – The complete solution for high quality salinity data Ũ IAPSO Standard Seawater Ũ High precision salinometers Ũ Operator training courses Ũ Salinometer service and repair Ũ Worldwide distribution & support T: +44 (0)2392 488240 / E: osil@osil.co.uk / www.osil.co.uk www.sea-technology.com AUGUST 2012 / st 17

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