Satellite
What Is a Satellite?
A satellite is anything that orbits a planet or a star. Earth is a satellite orbiting the Sun. The Moon is a satellite orbiting Earth. When you launch a spacecraft into orbit around Earth, that’s a satellite, too. This kind of satellite can help us learn about Earth and the universe.
But, did you know that our Earth and Moon are satellites, too? A satellite can be any object that orbits a planet, star, or moon. An orbit is a regular, repeating path that one object in space takes around another one. So, Earth is a satellite, because it orbits the Sun! The Moon is also a satellite because it orbits Earth.
But, usually, the word "satellite" refers to a machine that is launched into space and moves around Earth or another body in space.
Earth and the Moon are examples of natural satellites. Thousands of artificial, or man-made, satellites orbit Earth. Some take pictures of the planet that help meteorologists predict weather and track hurricanes. Some take pictures of other planets, the Sun, black holes, dark matter or faraway galaxies. These pictures help scientists better understand the solar system and universe.
Other satellites are used for communications, such as TV signals and phone calls around the world. Have you ever tried to find your house, or the closest ice cream shop, on a map on your phone? Satellites help us do this with GPS! A group of more than 20 satellites make up the Global Positioning System, or GPS. These satellites can help figure out your exact location.
A satellite is a body that orbits around another body in space. There are two different types of satellites – natural and man-made. Examples of natural satellites are the Earth and Moon. The Earth rotates around the Sun and the Moon rotates around the Earth. A man-made satellite is a machine that is launched into space and orbits around a body in space. Examples of man-made satellites include the Hubble Space Telescope and the International Space Station.
Man-made satellites come in many shapes and size and have different pieces of instruments on them to perform different functions while in space. Satellites are built by engineers and take months sometimes even years to build. The satellites have to endure many tests to make sure the satellite can withstand the launch and the harsh environment of space.
NASA establishes missions for a specific purpose and the engineers develop a satellite to perform the necessary functions for that mission. Once the satellite is launched into space, Space Communications and Navigation (SCaN) provides the channel of communications for the data to go to and from the Earth and the satellite. These communications include commands to the spacecraft as well as the scientific data coming to Earth.
SCaN supports over 100 satellites including:
- Satellites that observe Earth: Aqua and Aura
- Satellites that observe the Sun and see the effect of solar winds on the Earth: Parker Solar Probe, Solar Dynamics Observer (SDO) and Solar Terrestrial Relations Observatory (STEREO)
- Satellites that observe the Moon and the planets: Lunar Reconnaissance Orbiter (LRO) and the Mars Reconnaissance Orbiter (MRO)
- Satellites that look at the origins of the universe: Hubble Space Telescope
› SCaN Missions – complete list of missions supported by SCaN
How do Satellites Communicate?
Satellites communicate by using radio waves to send signals to the antennas on the Earth. The antennas then capture those signals and process the information coming from those signals. Information can include:
- scientific data (like the pictures the satellite took),
- the health of the satellite, and
- where the satellite is currently located in space.
Catherine G. Manning NASA What-is-a-satellite/
IN THIS ARTICLE ........
How does NASA use satellites today?
Brief history of artificial satellites
Catalog of Earth Satellite Orbits
The view that satellites like GPS have allows them to see large areas of Earth at one time. This means satellites can collect more data, more quickly, than instruments on the ground.
Satellites also can see into space better than telescopes at Earth's surface! That's because satellites fly above the clouds, dust, and molecules in the atmosphere that can block the view from the ground.
What are the parts of an artificial satellite?
Man-made satellites come in many shapes and sizes. But most have at least two parts in common - an antenna and a power source. The antenna sends and receives information, usually to and from Earth. Just like a toy that requires batteries to work here on Earth, satellites need power, too! There are several types of power sources for satellites, such as solar panels or batteries. Solar panels are cool because they power the satellite by turning sunlight into electricity.
Many NASA satellites carry cameras and scientific sensors. Sometimes, these instruments point toward Earth to gather information about its land, air and water. Other times, they face toward space to collect data from the solar system and universe.
How do satellites orbit Earth?
Most satellites are launched into space on rockets. A satellite orbits Earth when its speed is balanced by the pull of Earth's gravity. Without this balance, the satellite would fly in a straight line off into space or fall back to Earth.
Satellites orbit Earth at different heights, different speeds and along different paths. The two most common types of orbit are "geostationary" (jee-oh-STAY-shun-air-ee) and "polar."
A geostationary satellite travels from west to east over the equator. It moves in the same direction and at the same rate Earth is spinning. From Earth, a geostationary satellite looks like it is standing still since it is always above the same location.
From Earth, a geostationary satellite looks like it is always in the same place, because it moves in the same direction and at the same rate the Earth spins.
Polar-orbiting satellites travel in a north-south direction from pole to pole. As Earth spins underneath, these satellites can scan the entire globe, one strip at a time.
Why don't satellites crash into each other?
Actually, they can! In February 2009, two communications satellites – one American and one Russian – crashed in space! This, however, is believed to be the first time two man-made satellites have collided accidentally.
NASA and other organizations across the world keep track of satellites in space. Collisions usually don’t happen because when a satellite is launched, it is placed into an orbit designed to avoid other satellites. But orbits can change over time, and the chances of a crash increase as more and more satellites are launched into space.
What is the history of NASA satellites?
NASA has launched dozens of satellites into space, starting with the Explorer 1 satellite in 1958. Explorer 1 was America's first man-made satellite. The main instrument aboard was a sensor that measured high-energy particles in space called cosmic rays.
The first satellite picture of Earth came from NASA's Explorer 6 in 1959. TIROS-1 followed in 1960 with the first TV picture of Earth from space. These pictures did not show much detail. But they did show the potential satellites had to change how people view Earth and space.
How does NASA use satellites today?
NASA satellites help scientists study Earth, the other worlds of our solar system, and beyond.
Satellites looking toward Earth provide information about clouds, oceans, land and ice. They measure gases in the atmosphere, such as carbon dioxide. NASA's Orbiting Carbon Observatory 2, or OCO-2, launched in 2014 to measure carbon dioxide levels on Earth to better observe Earth's carbon cycle. NASA's OCO-2 also helps explore how measurements from space can predict future CO2 increases and its impact on Earth's climate.
Satellites also measure the amount of energy that Earth keeps inside the atmosphere, and the amount of energy the Earth sends back into space. And satellites monitor wildfires and volcanoes and their smoke.
An example is OCO-2, a satellite that studies carbon dioxide, taking carbon dioxide measurements on Earth all the way from space.
