The 3B40 is a merger of all available SSM/I and TMI microwave precipitation estimates. The 3B42 product is a merger of microwave plus estimates from geostationary infrared (IR) observations.
For more information, please visit: http://disc.sci.gsfc.nasa.gov/data/datapool/TRMM/01_Data_Products/index.html
Multimedia Items Available Relating to TRMM / 3B4XRT(a.k.a. TRMM 3-hour Rainmap)