Intercalibration of the GPM Microwave Radiometer Constellation

Abstract

The Global Precipitation Measurement (GPM) mission is a constellation-based satellite mission designed to unify and advance precipitation measurements using both research and operational microwave sensors. This requires consistency in the input brightness temperatures (Tb), which is accomplished by intercalibrating the constellation radiometers using the GPM Microwave Imager (GMI) as the calibration reference. The first step in intercalibrating the sensors involves prescreening the sensor Tb to identify and correct for calibration biases across the scan or along the orbit path. Next, multiple techniques developed by teams within the GPM Intersatellite Calibration Working Group (XCAL) are used to adjust the calibrations of the constellation radiometers to be consistent with GMI. Comparing results from multiple approaches helps identify flaws or limitations of a given technique, increase confidence in the results, and provide a measure of the residual uncertainty. The original calibration differences relative to GMI are generally within 2–3 K for channels below 92 GHz, although AMSR2 exhibits larger differences that vary with scene temperature. SSMIS calibration differences also vary with scene temperature but to a lesser degree. For SSMIS channels above 150 GHz, the differences are generally within ~2 K with the exception of SSMIS on board DMSP F19, which ranges from 7 to 11 K colder than GMI depending on frequency. The calibrations of the cross-track radiometers agree very well with GMI with values mostly within 0.5 K for the Sondeur Atmosphérique du Profil d’Humidité Intertropicale par Radiométrie (SAPHIR) and the Microwave Humidity Sounder (MHS) sensors, and within 1 K for the Advanced Technology Microwave Sounder (ATMS).

The Global Precipitation Measurement (GPM) Mission for Science and Society

“…Multiple independent approaches are compared during these steps, which help to identify flaws or limitations of a given approach, thus increasing confidence in the results and providing a measure of the uncertainty in the resulting calibration adjustments. After adjustments, residual differences between GMI channels and those on the constellation radiometers are generally smaller than 1 K (Berg et al 2016). This is a remarkable achievement that now allows the project to focus on the precipitation products rather than T B uncertainties.…”

“…Sensor intercalibration between GMI and the partner sensors involves several steps, as described in Wilheit (2013), Wilheit et al (2015), and Berg et al (2016). Multiple independent approaches are compared during these steps, which help to identify flaws or limitations of a given approach, thus increasing confidence in the results and providing a measure of the uncertainty in the resulting calibration adjustments.…”

The Global Precipitation Measurement (GPM) mission's scientific achievements and societal contributions: reviewing four years of advanced rain and snow observations

“…Subsequently, Wentz and Draper (2016) concluded that GMI is the most accurate precipitation radiometer currently in space. To provide unified global precipitation estimates a reliable transfer standard of brightness temperatures (Tbs) between the GPM-CO and the constellation partner precipitation sensors through the inter-calibration of all the radiometer sensors has been established (Berg et al, 2016). This inter-calibration effort ensures that the observed Tbs are consistent among the sensors allowing for expected differences due to variations in the observing frequencies, bandwidths, polarizations, and view angles (e.g.…”

“…Wilheit et al, 2015). After inter-calibration, residual differences between the GMI Tbs and those of the constellation radiometers are generally smaller than 1 K (Berg et al, 2016). These GPM inter-calibrated Tbs are made available on the NASA PPS and are the first step toward unified precipitation products.…”

Best Practices in Crafting the Calibrated, Enhanced-Resolution Passive-Microwave EASE-Grid 2.0 Brightness Temperature Earth System Data Record

“…As an ESDR, the CETB product leverages as input a new, (swath format) FCDR, which includes a completely reprocessed historical SSM/I and SSMIS record including new efforts to improve intersensor calibrations [29][30][31]. To ensure transparency and reproducibility, we populated provenance metadata in file-level attributes in each CETB file at runtime, to record the specific FCDR files used to derive the CETB image; software version tags for the gsx and core systems were automatically captured as file-level attributes.…”