All this information helps scientists predict weather and climate. The information also helps public health officials track disease and famine; it helps farmers know what crops to plant; and it helps emergency workers respond to natural disasters. We know today’s weather thanks to satellites!
Satellites that face toward space have many jobs. Some watch for dangerous rays coming from the Sun. Others explore asteroids and comets, the history of stars, and the origin of planets. Some satellites fly near or orbit other planets. These spacecraft may look for evidence of water on Mars or capture close-up pictures of Saturn's rings.
The largest satellite in orbit around Earth is the International Space Station.
Article Sections:
• History of artificial satellites
• Parts of a satellite
• What stops satellites from falling?
• What stops satellites from crashing?
• Moons around other worlds
A satellite is an object in space that orbits or circles around a bigger object. There are two kinds of satellites: natural (such as the moon orbiting the Earth) or artificial (such as the International Space Station orbiting the Earth).
There are dozens upon dozens of natural satellites in the solar system, with almost every planet having at least one moon. Saturn, for example, has at least 53 natural satellites, and between 2004 and 2017, it also had an artificial one — the Cassini spacecraft, which explored the ringed planet and its moons.
Artificial satellites, however, did not become a reality until the mid-20th century. The first artificial satellite was Sputnik, a Russian beach-ball-size space probe that lifted off on Oct. 4, 1957. That act shocked much of the western world, as it was believed the Soviets did not have the capability to send satellites into space.
Brief history of artificial satellites
Following that feat, on Nov. 3, 1957 the Soviets launched an even more massive satellite — Sputnik 2 — which carried a dog, Laika. The United States' first satellite was Explorer 1 on Jan. 31, 1958. The satellite was only 2 percent the mass of Sputnik 2, however, at 30 pounds (13 kg).
The Sputniks and Explorer 1 became the opening shots in a space race between the United States and the Soviet Union that lasted until at least the late 1960s. The focus on satellites as political tools began to give way to people as both countries sent humans into space in 1961. Later in the decade, however, the aims of both countries began to split. While the United States went on to land people on the moon and create the space shuttle, the Soviet Union constructed the world's first space station, Salyut 1, which launched in 1971. (Other stations followed, such as the United States' Skylab and the Soviet Union's Mir.)
Other countries began to send their own satellites into space as the benefits rippled through society. Weather satellites improved forecasts, even for remote areas. Land-watching satellites such as the Landsat series (on its ninth generation now) tracked changes in forests, water and other parts of Earth's surface over time. Telecommunications satellites made long-distance telephone calls and eventually, live television broadcasts from across the world a normal part of life. Later generations helped with Internet connections.
With the miniaturization of computers and other hardware, it's now possible to send up much smaller satellites that can do science, telecommunications or other functions in orbit. It's common now for companies and universities to create "CubeSats", or cube-shaped satellites that frequently populate low-Earth orbit.Click here for more Space.com videos...
These can be lofted on a rocket along with a bigger payload, or sent from a mobile launcher on the International Space Station (ISS). NASA is now considering sending CubeSats to Mars or to the moon Europa (near Jupiter) for future missions, although the CubeSats aren't confirmed for inclusion.
The ISS is the biggest satellite in orbit, and took over a decade to construct. Piece by piece, 15 nations contributed financial and physical infrastructure to the orbiting complex, which was put together between 1998 and 2011. Program officials expect the ISS to keep running until at least 2024.
Parts of a satellite
Every usable artificial satellite — whether it's a human or robotic one — has four main parts to it: a power system (which could be solar or nuclear, for example), a way to control its attitude, an antenna to transmit and receive information, and a payload to collect information (such as a camera or particle detector).
As will be seen below, however, not all artificial satellites are necessarily workable ones. Even a screw or a bit of paint is considered an "artificial" satellite, even though these are missing these parts.
What keeps a satellite from falling to Earth?
A satellite is best understood as a projectile, or an object that has only one force acting on it — gravity. Technically speaking, anything that crosses the Karman Line at an altitude of 100 kilometers (62 miles) is considered in space. However, a satellite needs to be going fast — at least 8 km (5 miles) a second — to stop from falling back down to Earth immediately.
If a satellite is traveling fast enough, it will perpetually "fall" toward Earth, but the Earth's curvature means that the satellite will fall around our planet instead of crashing back on the surface. Satellites that travel closer to Earth are at risk of falling because the drag of atmospheric molecules will slow the satellites down. Those that orbit farther away from Earth have fewer molecules to contend with.
There are several accepted "zones" of orbits around the Earth. One is called low-Earth-orbit, which extends from about 160 to 2,000 km (about 100 to 1,250 miles). This is the zone where the ISS orbits and where the space shuttle used to do its work. In fact, all human missions except for the Apollo flights to the moon took place in this zone. Most satellites also work in this zone.
Geostationary or geosynchronous orbit is the best spot for communications satellites to use, however. This is a zone above Earth's equator at an altitude of 35,786 km (22,236 mi). At this altitude, the rate of "fall" around the Earth is about the same as Earth's rotation, which allows the satellite to stay above the same spot on Earth almost constantly. The satellite thus keeps a perpetual connection with a fixed antenna on the ground, allowing for reliable communications. When geostationary satellites reach the end of their life, protocol dictates they're moved out of the way for a new satellite to take their place. That's because there is only so much room, or so many "slots" in that orbit, to allow the satellites to operate without interference.
While some satellites are best used around the equator, others are better suited to more polar orbits — those that circle the Earth from pole to pole so that their coverage zones include the north and south poles. Examples of polar-orbiting satellites include weather satellites and reconnaissance satellites.
Three small CubeSats float above the Earth after deployment from the International Space Station.
What stops a satellite from crashing into another satellite?
There are an estimated half-million artificial objects in Earth orbit today, ranging in size from paint flecks up to full-fledged satellites — each traveling at speeds of thousands of miles an hour. Only a fraction of these satellites are useable, meaning that there is a lot of "space junk" floating around out there. With everything that is lobbed into orbit, the chance of a collision increases.
Space agencies have to consider orbital trajectories carefully when launching something into space. Agencies such as the United States Space Surveillance Network keep an eye on orbital debris from the ground, and alert NASA and other entities if an errant piece is in danger of hitting something vital. This means that from time to time, the ISS needs to perform evasive maneuvers to get out of the way.
Collisions still occur, however. One of the biggest culprits of space debris was the leftovers of a 2007 anti-satellite test performed by the Chinese, which generated debris that destroyed a Russian satellite in 2013. Also that year, the Iridium 33 and Cosmos 2251 satellites smashed into each other, generating a cloud of debris.
