Aircraft comparisons of this type provide a mechanism for periodically testing the absolute calibration of spacecraft instruments with instrumentation for which the calibration can be carefully maintained on the ground. This capability is especially valuable for testing the long-term consistency and accuracy of climate observations, including those from the NASA EOS spacecrafts (Terra, Aqua and Aura) and the new complement of NPOESS operational instruments. The validation role for accurately calibrated aircraft spectrometers also includes application to broadband instruments and linking the calibrations of similar instruments on different spacecraft.
Both the AIRS and the S-HIS calibrations are expected to be very accurate (formal 3-sigma estimates are better than 1 K brightness temperature for a wide range of scene temperatures), because high spectral resolution offers inherent advantages for absolute calibration and because they make use of high emissivity cavity blackbodies as onboard radiometric references. AIRS has the added advantage of a cold space view, and the S-HIS calibration has benefited from the availability of a zenith view from high altitude flights on the Proteus aircraft. The S-HIS has also benefited from calibration techniques developed over many years in conjunction with the original HIS aircraft instrument and with the Atmospheric Emitted Radiance Interferometer (AERI) instruments developed for the DOE ARM Program. The absolute radiometric calibration is traceable to NIST, and in the future, we plan to check the calibration directly by inter-comparison to a NIST-maintained sensor (the TXR radiometer).
Aircraft flights of the S-HIS and its close cousin the NPOESS Atmopheric Sounder Testbed (NAST) will be used to check the long-term stability of AIRS over the life of the mission, and we expect to have new results from 2004 for the meeting.