GPM will extend TRMM's observations of precipitation to higher latitudes, with more frequent sampling, and with focused research on providing a more complete understanding of the global hydrological cycle. GPM will be capable of measuring rain rates as small as a hundredth of an inch per hour to as large as 4 inches an hour. GPM will be able to estimate the various sizes of precipitation particles, and will also discriminate between snow and rain. GPM will seek to achieve these measurements with a 3-hour average revisit time over 80% of the globe, and the data will be available to users within 3 hours of observation time.

NASA and the Japanese Aerospace Exploration Agency (JAXA) are working together to build and launch the GPM Core Satellite. The Core is the central precipitation-measuring observatory of GPM and will fly both a Dual-frequency Precipitation Radar (DPR) and a high-resolution, multi-channel passive microwave (PMW) rain radiometer known as the GPM Microwave Imager (GMI). The Core will also serve as the calibration reference system for a constellation of support satellites. As was the case with TRMM, JAXA will provide the weather radar and possibly a launch vehicle while NASA will provide the passive microwave radiometer, the satellite superstructure, and the ground control segment.

In addition to the Core, a constellation of up to eight satellites will comprise the GPM sensor web. NASA plans to provide a dedicated member of the constellation. This is conceived as a relatively small spacecraft that will carry a single radiometer on board. The radiometer will be identical to the GMI on the Core. Other vehicles in the constellation are called satellites of opportunity, contributed by domestic agency partners such as NOAA and the Department of Defense, and GPM international partners. One specific example of a potential satellite of opportunity is the proposed French/Indian mission known as Megha-Tropiques. Each satellite of opportunity has its own unique scientific mission but will also contribute precipitation measurements to GPM. Each satellite in the constellation will carry one or more precipitation sensing instruments. At a minimum, to be a support satellite for the GPM constellation, a mission has to carry some type of passive microwave radiometer measuring several precipitation frequencies.

The GPM Mission will also frequently sample the "diurnal" or 24-hour variation in rainfall due to the rising and setting of the sun, by capitalizing on some satellite orbits that are synchronized with the sun, and others that are not.