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The Defense Meteorological Satellite Program (DMSP) Special Sensor Microwave/Imager(SSM/I) became operational in July 1987 on the F-8 satellite. Subsequent SSM/I's have been flown on the F- 10 (November 1990), F-11 (December 1991), F-12 (August 1994), F-13 (March 1995), F-14 (April 1997) and most recently, F-15 (January 2000) satellites. At present, NESDIS receives data from the F-13, F-14 and F-15 satellites. The SSM/I is a seven channel passive microwave radiometer operating at four frequencies (19,35, 22,235, 37.0, and 85.5 GHz) and dual- polarization (except at 22.235 GHz which is V-polarization only). It should be noted that the SSM/I will be replaced by an advanced sensor, the SSMIS (Special Sensor Microwave Imager Sounder) on the F-16 satellite, which was launched in October 2003 and is still undergoing an extensive calibration/validation by the Naval Research Laboratory. However, there should be little impact on the suite of hydrological products described here, as the primary channels used
are very similar between the SSM/I and SSMIS. 
The SSM/I products are useful for evaluating the mean climate state, it's interannual and seasonal variations, and the detection of anomalies associated with ENSO and regional climatic variations. Researchers from the Cooperative Institute for Climate Studies (CICS) maintain a time series of the entire SSM/I archive, now entering it's 18th year, which includes data from July 1987 to the present. Monthly average products are produced for precipitation, cloud liquid water, total precipitable water, snow cover, sea-ice cover, and oceanic surface wind speed. This web page maintains imagery from the current year while the actual digital data can be obtained from the National
Climatic Data Center. A good overview of the products available from the SSM/I can be found in the May 1996 Bulletin of the American Meteorological Society, pages 891-905: "An Eight Year (1987-1994) Time Series of Rainfall, Clouds, Water Vapor, Snow-cover, and Sea-ice Derived from SSM/I Measurements" by R. Ferraro, F. Weng, N. Grody, and A. Basist"