Radiometric inter-calibration of SAPHIR using the Microwave Humidity Sounders

The Microwave Humidity Sounders (MHS) on NOAA and Metop satellites have been extensively inter-satellite cross-calibrated; thus these MHS radiometers serve as the radiometric calibration standards for other satellite sounder radiometers. Unfortunately, inter-satellite radiometric calibration (aka XCAL) between similar, but not identical, radiometers is not trivial. This paper explores the application of a “Double-Difference (DD)” XCAL technique to sounders, which has been successfully used for conical scanning microwave imagers. Specifically, the water vapor sounder Sondeur Atmosphérique du Profil d´Humidité Intertropicale par Radiométrie (SAPHIR) on the Megha-Tropiques satellite is inter-satellite calibrated using the MHS instruments on the NOAA-18, NOAA-19, and Metop-A satellites.