In this study, a new geolocation assessment tool for all scan angles (from -48.5 to 48.5 degree) has been developed based on previous experiences for Suomi NPP CrIS. Specifically, spatially collocated measurements from the Visible Infrared Imaging Radiometer Suite (VIIRS) band I5 are used to evaluate the geolocation performance of the CrIS SDR by taking advantage of high spatial resolution and accurate geolocation of VIIRS measurements. The basic idea is to find the best collocation position between VIIRS and CrIS measurements by perturbing CrIS line-of-sight (LOS) vector along the track and scan directions with small angles. The retrieved LOS vector with the best collocation position is then used to evaluate the CrIS geolocation performance.
In the second part, a correction model is built up to convert assessment results into correction angles that are used to update the parameters for further improving CrIS geolocation accuracy. The purpose is to perform post-launch geolocation calibration based on assessment results by refining calibration parameters that are used for geolocation computation. The observation equations are solved using a set of linearized collinearity equations to estimate correction biases for the satellite position, attitude, and/or scan mirror coefficients. Deterministic least squares (minimum variance) estimation is used to compute scan mirror coefficients parameters that best fit the assessment results.