Introduction and Brief History of Satellites and Communication Satellites

Introduction and Brief History of Satell ites 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 M oon. It is not always a simple matter to decide which is the 'satellite' in a pa ir of bodies. Because all objects exert gravity, the motion of the primary objec t is also affected by the satellite. If two objects are ufficiently similar in m ass, they are generally referred to as a binary system rather than a primary obj ect 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-ma de object that orbits the Earth or another body). In May, 1946, the Preliminary Design of an Experimental World-Circling S paceship stated, "A satellite vehicle with appropriate instrumentation can be ex pected to be one of the most potent scientific tools of the Twentieth Century. T he achievement of a satellite craft would produce repercussions comparable to th e 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, sci entists used balloons that went up to 30 km and radio waves to study the ionosph ere. From 1946 to 1952, upper-atmosphere research was conducted using V-2s and A erobee rockets. This allowed measurements of atmospheric pressure, density, and temperature up to 200 km. The U.S. had been considering launching orbital satell ites since 1945 under the Bureau of Aeronautics of the United States Navy. The A ir Force's Project RAND eventually released the above report, but did not believ e 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 Am erican Rocket Society, the National Science Foundation, and the International Ge ophysical 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 la unch 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 tri ggered the Space Race between the two nations. The largest artificial satellite currently orbiting the earth is the Int ernational Space Station, which can sometimes be seen with the unaided human eye . Types of satellites · Astronomical satellites: These are satellites used for o bservation of distant planets, galaxies, and other outer space objects. · Communications satellites: These are artificial satellit es stationed in space for the purposes of telecommunications using radio at micr owave frequencies. Most communications satellites use geosynchronous orbits or n ear-geostationary orbits, although some recent systems use low Earth-orbiting sa tellites. ·

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, ma p making etc. (See especially Earth Observing System.) · Navigation satellites are satellites which use radio tim e signals transmitted to enable mobile receivers on the ground to determine thei r exact location. The relatively clear line of sight between the satellites and receivers on the ground, combined with ever-improving electronics, allows satell ite navigation systems to measure location to accuracies on the order of a few m etres in real time. · Reconnaissance satellites are Earth observation satellit e or communications satellite deployed for military or intelligence applications . Little is known about the full power of these satellites, as governments who o perate them usually keep information pertaining to their reconnaissance satellit es classified. · Solar power satellites are proposed satellites built in high Earth orbit that use microwave power transmission to beam solar power to ve ry large antenna on Earth where it can be used in place of conventional power so urces. · Space stations are man-made structures that are designed for human beings to live on in outer space. A space station is distinguished fr om 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, mo nths, or even years. · Weather satellites are satellites that primarily are use d 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 satell ites: minisatellite (500-200 kg), microsatellite (below 200 kg), nanosatellite ( below 10 kg). Orbit types Many times satellites are characterized by their orbit. Although a satel lite may orbit at almost any height, satellites are commonly categorized by thei r altitude: ·

Low Earth Orbit (LEO: 200 - 1200km above the Earth's sur

·

Medium Earth Orbit (ICO or MEO: 1200 - 35286 km)

face)

· Geosynchronous Orbit (GEO: 35786 km above Earth's surfac e) and Geostationary Orbit ( zero inclination geosynchronous orbit). These orbit s 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 o

f the earth, a phenomenon known as apogee dwell. Molniya orbits are named after a series of Soviet/Russian Molniya communications satellites that have been usin g this class of orbits since the mid 1960s. · Heliosynchronous or sun-synchronous orbit: A heliosynchr onous orbit, or more commonly a sun-synchronous orbit is an orbit in which an ob ject 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 eart h's surface in visible or infrared wavelengths (e.g. weather, spy and remote sen sing 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 revol ution. · 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 m oves 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 directi on. Communication Satellites A communications satellite (sometimes abbreviated to comsat) is an artif icial satellite stationed in space for the purposes of telecommunications. Moder n communications satellites use geosynchronous orbits, Molniya orbits or low Ear th orbits. For fixed services, communications satellites provide a technology compl ementary to that of fiber optic submarine communication cables. For mobile appli cations, such as communications to ships and planes satellite based communicatio nis 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 stationa ry with respect to earth's surface and can be used for long-range radio communic ation, as it will over come the limitations imposed by earth curvature. Sputnik 1, The world's first artificial (non communication) satellite, was launched on O ctober 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 us ed to send a Christmas greeting to the world from President Eisenhower. NASA lau nched an Echo satellite in 1960. This 100-foot aluminized Mylar balloon served a s a passive reflector for radio communications. Courier 1B, (built by Philco) al so was launched in 1960, was the world's first active repeater satellite. Given below are the details of milestones in satellite communcation history: · onous orbit - 1928

