Suomi NPP/JPSS Cross-track Infrared Sounder (CrIS): Non-linearity Assessment and On-Orbit Monitoring

The path from research to operations for high spectral resolution infrared sounding has made a major step forward with the launch of Suomi NPP. Detailed characterization of the calibration and overall performance of the CrIS sensor is one of the key aspects needed for successful forecast model assimilation and impact on operations. The University of Wisconsin-Madison has promoted the use of hyperspectral infrared measurements of the top of the atmosphere radiance since the late 1970s. Numerous aircraft campaigns in the past 25 years have demonstrated the radiometric absolute accuracy and spectral stability required to provide vertical profile information of tempature and water vapor with greatly improved vertical resolution compared to the previous NOAA series of IR sensors (HIRS). The UW-Madison has developed extensive experience in the calibration of the hyperspectral radiances using a two point calibration method. For this method to be successful, any non-linearity of the sensor response to input radiance must first be corrected. While the expectation from the sensor vendor was that each of the 27 detectors in the three CrIS bands (longwave, midwave, and shortwave bands) would be linear, the pre-launch thermal vacuum testing revealed significant non-linearities in most detectors in the longwave and midwave bands. For these detectors, a non-linearity correction algorithm developed by UW-SSEC for use in UW aircraft and ground-based sensors was applied to CrIS T/V data to successfully meet the radiometric requirements. Quadratic non-linearity coefficients were determined pre-launch from the TV data and included in the initial launch configuration parameters. This paper will present a refinement of the CrIS pre-launch non-linearity coefficients based on on-orbit data which minimizes FOV-to-FOV radiometric errors among the nine fields of view within a 3x3 field of regard. This paper will also present a non-linearity coefficient monitoring methodology which can track the change in FOV-to-FOV radiometric differences in spectral regions sensitive to changes in non-linearity. This approach provides the JPSS CrIS Cal/Val team an approach to monitor changes in the sensor non-linearity across any potential instrument warm-up/cool-down events. This evaluation is in support of the JPSS SDR calibration/validation activities for the Suomi NPP satellite.