TRMM and GPM rely on passive and active measurements to measure the properties of precipitation. Active radars like GPM's Dual-Frequency Precipitation Radar (DPR) transmit and receive signals reflected back to the radar. The signal returned to the radar receiver (called radar reflectivity) provides a measure of the size and number of rain/snow drops at multiple vertical layers in the cloud (Fig. 1).

On the other hand, passive precipitation radiometers like the GPM Microwave Imager (GMI) measure natural thermal radiation (called brightness temperatures) from the complete observational scene including snow, rain, clouds, and the Earth's surface (Fig. 2).

Figure 1                    Figure 2

Precipitation retrievals using algorithms is the mathematical process of transforming the radar reflectivities (Z) and brightness temperatures (TB) into precipitation information. These scientific algorithms (that are converted to computer code) are designed by the PMM science team and processed (compiled and run) by the Precipitation Processing System (PPS).  The algorithm performance is verified against Ground Validation (GV) data. In the algorithm development stage, GV data may be used to improve the science of the retrievals.

There are 4 major algorithms associated with the TRMM and GPM observations.

1. Radar Algorithms
2. Radiometer Algorithms
3. Combined Radar+Radiometer Algorithms
4. Multi-Satellite Algorithms