Geosynchronous satellite
Encyclopedia
A geosynchronous Satellite is a satellite
whose orbit
on the Earth repeats regularly over points on the Earth over time. If such a satellite's orbit lies over the equator
, the orbit is circular and its angular velocity is the same as the earth's, then it is called a geostationary satellite. The orbits of the satellites are known as the geosynchronous orbit
and geostationary orbit
. Another type of geosynchronous orbit is the Tundra elliptical orbit
.
A geosynchronous network is a communication network based on communication with or through geosynchronous satellites.
, the orbital period of a satellite in a circular orbit
increases with increasing altitude. Space station
s and Shuttles
in Low Earth orbit
(LEO), typically two or four hundred miles above the Earth's surface make between fifteen and sixteen revolutions per day. The Moon
, at an altitude of about 238,900 miles (384,400 km), takes about 27 days 7 hours to make a complete revolution http://nssdc.gsfc.nasa.gov/planetary/factsheet/moonfact.html. Between those extremes lies the "magic" altitude of 22,236 miles (35,786 km) at which a satellite's orbital period
matches, or is an integral part of, the period at which the Earth rotates: once every sidereal day (23 hours 56 minutes 4 seconds). In that case, the satellite is said to be geosynchronous.
If a geosynchronous satellite's orbit is not exactly aligned with the equator
, the orbit is known as an inclined orbit
. It will appear (when viewed by someone on the ground) to oscillate daily around a fixed point. As the angle between the orbit and the equator decreases, the magnitude of this oscillation becomes smaller; when the orbit lies entirely over the equator, the satellite remains stationary relative to the Earth's surface – it is said to be geostationary.
Geostationary satellites appear to be fixed over one spot above the equator. Receiving and transmitting antenna
s on the earth do not need to track such a satellite. These antennas can be fixed in place and are much less expensive than tracking antennas. These satellites have revolutionized global communication
s, television broadcasting and weather forecasting
, and have a number of important defense
and intelligence
applications.
One disadvantage of geostationary satellites is a result of their high altitude: radio
signals take approximately 0.25 of a second to reach and return from the satellite, resulting in a small but significant signal delay
. This delay increases the difficulty of telephone
conversation and reduces the performance of common network protocols such as TCP/IP
, but does not present a problem with non-interactive systems such as television broadcasts. There are a number of proprietary satellite data protocols that are designed to proxy TCP/IP connections over long-delay satellite links—these are marketed as being a partial solution to the poor performance of native TCP over satellite links. TCP presumes that all loss is due to congestion, not errors, and probes link capacity with its "slow-start
" algorithm
, which only sends packets once it is known that earlier packets have been received. Slow start is very slow over a path using a geostationary satellite.
Another disadvantage of geostationary satellites is the incomplete geographical coverage, since ground stations at higher than roughly 60 degrees latitude have difficulty reliably receiving signals at low elevations. Satellite dish
es at such high latitudes would need to be pointed almost directly towards the horizon. The signals would have to pass through the largest amount of atmosphere, and could even be blocked by land topography, vegetation or buildings. In the USSR
, a practical solution was developed for this problem with the creation of special Molniya
/ Orbita
inclined path
satellite networks with elliptical
orbits. Similar elliptical orbits are used for the Sirius Radio satellites.
in 1928 and popularised by the science fiction author Arthur C. Clarke
in a paper in Wireless World in 1945. Working prior to the advent of solid-state electronics, Clarke envisioned a trio of large, manned space stations arranged in a triangle around the planet. Modern satellites are numerous, unmanned, and often no larger than an automobile.
Widely known as the "father of the geosynchronous satellite", Harold Rosen, an engineer at Hughes Aircraft Company, invented the first operational geosynchronous satellite, Syncom 2. It was launched on a Delta rocket
B booster from Cape Canaveral
July 26, 1963. A few months later Syncom 2 was used for the world's first satellite-relayed telephone
call. It took place between United States President John F. Kennedy
and Nigeria
n Prime minister Abubakar Tafawa Balewa
.