NASA, the European Space Agency and many other entities are considering measures to reduce the amount of orbital debris. Some suggest bringing down dead satellites in some way, perhaps using a net or air bursts to disturb the debris from its orbit and bring it closer to Earth. Others are thinking about refueling dead satellites for reuse, a technology that has been demonstrated robotically on the ISS.
Moons around other worlds
Most planets in our solar system have natural satellites, which we also call moons. For the inner planets: Mercury and Venus each have no moons. Earth has one relatively large moon, while Mars has two asteroid-sized small moons called Phobos and Deimos. (Phobos is slowly spiralling into Mars and will likely break apart or fall into the surface in a few thousand years.)
Beyond the asteroid belt, are four gas giant planets that each have a pantheon of moons. As of late 2018, Jupiter has 79 confirmed moons, Saturn has 53, Uranus has 27 and Neptune has 14. New moons are occasionally discovered – mainly by missions (either past or present, as we can analyze old pictures) or by performing fresh observations by telescope.
Saturn is a special example because it is surrounded by thousands of small objects that form a ring visible even in small telescopes from Earth. Scientists watching the rings close-up over 13 years, during the Cassini mission, saw conditions in which new moons might be born. Scientists were particularly interested in propellers, which are wakes in the rings created by fragments in the rings. Just after Cassini's mission ended in 2017, NASA said it's possible the propellers share elements of planet formation that takes place around young stars' gassy discs.
Even smaller objects have moons, however. Pluto is technically a dwarf planet. However, the people behind the New Horizons mission, which flew by Pluto in 2015, argue its diverse geography makes it more planet-like. One thing that isn't argued, however, is the number of moons around Pluto. Pluto has five known moons, most of which were discovered when New Horizons was in development or en route to the dwarf planet.
A lot of asteroids have moons, too. These small worlds sometimes fly close to the Earth, and the moons pop out in observations with radar. A few famous examples of asteroids with moons include 4 Vesta (which was visited by NASA's Dawn mission), 243 Ida, 433 Eros, and 951 Gaspra. There are also examples of asteroids with rings, such as 10199 Chariklo and 2060 Chiron.
Many planets and worlds in our solar system have human-made "moons" as well, particularly around Mars — where several probes orbit the planet doing observations of its surface and environment. The planets Mercury, Venus, Mars, Jupiter and Saturn all had artificial satellites observing them at some point in history. Other objects had artificial satellites as well, such as Comet 67P/Churyumov–Gerasimenko (visited by the European Space Agency's Rosetta mission) or Vesta and Ceres (both visited by NASA's Dawn mission.) Technically speaking, during the Apollo missions, humans flew in artificial "moons" (spacecraft) around our own moon between 1968 and 1972. NASA may even build a "Deep Space Gateway" space station near the moon in the coming decades, as a launching point for human Mars missions.
Fans of the movie "Avatar" (2009) will remember that the humans visited Pandora, the habitable moon of a gas giant named Polyphemus. We don't know yet if there are moons for exoplanets, but we suspect — given that the solar system planets have so many moons — that exoplanets have moons as well. In 2014, scientists made an observation of an object that could be interpreted as an exomoon circling an exoplanet, but the observation can't be repeated as it took place as the object moved in front of a star. However, a second exomoon might have been found very recently.
Bibliography
Joukowsky Institute, Brown University, "13 Things - space"
Amanda Barnett, NASA’s Jet Propulsion Laboratory for NASA’s Science Mission Directorate, "Basics of Space Flight - Section 1: Environment, Chapter 5: Planetary Orbits"
Astromaterials Research & Exploration Science, NASA, "The Orbital Debris Issue"
Elizabeth Howell Space.com Satellites
Catalog of Earth Satellite Orbits
Just as different seats in a theater provide different perspectives on a performance, different Earth orbits give satellites varying perspectives, each valuable for different reasons. Some seem to hover over a single spot, providing a constant view of one face of the Earth, while others circle the planet, zipping over many different places in a day.
There are essentially three types of Earth orbits: high Earth orbit, medium Earth orbit, and low Earth orbit. Many weather and some communications satellites tend to have a high Earth orbit, farthest away from the surface. Satellites that orbit in a medium (mid) Earth orbit include navigation and specialty satellites, designed to monitor a particular region. Most scientific satellites, including NASA’s Earth Observing System fleet, have a low Earth orbit.
The height of the orbit, or distance between the satellite and Earth’s surface, determines how quickly the satellite moves around the Earth. An Earth-orbiting satellite’s motion is mostly controlled by Earth’s gravity. As satellites get closer to Earth, the pull of gravity gets stronger, and the satellite moves more quickly. NASA’s Aqua satellite, for example, requires about 99 minutes to orbit the Earth at about 705 kilometers up, while a weather satellite about 36,000 kilometers from Earth’s surface takes 23 hours, 56 minutes, and 4 seconds to complete an orbit. At 384,403 kilometers from the center of the Earth, the Moon completes a single orbit in 28 days.
The higher a satellite’s orbit, the slower it moves. Certain orbital altitudes have special properties, like a geosynchronous orbit, in which a satellite travels around the Earth exactly once each day.
Changing a satellite’s height will also change its orbital speed. This introduces a strange paradox. If a satellite operator wants to increase the satellite’s orbital speed, he can’t simply fire the thrusters to accelerate the satellite. Doing so would boost the orbit (increase the altitude), which would slow the orbital speed. Instead, he must fire the thrusters in a direction opposite to the satellite’s forward motion, an action that on the ground would slow a moving vehicle. This change will push the satellite into a lower orbit, which will increase its forward velocity.
In addition to height, eccentricity and inclination also shape a satellite’s orbit. Eccentricity refers to the shape of the orbit. A satellite with a low eccentricity orbit moves in a near circle around the Earth. An eccentric orbit is elliptical, with the satellite’s distance from Earth changing depending on where it is in its orbit.
Inclination is the angle of the orbit in relation to Earth’s equator. A satellite that orbits directly above the equator has zero inclination. If a satellite orbits from the north pole (geographic, not magnetic) to the south pole, its inclination is 90 degrees.
Together, the satellite’s height, eccentricity, and inclination determine the satellite’s path and what view it will have of Earth.
Three Classes of Orbit
High Earth Orbit
When a satellite reaches exactly 42,164 kilometers from the center of the Earth (about 36,000 kilometers from Earth’s surface), it enters a sort of “sweet spot” in which its orbit matches Earth’s rotation. Because the satellite orbits at the same speed that the Earth is turning, the satellite seems to stay in place over a single longitude, though it may drift north to south. This special, high Earth orbit is called geosynchronous.
A satellite in a circular geosynchronous orbit directly over the equator (eccentricity and inclination at zero) will have a geostationary orbit that does not move at all relative to the ground. It is always directly over the same place on the Earth’s surface.
