military satellites

Artificial satellites used for a variety of military purposes. Of approximately 4000 satellites successfully launched between 1957 and 1999, about 50% have been either specifically military or usable for military purposes. Major functions of military satellites include communications, positioning and navigation, meteorology, reconnaissance and surveillance, early warning, remote sensing, geodesy, and research.

Although only certain satellites are used continuously for military purposes, all communication satellites, including commercial, may find use during conflict. All contain the necessary equipment to transmit a signal over great distances to assist in the command, control, administration, and logistic support of military forces. Military communication satellites differ from commercial satellites only in that they contain specialized components, certain capabilities, and multiple redundant systems designed to make them less vulnerable and more effective in a hostile environment. See also Communications satellite.

Military forces must be able to quickly and precisely determine their position on the ground, in the air, or at sea. The Navstar Global Positioning System (GPS) is the most accurate and reliable satellite navigation system available. It includes 25 spacecraft in semisynchronous (12-h) orbits inclined at 55° to the Equator at 11,600 mi (18,700 km) altitude. Of these, 21 are operational and 4 are spares. The inclined orbits provide worldwide coverage, including the Poles. Russia also maintains global navigation satellite systems. Its Tsikada/Nadezhda low-Earth-orbit system functions similar to the United States' decommissioned Transit system. In addition, Russia operates the GLONASS navigation system. Similar to GPS, the system is less complex, but its satellites have proven less reliable than the United States' version. Receivers are available that will accept navigational data from either GPS or GLONASS. See also Satellite navigation systems.

From orbit, it is possible to obtain a wide-field-of-view image of the Earth, its cloud formations, and their movements. This meteorological information is valuable for military planning and operations.

The Television Infrared Observation Satellites (TIROS) have, for decades, traveled in Sun-synchronous, low Earth orbits providing images of cloud cover, snow, ice, and the sea surface. The Defense Meteorological Satellite Program (DMSP) consists of several satellites in low Earth, Sun-synchronous, polar orbits at an altitude of 517 mi (833 km), spaced to provide complete coverage of the Earth at various times of the day and night.

Russia operates the Meteor system of satellites. As many as five satellites are in low Earth orbit similar to TIROS with an orbital inclination of 81.2°. They image in the infrared spectrum. Several other countries maintain a meteorological satellite capability that provides useful information for military operations, although not specifically designed for military use. See also Meteorological satellites.

Military reconnaissance and surveillance satellites offer near-real-time unrestricted access over almost any area on Earth. Operating in many parts of the electromagnetic spectrum, they can be used to observe weapons development and deployment of forces, and to provide warning of attack by ground forces as well as targeting intelligence, technical intelligence on enemy capabilities, electronic intelligence, and bomb damage assessment.

Early warning satellites provide information on missile launch and nuclear detonation that give governments time to make strategic military decisions.

Remote-sensing satellites afford a unique view of Earth, providing vital information to military forces. The images produced by these satellites are used to conduct routine reconnaissance, analyze waterways, assist in exercise and strike planning, and provide up-to-date maps for forces deploying to unfamiliar areas. Important information is gathered not only in the visible spectrum but in other bands of the spectrum. See also Remote sensing.

Geodesy



Geodesy is the study of the Earth's size and shape. Geodetic data are important to the military in that the data affect position determination, navigation, map making, and a variety of other missions. Almost all satellites can be used for geodesy, provided their position in space can be accurately determined by optical or electronic means from the Earth.

In addition to the missions discussed above, there have been hundreds of military research and technology spacecraft, as well as thousands of experimental investigations for military purposes on space vehicles launched by all space-faring nations.