Communications satellite

A communications satellite is an artificial satellite that helps send and boost radio signals. It creates a link between a sender and a receiver far apart on Earth. These satellites are important for many things we use every day, like television, telephone calls, radio, the internet, and even the military.

Some of these satellites are placed in a special path called geostationary orbit, very high above the equator. From there, they look like they stay in one place in the sky. This means satellite dishes on the ground can always point at the same spot without moving. But many satellites are in groups called satellite constellations that fly much closer to Earth in something called low Earth orbit. The dishes on the ground have to move to follow these satellites.

Radio waves, which carry our messages, can only travel in straight lines. Because the Earth is round, they can’t go around it. That’s why we need satellites—to bounce the signals around the Earth so people far apart can talk and share information. These satellites work on many different radio and microwave frequencies. Special groups make rules about which frequencies different users can use, so the signals don’t get mixed up.

History

Origins

In October 1945, Arthur C. Clarke published an article called "Extraterrestrial Relays" in a British magazine. The article explained how we could use satellites in special orbits to send radio signals around the world. Because of this, Arthur C. Clarke is often called the inventor of the idea of communications satellites. The special orbit he described is sometimes called the 'Clarke Belt'.

The first artificial Earth satellite was Sputnik 1, launched by the Soviet Union on 4 October 1957. It was built by Mikhail Tikhonravov and Sergey Korolev, and it carried a radio transmitter. Although Sputnik 1 wasn't meant to send messages from one place on Earth to another, its launch was an important step in space exploration and rocket development, marking the start of the Space Age.

Early active and passive satellite experiments

Communications satellites can be passive or active. Passive satellites only reflect signals from the source to the receiver, without making them stronger. Because these satellites are far from Earth, the signals they reflect are very weak. Active satellites make the signals stronger before sending them back to Earth. Passive satellites were the first type used, but active satellites are more common today.

In 1951, work began on a project called Communication Moon Relay, which used the Moon to send messages between Washington, D.C., and Hawaii. This was the longest communications circuit ever created at the time.

The first satellite built specifically to send messages was Project SCORE, launched on 18 December 1958. It carried a recorded message from U.S. President Dwight D. Eisenhower and could also send and receive live messages. It worked for eight hours before its batteries ran out.

Another early project was Courier 1B, launched on 4 October 1960. It was meant to test if we could create a global military communications network using satellites that store and send messages on command. It stopped working after 17 days.

NASA launched Echo 1 on 12 August 1960. This was a balloon satellite that reflected microwave signals between points on Earth, testing if we could send telephone, radio, and television signals worldwide.

More firsts and further experiments

Telstar was the first active satellite to send television signals across the Atlantic Ocean. It was launched on 10 July 1962 from Cape Canaveral and was a joint project between several companies and governments.

Project West Ford, led by Massachusetts Institute of Technology, tried to create a ring of tiny copper needles in space to reflect signals. Although not all the needles separated correctly, the project successfully tested communications using certain radio frequencies.

Syncom 2, launched on 26 July 1963, was the first satellite to orbit Earth once per day at the same speed, but it moved north and south, so special equipment was needed to follow it. Its successor, Syncom 3, launched on 19 July 1964, was the first satellite to stay in one place in the sky, making it easy for ground antennas to point at it constantly.

International commercial satellite projects

In 1962, the United States created the Communications Satellite Corporation (COMSAT). This led to the Intelsat Agreements, which resulted in the launch of Intelsat 1, or Early Bird, on 6 April 1965. It was the first commercial communications satellite in a special orbit, providing services to ships at sea and helping create a global network.

The Soviet Union launched its first communications satellite on 23 April 1965 as part of the Molniya program. This program used a special orbit that spent more time over Russia and Canada.

In the 2020s, new networks of satellites in low orbits made it easier and cheaper to provide internet services, reducing the need for new satellites in the higher geostationary orbits.

Satellite orbits

Communications satellites usually have one of three main types of orbit. MEO and LEO are non-geostationary orbits (NGSO).

• Geostationary satellites have a geostationary orbit (GEO), which is 22,236 miles (35,785 km) from Earth's surface. This orbit makes the satellite appear to "stand still" in the sky, so ground antennas can point permanently at that spot.
• Medium Earth orbit (MEO) satellites are closer to Earth, with altitudes ranging from 2,000 to 36,000 kilometres (1,200 to 22,400 mi) above Earth.
• Low Earth orbit (LEO) is about 160 to 2,000 kilometres (99 to 1,243 mi) above Earth.

