Scientists use a variety of techniques for measuring rain from space. Rainfall can be inferred from the temperature of cloud tops. Microwave radiation emitted by clouds can be used to discern rainfall intensity. And, in the case of TRMM, a weather radar can transmit a beam of microwave energy downward inside clouds; the energy scattered back to the radar by raindrops provides information on rain intensity and its vertical distribution. Each of these techniques has strengths and weaknesses. Recently, investigators have attempted to merge multiple satellite estimates to minimize the shortcomings and capitalize on the strengths of the various techniques. The TRMM satellite serves as a reliable calibrator of the merged rainfall. These integrated rain products provide a valuable testbed for developing the concept of the GPM constellation of satellites.

Yet, problems still loom. For instance, cloud top temperatures are imperfectly correlated with actual rainfall reaching the surface. Yet the cloud top data, which are derived from a globally continuous chain of geosynchronous satellites that simultaneously view the entire Earth, are necessary to fill in the gaps between the other rainfall measuring satellites (such as TRMM) which have relatively narrow orbits. Ideally, microwave-based techniques (utilizing either passive remote sensors, or weather radars) provide the most accurate measurement of rainfall. An all-microwave constellation of sensors, anchored by a "mother ship" with a weather radar to provide accurate calibration, is necessary for reliable, global coverage of precipitation in all of its liquid and solid forms. This is the measurement philosophy embodied by the Global Precipitation Measurement suite of sensors.

Above : January 2006 rainfall measurements from the TRMM-based Multi-Satellite Precipitation Analysis (T-MPA)