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Introduction and Brief History of Satellites and Communication Satellites
Introduction and Brief History of Satellites
A satellite is any object that orbits another object (which is known as its primary). All masses that are part of the solar system, including the Earth, are satellites either of the Sun, or satellites of those objects, such as the Moon. It is not always a simple matter to decide which is the 'satellite' in a pair of bodies. Because all objects exert gravity, the motion of the primary object is also affected by the satellite. If two objects are ufficiently similar in mass, they are generally referred to as a binary system rather than a primary object and satellite. The general criterion for an object to be a satellite is that the center of mass of the two objects is inside the primary object. In popular usage, the term 'satellite' normally refers to an artificial satellite (a man-made object that orbits the Earth or another body).
In May, 1946, the Preliminary Design of an Experimental World-Circling Spaceship stated, "A satellite vehicle with appropriate instrumentation can be expected to be one of the most potent scientific tools of the Twentieth Century. The achievement of a satellite craft would produce repercussions comparable to the explosion of the atomic bomb..."
The space age began in 1946, as scientists began using captured German V-2 rockets to make measurements in the upper atmosphere. Before this period, scientists used balloons that went up to 30 km and radio waves to study the ionosphere. From 1946 to 1952, upper-atmosphere research was conducted using V-2s and Aerobee rockets. This allowed measurements of atmospheric pressure, density, and temperature up to 200 km. The U.S. had been considering launching orbital satellites since 1945 under the Bureau of Aeronautics of the United States Navy. The Air Force's Project RAND eventually released the above report, but did not believe that the satellite was a potential military weapon; rather they considered it to be a tool for science, politics, and propaganda. Following pressure by the American Rocket Society, the National Science Foundation, and the International Geophysical Year, military interest picked up and in early 1955 the Air Force and Navy were working on Project Orbiter, which involved using a Jupiter C rocket to launch a small satellite called Explorer 1 on January 31, 1958.
On July 29, 1955, the White House announced that the U.S. intended to launch satellites by the spring of 1958. This became known as Project Vanguard. On July 31, the Soviets announced that they intended to launch a satellite by the fall of 1957 and on October 4, 1957 Sputnik I was launched into orbit, which triggered the Space Race between the two nations.
The largest artificial satellite currently orbiting the earth is the International Space Station, which can sometimes be seen with the unaided human eye.
Types of satellites
· Astronomical satellites: These are satellites used for observation of distant planets, galaxies, and other outer space objects.
· Communications satellites: These are artificial satellites stationed in space for the purposes of telecommunications using radio at microwave frequencies. Most communications satellites use geosynchronous orbits or near-geostationary orbits, although some recent systems use low Earth-orbiting satellites.
· Earth observation satellites are satellites specifically designed to observe Earth from orbit, similar to reconnaissance satellites but intended for non-military uses such as environmental monitoring, meteorology, map making etc. (See especially Earth Observing System.)
· Navigation satellites are satellites which use radio time signals transmitted to enable mobile receivers on the ground to determine their exact location. The relatively clear line of sight between the satellites and receivers on the ground, combined with ever-improving electronics, allows satellite navigation systems to measure location to accuracies on the order of a few metres in real time.
· Reconnaissance satellites are Earth observation satellite or communications satellite deployed for military or intelligence applications. Little is known about the full power of these satellites, as governments who operate them usually keep information pertaining to their reconnaissance satellites classified.
· Solar power satellites are proposed satellites built in high Earth orbit that use microwave power transmission to beam solar power to very large antenna on Earth where it can be used in place of conventional power sources.
· Space stations are man-made structures that are designed for human beings to live on in outer space. A space station is distinguished from other manned spacecraft by its lack of major propulsion or landing facilities -- instead, other vehicles are used as transport to and from the station. Space stations are designed for medium-term living in orbit, for periods of weeks, months, or even years.
· Weather satellites are satellites that primarily are used to monitor the weather and/or climate of the Earth.
