Sep 18, 2019 By Team YoungWonks *
How does satellite communication work? Given extensively satellites are used today for communication - among other - purposes, this is an increasingly relevant question. Which brings us to the topic of our latest blog. But before we look at how satellites help us communicate across huge distances, let us start at the beginning.
What is a Satellite?
A satellite is basically any object that revolves (or in other words, orbits) around another object in space. Some satellites are natural, while others are artificial (man-made). The moon is an example of a natural satellite that orbits the earth. In the solar system, there are six planetary satellite systems containing 185 known natural satellites.
Today, the term satellite typically refers to artificial objects flown in space. Much like their natural counterparts, these orbit a planet and the key difference is that they have been intentionally placed into orbit.
Sputnik 1 is the world’s first artificial satellite. It was launched into space on 4 October 1957 by the Soviet Union. Since then, about 8,900 satellites from more than 40 countries have been launched.
It is important to note here that satellites are used for several purposes. For instance, they can be used to make star maps and maps of planetary surfaces, and also take pictures of planets they are launched into. Common types include military and civilian earth observation satellites, communications satellites, navigation satellites, weather satellites, and space telescopes. Satellites are usually semi-independent computer-controlled systems. Satellite subsystems carry out many tasks, such as power generation, thermal control, telemetry, attitude.
In fact, space stations and human spacecraft in orbit are also satellites. Satellite orbits differ greatly, depending on the purpose of the satellite, and are classified in a number of ways. Well-known (overlapping) classes include low earth orbit, polar orbit, and geostationary orbit.
How are these satellites launched into space? This is done with a launch vehicle, basically a rocket that places the satellite into orbit. More often than not, the rocket lifts off from a launch pad on land but there are some that have been launched at sea from a submarine or a mobile maritime platform, or aboard a plane.
In this blog, we shall take a close look at communications satellites. They are called so because they are used for communication purposes.
What is a Communications Satellite?
A communications satellite is an artificial satellite that relays and amplifies radio telecommunications signals through a transponder. It basically creates a communication channel between a source transmitter and a receiver at different locations on earth. Communications satellites are used for television, telephone, radio, internet, and military applications. There are currently 2,134 communications satellites in the earth’s orbit and these comprise both private and government organizations. Several are in geostationary orbit 22,236 miles (35,785 km) above the equator, so that the satellite appears stationary at the same point in the sky. The orbital period of these satellites is the same as the rotation rate of the Earth, which in turn allows the satellite dish antennas of ground stations to be aimed permanently at that spot; they do not have to move along and track it. Since the high frequency radio waves used for telecommunications links travel by line of sight, they get obstructed by the curve of the earth. What these communications satellites do is they relay the signal around the curve of the earth thus making possible communication between widely removed geographical points. Communications satellites use a wide range of radio and microwave frequencies. To avoid signal interference, international organizations have regulations stating which frequency ranges (or bands) certain organizations are permitted to use. This allocation of bands reduces the chances of signal interference.
Satellites can be classified as per their orbits. As mentioned earlier, many are geostationary satellites, as they have a Geostationary Orbit (GEO), which is 22,236 miles (35,785 km) from the earth’s surface. Here the satellite appears to be in the same position in the sky when viewed by ground observers. So here ground antennas do not have to track the satellite across the sky.
Medium Earth Orbit (MEO) satellites are the ones that are closer to the earth; their orbital altitudes vary from 2,000 to 36,000 kilometres (1,200 to 22,400 mi) above the earth. The region below medium orbits is around 160 to 2,000 kilometres (99 to 1,243 mi) above the earth and is called Low Earth Orbit (LEO).
With MEO and LEO satellites orbiting the earth faster, they are not continually visible in the sky at a fixed point on the earth. Instead, they appear to cross the sky and “set” when they go behind the earth. This means that offering continuous communications services with these lower orbit satellites would need a bigger number of satellites, thus ensuring that at least one of them is always in the sky to facilitate transmission of communication signals. But it is also important to note that due to their relatively shorter distance to the earth, their signals are much stronger.
