Special Sensor Microwave/imager - Instrument Characteristics

Instrument Characteristics

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.

The spin rate of the SSM/I provides a period of 1.9 sec during which the DMSP spacecraft sub-satellite point travels 12.5 km. Each 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 5 lower frequency channels. The resolution is determined by the Nyquist limit and the Earth surface's contribution of 3 dB bandwidth of the signal at a given frequency (see Table). The radiometer direction intersects the Earth’s surface at a nominal incidence angle of 53.1 degrees, as measured from the local Earth normal.

Table 1 Radiometric characteristics of the SSM/I (Hollinger 1989).



Along track

resolution (km)


resolution (km)


Sampling (km)


Noise (K)

19.35 horizontal 69 43 25 0.42
19.35 vertical 69 43 25 0.45
22.235 vertical 50 40 25 0.74
37.0 horizontal 37 28 25 0.38
37.0 vertical 37 28 25 0.37
85.5 horizontal 15 13 12.5 0.73
85.5 vertical 15 13 12.5 0.69

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