· Miniaturized satellites are satellites of unusually low weights and small sizes. New classifications are used to categorize these satellites: minisatellite (500-200 kg), microsatellite (below 200 kg), nanosatellite (below 10 kg).
Many times satellites are characterized by their orbit. Although a satellite may orbit at almost any height, satellites are commonly categorized by their altitude:
· Low Earth Orbit (LEO: 200 - 1200km above the Earth's surface)
· Medium Earth Orbit (ICO or MEO: 1200 - 35286 km)
· Geosynchronous Orbit (GEO: 35786 km above Earth's surface) and Geostationary Orbit ( zero inclination geosynchronous orbit). These orbits are of particular interest for communication satellites and will be discussed in detail later.
· High Earth Orbit (HEO: above 35786 km)
The following orbits are special orbits that are also used to categorize satellites:
· Molniya orbits: Is a class of a highly elliptic orbit. A satellite placed in this orbit spends most of its time over a designated area of the earth, a phenomenon known as apogee dwell. Molniya orbits are named after a series of Soviet/Russian Molniya communications satellites that have been using this class of orbits since the mid 1960s.
· Heliosynchronous or sun-synchronous orbit: A heliosynchronous orbit, or more commonly a sun-synchronous orbit is an orbit in which an object always passes over any given point of the Earth's surface at the same local solar time. This is a useful characteristic for satellites that image the earth's surface in visible or infrared wavelengths (e.g. weather, spy and remote sensing satellites).
· Polar orbit : A satellite in a polar orbit passes above or nearly above both poles of the planet (or other celestial body) on each revolution.
· Hohmann transfer orbit: For this particular orbit type, it is more common to identify the satellite as a spacecraft. In astronautics and aerospace engineering, the Hohmann transfer orbit is an orbital maneuver that moves a spacecraft from one orbit to another.
· Supersynchronous orbit or drift orbit : orbit above GEO. Satellites will drift in a westerly direction.
· Subsynchronous orbit or drift orbit: orbits close to but below GEO. Used for satellites undergoing station changes in an eastern direction.
A communications satellite (sometimes abbreviated to comsat) is an artificial satellite stationed in space for the purposes of telecommunications. Modern communications satellites use geosynchronous orbits, Molniya orbits or low Earth orbits.
For fixed services, communications satellites provide a technology complementary to that of fiber optic submarine communication cables. For mobile applications, such as communications to ships and planes satellite based communicationis only the viable means of communications as application of other technologies, such as cable, are impractical or impossible.
Early missions: The origin of satellite communication can be traced to an article written by Arthur C. Clarke in 1945. He suggested that a radio relay satellite in an equatorial orbit with a period of 24 hours would remain stationary with respect to earth's surface and can be used for long-range radio communication, as it will over come the limitations imposed by earth curvature. Sputnik 1, The world's first artificial (non communication) satellite, was launched on October 4, 1957. The first satellite to relay communications was Project SCORE in 1958, which used a tape recorder to store and forward voice messages. It was used to send a Christmas greeting to the world from President Eisenhower. NASA launched an Echo satellite in 1960. This 100-foot aluminized Mylar balloon served as a passive reflector for radio communications. Courier 1B, (built by Philco) also was launched in 1960, was the world's first active repeater satellite. Given below are the details of milestones in satellite communcation history: -
· Herman Potocnik - describes a space station in geosynchronous orbit - 1928
· Arthur C. Clarke - proposes a station in geosynchronous orbit to relay communications and broadcast television - 1945
· Project SCORE - first communications satellite - 1958
· Echo I - first passive reflector satellite - August 1960
· Courier 1B - first active repeater satellite - October 1960
· Telstar - the first active direct relay satellite designed to transmit television and high-speed data communications. Telstar was placed in an elliptical orbit (completed once every 2 hours and 37 minutes), rotating at a 45° angle above the equator. July 1962
