An Intercalibrated Constellation of Microwave Radiometers Based on a New TRMM-GPM Calibration Reference Dataset

The Global Precipitation Measurement (GPM) mission is a constellation-based satellite mission designed to produce unified precipitation retrievals from all of the available microwave radiometers. The constellation approach is a unique and challenging aspect of the mission that has implications for science applications far beyond global precipitation monitoring. The GPM inter-satellite calibration working group, or XCAL team, is tasked with producing intercalibrated brightness temperature (Tb) data, referred to as Level 1C, for each of the constellation radiometers. This involves identifying calibration issues, developing corrections, and applying adjustments for residual calibration differences relative to a calibration reference in order to make the radiometric calibrations physically consistent after accounting for sensor differences in channel frequencies, polarizations, and view angles. A variety of issues impacting sensor calibration limit the usefulness of these data for many applications such as long-term climate monitoring. Some of the issues impacting sensor calibration include emissive reflectors, solar and lunar intrusions into the warm targets or cold-sky mirrors, obstructions impacting spillover regions, uncertainties in antenna patterns and thus the resulting corrections, pointing and geolocation errors etc.

After the launch of the GPM core satellite, the XCAL team initially focused on the calibration of the GMI instrument to determine if it was a suitable reference for the other constellation radiometers. Several deep space calibration maneuvers were performed and the resulting data were used to develop and refine calibration corrections and to check for issues found to impact prior radiometers such as polarization shifts, reflector emissivity, and solar intrusions into the warm load. In addition, corrections were developed for magnetic-induced anomalies that were discovered and the antenna spillover corrections were updated. The resulting stability and absolute accuracy of the GMI calibration represents a significant step forward for conical-scanning imagers, thus providing an excellent reference sensor for both conically-scanning microwave imagers and cross-track scanning moisture sounders with channels near the 183 GHz water vapor line. As a result, the decision was made to use GMI as an absolute calibration reference not only for the GPM constellation, but also for the long-term radiometer data record. To create a high quality long-term reference dataset, the calibration of the TRMM Microwave Imager (TMI) was revisited. An overlap period of just over a year is available after the launch of GPM in February of 2014 and before the TMI instrument was turned off in April of 2015. The availability of coincident GMI observations during this period, on-orbit calibration maneuvers at the beginning and end of the TRMM mission, and lessons from the GMI calibration led to the development of several improvements to the TMI calibration. These include updated corrections associated with an emissive main reflector, solar intrusions into the hot load, antenna pattern corrections, cross-track biases, and spacecraft attitude and geolocation. Coincident observations between TRMM and GPM during the overlap period were used to intercalibrate TMI to GMI.

The combined TMI-GMI reference dataset was then used to compare with the constellation of available microwave radiometers going back to the start of the TRMM mission in December of 1997. This includes a total of twelve conical scanning imagers and nine cross-track scanning moisture sounders with observations near the 183 GHz water vapor line. The approach used to intercalibrate these sensors follows that done for the GPM radiometer constellation, which involves multiple techniques from different groups within the XCAL team. For the imagers, different techniques are used for cold scene (ocean) versus warm scene (vegetated land) to provide information on scene temperature dependence of calibration differences versus GMI. While GMI has channels near 183 GHz, TMI does not, making it unsuitable as a reference for the sounders. Comparisons with GMI indicate the Microwave Humidity Sounders (MHS) instruments on board NOAA 18 and 19 and MetOp-A and B are all very well calibrated. These sensors are used for the period available, and then the calibration is “daisy-chained” back in time using the best available reference. For the imagers, TMI provides the high-quality reference. The resulting V05 Level 1C intercalibrated Tb dataset was publicly released for the GPM constellation (February 2014 to the present) in May of 2017 with the data for the TRMM constellation sensors (December 1997 – January 2014) released in October of 2017.