Absolute Intercalibration of Passive Microwave Imagers

Absolute calibration of spaceborne microwave radiometer observations consists in accurate determination of antenna spillover, cross-polarization contamination and non-linearity coefficients of the receivers. The GMI sensor onboard GPM is the best calibrated microwave radiometer. It reaches absolute calibration standards due to the superior design of its calibration system, which combines external and internal calibration targets. The calibration of the GMI brightness temperatures (TB) can be entirely based on pre-launch measurements and a series of on-orbit calibration maneuvers. It does not depend on a specific radiative transfer model (RTM). GMI’s 65-degree inclined orbit is not sun-synchronous, and it allows for tight collocation windows with other satellite sensors. The goal of the absolute intercalibration method is to transfer the GMI calibration to other radiometers, whose operations have a sufficient time overlap with GMI. Specifically, we will show results for WindSat and AMSR2. The sensor intercalibration is based on brightness temperature match-ups between GMI and the other two instruments over a large number of open ocean and rain forest scenes. In all instances of inter-calibration, the calibration parameters are found by minimizing the root mean square difference between the TBs measured by the uncalibrated sensor (WindSat, AMSR2) and the TBs that are measured by the absolutely-calibrated sensor (GMI) . Small differences in center frequencies and Earth incidence angles between the sensors can be corrected for using the RTM double differences. In order to assess the calibration accuracy, we compare the intercalibrated brightness temperatures with radiative transfer model calculations. In addition, we provide in-situ validation results for wind speed, water vapor and cloud water retrievals from the intercalibrated sensors. Two new microwave sensors will be launched in the 2024-2025 time-frame: AMSR3 onboard GOSAT-GW and MWI (Microwave Imager) onboard WSF-M (Weather System Follow On Mission). Our presentation will discuss the intercalibration framework for AMSR3 and MWI that is based upon the intercalibration of AMSR2 and WindSat to GMI. The presented intercalibration methodology is the backbone for the creation of a multi-decadal climate data record from passive microwave satellite observations. This record will consist of SST, wind speed, water vapor and cloud liquid water.