NASA and the Japan Aerospace Exploration Agency (JAXA) collaborated in launching the Tropical Rainfall Measuring Mission (TRMM) satellite in November 1997 to study global rainfall. The major rain sensors for the rainfall measurement onboard TRMM are Precipitation Radar (PR), TRMM Microwave Imager (TMI) and Visible Infrared Scanner (VIRS). Kummerow et al. (2000) provided a good description of the TRMM project and instrument performance.