The MODIS/ASTER Airborne Simulator was developed for the Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) and Moderate Resolution Imaging Spectroradiometer (MODIS) projects. ASTER and MODIS are both spaceborne imaging instruments on the Terra platform launched in the fall of 1999. Currently MASTER is flown on the Department of Energy (DOE) King Air Beachcraft B200 aircraft and the NASA DC-8. In order to validate the in-flight performance of the instrument, the Jet Propulsion Laboratory and the University of Arizona conducted a joint experiment in December 1998. The experiment involved overflights of the MASTER instrument at two sites at three elevations (2000, 4000, and 6000 m). The two sites: Ivanpah Playa, California, and Lake Mead, Nevada, were selected to validate the visible±shortwave infrared and thermal infrared (TIR) channels, respectively. At Ivanpah Playa, a spectrometer was used to determine the surface reflectance and a sun photometer used to obtain the optical depth. At Lake Mead contact and radiometric surface lake temperatures were measured by buoy-mounted thermistors and self-calibrating radiometers, respectively. Atmospheric profiles of temperature, pressure, and relative humidity were obtained by launching an atmospheric sounding balloon. The measured surface radiances were then propagated to the at-sensor radiance using radiative transfer models driven by the local atmospheric data. There was excellent agreement between the predicted radiance at sensor and the measured radiance at sensor at all three altitudes. The percent difference between the channels not strongly affected by the atmosphere in the visible±shortwave infrared was typically 1±5% and the percent difference between the TIR channels not strongly affected by the atmosphere was typically less than 0.5%. These results indicate the MASTER instrument should provide a well-calibrated instrument for Earth Science Studies. It should prove particularly valuable for those studies that leverage information across the electromagnetic spectrum from the visible to the TIR.