Instrument Calibration and Characterization Ancillary Files

Spectral Response Functions

The Spectral Response Functions (SRFs) describe the wavelength at which each AIRS channel responds. Dr. Larrabee Strow at the University of Maryland Baltimore County created these SRF data sets under contract to the NASA AIRS Project.

The L1B and L1C data products have separate SRF data sets, since the L1C channels are moved onto a regularly-spaced grid. The SRFs of the 2378 L1B channels are based on the analysis of AIRS pre-flight data measured with a Fourier Transform Spectrometer (FTS), as described in the Strow et al. 2003 IEEE special issue paper cited below.

The L1C product uses 2,645 channels. L1C was created circa 2012 to produce a more uniform product. About 2,350 channels are copied from L1B, including their SRFs. The other channels are based on the grating model, which is extended to eliminate the gaps between detector modules. L1C uses Principal Component (PC) reconstruction to fill the gaps between the detector modules. The L1C SRFs for the "fill" channels are thus based on a model.

The data files and descriptions are found at:

References:

Strow, Larrabee L., et al. "Prelaunch spectral calibration of the Atmospheric Infrared Sounder (AIRS)." IEEE transactions on geoscience and remote sensing 41.2 (2003): 274-286. DOI:10.1109/TGRS.2002.808245

Gaiser, Steven L., et al. "In-flight spectral calibration of the atmospheric infrared sounder." IEEE Transactions on Geoscience and Remote Sensing 41.2 (2003): 287-297. DOI:10.1109/TGRS.2003.809708

Strow, L. L., et al. "Validation of the Atmospheric Infrared Sounder radiative transfer algorithm." Journal of Geophysical Research: Atmospheres 111.D9 (2006). DOI:10.1029/2005JD006146

Channel Frequency Centroids vs. y0

The L1B provides a metric of the spectral calibration relative to a ‘grating’ model of the instrument frequencies vs their position on the AIRS Focal Plane Array (FPA). The metric is a representation of the positional shift of the FPA as a whole relative to their position seen pre-flight. The metric, y0, can be used to shift the frequencies of the centroids of the 2378 AIRS spectral response functions (SRFs) according to the grating model, and is provided in the V5 L1B as y0=spec_shift_upwell, and V8 L1B as y0 = spec_shift_y0. The L1B does not provide a dynamic estimate of the frequency centroids because the y0 term is slightly noisy and centroid determination is best done by averaging the y0 over time. In lieu of providing the grating model, we provide a table (linked below) of frequency centroids vs. y0 for a range of y0’s bounding those seen during the AIRS mission, i.e. [-14.5 -14.0 -13.5 -13.0 -12.5 -12.0 -11.5 µm].

Table of frequency centroids

Users can linearly interpolate the y0’s and frequency centroids in the table to the y0 provided in the L1B for each channel. The linear interpolation method is simple, stable and provides frequency centroids accurate to 0.5% of the width of the SRF. Users are encouraged to use the L1C product, if possible, that interpolates the frequency centroids to a fixed grid and performs many other useful functions to more easily utilize AIRS radiances [Level 1 | About the Products – AIRS].