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Instrument: VISSR-GMS : Visible and Infrared Spin Scan Radiometer (GMS Series)
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[Text Source: Gunter's Space Page, ]

The principal instrument on board all satellites in the GMS series is the visible and infrared spin scan radiometer (VISSR), which is produced by Hughes' Santa Barbara Research Center. The spacecraft body carries the VISSR and spins at 100 rpm, while the antennas are despun and remain pointed toward Earth.

The spinning motion of the satellite carries the west-east scan. The north-south scan is produced by the VISSR scan mirror, which steps approximately 0.008 with each satellite revolution.

Visible spectrum information consists of reflected sunlight and is obtained when Earth's surface is illuminated by the sun. Infrared spectrum information consists of heat radiation from Earth's surface and cloud tops. Since this information contains very little sunlight reflection, it can be obtained day and night. Extremely sensitive detectors, which are kept cold by a radiative cooler, convert Earth's infrared radiation into analog signals.

The VISSR senses radiation and produces images of Earth and its atmosphere one scan line at a time, similar to the way television images are generated. About 2500 scan mirror steps or line scans are required to make a full image of the portion of Earth's disk as seen by the satellite. With its specially designed optical telescope and detectors, the VISSR obtains a complete 20 by 20 scan, produces an image of the full Earth disk every 25 minutes, and transmits that image back to Earth as weather facsimile pictures, showing different portions of the hemisphere. This enables meteorologists to identify, monitor, and track cataclysmic weather events such as windstorms, heavy rainfall, and typhoons, and to predict weather dangers long before storms reach densely populated areas.

Unlike its predecessors, which carried one infrared channel each, GMS-5 carries three. Two perform the same function as the single infrared on the previous satellites. The third determines atmospheric water vapor distribution.

Many aspects of weather that affect the daily lives of people exist for such relatively short periods that polar-orbit weather satellites fail to observe them. However, the GMS, from its stationary vantage point of constant observation, acquires almost instantaneous information on rapidly changing weather patterns and regional weather phenomena.

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