THE DEVIL'S DUKE

Satellites
What is a Satellite?
Any body that orbits another body is regarded as a satellite, whether it is a natural body (ex. a planet, a moon), or an artificial one (ex. a space craft, a space probe, a telescope). The satellites referred to in this article are actually the artificial ones. All satellites share almost the same basic instruments. However, their uses range from monitoring of weather conditions, facilitating communications to observation of to astronomical phenomena and military sites.
Fact: By 1995, the number of satellites orbiting the Earth reached about 350. Each of these sent and received signals according to its particular purpose.
Satellite Uses
According to their uses, satellites are classified as follows:

Communications Satellites: Nearly all communications satellites rotate around the earth in a geostationary orbit, i.e above the equator. Satellite dishes on earth are responsible for sending telephone or TV signals up to the satellite, which processes and sends them back down to another ground station. Depending on the way they function, satellites can send signals to a whole continent or to a localized region. Direct-broadcast satellites are designed to send signals to individual dish receivers or to televisions that are connected by cable to larger dishes.

Weather Satellites: There are two types of weather satellites; those that rotate around the earth in a geostationary orbit, thus maintain a constant watch over about one third of the earth's surface, and those that move in polar orbits enabling them to cover the whole surface of the earth every 12 hours. The information weather satellites send to ground stations encloses measurements of ground and air temperatures, records of wind speeds, cloud movements and rainfalls. This information allows meteorologists to forecast the weather.

Fact: Satellites following a geostationary orbit fly at an altitude of 36, 000 km above the earth's equator. At such an altitude, the time taken by the satellite to orbit the earth is the same as that taken by the earth to rotate. This phenomenon results in the satellite facing a fixed spot on the surface of the earth. On the other hand, those moving in polar orbits fly at an altitude as low as 600 km and as high as 1,600 km.

Earth-Resources Satellites: "Earth-Resources Satellites monitor conditions on the earth." They follow a near-polar orbit, from which they can observe the whole surface on a regular basis. These satellites provide information including "measurements for mapping purposes, geological surveys, mining activities, oil exploration"and agriculture-related records. They can also detect oil spills, forest fires and various forms of pollution.

Fact: Earth-Resources satellites have served zoologists in tracking the movements of large animals that were fitted with special collars that transmit radio signals. Archeologists have been able to find and study ancient sites in regions that are inaccessible by road through the help of satellites.

Astronomical Satellites: Astronomical satellites help provide information about space ad the universe that may not be obtained by observatories on earth. Not only do these satellites send data about the earth's magnetic field, its radiation belts and the solar wind, they also detect radiation (ex. gamma rays, infrared rays, X-rays and ultraviolet rays) from outer space that indicates the existence of various objects, ranging from stars and galaxies to quasars, invisible gas clouds, black holes and the remains of exploding stars.

Navigation Satellites: Navigation Satellites help people pinpoint their position. Such satellites are developed to provide guidace to warplaes, warships as well as commercial aircraft and ships. The American Navstar Global Positioning System has 24 satellites. "On the Earth, a receiver uses the signals from four satellites to calculate its own position, height, and speed, if moving".

Military Satellites: "Military satellites provide an ideal vantage point for spying." These satellites are characterized by such powerful cameras that can take detailed photos of military sites and troop movements. Some satellites can give early warning of enemy missiles. These are termed "early-warning satellites". "Other satellites can listen in on enemy communications." However, spy satellites can easily be attacked by enemy missiles launched from the ground or by "killer satellites" that use lasers or explosives.