The National Polar-orbiting Operational Environmental Satellite System (NPOESS) Atmospheric Sounding Testbed (NAST) is a suite of airborne infrared and microwave spectrometers, being developed for the Integrated Program Office (IPO), that will be flown on the NASA high altitude ER-2 aircraft as part of the risk reduction effort for NPOESS. In addition to their stand-alone scientific value, data from these airborne instruments will be used to simulate possible satellite-based radiance measurements, therefore enabling experimental validation of instrument system specifications and data processing techniques for future advanced atmospheric remote sensors (e.g., the proposed sounder component for NPOESS).