DMSP 5D-2/F6 was one of a series of meteorological satellites developed and operated by the Air Force under the Defense Meteorological Satellite Program (DMSP). This program, previously known as DAPP (Data Acquisition and Processing Program), was classified until March 1973. The objective of this program was to provide global visual and infrared cloudcover data and specialized environmental data to support Department of Defense requirements for operational weather analysis and forecasting. Operationally, the program consisted of two satellites in planned 830-km, sun-synchronous polar orbits, with the ascending node of one satellite in early morning and the other at local noon. The 6.4-m-long spacecraft was divided into four sections: (1) a precision mounting platform for sensors and equipment requiring precise alignment; (2) an equipment support module containing the electronics, reaction wheels, and some meteorological sensors; (3) a reaction-control-equipment support structure containing the spent third-stage rocket motor, and supporting the ascent-phase reaction-control equipment; and (4) a 9.29-sq-m solar cell panel. The spacecraft stabilization was controlled by a combination flywheel and magnetic control coil system so sensors were maintained in the desired "earth-looking" mode. One feature was the precision-pointing accuracy of the primary imager to 0.01 deg, provided by a star sensor and an updated ephemeris navigation system. This allowed automatic geographical mapping of the digital imagery to the nearest picture element. The operational linescan system was the primary data acquisition system that provided real-time or stored, multi-orbit, day-and-night, visual and infrared imagery. A supplementary sensor package contained four special sensors: (1) an infrared temperature/humidity sounder, (2) a scanning X-ray spectrometer, (3) an ionospheric plasma monitor, and (4) a precipitating electron/ion spectrometer. Either recorded or real-time data were transmitted to ground-receiving sites by two redundant S-band transmitters. Recorded data were read out to tracking sites located at Fairchild AFB, Washington, and at Loring AFB, Maine, and relayed by SATCOM to Air Force Global Weather Central, Offutt AFB, Nebraska. Real-time data were read out at mobile tactical sites located around the world. A more complete description of the satellite can be found in the report by D. A. Nichols, "The Defense Meteorological Satellite Program," Optical Engineering, v. 14, n. 4, July-August 1975.
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