Herman Potocnik - describes a space station in geosynchr

· 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 1

960 · Telstar - the first active direct relay satellite design ed 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 geosynchronou s 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 · - March 1965

OSCAR-III - first amateur radio communications satellite

· Molniya - first Soviet communication satellite, highly e lliptic orbit - October 1965 · ervice - April 1965

Early Bird - INTELSAT's first satellite for commercial s

· Orbita - first national TV network based on satellite te levision - November 1967 · Canada, - 1973

Anik 1 - the first national satellite television system,

· Westar 1, the USA's first geosynchronous communications satellite - April 1974 · ellite 1976

Ekran - first serial Direct-To-Home TV communication sat

· uly 8 1976

Palapa A1 - first Indonesia communications satellite - J

· TDRSS - first satellite designed to provide communicatio ns 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 Huyge ns probe as it lands on Saturn's moon, Titan, the longest relay to date. -- Janu ary 14, 2005 Depending on the need the communication satellites can be placed in vari ous 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 geostation ary satellite revolves around the earth at a constant speed once per day over th e equator. The geostationary orbit is useful for communications applications bec ause ground based antennae, which must be directed toward the satellite, can ope rate effectively without the need for expensive equipment to track the satellite

's motion. Especially for applications that require a large number of ground ant ennae (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 prop osed 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 B efahrung 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 sat ellites in geostationary orbits for the purpose of relaying radio signals. Thus Arthur C. Clarke is often quoted as being the inventor of the communications sat ellite. The first geostationary communications satellite was Anik 1, a Canadian satellite launched in 1972. The United States launched their own geostationary c ommunication satellites afterward, with Western Union launching their Westar 1 s atellite in 1974, and RCA Americom (later GE Americom, now SES Americom) launchi ng 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 fir st 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 station s. The reason that Satcom 1 was so widely used is that it had twice the communic ations 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) h ad built nearly 40 percent of the satellites in service worldwide. Other major s atellite 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, corresponding ly, 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 ro ughly 1000 kilometers from the sub-satellite point. In addition, satellites in l ow 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 mis sion requires uninterrupted connectivity. Low earth orbiting satellites are less expensive to position in space th an geostationary satellites and, because of their closer proximity to the ground , require lower signal strength. So there is a trade off between the number of s atellites and their cost. In addition, there are important differences in the on board and ground equipment needed to support the two types of missions. A group of satellites working in concert thus is known as a satellite co nstellation. Two such constellations which were intended for provision for hand held telephony, primarily to remote areas, were the Iridium and Globalstar. The Iridium system has 66 satellites. Another LEO satellite constellation, with back ing from Microsoft entrepreneur Paul Allen, was to have as many as 720 satellite s. It is also possible to offer discontinuous coverage using a low Earth orbit s atellite capable of storing data received while passing over one part of Earth a nd transmitting it later while passing over another part. This will be the case with the CASCADE system of Canada's CASSIOPE communications satellite.

