Estimating errors in IceBridge freeboard at ICESat Scales

The Airborne Topographic Mapping (ATM) system flown on NASA Operation IceBridge allows for estimation of sea ice thickness from surface elevations in the Bellingshausen - Amundsen Seas. The estimation of total freeboard is based on the accuracy of local sea level estimations and the footprint size. We used the high density of ATM L1B (~1 m footprint) observations at varying spatial resolutions to assess errors associated with averaging over larger footprints and deviation of local sea level from the WGS-84 geoid over longer segment lengths The ATM data sets allow for a comparison between IceBridge (2009-2014) and ICESat(2003-2009)derived freeboards by comparing the ATM L2 (~70 m footprint) data, similar to the IceSAT footprint. While The average freeboard estimates for the L2 data in 2009 underestimate total freeboard by only 5 cm at 5 km segment lengths the error increases to 49 cm at the 50 km segment lengths typical of IceSAT analyses. Since the error in freeboard estimation greatly increases at the segment lengths used for IceSAT analyses, some caution may be required in comparing IceSAT thickness estimates with later IceBridge estimates over the same region.