A geostationary orbit is extremely valuable for weather monitoring because satellites in this orbit provide a constant view of the same surface area. When you log into your favorite weather web site and look at the satellite view of your hometown, the image on the link comes from a satellite in geostationary orbit. Every few minutes, geostationary satellites like the Geostationary Operational Environmental Satellite (GOES) satellites send information about clouds, water vapor, and wind, and this near-constant stream of information serves as the basis for most weather monitoring and forecasting.
Because geostationary satellites are always over a single location, they can also be useful for communication (phones, television, radio). Built and launched by NASA and operated by the National Oceanic and Atmospheric Administration (NOAA), the GOES satellites provide a search and rescue beacon used to help locate ships and airplanes in distress.
Finally, many high Earth orbiting satellites monitor solar activity. The GOES satellites carry a large contingent of “space weather” instruments that take images of the Sun and track magnetic and radiation levels in space around them.
Other orbital “sweet spots,” just beyond high Earth orbit, are the Lagrange points. At the Lagrange points, the pull of gravity from the Earth cancels out the pull of gravity from the Sun. Anything placed at these points will feel equally pulled toward the Earth and the Sun and will revolve with the Earth around the Sun.
Of the five Lagrange points in the Sun-Earth system, only the last two, called L4 and L5, are stable. A satellite at the other three points is like a ball balanced at the peak of a steep hill: any slight perturbation will push the satellite out of the Lagrange point like the ball rolling down the hill. Satellites at these three points need constant adjustments to stay balanced and in place. Satellites at the last two Lagrange points are more like a ball in a bowl: even if perturbed, they return to the Lagrange point.
The first Lagrange point is located between the Earth and the Sun, giving satellites at this point a constant view of the Sun. The Solar and Heliospheric Observatory (SOHO), a NASA and European Space Agency satellite tasked to monitor the Sun, orbits the first Lagrange point, about 1.5 million kilometers away from Earth.
The second Lagrange point is about the same distance from the Earth, but is located behind the Earth. Earth is always between the second Lagrange point and the Sun. Since the Sun and Earth are in a single line, satellites at this location only need one heat shield to block heat and light from the Sun and Earth. It is a good location for space telescopes, including the future James Webb Space Telescope (Hubble’s successor, scheduled to launch in 2014) and the current Wilkinson Microwave Anisotropy Probe (WMAP), used for studying the nature of the universe by mapping background microwave radiation.
The third Lagrange point is opposite the Earth on the other side of the Sun so that the Sun is always between it and Earth. A satellite in this position would not be able to communicate with Earth. The extremely stable fourth and fifth Lagrange points are in Earth’s orbital path around the Sun, 60 degrees ahead of and behind Earth. The twin Solar Terrestrial Relations Observatory (STEREO) spacecraft will orbit at the fourth and fifth Lagrange points to provide a three-dimensional view of the Sun.
Medium Earth Orbit
Closer to the Earth, satellites in a medium Earth orbit move more quickly. Two medium Earth orbits are notable: the semi-synchronous orbit and the Molniya orbit.
The semi-synchronous orbit is a near-circular orbit (low eccentricity) 26,560 kilometers from the center of the Earth (about 20,200 kilometers above the surface). A satellite at this height takes 12 hours to complete an orbit. As the satellite moves, the Earth rotates underneath it. In 24-hours, the satellite crosses over the same two spots on the equator every day. This orbit is consistent and highly predictable. It is the orbit used by the Global Positioning System (GPS) satellites.
The second common medium Earth orbit is the Molniya orbit. Invented by the Russians, the Molniya orbit works well for observing high latitudes. A geostationary orbit is valuable for the constant view it provides, but satellites in a geostationary orbit are parked over the equator, so they don’t work well for far northern or southern locations, which are always on the edge of view for a geostationary satellite. The Molniya orbit offers a useful alternative.
The Molniya orbit is highly eccentric: the satellite moves in an extreme ellipse with the Earth close to one edge. Because it is accelerated by our planet’s gravity, the satellite moves very quickly when it is close to the Earth. As it moves away, its speed slows, so it spends more time at the top of its orbit farthest from the Earth. A satellite in a Molniya orbit takes 12 hours to complete its orbit, but it spends about two-thirds of that time over one hemisphere. Like a semi-synchronous orbit, a satellite in the Molniya orbit passes over the same path every 24 hours. This type of orbit is useful for communications in the far north or south.
Low Earth Orbit
Most scientific satellites and many weather satellites are in a nearly circular, low Earth orbit. The satellite’s inclination depends on what the satellite was launched to monitor. The Tropical Rainfall Measuring Mission (TRMM) satellite was launched to monitor rainfall in the tropics. Therefore, it has a relatively low inclination (35 degrees), staying near the equator.
Many of the satellites in NASA’s Earth Observing System have a nearly polar orbit. In this highly inclined orbit, the satellite moves around the Earth from pole to pole, taking about 99 minutes to complete an orbit. During one half of the orbit, the satellite views the daytime side of the Earth. At the pole, satellite crosses over to the nighttime side of Earth.
As the satellites orbit, the Earth turns underneath. By the time the satellite crosses back into daylight, it is over the region adjacent to the area seen in its last orbit. In a 24-hour period, polar orbiting satellites will view most of the Earth twice: once in daylight and once in darkness.
Just as the geosynchronous satellites have a sweet spot over the equator that lets them stay over one spot on Earth, the polar-orbiting satellites have a sweet spot that allows them to stay in one time. This orbit is a Sun-synchronous orbit, which means that whenever and wherever the satellite crosses the equator, the local solar time on the ground is always the same. For the Terra satellite for example, it’s always about 10:30 in the morning when the satellite crosses the equator in Brazil. When the satellite comes around the Earth in its next overpass about 99 minutes later, it crosses over the equator in Ecuador or Colombia at about 10:30 local time.
The Sun-synchronous orbit is necessary for science because it keeps the angle of sunlight on the surface of the Earth as consistent as possible, though the angle will change from season to season. This consistency means that scientists can compare images from the same season over several years without worrying too much about extreme changes in shadows and lighting, which can create illusions of change. Without a Sun-synchronous orbit, it would be very difficult to track change over time. It would be impossible to collect the kind of consistent information required to study climate change.
The path that a satellite has to travel to stay in a Sun-synchronous orbit is very narrow. If a satellite is at a height of 100 kilometers, it must have an orbital inclination of 96 degrees to maintain a Sun-synchronous orbit. Any deviation in height or inclination will take the satellite out of a Sun-synchronous orbit. Since the drag of the atmosphere and the tug of gravity from the Sun and Moon alter a satellite’s orbit, it takes regular adjustments to maintain a satellite in a Sun-synchronous orbit.
