To measure precipitation with improved accuracy and to better understand its role in climate, the National Aeronautics and Space Administration (NASA) and the National Space Development Agency of Japan (NASDA) planned and carried out the Tropical Rainfall Measuring Mission (TRMM). TRMM was the first satellite dedicated to rainfall measurement, and is the only satellite that carries a weather radar. Now in its eighth year, the TRMM mission has provided a wealth of knowledge on severe tropical storms such as hurricanes and short-duration climate shifts such as El Nino.

Though TRMM has exceeded expectations, the mission is still inherently limited for fully understanding the complete role of precipitation in the hydrologic cycle. A critical element driving the scientific objectives of GPM is to understand what scientific problems TRMM has not been able to address. TRMM has a limited view of the Earth, ranging from 36° N to 36° S, and samples rain relatively infrequently, passing over the same location about once a day. TRMM also cannot measure frozen precipitation and is insensitive to light rainfall.

Scientists seek more direct measurements on the microscopic nature of rainfall (such as the relative sizes of raindrops), and longer duration datasets are needed to understand the impact of climate shifts on global precipitation.