Channel alignment and radiometry in hyperspectral atmospheric infrared sounders

The modern generation of hyperspectral infrared atmospheric sounders features high spectral resolution and very low sensor noise. The improved performance of such instruments places much stiffer requirements on certain aspects of instrument design compared to earlier sounders. In this paper we will concentrate on spatial response issues-in particular, relative boresight alignments of different channels. An overview of the AIRS instrument and pre-flight test program and the pre-launch performance characteristics can be found in the literature (reference 1 and 2). We show that the degree of radiometric error in the difference between two channels is proportional to the amount of misalignment between channels. This paper uses the Atmospheric Infrared Sounder (AIRS) on NASA's EOS Aqua spacecraft to demonstrate our point. AIRS had a very stringent specification for boresight alignment. For every possible pair of channels, the spatial responses had to agree to 99% or better. This paper discusses results of the pre-launch spatial alignment tests relative to the requirements. The AIRS design enables quantitative analysis of channel misalignments using on-orbit data. This feature is available on any hyperspectral infrared sounder. There are pairs of channels in which the two detectors are sensitive to the same frequency, or to similar frequencies that have closely matching infrared transmission properties, but have quite different locations on the focal plane. This paper uses such pairs, and a typical scene of the Indian Ocean at night, to perform quantitative analysis of the effect of misalignment on radiometry.