Other Orbits
Molniya orbit
Geostationary satellites must operate above the equator and therefore appear lower on the horizon as the receiver gets farther from the equator. This will cause problems for extreme northerly latitudes, affecting connectivity and causing multipath interference (caused by signals reflecting off the ground and into the ground antenna).
Thus, for areas close to the North (and South) Pole, a geostationary satellite may appear below the horizon. Therefore, Molniya orbit satellites have been launched, mainly in Russia, to alleviate this problem.
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.)
The Molniya orbit is designed so 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 targeted region for six to nine hours every second revolution. In this way a constellation of three Molniya satellites (plus in-orbit spares) can provide uninterrupted coverage.
The first satellite of the Molniya series was launched on 23 April 1965 and was used for experimental transmission of TV signals from a Moscow uplink station to downlink stations located in Siberia and the Russian Far East, in Norilsk, Khabarovsk, Magadan and Vladivostok. In November 1967 Soviet engineers created a unique system of national TV network of satellite television, called Orbita, that was based on Molniya satellites.
Polar orbit
In the United States, the National Polar-orbiting Operational Environmental Satellite System (NPOESS) was established in 1994 to consolidate the polar satellite operations of NASA (National Aeronautics and Space Administration) NOAA (National Oceanic and Atmospheric Administration). NPOESS manages a number of satellites for various purposes; for example, METSAT for meteorological satellite, EUMETSAT for the European branch of the program, and METOP for meteorological operations.
These orbits are Sun synchronous, meaning that they cross the equator at the same local time each day. For example, the satellites in the NPOESS (civilian) orbit will cross the equator, going from south to north, at times 1:30 P.M., 5:30 P.M., and 9:30 P.M.
Beyond geostationary orbit
There are plans and initiatives to bring dedicated communications satellite beyond geostationary orbits. NASA proposed LunaNet as a data network aiming to provide a „Lunar Internet for cis-lunar spacecraft and Installations. The Moonlight Initiative is an equivalent ESA project that is stated to be compatible and providing navigational services for the lunar surface. Both programmes are satellite constellstions of several satellites in various orbits around the Moon.
Other orbits are also planned to be used. Positions in the Earth-Moon-Libration points are also proposed for communication satellites covering the Moon alike communication satellites in geosynchronous orbit cover the Earth. Also, dedicated communication satellites in orbits around Mars supporting different missions on surface and other orbits are considered, such as the Mars Telecommunications Orbiter.
Achieving and Maintaining Orbit
Launch
The amount of energy required to launch a satellite into orbit depends on the location of the launch site and how high and how inclined the orbit is. Satellites in high Earth orbit require the most energy to reach their destination. Satellites in a highly inclined orbit, such as a polar orbit, take more energy than a satellite that circles the Earth over the equator. A satellite with a low inclination can use the Earth’s rotation to help boost it into orbit. The International Space Station orbits at an inclination of 51.6397 degrees to make it easier for the Space Shuttle and Russian rockets to reach it. A polar-orbiting satellite, on the other hand, gets no help from Earth’s momentum, and so requires more energy to reach the same altitude.
Maintaining Orbit
Once a satellite is in orbit, it usually takes some work to keep it there. Since Earth isn’t a perfect sphere, its gravity is stronger in some places compared to others. This unevenness, along with the pull from the Sun, Moon, and Jupiter (the solar system’s most massive planet), will change the inclination of a satellite’s orbit. Throughout their lifetime, GOES satellites have to be moved three or four times to keep them in place. NASA’s low Earth orbit satellites adjust their inclination every year or two to maintain a Sun-synchronous orbit.
Satellites in a low Earth orbit are also pulled out of their orbit by drag from the atmosphere. Though satellites in low Earth orbit travel through the uppermost (thinnest) layers of the atmosphere, air resistance is still strong enough to tug at them, pulling them closer to the Earth. Earth’s gravity then causes the satellites to speed up. Over time, the satellite will eventually burn up as it spirals lower and faster into the atmosphere or it will fall to Earth.
Atmospheric drag is stronger when the Sun is active. Just as the air in a balloon expands and rises when heated, the atmosphere rises and expands when the Sun adds extra energy to it. The thinnest layer of atmosphere rises, and the thicker atmosphere beneath it lifts to take its place. Now, the satellite is moving through this thicker layer of the atmosphere instead of the thin layer it was in when the Sun was less active. Since the satellite moves through denser air at solar maximum, it faces more resistance. When the Sun is quiet, satellites in low Earth orbit have to boost their orbits about four times per year to make up for atmospheric drag. When solar activity is at its greatest, a satellite may have to be maneuvered every 2-3 weeks.
The third reason to move a satellite is to avoid space junk, orbital debris, that may be in its path. On February 11, a communication satellite owned by Iridium, a U.S. company, collided with a non-functioning Russian satellite. Both satellites broke apart, creating a field of debris that contained at least 2,500 pieces. Each piece of debris was added to the database of more than 18,000 manmade objects currently in Earth orbit and tracked by the U.S. Space Surveillance Network.
NASA satellite mission controllers carefully track anything that may enter the path of their satellites. As of May 2009, Earth Observing satellites had been moved three separate times to avoid orbital debris.
The debris field generated by the Iridium collision is of particular concern to the Earth Observing System because the center of the debris field will eventually drift through the EOS satellites’ orbits. The Iridium and Russian satellites were 790 kilometers above the Earth, while EOS satellites orbit at 705 kilometers. Many pieces of debris from this collision were propelled to lower altitudes and are already causing issues at 705 kilometers.
Mission control engineers track orbital debris and other orbiting satellites that could come into the Earth Observing System’s orbit, and they carefully plan avoidance maneuvers as needed. The same team also plans and executes maneuvers to adjust the satellite’s inclination and height. The team evaluates these planned maneuvers to ensure that they do not bring the EOS satellites into close proximity to catalogued orbital debris or other satellites. To peek in on a day in the mission control center during one such maneuver, see the related article Flying Steady: Mission Control Tunes UpAqua’s Orbit.
References
- Air University. (2003, August). Orbital Mechanics. Space Primer. Accessed May 22, 2009.
- Blitzer, L. (1971, August). Satellite orbit paradox: A general view. American Journal of Physics. 39, 882-886.
- Cornish, N. J. (2008). The Lagrange Points. Wilkinson Microwave Anisotropy Probe (WMAP), National Aeronautics and Space Administration. Accessed June 4, 2009.
- European Space Agency. (2009, February 12). What are Lagrange Points? Accessed June 4, 2009.
