Instrument: Special Sensor Microwave Imager

The Special Sensor Microwave Imager (SSM/I) is a passive microwave radiometer that measures atmospheric, oceanic and terrestrial microwave brightness temperatures at 19.35, 22.2, 37.0, and 85.5 GHz, from which ocean surface wind speed, atmospheric water vapour, cloud liquid water and rain rate can be derived. The SSM/I consists of an offset parabolic reflector of dimensions 24 x 26 inches, fed by a corrugated, broad-band, seven-port horn antenna. The reflector and feed are mounted on a drum that contains the radiometers, digital data subsystem, mechanical scanning subsystem, and power subsystem. The reflector-feed-drum assembly is rotated about the axis of the drum by a coaxially mounted bearing and power transfer assembly (BAPTA). All data, commands, timing and telemetry signals, and power pass through the BAPTA on slip ring connectors to the rotating assembly. The absolute brightness temperature of the scene incident upon the antenna is received and spatially filtered by the antenna to produce an effective input signal or antenna temperature at the input of the feed horn antenna. The SSM/I has seven channels, four frequencies, and is orthogonally polarised. It rotates continuously about an axis parallel to the local spacecraft at 31.6 rpm and measures the upwelling scene brightness temperature over an angular range of 102.4 degrees at nadir. The active scene measurements lie +51.2 to -51.2 degrees about the forward (F10, F11) or aft (F8) direction. This results in a swath width of approximately 1400 kilometres. During each 1.9 second scan, 128 discrete uniformly spaced radiometric samples are taken at the two 85 GHz channels and, on alternate scans, 64 discrete samples are taken at the remaining lower frequency channels. The antenna beam intersects the Earth's surface at an incidence angle of 53.1 degrees. Flown on F08, F10, F11, F13, F14, F15, F16, F17, F18 F19. https://nsidc.org/data/docs/daac/ssmi_instrument.gd.html