The CrIS/ATMS instruments used for this product are on board the Suomi National Polar-orbiting Partnership (SNPP) platform and use the Normal Spectral Resolution (NSR) data. The CrIS instrument is a Fourier transform spectrometer with a total of 1305 NSR infrared sounding channels covering the longwave (655-1095 cm-1), midwave (1210-1750 cm-1), and shortwave (2155-2550 cm-1) spectral regions. The ATMS instrument is a cross-track scanner with 22 channels in spectral bands from 23 GHz through 183 GHz. Infrared temperature sounders generate a large amount of Level-1B spectral data.

The purpose of the Calibration Data Subsets is extract key information from these data into a few daily files to:

1. Facilitate a quick evaluation of the absolute calibration of the instruments.
2. Facilitate an assessment of the instrument performance under clear, cloudy, and extreme hot and cold conditions.
3. Facilitate the evaluation of instrument trends and their significance relative to climate trends.
4. Facilitate the comparison of AIRS with CrIS using their equivalent data subsets.

The “summary” product includes a large set of cases of interest, including all identified spectra that match selection criteria detailed below for clear, special cloud classes, etc. These amount to about 10% of all spectra. But for each selected case only brightness temperatures (BTs) for selected key channels are saved.

The output files are constructed from Level-1B and MW brightness or antenna temperatures. Each file contains selected observations taken from a nominal 24-hour period.

The S-NPP CrIS summary subset contains Level-1B BTs for all selection types but only for selected channels, while the S-NPP CrIS random calibration subset contains full Level-1B spectra for only the randomly selected observations.