Validation of GLAS calibration using ground- and satellite-based data

A key aspect to the recently-launched GLAS instrument is understanding its radiometric performance using on-orbit verification approaches. One of these approaches compares the reported backscattered signal from a surface to a predicted signal from that same surface. This returned signal depends on both the atmospheric transmittance and the surface reflectance in the backscatter direction. This work describes three methods that have been used to derive the backscatter surface reflectance for the prediction of the hard-target signal at the GLAS sensor. The first relies on a portable field spectrometer carried over a representative area of the surface and the reflected solar irradiance is measured with reference to a known field standard of reflectance. The difficulty with this approach is that it does not measure the backscatter reflectance. Thus, a second method using a laser-based system operating on the ground has been used to determine the surface reflectance. Both of these cases require that ground personnel be present near in time to the GLAS overpass. A third method, relying on reflectances derived from the MODIS sensor increases the available number of data sets for comparison to GLAS. Early results from all three approaches indicate agreement at better than 10% with the GLAS sensor.