The National Polar-orbiting Operational Environmental Satellite System (NPOESS) Airborne Sounding Testbed-Interferometer (NAST-I), is the infrared component of a suite of airborne infrared and microwave spectrometers, developed for the Integrated Program Office (IPO), that has been flying on high-altitude aircraft as part of the risk reduction effort for NPOESS. It is a high spectral resolution (0.25 cm-1, unapodized) and high spatial resolution (~2 km, nadir) cross-track scanning (~ 45 km swath width, at 20 km aircraft altitude) Fourier Transform Spectrometer (FTS) observing within the 3.7 - 16.1 micron spectral range. In addition to characterizing the atmospheric thermal and moisture structure and providing information on radiatively active trace gases (e.g. O3 & CO) during field experiments, NAST-I measurements greatly contribute toward instrument and forward model pre-launch specification optimization (i.e., for the Cross-track Infrared Sounder, CrIS, to fly on NPOESS) and will enhance post-launch calibration/validation activities for CrIS as well as for other future advanced atmospheric spaceborne sensors (e.g., the EOS AIRS, CERES, MODIS, MOPITT, & TES instruments). In this paper, we investigate some of the challenges associated with validating infrared spectral radiances obtained from remote sensing measurements and forward model simulations. Specifically, measured infrared spectral radiances are compared with radiance calculations performed using a Line by Line forward radiative transfer model based on nearly-coincident temperature and water vapor profiles observed with several independent in-situ, passive, and active measurement systems.