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Communications satellite

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U.S. military MILSTAR communications satellite

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, for which application of other technologies, such as cable, are impractical or impossible.

History

Early missions

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.

Telstar was the first active, direct relay communications satellite. Belonging to AT&T as part of a multi-national agreement between AT&T, Bell Telephone Laboratories, NASA, the British General Post Office, and the French National PTT (Post Office) to develop satellite communication, it was launched by NASA from Cape Canaveral on July 10, 1962, the first privately sponsored space launch. Telstar was placed in an elliptical orbit (completed once every 2 hours and 37 minutes), rotating at a 45° angle above the equator.

An immediate antecedent of the geostationary satellites was Hughes’ Syncom 2, launched on July 26 1963.

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.

Geostationary orbits

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), Alcatel Space and EADS Astrium.

Low-Earth-orbiting satellites

A low Earth orbit typically is a circular orbit about 150 kilometres 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 kilometres 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.

Low earth orbiting satellites are less expensive to position in space than 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 satellites and their cost. In addition, there are important differences in the onboard and ground equipment needed to support the two types of missions.

A group of satellites working in concert thus is known as a satellite constellation. 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 backing from Microsoft entrepreneur Paul Allen, was to have as many as 720 satellites.

It is also possible to offer discontinuous coverage using a low Earth orbit satellite capable of storing data received while passing over one part of Earth and transmitting it later while passing over another part. This will be the case with the CASCADE system of Canada’s CASSIOPE communications satellite.

Molniya satellites

As mentioned, geostationary satellites are constrained to operate above the equator. As a consequence, they are not always suitable for providing services at high latitudes: for at high latitudes a geostationary satellite may appear low on (or even below) the horizon, affecting connectivity and causing multipathing (interference caused by signals reflecting off the ground into the ground antenna). The first satellite of Molniya series was launched 04.23.1965 and was used for experimental transmission of TV signal from Moscow up-link station to down-link stations, located in Russian Far East, in Khabarovsk and Vladivostok. In 1967 Soviet engineers created an unique system of national TV network called Orbita that was based on Molniya satellites.

Molniya orbits can be an appealing alternative in such cases. The Molniya orbit is highly inclined, guaranteeing good elevation over selected positions during the northern portion of the orbit. (Elevation is the extent of the satellite’s position above the horizon. Thus a satellite at the horizon has zero elevation 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 moves only slightly. Its period is one half day, so that the satellite is available for operation over the targetted region for eight hours every second revolution. 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 lower latitudes) since cars travelling through urban areas need access to satellites at high elevation in order to secure good connectivity, e.g. in the presence of tall buildings.


Applications

Telephony

File:Astra 2A 2C Boeing 601.JPG
A BSS 601 model, owned by SES Astra, used for DTH television broadcasting in Europe

The first and still, arguably, most important application for communication satellites is in international telephony. Fixed-point telephones relay calls to an earth station, where they are then transmitted to a geostationary satellite. An analogous path is then followed on the downlink. In contrast, mobile telephones (to and from ships and airplanes) must be directly connected to equipment to uplink the signal to the satellite, as well as being able to ensure satellite pointing in the presence of disturbances, such as waves onboard a ship.

Hand held telephony (cellular phones) used in urban areas do not make use of satellite communications. Instead they have access to a ground based constellation of receiving and retransmitting stations.

Television and Radio

There are two types of satellites used for television and radio: 

   * Direct Broadcast Satellite (DBS), and
   * Fixed Service Satellite (FSS).

A direct broadcast satellite is a communications satellite that transmits to small DBS satellite dishes (usually 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-To-Home) satellite tv services, such as DirecTV and Dish Network in the United States, ExpressVu in Canada, and Sky Digital in the UK.

Fixed Service Satellites use the C band, and the lower portions of the Ku bands. They are normally used for broadcast feeds to and from television networks and local affiliate stations (such as program feeds for network 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 satellites 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 originally 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 used in its Ku band form for the now-defunct Primestar satellite TV service.

This all changed when the first American DBS provider, DirecTV, was established in 1994, stealing the limelight from FSS satellite technology for DTH programming (due to DirecTV's smaller 18"-diameter dishes and lower cost of equipment). However, FSS satellites on the C and Ku bands still are used by cable & satellite channels such as CNN, The Weather Channel, HBO, Starz, and others, for distribution to cable tv headends (as mentioned earlier), and to the DBS providers themselves such as DirecTV and Dish Network who then re-distribute these channels over their own DBS systems.

The fact that these channels still exist on FSS satellites (more so for reception and re-distribution by cable tv and DBS systems, instead of for DTH viewers) makes TVRO systems for DTH viewing a still-viable option for satellite tv, often being a much-cheaper alternative to DBS, as far as monthly subscription fees are concerned. TVRO-oriented programming packages sold by companies such as National Programming Services[1], Bigdish.com[2], and Skyvision[3], are often quite a bit cheaper than their DBS equivalents. Motorola still makes digital 4DTV receivers for DTH TVRO use, and analog TVRO receivers are still available.

However, the hardware for a brand-new TVRO system (dish & receiver, along with a VideoCipher or DigiCipher descrambler, or an integrated receiver/decoder or IRD instead of a separate receiver & descrambler/decoder) nowadays costs quite a bit more than a DBS system (about $1500-2000 USD, including installation). But most older used TVRO systems can be had almost for free, due to most people converting over to DBS systems over the years. Unlike DBS, big-dish TVRO satellite TV also provides a plethora of unscrambled & unencrypted channels such as Classic Arts Showcase, and feeds of syndicated TV shows for reception by local TV stations.

Free-to-air satellite TV channels are also usually distributed on FSS satellites in the Ku band. The Intelsat America 5 and AMC 3 satellites over North America provide a quite large amount of FTA channels on their Ku-band transponders.

See broadcast satellites for further information on FSS and DBS satellites in orbit.

Initially available for broadcast to stationary TV receivers, by 2004 popular mobile direct broadcast applications 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, and are popular with recreational vehicle owners. Such mobile DBS antennas are also used by JetBlue Airlines for DirecTV, which passengers can view on-board on LCD screens mounted in the seats.

In the near future, satellites for communication will be launched that have transponders in the Ka band. NASA has launched experimental satellites utilizing the Ka band recently.

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 spaceborne repeaters, and are generally accessed by amateurs equipped with UHF or VHF radio equipment and highly directional antennas such as Yagis or dish antennas. Due to the limitations of ground-based amateur equipment, most amateur satellites are launched into fairly low Earth orbits, and are designed to deal with only a limited number of brief contacts at any given time. Some satellites also provide data-forwarding services using the AX.25 or similar protocols.

See also

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