A group of satellites working together is called a satellite constellation. Two such constellations, that are supposed to offer satellite phone services (mainly to remote areas), are the Iridium and Globalstar systems. The Iridium system has 66 satellites. It is also possible today to provide discontinuous coverage using a low-earth-orbit satellite that can store data received while passing over one part of earth and transmitting it later while passing over another part. The CASCADE system being used by Canada’s CASSIOPE communications satellite is an apt example.
Rise of Communication Satellites
Sputnik 1 was the world’s first artificial earth satellite; it was placed into orbit by the Soviet Union on October 4, 1957. At the time, it was fitted with an on-board radio-transmitter that worked on two frequencies: 20.005 and 40.002 MHz. Sputnik 1 was launched as a major step in the exploration of space and rocket development.
That said, it was not placed in orbit to send data from one point on earth to another. The first satellite to relay communications was in fact Pioneer 1, an intended lunar probe. The spacecraft made it halfway to the moon, and flew high enough to carry out the proof of concept relay of telemetry across the world: first from Cape Canaveral to Manchester, England; then from Hawaii to Cape Canaveral; and finally, across the world from Hawaii to Manchester.
Applications/ Uses of Communications Satellites
1. Satellite phones:
They are the first and historically most important use of communications satellites. The fixed Public Switched Telephone Network carries telephone calls from landline phones to an earth station, from where they are transmitted to a geostationary satellite. The downlink follows an analogous path. With significant improvements in submarine communications cables through the use of fiber-optics, satellites are no longer being used for fixed telephony on the same scale. But that doesn’t mean that satellites are no longer used for communication. Remote places such as Ascension Island, Saint Helena, Diego Garcia, and Easter Island have no submarine cables in service, so those areas need satellite telephones. Satellite communication is also needed in continents and countries where landline telecommunications are rare to nonexistent - say, in Antarctica, Greenland large regions of South America, Africa, Canada, China, Russia, and Australia.
Other land use for satellite phones include ships at sea, rigs at sea, back up for hospitals, military and recreation. Typically, satellite phone systems function through a local telephone system in an isolated area with a link to the telephone system in a main land area. There are also services that will send a radio signal to a telephone system. In this example, almost any type of satellite can be used. Satellite phones reach out directly to a constellation of either geostationary or low-earth-orbit satellites. Calls are then forwarded to a satellite teleport connected to the Public Switched Telephone Network.
2. Satellite Television:
Satellite television is when television programming is delivered to viewers by relaying it from a communications satellite orbiting the earth directly to the viewer’s location. The signals are received through an outdoor parabolic antenna called a satellite dish and a low-noise block downconverter. A satellite receiver - either an external set-top box, or a built-in television tuner - decodes the desired television programme for viewing on a television set. Satellite television offers a wide range of channels and services. It is the only television available in many remote areas that do not have terrestrial television or cable television service. Modern systems signals are passed on from a communications satellite on the Ku band frequencies (12–18 GHz) that need only a small dish less than a meter in diameter.
Also unlike early systems that used analog signals, modern ones use digital signals which allow transmission of the modern television standard high-definition television, thanks to the much improved spectral efficiency of digital broadcasting. As of 2018, the only channels relying on satellite broadcasting in analog signals are Brazil’s Star One C2 and American channel C-SPAN on AMC-11. Different receivers are required for the two types. Some transmissions and channels are unencrypted and thus free-to-air or free-to-view. Other channels are transmitted with encryption (pay television), needing the viewer to subscribe and pay a monthly fee to receive the programming. Satellite TV consumption now has a lot less takers due to the cord-cutting trend where people are preferring to watch internet based streaming television.
3. Satellite Radio:
A satellite radio or subscription radio (SR) is basically a digital radio signal that is relayed by a communications satellite and this typically covers a wider geographical range than terrestrial radio signals. Satellite radio provides audio broadcast services in some countries, among them is the US. Mobile services, like SiriusXM, and Worldspace, let listeners travel across the continent and tune in to the same audio programming anywhere. Services, such as Music Choice or Muzak’s satellite-delivered content, need a fixed-location receiver and a dish antenna. In all instances, the antenna should have a clear view to the satellites. In places that have tall buildings, bridges, or even parking garages obscuring the signal, repeaters can be used to make the signal available to listeners.
