Deriving CrIS Detector Nonlinearity from Changing Sensor Background

The Cross-track Infrared Sounder (CrIS) is a Michelson Interferometer that operates in the infrared long wave, mid wave and short wave spectral bands. One CrIS sensor is presently operating on the Suomi Polar-orbiting Partnership (S-NPP) spacecraft, and a second sensor is scheduled to be launched in early 2017 on the Joint Polar Satellite System (JPSS-1) spacecraft. The focal plane for each CrIS spectral band has 9 separate detectors arranged in 3 x 3 configuration. These detectors have the same nonlinearity correction form, which was characterized during subsystem level testing. The nonlinearity correction coefficients were then derived during thermal-vacuum (TVAC) testing using the sensor’s external calibration target (ECT) with multiple temperature settings to change the radiance on the detector throughout its operating range. While this is the traditional method of determining the correction coefficients, it does require knowledge of the absolute radiance of the ECT to use the nonlinearity correction on orbit, and involves extensive and costly characterization of the ECT.  In an effort to reduce the dependency of this correction on the external target, the Space Dynamics Laboratory (SDL) developed an alternative method to determine the CrIS nonlinearity correction coefficients that does not require absolute radiance knowledge of the ECT.  It only requires that the temperature of the ECT is independent of the varying sensor background.  This method was developed and tested on the CrIS JPSS-1 sensor using data from the instrument’s TVAC test.  During testing, the ECT was maintained at a constant temperature while the sensor background temperature was varied to change the radiance on the detector. An iterative method was then used to adjust the nonlinearity correction coefficients to minimize the change in calibrated radiance for the ECT under changing background conditions. During each iteration, the normal on-orbit calibration procedure was performed using preliminary nonlinearity coefficients. Iterations of the nonlinearity correction coefficients were made until the observed calibrated radiance variation of the ECT with changing sensor temperature was minimized. If the nonlinearity correction coefficients are correct, the calibrated radiance measurement of the ECT will remain constant even with a changing sensor background. This poster describes this alternative method and provides a comparison to the coefficients determined by the traditional method.