History of Satellite Communications
The first idea of satellite communication came from an article in 1945 named Wireless World, where Author C. Clarke described the use of manned satellites in 24 hour orbits to distribute television programs. However, the first person to carefully evaluate the technical and financial aspects of such a venture was John R. Pierce of Bell Telephone Laboratories (Whalen, n.d.). In a 1954 speech and 1955 article, Pierce described the usefulness of a communications "mirror" in space, a medium-orbit "repeater" and a 24-hour-orbit "repeater." In comparing the communications capacity of a satellite, which he estimated the capacity at 1,000 simultaneous telephone calls, and the capacity of the first trans-atlantic telephone cable, which could carry 36 simultaneous telephone calls at a cost of 30-50 million dollars, Pierce wondered if a satellite would be worth a billion dollars (Whalen, n.d.). By the middle of 1961, RCA had a contract with NASA to build, a 4000 mile high, medium-orbit, active communications satellite called RELAY, AT&T was working on its own medium-orbit satellite called TELSTAR, and Hughes Aircraft Company had an exclusive contract to build a 24-hour orbit, 20,000 mile high satellite, called SYNCOM. By 1964, two TELSTARs, two RELAYs, and two SYNCOMs had operated successfully in space. The transponder technology used by AT&T in the TELSTAR I satellite is current technology in use today (Whalen, n.d.). On April 6, 1965, a new company called COMSAT launched its first satellite, EARLY BIRD, from Cape Canaveral beginning Global satellite communications. The EARLY BIRD satellite provided almost 10 times the capacity of submarine telephone cables for almost 1/10th the price. Satellites are still competitive with cable for point-to-point communications, but the future advantage may lie with fiber-optic cable (Whalen, n.d.).
How Satellites Work
First, as one would guess, satellites are launched into orbit. There are several types of orbit for satellites to follow but the main three are Low Earth Orbit (LEO), Medium Earth Orbit (MEO), and Geosynchronous Orbit (GEO). Satellites in a Low Earth Orbit are 100-300 miles above the earth's surface, and it must travel around 17,500 mph, circling the planet in about 10 minutes, to avoid gravity pulling them back to earth. In a Medium Earth Orbit, a satellite is 6,00012,000 miles above the earth and will circle the planet 4 to 6 hours. The Geosynchronous Orbit is a bit more complex, this orbit is 22,282 miles above the earth. These satellites are positioned over the equator and rotate at the same speed as the earth. This makes a satellite in a geosynchronous orbit remain in the same position above the earth at all times (Boeing, 2002, p. 3). Once a communication satellite is place in orbit, it receives signals and information from earth. The signal being sent to the satellite is called the uplink. The satellite's transponder then amplifies the signal, converts it to a different frequency, and sends it to another satellite or to a ground station on earth. The signal sent back from the satellite is the downlink (Boeing, 2002, p. 2). Technology involved
There are many different types of technology involved with satellite communications. There is technology used with the satellites themselves, to control the satellites and keep them where they should be, as well as, the technology used for them relay signals back and forth to earth. There are also numerous technologies used on the ground to interact with these communication satellites to send and receive information. The Satellite Technology
Communications satellites are comprised of many different subsystems to make a complete system. Some of these subsystems are as follows:
Propulsion subsystem is electric or chemical motors and thrusters used to reposition the satellite or keep it in its proper orbit.
Power subsystems comprised of solar panels that...
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