Geolocation Assessment Tool and Correction Model for JPSS CrIS

The Cross-track Infrared Sounder (CrIS) on Suomi National Polar-orbiting Partnership (SNPP) and future Joint Polar Satellite System (JPSS) is a Fourier transform spectrometer that provides soundings of the atmosphere with 1305 spectral channels, over 3 wavelength ranges: LWIR (9.14 - 15.38 μm); MWIR (5.71 - 8.26 μm); and SWIR (3.92 - 4.64 μm). Just as important as spectral and radiometric calibration, geometric calibration is also one of the requisites of the CrIS Sensor Data Records (SDR). For instance, accurate and precise geolocation is required to coalign CrIS with ATMS in order to combine the ATMS and CrIS SDRs to atmospheric profile retrievals and data assimilation. Furthermore, good geometric calibration of the CrIS SDR is fundamental to potentially use VIIRS measurements for cloud flagging and scene feature detection. The spatial resolution of CrIS field of view (FOV) is 14.0 km at nadir. The designed specification for CrIS geolocation accuracy is less than 1.5 km for all the FOVs along scan angles, which are from a tenth to a hundredth of the FOV sizes varying with the scan angles. Therefore, evaluation of the postlaunch geolocation performance of the CrIS SDR has been listed as one of core tasks by the CrIS SDR team.

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.