The Moderate Resolution Imaging Spectroradiometer (MODIS) Airborne Simulator (MAS) was developed for NASA's high-altitude ER-2 research aircraft by Daedalus Enterprises, Inc., in support of MODIS remote sensing algorithm development. The overall goal was to modify the spectral coverage and gains of MAS to emulate as many of the MODIS spectral channels as possible. With its much higher spatial resolution (50 m vs. 250-1000 m for MODIS), MAS provides unique information on the small-scale distribution of various geophysical parameters. The MAS instrument has been deployed on multiple platforms for many field campaigns since its first mission in 1991.