- Gleick, J. (2003). Isaac Newton. New York: Vintage Books.
- Hawking, S. (2004). The Illustrated on the Shoulders of Giants. Philadelphia: Running Press.
- Iannotta, B. and Malik, T. (2009, February 11). U.S. satellite destroyed in space collision. Accessed May 22, 2009.
- The James Webb Space Telescope. About JWST’s Orbit. National Aeronautics and Space Administration. Accessed June 4, 2009.
- KomomaGPS Satellite Orbits. Accessed May 22, 2009.
- Pisacane. V. (2005). Fundamentals of Space Systems, p. 568. New York: Oxford University Press US. [Online]. Accessed September 4, 2009.
- Solar and Heliospheric Observatory. (2006). SOHO’s Orbit. National Aeronautics and Space Administration and European Space Agency. Accessed June 4, 2009.
1 Elizabeth Howell, Ben Biggs NASA Earth Observatory Orbits Catalog
Satellite Communication
Satellite communication refers to any communication link that involves the use of an artificial satellite in its propagation path. Satellite communications play a vital role in modern life. There are over 2000 artificial satellites in use. They can be found in geostationary, Molniya, elliptical, and low Earth orbits and are used for traditional point-to-point communications, mobile applications, and the distribution of TV and radio programs.
A communications satellite is an artificial satellite that relays and amplifies radio telecommunication signals via a transponder; it 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. Many communications satellites are in geostationary orbit 22,300 miles (35,900 km) above the equator, so that the satellite appears stationary at the same point in the sky; therefore the satellite dish antennas of ground stations can be aimed permanently at that spot and do not have to move to track the satellite. Others form satellite constellations in low Earth orbit, where antennas on the ground have to follow the position of the satellites and switch between satellites frequently.
The radio waves used for telecommunications links travel by line of sight and so are obstructed by the curve of the Earth. The purpose of communications satellites is to relay the signal around the curve of the Earth allowing communication between widely separated geographical points. Communications satellites use a wide range of radio and microwavefrequencies. To avoid signal interference, international organizations have regulations for which frequency ranges or "bands" certain organizations are allowed to use. This allocation of bands minimizes the risk of signal interference.
Satellite communications tend to use high frequency signals: Ultra High Frequency (UHF), 300 MHz - 3 GHz and Super High Frequency (SHF), 3 - 30 GHz. Radio signals propagating to and from a satellite in orbit are affected by the environmental conditions along the propagation path. In a vacuum, radio signals propagate at the speed of light, but in the presence of plasma in the ionosphere, the signals are affected by group delay and phase advance and attenuation due to absorption and scintillation. The environment's effect on the signal is frequency dependent and to a first approximation is proportional to the amount of structure in the plasma present along the propagation path.
Due to ionospheric variability (Space Weather), the effects on propagating signals are highly variable. Up to some level, the effects of Space Weather on propagation can be mitigated through engineering design solutions, but space weather can lead to a total loss of communication due to attenuation and/or severe scintillation when the broadcast signals cross the ionosphere. In trans-ionosphere propagation, scintillation refers to the rapid variation of the amplitude and phase of a received signal. Scintillation is produced by structure in the ionosphere. The severity of scintillation depends on the frequency of the used signal and the spatial structure of plasma density and plasma drifts along the propagation path. Specifically, scintillation at the receiver is produced by constructive and destructive interference of refracted and diffracted components of the broadcast signal.
Bibliography
Basu et al., Specification of the occurrence of equatorial ionospheric scintillations during the main phase of large magnetic storms within solar cycle 23, RADIO SCIENCE, VOL. 45, RS5009, doi:10.1029/2009RS004343, 2010.
Bruce R. Elbert, Introduction to Satellite Communications, 3rd ed. (2008).
Virgil S. Labrador and Peter I. Galace, Heavens Fill with Commerce: A Brief History of the Communications Satellite Industry (2005).
Virgil S. Labrador et al., The Satellite Technology Guide for the 21st Century (2008).
Joseph N. Pelton, The Basics of Satellite Communications 2nd ed. (2006).
David J. Whalen, The Origins of Satellite Communications 1945–1965 (2002).
swpc.noaa.gov Satellite-communications
Applications
Telephony
The first and historically most important application for communication satellites was in intercontinental long distance telephony. The fixed Public Switched Telephone Network relays telephone calls from land line telephones to an Earth station, where they are then transmitted to a geostationary satellite. The downlink follows an analogous path. Improvements in submarine communications cables through the use of fiber-optics caused some decline in the use of satellites for fixed telephony in the late 20th century.
Satellite communications are still used in many applications today. Remote islands such as Ascension Island, Saint Helena, Diego Garcia, and Easter Island, where no submarine cables are in service, need satellite telephones. There are also regions of some continents and countries where landline telecommunications are rare to non existent, for example large regions of South America, Africa, Canada, China, Russia, and Australia. Satellite communications also provide connection to the edges of Antarctica and Greenland. Other land use for satellite phones are rigs at sea, a backup for hospitals, military, and recreation. Ships at sea, as well as planes, often use satellite phones.
Satellite phone systems can be accomplished by a number of means. On a large scale, often there will be 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 patch a radio signal to a telephone system. In this example, almost any type of satellite can be used. Satellite phones connect 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 .
Television
As television became the main market, its demand for simultaneous delivery of relatively few signals of large bandwidth to many receivers being a more precise match for the capabilities of geosynchronous comsats. Two satellite types are used for North American television and radio: Direct broadcast satellite (DBS), and Fixed Service Satellite (FSS).
The definitions of FSS and DBS satellites outside of North America, especially in Europe, are a bit more ambiguous. Most satellites used for direct-to-home television in Europe have the same high power output as DBS-class satellites in North America, but use the same linear polarization as FSS-class satellites. Examples of these are the Astra, Eutelsat, and Hotbird spacecraft in orbit over the European continent. Because of this, the terms FSS and DBS are more so used throughout the North American continent, and are uncommon in Europe.
Fixed Service Satellites use the C band, and the lower portions of the Ku band. 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 and universities, business television (BTV), Videoconferencing, and general commercial telecommunications. FSS satellites are also used to distribute national cable channels to cable television headends.
Free-to-air satellite TV channels are also usually distributed on FSS satellites in the Ku band. The Intelsat Americas 5, Galaxy 10R and AMC 3 satellites over North America provide a quite large amount of FTA channels on their Ku band transponders.
The American Dish NetworkDBS service has also recently used FSS technology as well for their programming packages requiring their SuperDish antenna, due to Dish Network needing more capacity to carry local television stations per the FCC's "must-carry" regulations, and for more bandwidth to carry HDTV channels.
