SSMI

The special sensor microwave/imager (SSM/I) is a seven-channel, four-frequency, linearly polarized passive microwave radiometer system. It is flown on board the United States Air Force Defense Meteorological Satellite Program (DMSP) Block 5D-2 satellites. The instrument measures surface/atmospheric microwave brightness temperatures (TBs) at 19.35, 22.235, 37.0 and 85.5 GHz. The four frequencies are sampled in both horizontal and vertical polarizations, except the 22 GHz which is sampled in the vertical only.

The SSM/I has been a very successful instrument, superseding the across-track and Dicke radiometer designs of previous systems. Its combination of constant-angle rotary-scanning and total power radiometer design has become standard for passive microwave imagers, e.g. TRMM Microwave Imager, AMSR.

Its predecessor, the scanning multichannel microwave radiometer (SMMR), provided similar information. Its successor, the Special Sensor Microwave Imager / Sounder (SSMIS), is an enhanced eleven-channel, eight-frequency system.

Along with its predecessor SMMR, the SSM/I contributes to an archive of global passive microwave products from late 1978 to present.

Information within the SSM/I TBs measurements allow the retrieval of four important meteorological parameters over the ocean: near-surface wind speed (note scalar not vector), total columnar water vapor, total columnar cloud liquid water (liquid water path) and precipitation. Accurate and quantitative measurement of these parameters from the SSM/I TBs is, however, a non-trivial task. Variations within the meteorological parameters significantly modify the TBs. As well as open ocean retrievals, it is also possible to retrieve quantitatively reliable information on sea ice, land snow cover and over-land precipitation.

The Block 5D-2 satellites are in circular or near-circular Sun-synchronous and near-polar orbits at altitudes of 833 km with inclinations of 98.8° and orbital periods of 102.0 minutes, each making 14.1 full orbits per day. The scan direction is from the left to the right with the active scene measurements lying ± 51.2 degrees about when looking in the F8 forward (F10–F15) or aft (F8) direction of the spacecraft travel. This results in a nominal swath width of 1394 km allowing frequent ground coverage, especially at higher latitudes. All parts of the globe at latitudes greater than 58° are covered at least twice daily except for small unmeasured circular sectors of 2.4° about the poles. Extreme polar regions (> 72° N or S) receive coverage from two or more overpasses from both the ascending and descending orbits each day.

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Wikipedia