GPM is broader than a fleet of satellites. A comprehensive ground validation, research and calibration program will be employed to ensure proper accuracy and precision of the global precipitation measurements. A robust measure of accuracy is required by the modeling community, algorithm developers and GPM stakeholders (such as those involved with agriculture, public health, water resources management, aviation safety, etc). From an end user perspective, GPM will almost seamlessly advance precipitation products from the tropical-focused TRMM era, to an era where high frequency sampling, complete global coverage, measurement of both rain and snow, and thorough documentation of measurement accuracy become a reality.

The GPM Ground Validation (GV) program is designed to go beyond direct comparisons of surface precipitation rates between ground and satellite measurements. But the primary goal is to assess the accuracy of satellite products and to improve the algorithms that generate these products. (Algorithms are sets of computer programs that convert satellite measurements of microwave energy into useful characteristics of precipitation, such as rain intensity).

Above : Diverse GPM ground validation sites

The GV program is composed of three strategies, two of which comprise comprehensive ground measurement sites. The first type of site will be concerned with the statistical validation of rainfall, and co-located with existing or upgraded national weather radars (i.e. the National Weather Service NEXRAD array) and dense networks of rain gauges. The sites will enable scientists to identify and resolve significant discrepancies between the national radar network and satellite estimates. The second category of GV sites are the precipitation process sites. Several locations will collect observations to improve our understanding and modeling of precipitation physics, and to advance the precipitation retrieval algorithms. Sites will be chosen to highlight geographical diversity and the impact of varying regional climates on precipitation processes. They will include: (1) continental tropical, mid- and high-latitude sites (including both mountain and coastal locations) and (2) oceanic tropical and mid-latitude sites. The third prong of the GV strategy will use research aircraft to directly sample the upper levels of precipitating clouds, beyond the reach of conventional surface measurement systems.

Additionally, the GV program will leverage off existing hydrological monitoring sites in watersheds maintained by U.S. agencies and international research programs.Ê These hydrological basins will enable GPM scientists to the study the hydrological cycle as a whole - recognizing that precipitation is a key input to water cycle dynamics.