4. Amatuer Radio Satellite:
Amateur radio operators make use of amateur satellites, that have been created specifically for amateur radio traffic. Most of these satellites function as spaceborne repeaters, and are generally used by amateurs equipped with UHF or VHF radio equipment and highly directional antennas like Yagis or dish antennas. Due to launch costs, most amateur satellites are launched into low earth orbits, and are designed to deal with only a few brief contacts at a given time.
5. Satellite Internet:
Satellite Internet access refers to Internet access made possible through communications satellites. Today, consumer grade satellite Internet service is typically offered to people through geostationary satellites that can provide relatively high data speeds, especially thanks to newer satellites using Ku band to achieve downstream data speeds up to 506 Mbit/s. After the 1990s, satellite communication technology has been used as a means to connect to the Internet using broadband data connections. This is particularly useful for people in remote areas who cannot avail a broadband connection.
6. Satellites used for Military Purposes:
Communications satellites are also used for military communications applications, such as Global Command and Control Systems. Military systems that use communication satellites are the MILSTAR, the DSCS, and the FLTSATCOM of the United States, NATO satellites, UK satellites (for example, Skynet), and satellites of the former Soviet Union. India too has a first military communication satellite GSAT-7, its transponders operate in UHF, F, C and Ku band bands. Military satellites usually operate in the UHF, SHF or EHF (also known as Ka band) frequency bands.
Satellite Communication Today
Since the launch of the first satellite Sputnik 1, around 8,900 satellites from more than 40 countries have been launched. According to a 2018 estimate, 5,000 are in orbit. Out of these 5,000, 63% of operational satellites are in low-earth orbit, 6% are in medium-earth orbit (at 20,000 km), 29% are in geostationary orbit (at 36,000 km) and the remaining 2% are in elliptic orbit. Some large space stations have in fact been launched in parts and assembled in orbit. It is important to note that out of the 5,000 satellites in orbit, only 1,900 were operational in 2018, while the rest have now become space debris.
Indeed, the space pollution caused by this space debris is a big problem today. So what is space debris? Space debris refers to the natural debris found in the solar system, so it includes asteroids, comets, and meteoroids. But it is no longer limited just to these bodies. Since the 1979 beginning of the NASA Orbital Debris Program, the term also refers to space waste or space garbage that is generated from the mass of defunct, artificially created objects in space, especially in the earth orbit. These include old satellites and spent rocket stages and the fragments from their disintegration and collisions. As of December 2016, five satellite collisions have generated space debris. Space debris is also known as space trash, space litter, space junk or orbital debris.
Today, several steps are being taken to deal with such debris. The U.S. has a set of standard practices for civilian (NASA) and military (DoD and USAF) orbital-debris mitigation, much like the European Space Agency. In 2007, the International Organization for Standardization (ISO) began preparing an international standard for space-debris mitigation. Germany and France have posted bonds to safeguard property from debris damage.
Another approach to debris mitigation is designing the mission architecture so as to always leave the rocket second-stage in an elliptical geocentric orbit with a low-perigee, thus ensuring rapid orbital decay and avoiding long-term orbital debris from spent rocket bodies. External removal of space debris has not seen many takers primarily because it has been found to be not cost effective.
April 2018 saw the launch of the RemoveDEBRIS mission. This mission aims to test the efficiency of several Active Debris Removal (ADR) technologies on mock targets in low earth orbit. It will do so by carrying out several planned experiments and the platform is accordingly fitted with a net, a harpoon, a laser ranging instrument, a dragsail, and two CubeSats (miniature research satellites). RemoveDEBRIS was launched aboard the SpaceX Dragon refill spacecraft as part of the CRS-14 mission and it arrived at the International Space Station (ISS) on April 4, 2018.
*Contributors: Written by Vidya Prabhu; Lead image by: Leonel Cruz