Definition and Explanations of Satellite

 A telecommunication satellite is an artificial satellite positioned in space for telecommunications
needs. It can use geostationary orbit, low polar orbit, or Molniya orbit.

For fixed services, communications satellites provide complementary technology to the optical fiber that makes up the submarine cables. They are also used for mobile applications, such as communications to ships or aircraft, to which it would be impossible to use cable

Historical

Telstar 1 was the first active satellite to directly relay intercontinental communications. It was created by a
multinational group made up of AT&T, Bell Laboratories, NASA, EnglishGeneral Post Office(ancestor of British Telecom), and French PTT, to develop satellite communications. It was launched by NASA from Cape Canaveral on July 10, 1962, it was also the first private launch in history. It was placed in an elliptical orbit, circling the Earth in 157 minutes, inclined at 45° to the equator. It enabled the first television transmission on July 11live between the stations of Andover (United States) and Pleumeur-Bodou in France. The first satellite actually in geostationary orbit was the Syncom 3 satellite, launched on August 19, 1964, by Howard Hughes ‘ Hughes Aircraft Company.

Geostationary orbit 

A satellite in a geostationary orbit appears stationary to an observer on the Earth’s surface. It circles the Earth in 23h 56 min, at a constant speed, vertically above the equator.

Geostationary orbit is very practical for communication applications because the antennas on the ground, which absolutely must be pointed towards the satellite, can operate efficiently without having to be satellite equipped with a system for tracking the movements of the satellite, a system which is expensive and complicated to exploit. In the case of applications requiring a very large satellite number of antennas on the ground (such as the broadcasting of digital television packages), the savings made on ground equipment largely justify the technological complexity of the satellite and the additional cost of placing on a relatively high orbit (nearly 36,000 km).

The concept of a geostationary communication satellite was first exposed by Arthur C. Clarke, based on the work of Constantine Tsiolkovsky and an article by Herman Poto? Nik, written in 1929 under the pseudonym of Herman Noordung, with the title Das Problem der Befahrung des Weltraums — der Raketen-motor. In October 1945 Clarke published an article entitled ”  Extra-terrestrial Relays  ” in the British magazine Wireless World. The article describes the fundamental laws satellite governing the deployment of artificial satellites in geostationary orbit to relay radio signals. For this reason, Arthur C. Clarke is considered the inventor of the communications satellite.

The first geostationary communication satellite was Canadian, it was the Anik 1, launched on November 9, 1972; it would remain in operation until July 15, 1982. The United States of America would follow soon after, with Western Union launching the Westar 1 satellite on April 13, 1974, and RCA Americom (now SES Americom) launching the satellite Satcom 1 on December 12, 1975.

Satcom 1 was the source of the success of American cable channels like WTBS, HBO, CBN, The Weather Channel, etc. allowing them to reach the heads of all local satellite networks. In addition, this satellite
was the first used by the major television networks, such as ABC, NBC, or CBS to supply their local subsidiaries with programs. Satcom 1 was widely satellite used because it offered twice as much bandwidth (24 transponders instead of 12 for Westar 1 ), and therefore had much lower operating costs.

The first three-axis stabilized geostationary telecommunications satellite was Symphonie-A, the first of the
Franco-German program, launched on December 19, 1974.

By 2000, Hughes Space and Communications satellite, since acquired by Boeing Satellite Development Center, had built nearly 40% of the satellites in operation worldwide. Other important manufacturers are Loral
Space Systems, Lockheed Martin Space Systems, Northrop Grumman, Thales Alenia Space, and EADS Astrium.

Satellites in low earth orbit

A low Earth orbit is a circular orbit between 350 and 1400 km from the Earth’s surface; consequently, the period of revolution of the satellites is between 90 minutes and 2 hours. Due to their low altitude, these satellites are only visible within a radius of a few  satellite hundred kilometers around the point above which the satellite is located. Additionally, low-orbit satellites move rapidly relative to a fixed point on Earth, so even for local uses, a large number of satellites are required if the application requires permanent connectivity.

Satellites in low Earth orbit are much cheaper to put into orbit than geostationary satellites, and due to their proximity to the ground, require less signal satellite strength [ 1 ]. The cost of each satellite being much lower, it may be interesting to launch more of them, the launch being also less expensive, as well as the equipment necessary for ground operation.

A set of satellite working in concert is known as a satellite constellation. Several of these constellations provide wireless telephony services via satellite, initially to isolated areas. The Iridium network for example uses 66 satellites. The Globalstar network consists of 60 satellites.

Another possible use of these systems is the recording of data received fro satellite when passing over a terrestrial area, and its retransmission when passing over another area. This will be the case with the
CASCADE system, from the Canadian CASSIOPE satellite communication project.

Molniya Orbiting Satellite

Geostationary satellite are necessarily vertical to the equator. As a result, they are fairly unattractive at high latitudes: in such regions, a geostationary satellite will appear very low on the horizon; the connection could then be disturbed by the lower layers of the atmosphere. The first Molniya satellite was launched on April 23, 1965, and was used for experimental  satellite television transmissions, broadcasting from Moscow, and various receptions in Siberia and the Far EastRussian, in Norilsk, Khabarovsk,
Magadan, and Vladivostok. In November 1967, Soviet engineers created a unique
national satellite television system, called Orbita, based on Molniya.

Molniya’s orbit is characterized by an apogee of the order of 40,000 km located above the northern hemisphere and a perigee of the order of 1,000 km east above the southern hemisphere. Moreover,
its inclination on the equator is strong, 63.4°. The properties of this orbit satellite  ensure that the satellite spends most of its orbit above the most northern latitudes, during which time its footprint changes relatively little as it moves more slowly. His pursuit is thus facilitated.  satellite The period of this orbit is half a day (12 hours), which makes the satellite usable for 8 hours at each revolution. Thus, a constellation of three Molniya satellite (plus a spare in orbit) could provide permanent coverage of northern latitudes.

Molniya’s orbiting satellites are primarily used for telephony and television services over Russia. Another application allows them to be used for mobile radio systems (even in lower latitudes) because vehicles circulating in highly urbanized areas need satellites with high elevations to guarantee good connectivity even in the presence of high buildings…

The United States DoD also uses such an orbit for surveillance and communications satellites.