Satellites in MEO and LEO orbit Earth faster, so they appear to move across the sky and "set" when they go behind Earth. To provide continuous communications, many satellites are needed so that one is always visible for transmission. However, because they are closer to Earth, LEO and MEO satellites can communicate with less delay and lower power than geostationary satellites.

Low Earth orbit (LEO)

Main article: Low Earth orbit

  Low Earth orbit

A low Earth orbit (LEO) is a circular orbit about 160 to 2,000 kilometres (99 to 1,243 mi) above Earth, taking about 90 minutes to revolve around Earth.

Because of their low altitude, these satellites are only visible from within about 1,000 kilometres (620 mi) from the point directly below them. They move quickly relative to the ground, so many satellites are needed for uninterrupted connectivity.

LEO satellites are less expensive to launch and, due to their proximity, do not require as strong a signal as geostationary satellites.

Satellite constellation

Main article: Satellite constellation

A group of satellites working together is known as a satellite constellation. Two such constellations, the Iridium and Globalstar systems, provide satellite phone and data services, mainly to remote areas. The Iridium system has 66 satellites. Starlink is a satellite internet constellation operated by SpaceX, aiming for global satellite Internet access coverage.

It is also possible to offer discontinuous coverage using a low-Earth-orbit satellite that stores data while passing over one part of Earth and transmits it later while passing over another part.

Medium Earth orbit (MEO)

Main article: Medium Earth orbit

A medium Earth orbit is a satellite orbit between 2,000 and 35,786 kilometres (1,243 and 22,236 mi) above Earth. MEO satellites are similar to LEO satellites in function. They are visible for longer periods (2 to 8 hours) and cover a larger area than LEO satellites, so fewer satellites are needed. However, MEO satellites have a longer delay and weaker signal than LEO satellites.

Like LEO satellites, MEO satellites do not stay fixed over one spot, unlike geostationary satellites which are always 35,786 kilometres (22,236 mi) from Earth.

Typically, a MEO satellite orbits about 16,000 kilometres (10,000 mi) above Earth, completing a trip around Earth in 2 to 8 hours.

Examples of MEO

• In 1962, the communications satellite Telstar was launched. It was a medium Earth orbit satellite designed to help telephone signals. However, because its orbit did not match Earth's rotation, continuous coverage was impossible without multiple MEO satellites.
• In 2013, the first four of a constellation of 20 MEO satellites were launched. The O3b satellites provide broadband internet services, especially to remote locations and maritime and in-flight use, orbiting at 8,063 kilometres (5,010 mi).

Geostationary orbit (GEO)

Main article: Geostationary orbit

To an observer on Earth, a satellite in a geostationary orbit appears motionless in a fixed position in the sky because it revolves around Earth at the same rate as Earth's rotation.

A geostationary orbit is useful for communications because ground antennas can point at the satellite without tracking its motion. This is cost-effective.

In applications requiring many ground antennas, such as DirecTV distribution, the savings in ground equipment can outweigh the cost of placing a satellite in orbit.

Examples of GEO

• The first geostationary satellite was Syncom 3, launched on 19 August 1964, used for communication across the Pacific, including television coverage of the 1964 Summer Olympics. Shortly after, Intelsat I, aka Early Bird, was launched on 6 April 1965 and placed in orbit at 28° west longitude, the first geostationary satellite for telecommunications over the Atlantic Ocean.
• On 9 November 1972, Canada's first geostationary satellite, Anik A1, was launched by Telesat Canada, with the United States following with the launch of Westar 1 by Western Union on 13 April 1974.
• On 30 May 1974, the first geostationary communications satellite in the world to be three-axis stabilized was launched: the experimental satellite ATS-6 built for NASA.
• After Telstar through Westar 1, RCA Americom (later GE Americom, now SES) launched Satcom 1 in 1975. It helped early cable TV channels such as WTBS (now TBS), HBO, CBN (now Freeform) and The Weather Channel become successful by distributing programming to local cable TV headends. It was also the first satellite used by U.S. broadcast networks like ABC, NBC, and CBS to distribute programming to local stations. Satcom 1 was widely used because it had twice the communications capacity of Westar 1 (24 transponders as opposed to Westar 1's 12), resulting in lower costs.

