The National Polar-orbiting Operational Satellite System (NPOESS) Aircraft Sounder Testbed-Interferometer (NAST-I) is one of two airborne infrared sounder systems currently being used to evaluate future spaceborne advanced sounder designs. The NAST-I instrument is a cross-track scanning Fourier Transform Spectrometer (FTS) that measures the upwelling radiation in the infrared spectrum between 645 - 2700 cm-1 (15.5 - 3.7 micrometer) at a high-spectral resolution of 0.25 cm-1. Each observation has a spatial resolution of 2.6 km from NASA's ER-2 high-altitude aircraft, which operates 20 km above the surface. Measurements from this instrument in its first year of operation have not only contributed to risk reduction studies for future IR sounders but have also provided valuable datasets from three different climate regimes. The spatial coverage, 40 km swath width, has facilitated evaluation of non-linear retrieval algorithms using high-spectral resolution information content and provided a means for further validation of infrared radiative transfer models. The capabilities of the NAST-I instrument have already been tested under varied field conditions such as tropical, mid-latitude summer, and mid- latitude winter regimes. Preliminary results from these field experiments have demonstrated favorable sounding capability under such conditions, including intensive tropical cyclone environments (Hurricane Bonnie, August 1998 and Hurricane Georges, September 1998). In addition, repeated observations over the same geographic location near Andros Island in the Bahamas have provided additional information on the temporal change and spatial distribution of water vapor responding to complex mesoscale and large-scale dynamic processes. The framework for future spaceborne IR sounders will be well established by current and future observations made by the NAST-I instrument with its capability to remotely sense atmospheric state variables and cloud radiative properties.