The Special Sensor Microwave Imager (SSM/I) has been carried onboard Defense Meteorological Satellite Program (DMSP) satellites since 1987. The DMSP satellites are in a Sun synchronous orbit with equatorial crossings at 1811 local time for the ascending node and 0611 local time for the descending node.
The SSM/I sensor is a seven channel device, measuring microwave radiation emitted by Earth's surface and atmosphere at four frequencies (19.3, 22.2, 37.0, and 85.5 GHz) and two polarizations (horizontal and vertical) at each frequency (except 22.2 GHz). The SSM/I sensor provided complete global coverage every 6 days. Polar regions (above latitudes of 72°) have complete coverage for every day the sensor was in normal operation (NSIDC, 1992).