A direct broadcast satellite is a communications satellite that transmits to small DBS satellite dishes (usually 18 to 24 inches or 45 to 60 cm in diameter). Direct broadcast satellites generally operate in the upper portion of the microwave Ku band. DBS technology is used for DTH-oriented (Direct-To-Home) satellite TV services, such as DirecTV, DISH Network and Orby TV in the United States, Bell Satellite TV and Shaw Direct in Canada, Freesat and Sky in the UK, Ireland, and New Zealand and DSTV in South Africa.
Operating at lower frequency and lower power than DBS, FSS satellites require a much larger dish for reception (3 to 8 feet (1 to 2.5 m) in diameter for Ku band, and 12 feet (3.6 m) or larger for C band). They use linear polarization for each of the transponders' RF input and output (as opposed to circular polarization used by DBS satellites), but this is a minor technical difference that users do not notice. FSS satellite technology was also originally used for DTH satellite TV from the late 1970s to the early 1990s in the United States in the form of TVRO (Television Receive Only) receivers and dishes. It was also used in its Ku band form for the now-defunct Primestar satellite TV service.
Some satellites have been launched that have transponders in the Ka band, such as DirecTV's SPACEWAY-1 satellite, and Anik F2. NASA and ISRO have also launched experimental satellites carrying Ka band beacons recently.
Some manufacturers have also introduced special antennas for mobile reception of DBS television. Using Global Positioning System (GPS) technology as a reference, these antennas automatically re-aim to the satellite no matter where or how the vehicle (on which the antenna is mounted) is situated. These mobile satellite antennas are popular with some recreational vehicle owners. Such mobile DBS antennas are also used by JetBlue Airways for DirecTV (supplied by LiveTV, a subsidiary of JetBlue), which passengers can view on-board on LCD screens mounted in the seats.
Radio broadcasting
Satellite radio offers audio broadcast services in some countries, notably the United States. Mobile services allow listeners to roam a continent, listening to the same audio programming anywhere.
A satellite radio or subscription radio (SR) is a digital radio signal that is broadcast by a communications satellite, which covers a much wider geographical range than terrestrial radio signals.
Amateur radio
Amateur radio operators have access to amateur satellites, which 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 launch costs, most current 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 X.25 or similar protocols.
Internet access
After the 1990s, satellite communication technology has been used as a means to connect to the Internet via broadband data connections. This can be very useful for users who are located in remote areas, and cannot access a broadband connection, or require high availability of services.
Military
Communications satellites are used for military communications applications, such as Global Command and Control Systems. Examples of military systems that use communication satellites are the MILSTAR, the DSCS, and the FLTSATCOM of the United States, NATO satellites, United Kingdom satellites (for instance Skynet), and satellites of the former Soviet Union. India has launched its first Military Communication satellite GSAT-7, its transponders operate in UHF, F, C and Ku band bands. Typically military satellites operate in the UHF, SHF (also known as X-band) or EHF (also known as Ka band) frequency bands.
Data collection
Near-ground in situenvironmental monitoring equipment (such as tide gauges, weather stations, weather buoys, and radiosondes), may use satellites for one-way data transmission or two-way telemetry and telecontrol. It may be based on a secondary payload of a weather satellite (as in the case of GOES and METEOSAT and others in the Argos system) or in dedicated satellites (such as SCD). The data rate is typically much lower than in satellite Internet access.
Wikipedia Communications Satellite
Abbreviations
Orbit:
In physics, an orbit is the gravitationally curved trajectory of an object, such as the trajectory of a planet around a star or a natural satellite around a planet. In the context of satellite communication, it usually refers to an artificial satellite built and launched by humans, and orbit is around the earth.
Satellite:
In the context of spaceflight, a satellite is an object that has been intentionally placed into orbit, with human endeavor. The very first satellite was “Sputnik” launched in 1957 sending signals to the earth.
Kepler’s laws of planetary motion:
There are three laws by Johannes Kepler. (1) First one describes how planets move in elliptical orbits with the Sun as a focus, (2) The second law states—a planet covers the same area of space in the same amount of time no matter where it is in its orbit, and (3) the third law states—a planet's orbital period is proportional to the size of its orbit (its semi-major axis).
Perigee and apogee:
In the context of artificial satellites around the earth, the point of the orbit closest to Earth is called perigee, while the point farthest from Earth is known as apogee.
Escape velocity:
The velocity required to escape from the gravitational pull of the earth. This escape velocity is about 11.2 km per second, or approximately 33 times the speed of sound: MACH 33.
Injection velocity:
The velocity at which a satellite is injected into orbit. This is always less than escape velocity. Higher the orbit (farther from earth) lower will be the injection velocity needed, since earth’s gravitational pull reduces as satellites are placed farther away.
A.U Astronomical Unit:
The astronomical unit (symbol: au, or AU or AU) is a unit of length representing the distance from Earth to the Sun and is physically about 150 million kilometers, or ~8 light minutes (since light takes 8 min to travel from the Sun to the Earth).
LEO:
Low Earth Orbit, generally considered as 200–2000 km from the earth surface and apogee of up to 1000 km, where many remote sensing satellites and space stations are parked. The inclination within this orbit can vary considerably, including polar orbit which is 90° (over north and south pole), that allows “sun synchronous” orbit, meaning satellite visits the same spot on earth every few days.
MEO:
Medium Earth Orbit, considered as 2000–35,000 km from the earth surface and apogee of up to 20,000 km. Many satellites including the GPS are in this orbit and highly inclined orbits that cover the globe (Molniya and Tundra) in which geosynchronous orbits (satellites take 12 h or 24 h to go around the earth, but are highly inclined and require antenna tracking) are in this orbit as well.
GEO:
Geostationary orbit, is a popular orbit in the equatorial plane at 35,786 km from the earth. When satellites are parked in this orbit, they appear stationary to viewers on earth. The antenna remains almost stationary with very little or no tracking. This feature helps communication satellites.
Navigation satellites:
Satellites that help in location and directions for observers on earth, three major satellite systems used for navigation are GPS, GLONASS, and Galileo.
Remote sensing satellites:
Satellites that continuously scan the earth surface and send pictures back to the earth. They can be programmed to highlight features such as minerals, forestry, ocean, and others depending on user needs. They use infra-red cameras to avoid the problems of cloud cover.
PAA—phased array antenna:
A phased array refers to an electronically scanned array of antennas, which creates a beam of radio waves that can be steered in a specific direction without physically moving the antennas. Such steering is usually accomplished using multiple antennas and by dynamically changing the phase of signal to each antenna element.
