ICESat Geolocation Validation Using Airborne Photography

NASA´s ICESat satellite launched in January of 2003, carrying the Geoscience Laser Altimeter. During the initial phase of this mission, many validation procedures were implemented to verify the accuracy associated with a variety of altimetry-derived data products. Of specific interest was the need to validate the geodetic position of the ICESat footprints, which is a convolution of laser-pointing determination, satellite position, and ranging measurements. This paper describes the methodology and implementation of one effort using aerial photography to image the laser spots on the surface during a satellite overflight. The spot locations are determined based on the relative positions of accurately placed geodetic infrared-emitting markers within the overflight area and apparent in the aerial photograph. One specific overflight opportunity captured six successive ICESat footprints with the airborne camera system. The mean geolocation predictions of those spots using the ground fiducial placement in the image provide a data product validation to better than 3.1-m rms on the surface with an estimated accuracy of ??3.6 m when compared to the ICESat solution. These results are within the ICESat mission requirement of 4.5 m on the surface (1.5-arcsecond pointing knowledge) for geodetic position determination.