The first geostationary communication satellite was Syncom 3, launched on August 19, 1964 with a Delta D launch vehicle from Cape Canaveral. The satellite, in orbit near the International Date Line
, was used to telecast the 1964 Summer Olympics
in Tokyo to the United States. It was the first television program to cross the Pacific ocean
.
Satellite
In the context of spaceflight, a satellite is an object which has been placed into orbit by human endeavour. Such objects are sometimes called artificial satellites to distinguish them from natural satellites such as the Moon....
whose orbit
Orbit
In physics, an orbit is the gravitationally curved path of an object around a point in space, for example the orbit of a planet around the center of a star system, such as the Solar System...
on the Earth repeats regularly over points on the Earth over time. If such a satellite's orbit lies over the equator
Equator
An equator is the intersection of a sphere's surface with the plane perpendicular to the sphere's axis of rotation and containing the sphere's center of mass....
, the orbit is circular and its angular velocity is the same as the earth's, then it is called a geostationary satellite. The orbits of the satellites are known as the geosynchronous orbit
Geosynchronous orbit
A geosynchronous orbit is an orbit around the Earth with an orbital period that matches the Earth's sidereal rotation period...
and geostationary orbit
Geostationary orbit
A geostationary orbit is a geosynchronous orbit directly above the Earth's equator , with a period equal to the Earth's rotational period and an orbital eccentricity of approximately zero. An object in a geostationary orbit appears motionless, at a fixed position in the sky, to ground observers...
. Another type of geosynchronous orbit is the Tundra elliptical orbit
Tundra orbit
Tundra orbit is a type of highly elliptical geosynchronous orbit with a high inclination and an orbital period of one sidereal day . A satellite placed in this orbit spends most of its time over a chosen area of the Earth, a phenomenon known as apogee dwell...
.
A geosynchronous network is a communication network based on communication with or through geosynchronous satellites.
Definition
According to Kepler's Third LawKepler's laws of planetary motion
In astronomy, Kepler's laws give a description of the motion of planets around the Sun.Kepler's laws are:#The orbit of every planet is an ellipse with the Sun at one of the two foci....
, the orbital period of a satellite in a circular orbit
Orbit
In physics, an orbit is the gravitationally curved path of an object around a point in space, for example the orbit of a planet around the center of a star system, such as the Solar System...
increases with increasing altitude. Space station
Space station
A space station is a spacecraft capable of supporting a crew which is designed to remain in space for an extended period of time, and to which other spacecraft can dock. A space station is distinguished from other spacecraft used for human spaceflight by its lack of major propulsion or landing...
s and Shuttles
Space Shuttle
The Space Shuttle was a manned orbital rocket and spacecraft system operated by NASA on 135 missions from 1981 to 2011. The system combined rocket launch, orbital spacecraft, and re-entry spaceplane with modular add-ons...
in Low Earth orbit
Low Earth orbit
A low Earth orbit is generally defined as an orbit within the locus extending from the Earth’s surface up to an altitude of 2,000 km...
(LEO), typically two or four hundred miles above the Earth's surface make between fifteen and sixteen revolutions per day. The Moon
Moon
The Moon is Earth's only known natural satellite,There are a number of near-Earth asteroids including 3753 Cruithne that are co-orbital with Earth: their orbits bring them close to Earth for periods of time but then alter in the long term . These are quasi-satellites and not true moons. For more...
, at an altitude of about 238,900 miles (384,400 km), takes about 27 days 7 hours to make a complete revolution http://nssdc.gsfc.nasa.gov/planetary/factsheet/moonfact.html. Between those extremes lies the "magic" altitude of 22,236 miles (35,786 km) at which a satellite's orbital period
Orbital period
The orbital period is the time taken for a given object to make one complete orbit about another object.When mentioned without further qualification in astronomy this refers to the sidereal period of an astronomical object, which is calculated with respect to the stars.There are several kinds of...
matches, or is an integral part of, the period at which the Earth rotates: once every sidereal day (23 hours 56 minutes 4 seconds). In that case, the satellite is said to be geosynchronous.
