In olden times, priests and sages scanned the skies and anticipated the destiny of humankind according to the position of the stars and planets. Today, thousands of satellites travel across that same sky. Satellites are designed to perform many diverse, specific jobs. There are military and industrial research satellites, spy and intelligence satellites, satellites for meteorological data and satellites for intercontinental television and telephone connections. Satellites are particularly useful in communications; they can act as relays between transmitting and receiving stations on Earth.
They are also used to search outer space free of the boundary of our planet, directing their antennas toward whatever might lie in the most distant and hidden reaches of the universe, because the human desire to know and to communicate is never fulfilled.
A communications satellite, is basically a radio relay in the sky. Signals are transmitted from antennae on earth, amplified, and retransmitted back to an earth station. The history of satellite communications is directly connected to man's quest to conquer outer space. Experiments involving artificial satellites began in the late 1950's. Experiments demonstrated that using a large earth station, a radio signal could be sent to a satellite thousands of miles out in space. Using metallized balloons, placed in orbit, a passive satellite system became possible. There are two basic types of satellite orbits, medium or low and geosynchronous. The low-medium satellite orbits the earth at an altitude of 6,000 to 12,000 miles. Geosynchronous satellites orbit at 22,300 miles in space.
With the launch of SPUTNIK by the USSR in 1957, the race for space began. A successful launch of SCORE (Signal Communicating by Orbital Relay Equipment) on December 18, 1958 was an effort by the United States to catch up to the USSR. Placed in a low elliptical orbit, that took one hundred and one minutes to complete, SCORE broadcast a four minute taped message from President Eisenhower. The transponder consisted of a modified FM pocket pager for receiving and a handy-talky for transmitting. The satellite's batteries lasted for thirty-five days before failing.
The first communication satellites were the AT&T ECHO I (1960) and ECHO II (1964). These satellites were passive reflectors. There were no batteries to run down and they did not require a strict frequency channel on uplink signals. The ECHOs were in very low orbit, making them visible to two widely separated ground stations for only a few minutes per pass.
In 1962 Bell Laboratories developed TELSTAR, the first commercial communication satellite. Satellites had developed from large balloon-like reflectors to active repeaters. TELSTAR I and II (1963) were the first successful broadband, real time transponders. In a low elliptical orbit they provided a 50 MHz bandwidth for analog FM signals.
Arthur C. Clarke, a British science fiction writer, is credited with originating the idea of the geosynchronous communication satellite. In 1945, Clarke wrote that a satellite in a circular equatorial orbit with a radius of about 42242 Km would have an angular velocity that matched the earth's. A satellite meeting these parameters, according to Clarke, would always be above the same point on the earth's surface and could send and receive signals from most of a hemisphere and three satellites placed one hundred and twenty degrees apart could cover the entire world. In geosynchronous orbit, the orbital period is the same as the Earth's period for one complete rotation about its axis. The orbit is approximately 22,300 miles above the earth. When the satellite is positioned above the equator and travels in the same direction as the earth spins, the satellite appears to be stationary over one point on earth.
Several advantages that geosynchronous orbits have over low altitude orbits are:
1. Since the satellite is stationary, the cost of computer tracking equipment at the earth station is avoided.
2. It is not necessary to switch to another satellite as one disappears over the horizon.
3. There are no breaks in transmission, because the satellite is always in view.
4. Because of the distance, the satellite is in line-of-sight to 42% of the earth's surface.
5. Three satellites, if placed properly, provide global coverage, excepting the Polar Regions.
6. There is almost no Doppler shift (a change in the frequency resulting from motion of the transmitter or receiver).
Geosynchronous satellites are not without disadvantages:
1. Latitudes greater than 81.25 degrees north and south are not covered.
2. Due to the distance of the satellite, the received signal is weak and the delay in signal propagation is approximately 250 milliseconds (one-quarter second).
The first successful geosynchronous satellite was SYNCOM II in 1963. The transponder could support two carriers at the same time and used full duplex operation. Two years later, in 1965, INTELSAT I, the first commercial geosynchronous satellite was launched. On June 28th it began routine operations between Europe and the United States. With the launch of INTELSAT III in 1969 a fully global satellite system became operational.
In 1972 the FCC released the Open Skies Policy. As a result of this new policy, Western Union launched two WESTAR satellites in 1974. These were the first domestic satellites. Even with the placement of transoceanic fiber-optic cables, communication satellites are still a viable communication system. Many different countries have placed satellites in orbit, making global communications very commonplace.
Space communications are the most developed sector of space services industry. It is difficult to overestimate the value of space communications for Russia taking into account very large extent of its communication lines and travelling distances, with existence of extended and difficult-to- access territories with sparse population and poorly developed telecommunication systems.
The beginning of space communication in the USSR and Russia was laid down by launch of communication satellite Gorizont at the end of the seventies.
At present, there are number of satellite communication and broadcasting systems in Russia that play substantial role in the countries operating communication systems.
These systems provide satellite communication, TV and audio broadcasting, transmission of news columns, data transmission to many Russian regions and a number of other countries. These systems are based on Gorizont, Ekran-M, Express and Gals communication satellites. There are about 30 such satellites now in orbit.
In addition to fixed communication systems mobile satellite communication systems are being developed. One such system named Marathon is based on Arcos geostationary satellite orbit and another one on a Mayak satellite which is set for use a high elliptical orbit of Molniya type. A first step to provide person-to-person communications is launching experimental low-Earth-orbit Gonets type satellites. In the near future current communication and broadcasting satellites will be replaced by new generation ones.
During development of new communication satellites such as Express-D, Gals-P16, Yamal-2000, Arcos and others it is planned to use edge technologies which permit to increase the capacity of communication channels and power of on-board transponder systems and to bring active life span of spacecraft to 10-12 years.