· Syncom - first communications satellite in geosynchronous orbit. Syncom 2 revolved around the earth once per day at constant speed, but because it still had north-south motion special equipment was needed to track it. 1963
· OSCAR-III - first amateur radio communications satellite - March 1965
· Molniya - first Soviet communication satellite, highly elliptic orbit - October 1965
· Early Bird - INTELSAT's first satellite for commercial service - April 1965
· Orbita - first national TV network based on satellite television - November 1967
· Anik 1 - the first national satellite television system, Canada, - 1973
· Westar 1, the USA's first geosynchronous communications satellite - April 1974
· Ekran - first serial Direct-To-Home TV communication satellite 1976
· Palapa A1 - first Indonesia communications satellite - July 8 1976
· TDRSS - first satellite designed to provide communications relay services for other spacecraft. - 1983
· Mars Global Surveyor - first communications satellite in orbit around another planet (Mars) - 1997
· Cassini spacecraft relays to Earth images from the Huygens probe as it lands on Saturn's moon, Titan, the longest relay to date. -- January 14, 2005
Depending on the need the communication satellites can be placed in various types of orbits. We discuss few common types: -
(a) Geostationary orbits Satellites: A satellite in a geostationary orbit appears to be in a fixed position to an earth-based observer. A geostationary satellite revolves around the earth at a constant speed once per day over the equator. The geostationary orbit is useful for communications applications because ground based antennae, which must be directed toward the satellite, can operate effectively without the need for expensive equipment to track the satellite's motion. Especially for applications that require a large number of ground antennae (such as direct TV distribution), the savings in ground equipment can more than justify the extra cost and onboard complexity of lifting a satellite into the relatively high geostationary orbit.
The concept of the geostationary communications satellite was first proposed by Arthur C. Clarke, building on work by Konstantin Tsiolkovsky and on the 1929 work by Herman Potočnik (writing as Herman Noordung) Das Problem der Befahrung des Weltraums - der Raketen-motor. In October 1945 Clarke published an article titled "Extra-terrestrial Relays" in the British magazine Wireless World. The article described the fundamentals behind the deployment of artificial satellites in geostationary orbits for the purpose of relaying radio signals. Thus Arthur C. Clarke is often quoted as being the inventor of the communications satellite.
The first geostationary communications satellite was Anik 1, a Canadian satellite launched in 1972. The United States launched their own geostationary communication satellites afterward, with Western Union launching their Westar 1 satellite in 1974, and RCA Americom (later GE Americom, now SES Americom) launching Satcom 1 in 1975.
It was Satcom 1 that was instrumental in helping early cable TV channels such as WTBS (now TBS Superstation), HBO, CBN (now ABC Family), and The Weather Channel become successful, because these channels distributed their programming to all of the local cable TV headends using the satellite. Additionally, it was the first satellite used by broadcast TV networks in the United States, like ABC, NBC, and CBS, to distribute their programming to all of their local affiliate stations. The reason that Satcom 1 was so widely used is that it had twice the communications capacity of Westar 1 (24 transponders as opposed to Westar 1's 12), which resulted in lower transponder usage costs.
By 2000 Hughes Space and Communications (now Boeing Satellite Systems) had built nearly 40 percent of the satellites in service worldwide. Other major satellite manufacturers include Space Systems/Loral, Lockheed Martin (owns former RCA Astro Electronics/GE Astro Space business), Northrop Grumman, Alcatel Space and EADS Astrium.
(b) Low-Earth-orbiting satellites: A low Earth orbit typically is a circular orbit about 150 kilometers above the earth's surface and, correspondingly, a period (time to revolve around the earth) of about 90 minutes. Because of their low altitude, these satellites are only visible from within a radius of roughly 1000 kilometers from the sub-satellite point. In addition, satellites in low earth orbit change their position relative to the ground position quickly. So even for local applications, a large number of satellites are needed if the mission requires uninterrupted connectivity.
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