ICESat laser altimetry in the Great Lakes

The primary goal of NASA's ICESat laser altimetry mission, launched on Jan 12 2003, is to measure ice sheet elevation and changes in the polar regions. The laser altimeter on-board, the Geoscience Laser Altimeter System (GLAS) also delivers valuable information of land elevations, water surface elevations, and canopy heights. In this study, GLAS elevation data are used to monitor the lake levels of the Great Lakes in North America. Existing data include lake level measurements from a network of 52 tide gauges (25 Canadian MEDS station and 27 US NOAA stations) and observations from satellite radar altimeter missions of Topex, Jason-1 and ENVISAT. ICESat observations, based on GLAS data, are available during Sep-Nov 2003, and Feb-Mar 2004. The different altimetry missions collect data simultaneously, but with different orbital patterns. We compare the three data sets (tide gauge, radar and laser altimetry) for determining inter-mission biases and estimate absolute biases of ICESat with respect to tide gauge data. The comparison of radar altimetry data with footprint diameters of several kilometers and ICESat with an average footprint diameter of only 70 meters is difficult, since the illuminated surface area can be very different. Over the ocean, the difference in footprint diameter, waves and ocean tides may introduce additional biases. Since the Great Lakes don't have prevalent ocean tides or high waves, the reflecting surface is rather smooth and flat at the scale comparable with the footprint size of the altimetry systems. This fact makes the Great Lakes an excellent site for validation studies. Six out of the 52 tide gauges are within 3 km of ICESat observations. Unlike radar altimetry, ICESat is capable of measuring precise topography near the coast and therefore ICESat measurements can be directly compared with coastal tide gauge data. Over open water, the lake level is compared with radar altimetry at crossover points.