(c) Molniya satellites: As mentioned, geostationary satellites are c onstrained to operate above the equator. As a consequence, they are not always s uitable for providing services at high latitudes: for at high latitudes a geosta tionary satellite may appear low on (or even below) the horizon, affecting conne ctivity and causing multipathing (interference caused by signals reflecting off the ground into the ground antenna). The first satellite of Molniya series was l aunched on April 23, 1965 and was used for experimental transmission of TV signa l from Moscow uplink station to downlink stations, located in Russian Far East, in Khabarovsk, Magadan and Vladivostok. In November of 1967 Soviet engineers cre ated a unique system of national TV network of satellite television, called Orbi ta that was based on Molniya satellites. Molniya orbits can be an appealing alternative in such cases. The Molniy a orbit is highly inclined, guaranteeing good elevation over selected positions during the northern portion of the orbit. (Elevation is the extent of the satell ite's position above the horizon. Thus a satellite at the horizon has zero eleva tion and a satellite directly overhead has elevation of 90 degrees). Furthermore , the Molniya orbit is so designed that the satellite spends the great majority of its time over the far northern latitudes, during which its ground footprint m oves only slightly. Its period is one half day, so that the satellite is availab le for operation over the targeted region for eight hours every second revolutio n. In this way a constellation of three Molniya satellites (plus in-orbit spares ) can provide uninterrupted coverage. Molniya satellites are typically used for telephony and TV services over Russia. Another application is to use them for mobile radio systems (even at lo wer latitudes) since cars traveling through urban areas need access to satellite s at high elevation in order to secure good connectivity, e.g. in the presence o f tall buildings. Applications of Satellites (a) Telephony: One of the major applications of a communication sate llite is in provision of long distance telephone services. The connectivity is t hrough frequency division multiple access (FDMA) or time division multiple acce ss(TDMA) predominantly. Telephone subscribers can be connected through a network of exchanges which are in turn connected to satellite earth stations which upli nk the traffic to satellite for further processing. (b) Television and Radio: There are two types of satellites used for television and radio: (i) Direct Broadcast Satellite (DBS): A direct broadcast satellite i s a communications satellite that transmits to small DBS satellite dishes (usual ly 18" to 24" in diameter). Direct broadcast satellites generally operate in the upper portion of the Ku band. DBS technology is used for DTH-oriented (Direct-T o-Home) satellite TV services, such as DirecTV and Dish Network in the United St ates, ExpressVu in Canada, and Sky Digital in the UK. (ii) Fixed Service Satellite (FSS): Use the C band, and the lower por tions of the Ku bands. They are normally used for broadcast feeds to and from te levision networks and local affiliate stations (such as program feeds for networ k and syndicated programming, live shots, and backhauls), as well as being used for distance learning by schools & universities, business television (BTV), videoconferencing, and general commercial telecommunications. FSS satellites are also used to distribute national cable channels to cable TV headends. FSS satel lites differ from DBS satellites in that they have a lower RF power output than the latter, requiring a much larger dish for reception (3 to 8 feet in diameter for Ku band, and 12 feet on up for C band). FSS satellite technology was also or

iginally used for DTH satellite TV from the late 1970s to the early 1990s in the USA in the form of TVRO (TeleVision Receive Only) receivers and dishes (a.k.a. big-dish, or more pejoratively known as big ugly dish, systems). It was also use d in its Ku band form for the now-defunct Primestar satellite TV service. (c) Mobile satellite technologies: Initially available for broadcas t to stationary TV receivers, by 2004 popular mobile direct broadcast applicatio ns made their appearance with that arrival of two satellite radio systems in the United States: Sirius and XM Satellite Radio Holdings. Some manufacturers have also introduced special antennas for mobile reception of DBS television. Using G PS technology as a reference, these antennas automatically re-aim to the satelli te no matter where or how the vehicle (that the antenna is mounted on) is situat ed. These mobile satellite antennas are popular with some recreational vehicle o wners. Such mobile DBS antennas are also used by JetBlue Airways for DirecTV (su pplied by LiveTV, a subsidiary of JetBlue), which passengers can view on-board o n LCD screens mounted in the seats. (d) Amateur radio: Amateur radio operators have access to the OSCAR satellites that have been designed specifically to carry amateur radio traffic. Most such satellites operate as space borne repeaters, and are generally access ed by amateurs equipped with UHF or VHF radio equipment and highly directional a ntennas such as Yagis or dish antennas. Due to the limitations of ground-based a mateur equipment, most amateur satellites are launched into fairly low Earth orb its, and are designed to deal with only a limited number of brief contacts at an y given time. Some satellites also provide data-forwarding services using the X. 25 or similar protocols. Satellite Broadband Services: In recent years, satellite communication t echnology has been used as a means to connect to the Internet via broadband data connections. This is can be very useful for users to test who are located in ve ry remote areas, and can't access a wireline broadband or dialup connection. Countries with satellite launch capability This list includes counties with an independent capability to place sate llites in orbit, including production of the necessary launch vehicle. Many more countries have built satellites that were launched with the aid of others. The French and British capabilities are now subsumed by the European Union under the European Space Agency. First launch by country Country

Year of first launch

First satellite

Russia

1957

"Sputnik 1"

United States

1958

"Explorer 1"

France

1965

"Asterix"

Japan

1970

"Osumi"

China

1970

"Dong Fang Hong I"

United Kingdom 1971

"Prospero X-3"

European Union 1979

"Ariane 1"

India

"Rohini"

1980

Israel

1988

"Ofea 1"

Iran

2005

"Sina 1"

In 1998, North Korea claimed to have launched a satellite, but this was never confirmed, and widely believed to be a cover for the test launch of the Ta epodong-1 missile over Japan (See Kwangmyongsong).

Author has 28 years of experience in the field of Teaching and Management. He is M. Tech from IIT Kanpur and has worke d in different capacities including Signal corps Indian Army, Regional Manager f or a Telecom Company. Currently he is Associate Professor with ITM, Gurgaon that is rated as best Engineering colleges of North India.

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