Intercalibration of the GPM Constellation Using GMI as a Reference

Abstract:

The GPM core satellite, launched in February of 2014, provides a number of advances for satellite monitoring of precipitation including a dual-frequency radar, high frequency channels on the GMI radiometer, and coverage over middle and high latitudes. The GPM concept, however, is also about producing unified precipitation retrievals from a constellation of microwave radiometers to provide ~3 hourly global sampling. The GPM intersatellite calibration working group, or XCAL team, is tasked with developing adjustments to the brightness temperature data from the constellation radiometers to make the radiometric calibrations physically consistent by accounting for differences in channel frequencies, polarizations, and view angles. The GPM Microwave Imager (GMI) is used as both a transfer standard as well as the calibration reference given that it has been shown to be extremely well calibrated and stable. After launch, several deep space calibration maneuvers were performed, and the resulting data was used to develop and/or refine calibration corrections and to check for issues found to impact prior radiometers such as an emissive reflector, polarization errors, and solar intrusion effects. Once the GMI calibration was finalized, the XCAL team focused on developing calibration adjustments for the other constellation radiometers. These corrections are developed independently by different team members, compared to identify potential issues, and merged to create calibration adjustment tables. The subsequent reprocessing of the GPM constellation datasets marks the transition from the at-launch calibrations and retrieval algorithms, which were based on TRMM data, to ones developed from GPM observations. The focus of the XCAL team now shifts towards improving corrections of sensor calibration issues, refining intercalibration techniques, and developing calibration adjustments for the TRMM-era constellation radiometers for reprocessing back to the TRMM launch in late 1997.