By 2000, Hughes Space and Communications (now Boeing Satellite Development Center) had built nearly 40 percent of the more than one hundred satellites in service worldwide. Other major manufacturers include Space Systems/Loral, Orbital Sciences Corporation with the Star Bus series, Indian Space Research Organisation, Lockheed Martin, Northrop Grumman, Alcatel Space, now Thales Alenia Space, with the Spacebus series, and Astrium.

Molniya orbit

Main article: Molniya orbit

Geostationary satellites must operate above the equator, so they appear lower on the horizon for viewers farther from the equator, causing connectivity issues and multipath interference.

For areas near the North (and South) Pole, geostationary satellites may appear below the horizon. Molniya orbit satellites, mainly launched by Russia, solve this problem.

The Molniya orbit is highly inclined, ensuring good coverage over northern latitudes. The satellite spends most of its time over far northern regions, moving only slightly. With a period of half a day, a Molniya satellite is available for six to nine hours every second revolution. A constellation of three Molniya satellites (plus spares) can provide uninterrupted coverage.

The first Molniya satellite was launched on 23 April 1965 for experimental transmission of TV signals from a Moscow uplink station to downlink stations in Siberia and the Russian Far East, including Norilsk, Khabarovsk, Magadan, and Vladivostok. In November 1967, Soviet engineers created a national TV network called Orbita, based on Molniya satellites.

Polar orbit

Main article: Polar orbit

In the United States, the National Polar-orbiting Operational Environmental Satellite System (NPOESS) was established in 1994 to manage polar satellite operations for NASA and NOAA.

These orbits are Sun synchronous, crossing the equator at the same local time each day, such as 1:30 P.M., 5:30 P.M., and 9:30 P.M.

Beyond geostationary orbit

There are plans to place dedicated communications satellites beyond geostationary orbits. NASA proposed LunaNet as a data network for a "Lunar Internet" for spacecraft near the Moon. The Moonlight Initiative is an ESA project for navigational services on the lunar surface. Both are satellite constellations in various orbits around the Moon.

Other orbits are planned, such as positions in the Earth-Moon-Libration points for Moon coverage, and orbits around Mars to support missions on the surface.

Structure

Communications satellites have several important parts that work together. They include special equipment for sending and receiving signals, engines to move the satellite to the right place, systems to keep the satellite steady, and power sources like solar cells to keep everything running.

These satellites also have a way to stay in touch with control stations on Earth, which watch over the satellite and help manage its work. The amount of space for sending signals depends on how many special channels, called transponders, the satellite has. Different services like TV, phone calls, internet, and radio need different amounts of space for their signals.

Frequency allocation for satellite systems

Allocating frequencies for satellite services is a detailed process that needs planning and teamwork across the world. This work is guided by the International Telecommunication Union. To help organize these frequencies, the world is split into three areas:

• Region 1: Europe, Africa, the Middle East, parts of past Soviet Union countries, and Mongolia
• Region 2: North and South America and Greenland
• Region 3: Most of Asia, Australia, and the southwest Pacific

In these areas, different frequency bands are assigned to various satellite services. Some of these services include:

• Fixed satellite service (FSS)
• Broadcasting satellite service (BSS)
• Mobile-satellite service
• Radionavigation-satellite service
• Meteorological-satellite service

Applications

Telephony

Main article: Satellite phone

The first big use of communication satellites was for long-distance phone calls. These satellites help connect phone calls from one place on Earth to another. Before satellites, phone calls traveled through wires, but now satellites can help places far apart talk to each other. This is very useful for places like islands or remote areas that don't have wires connecting them.

Television

Main article: Satellite television

Satellites are also used to send television signals. This lets many people watch the same shows at the same time. There are two main types of satellites used for TV: one type sends signals directly to small dishes on homes, and another type is used to send signals to bigger stations that then share the TV channels.

Radio broadcasting

Main article: Satellite radio

Satellites can also send radio signals. This lets people listen to the same radio stations no matter where they are, even when they are far from regular radio towers.

Amateur radio

Main article: Amateur radio satellite

People who enjoy amateur radio can also use satellites to talk to each other. These special satellites help amateur radio operators send messages even when they are far apart.

Internet access

Main article: Satellite Internet access

Satellites can help people get online, especially in places where normal internet wires are hard to reach. This helps people in remote areas stay connected to the world.

Military

Main article: Military communications satellite

Further information: X Band Satellite Communication

Satellites are also used by armies and governments to send important messages around the world. These special satellites help keep everyone safe and connected, no matter where they are.