In addition to the PMM satellites, TRMM and GPM, roughly a dozen other satellites carry precipitation-relevant sensors. The goal of multi-satellite algorithms is to use “all” of the available quasi-global precipitation estimates computed from this international constellation of satellites to create a High-Resolution Precipitation Product with complete coverage over the chosen domain and period of record (currently 50°N-50°S, 1998-present).
Estimates based on microwave and combined radar/radiometer input data have higher quality due to the physically direct relationships that exist between precipitation and satellite data. Estimates based on thermal infrared sensors are lower quality due to the indirect relationship, but they provide much more frequent coverage due to the sensors' position in geosynchronous orbit. One of the higher-quality data sets, currently the Combined Radar+Radiometer algorithm described above, is chosen as a standard, then all of the other precipitation data sets are calibrated to that standard. In the combination (currently on a 0.25°x0.25° latitude/longitude grid every 3 hours), the higher-quality data are used as much as possible, while lower-quality data take a secondary role. Finally, in locations where they exist, primarily land areas, monthly precipitation gauge data are used to control the biases that satellite data sets can exhibit. The multi-satellite algorithm is run within several hours of the observation time, and then again after all data, including the monthly gauge, are received. The first product is more approximate, but enables important short-fuse uses such as flood analysis and crop forecasting. The second product is considered the research-grade archival product. For the theoretical and mathematical details of the multi-satellite algorithm please review its Algorithm Theoretical Basis Document (ATBD) found at the Precipitation Processing System website.