TRMM is a joint mission between the National Aeronautics and Space Administration (NASA) of the United States and the Japan Aerospace Exploration Agency (JAXA). The satellite was launched in November of 1997 and is currently continuing to operate. It is in a low inclination orbit covering the tropics between approximately 40S to 40N latitude. The primary rainfall sensors on board the TRMM spacecraft include the 13.8 GHz Precipitation Radar (PR) and the TRMM Microwave Imager (TMI). In addition, TRMM carries the Visible and Infrared Radiometer (VIRS), the Clouds and Earth's Radiant Energy System (CERES), and the Lightning Imaging System (LIS). The CERES instrument failed after only a few months of operation, but the other instruments are continuing to operate providing detailed information of rainfall over the tropics.
The TRMM PR is the first and currently the only precipitation radar in space. It provides detailed information on the three-dimensional structure of rain systems with a horizontal resolution of approximately 4-km and a total of 80 levels in the vertical with a resolution of 250 meters. The PR is a cross-track scanner with a relatively narrow swath width (~215 km), however, which results in limited sampling for climate. A PR surface rainfall map for January 1, 1998 shows the coverage by the PR sensor for one day. Although the sampling of the PR is much more limited than most satellite rain sensors, the PR provides a far more detailed look at tropical rain systems than is available from any other current or previous spaceborne rainfall sensor.
The other primary rain sensor on board TRMM is the TMI, which is a 9-channel passive microwave radiometer. The TMI observed brightness temperatures are sensitive to integrated quantities of water vapor, liquid water, and ice in the atmosphere, as well as surface temperature and wind speed over ocean regions. As a result, TMI does not directly provide information on the vertical structure of rain systems. The horizontal resolution of the sensor is also much lower, varying from around 5 km for the highest frequencies, which are sensitive to precipitation-sized ice particles, to around 40 km for the lowest frequency channels, which are sensitive to liquid water droplets. In contrast to the PR, however, the TMI sensor has a swath width over three times larger (~759 km), providing much better sampling of rain systems for climate applications. A corresponding TMI surface rainfall map for January 1, 1998 shows the sampling provided by the TMI sensor. More details regarding the TRMM instrument specifications are given by Kummerow et al. [1998, 2000].