Abstract

The airborne NPOESS Aircraft Sounding Test-bed Interferometer (NAST-I) has flown on numerous flights under the Aqua satellite in order to validate AIRS radiance measurements and profile retrievals. The NAST-I is an excellent AIRS airborne validation tool since it possesses relatively high spectral and spatial resolution as well as relatively low radiance measurement noise, especially after the data are spatially averaged to the footprint size of the AIRS sensor. The results of these airborne missions have shown that the AIRS calibration validates well against airborne interferometer radiance measurements. Here, it is shown that the retrievals obtained using the same retrieval algorithm are in general agreement, although the airborne NAST-I result possesses higher vertical resolution as a consequence of it’s higher spectral resolution and lower “effective” radiance noise level. The “effective” radiance noise level is a function of the single spectral sample radiance measurement noise, the number of spectral channels used for the retrieval, and the number of spatial samples averaged to produce the final profile result. For the NAST-I aircraft instrument, the effective radiance noise level is extremely small, particularly after reducing the horizontal linear resolution to that of the satellite AIRS instrument (i.e., ~14 km for AIRS as opposed to ~2 km for NAST-I), and as a result its relatively large spectral range and high spectral resolution (i.e., NAST-I possess nearly four times the number of spectral channels as does the AIRS). It is shown here that spatial averaging of the AIRS data, which decreases the noise level of the radiances used for the retrieval, improves the vertical resolution of the profile results.