Sentinel-2 mission:
Copernicus Sentinel-2 mission comprises a constellation of two polar-orbiting satellites placed in the same sun-synchronous orbit, phased at 180° to each other. It aims at monitoring variability in land surface conditions, and its wide swath width (290 km) and high revisit time (10 days at the equator with one satellite, and 5 days with two satellites under cloud-free conditions, which results in 2–3 days at mid-latitudes) will support monitoring of Earth's surface changes.
HEO, highly elliptical orbit:
Indicates an elliptic orbit with high eccentricity. This usually refers to eccentric angle around Earth. Such extremely elongated orbits have the advantage of long dwell times at a point in the sky during the approach to, and descent from, apogee.
Molniya orbit:
A highly elliptical orbit extensively used by the erstwhile Soviet Union (Russia). This orbit is geosynchronous since each satellite offer 12 h of continuous coverage of a region on earth. It is not geostationary and requires tracking antennas from ground.
Tundra orbit:
A highly elliptical orbit but with “apogee dwelling” feature. This orbit also is geosynchronous and with multiple satellites it covers a region on earth for 24 h. Sirius satellite radio used this orbit to provide broadcast capability for the North American region.
Station keeping:
It is an orbital control process to maintain a satellite stationary within a given orbit by using small control thruster rockets on satellites. This also refers to a routine exercise performed regularly by all satellite agencies, to keep satellite in an allocated position in space.
GEO belt:
A term commonly used to refer to a library listing by the planetary society that lists all the satellites stationed in the geostationary GEO orbit. This 10-frame mosaic of Earth's geostationary satellite belt covers approximately 35° of sky around the GEO orbit.
Uplink limited:
Normally refers to the noise of the uplink that limits the bandwidth available in a communication system. This term refers to both satellite communication and cellular communication.
Cryogenically cooled amplifier (LNA):
Low Noise Amplifier uses Cryogenic cooling of receivers to reduce their noise temperature, which is especially important in radio astronomy and in satellite and deep space communication. An antenna is “looking up “ into the sky and, in the absence of strong celestial sources (Sun, Moon, planets, Cassiopeia, Cygnus, Taurus, Virgo, Orion, and the galactic plane) in the antenna beam “sees” a very cold sky: 2.725 K of the cosmic microwave background radiation modified by the presence of atmosphere. The antenna temperature required is, therefore, one order of magnitude less than those seen in terrestrial applications (300 K). Reduction of receiver noise by cryogenic cooling offers an effective way of improving radio astronomy system sensitivity.
Antenna quality factor G/T:
Antenna gain-to-noise-temperature (G/T) is a figure of merit in the characterization of antenna. It is the ratio of gain of the antenna divided by the antenna temperature (or system temperature if a receiver is specified).
VSAT:
Very Small Aperture Terminal, is a small-sized earth station used to transmit/receive data, voice, and video signals over a satellite communication network, excluding broadcast television. VSAT antenna is 1.2 m–3 m diameter and operates in the Ku, C, and Ka bands. It offers high-bandwidth, bidirectional VSAT services for enterprise, government since many users will increasingly migrate to VSAT satellites.
Transponder:
It is a device for receiving a radio signal and automatically transmitting a different signal (usually at a different frequency). In communication satellite, transponder refers to the series of interconnected units that form a communications channel between the receiving and the transmitting antennas.
EIRP:
Equivalent Isotropic Radiated Power, represents the actual RF power in the air in a specific direction It considers transmitted power, effective antenna gain, and cable losses into account. EIRP is most commonly indicated in decibels over isotropic, dBi.
HPA:
High Power Amplifier: In satellite communication refers to the high-power amplifier (HPA) provides the RF power for a payload downlink from the satellite. Before the signal goes into the HPA, there is a preamplifier that boosts the signal to a level proper for input to the HPA. These two together form the HPA subsystem. There are two types of HPA subsystem: the traveling-wave tube amplifier (TWTA) subsystem and the solid-state power amplifier (SSPA). Satellite downlink beams have very high power (up to 100, 000 watts of EIRP in spot beams). TWTA is more often used. Some ship-borne heavy duty systems also uses simpler HPA in the uplink (about 100 W EIRP).
DRM:
Digital Radio Mondiale, is a set of digital audio broadcasting technologies designed to work over the bands (<30 MHz) currently used for analog radio broadcast including AM, shortwave, and FM bands. DRM is spectrally efficient compared to AM and FM and supports more stations with high-quality digital sound. It also provides two-way communication to users (in lower frequency bands). DRM is also the name of the international non-profit consortium that designed the platform and now promotes its introduction. Radio France Internationale, TéléDiffusion de France, BBC World Service, Deutsche Welle, Voice of America, Telefunken (now Transradio) and Thomcast (now Ampegon) are part of DRM consortium.
GLONASS:
GLONASS refers to a system of navigation satellites and consists of an orbital constellation of 24 satellites covering the entire Russian territory, since 2011.
GPS:
Global Positioning System consists of satellite constellation arranged into six equally spaced orbital planes surrounding the Earth. The U.S. Space Force has been flying 31 operational GPS satellites for well over a decade.
Galileo:
The Galileo System is a global navigation satellite system that went live in 2016. It was created by the European Union through the European Space Agency and operated by the European Union Agency for the Space Program.
BeiDou:
BDS is a global navigation system constructed and operated independently by China. The BeiDou navigation satellite system BDS has completed the constellation deployment and started to provide global services.
Telemetry:
Telemetry is measurement at a distance. In the context of satellite communication, it refers to measurement made on satellites and sent back to the earth (downlink only). It is used as a health monitoring system as well as a method to confirm commands sent from earth are implemented.
Telecommand:
In the context of satellite communication, Telecommand represents a system that sends commands to the satellite (uplink only).
Tracking:
Satellite tracking consists of a system that involves multiple methods such as tone ranging, use of PN code to confirm the actual position and movement of the satellite system. In recent years, Satellite laser ranging has become a proven geodetic technique. It is the most accurate technique currently available to determine the geocentric position of an Earth satellite.
Drone of UAV:
Unmanned Aerial Vehicle is an aircraft without a human pilot, it is often referred to as a drone. UAV is a part of the system that consists of ground control station for remote control of UAV. They are used in a variety of applications, reconnaissance, agricultural operations, and others.
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44. SiriusXM satellite radio (Info for Techies) http://www3.sympatico.ca/n.rieck/docs/sirius.html
Krishnamurthy Raghunandan Springer.com Satellite Communication Abbreviations
- Read about some of the satellites that NASA has in orbit around Earth.
- Discover how satellites can prepare us for the increasing frequency of flood events around the world.
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