If a geosynchronous satellite's orbit is not exactly aligned with the equator
Equator
An equator is the intersection of a sphere's surface with the plane perpendicular to the sphere's axis of rotation and containing the sphere's center of mass....
, the orbit is known as an inclined orbit
Inclined orbit
A satellite is said to occupy an inclined orbit around the Earth if the orbit exhibits an angle other than zero degrees with the equatorial plane. This angle is called the orbit's inclination...
. It will appear (when viewed by someone on the ground) to oscillate daily around a fixed point. As the angle between the orbit and the equator decreases, the magnitude of this oscillation becomes smaller; when the orbit lies entirely over the equator, the satellite remains stationary relative to the Earth's surface – it is said to be geostationary.
Application
There are approximately 300 operational geosynchronous satellites.Geostationary satellites appear to be fixed over one spot above the equator. Receiving and transmitting antenna
Antenna (radio)
An antenna is an electrical device which converts electric currents into radio waves, and vice versa. It is usually used with a radio transmitter or radio receiver...
s on the earth do not need to track such a satellite. These antennas can be fixed in place and are much less expensive than tracking antennas. These satellites have revolutionized global communication
Telecommunication
Telecommunication is the transmission of information over significant distances to communicate. In earlier times, telecommunications involved the use of visual signals, such as beacons, smoke signals, semaphore telegraphs, signal flags, and optical heliographs, or audio messages via coded...
s, television broadcasting and weather forecasting
Weather forecasting
Weather forecasting is the application of science and technology to predict the state of the atmosphere for a given location. Human beings have attempted to predict the weather informally for millennia, and formally since the nineteenth century...
, and have a number of important defense
Military
A military is an organization authorized by its greater society to use lethal force, usually including use of weapons, in defending its country by combating actual or perceived threats. The military may have additional functions of use to its greater society, such as advancing a political agenda e.g...
and intelligence
Intelligence
Intelligence has been defined in different ways, including the abilities for abstract thought, understanding, communication, reasoning, learning, planning, emotional intelligence and problem solving....
applications.
One disadvantage of geostationary satellites is a result of their high altitude: radio
Radio Wave
Radio Wave may refer to:*Radio frequency*Radio Wave 96.5, a radio station in Blackpool, UK...
signals take approximately 0.25 of a second to reach and return from the satellite, resulting in a small but significant signal delay
Latency (engineering)
Latency is a measure of time delay experienced in a system, the precise definition of which depends on the system and the time being measured. Latencies may have different meaning in different contexts.-Packet-switched networks:...
. This delay increases the difficulty of telephone
Telephone
The telephone , colloquially referred to as a phone, is a telecommunications device that transmits and receives sounds, usually the human voice. Telephones are a point-to-point communication system whose most basic function is to allow two people separated by large distances to talk to each other...
conversation and reduces the performance of common network protocols such as TCP/IP
Internet protocol suite
The Internet protocol suite is the set of communications protocols used for the Internet and other similar networks. It is commonly known as TCP/IP from its most important protocols: Transmission Control Protocol and Internet Protocol , which were the first networking protocols defined in this...
, but does not present a problem with non-interactive systems such as television broadcasts. There are a number of proprietary satellite data protocols that are designed to proxy TCP/IP connections over long-delay satellite links—these are marketed as being a partial solution to the poor performance of native TCP over satellite links. TCP presumes that all loss is due to congestion, not errors, and probes link capacity with its "slow-start
Slow-start
Slow-start is part of the congestion control strategy used by TCP, the data transmission protocol used by many Internet applications. Slow-start is used in conjunction with other algorithms to avoid sending more data than the network is capable of transmitting, that is, to avoid causing network...
" algorithm
Algorithm
In mathematics and computer science, an algorithm is an effective method expressed as a finite list of well-defined instructions for calculating a function. Algorithms are used for calculation, data processing, and automated reasoning...
, which only sends packets once it is known that earlier packets have been received. Slow start is very slow over a path using a geostationary satellite.
Another disadvantage of geostationary satellites is the incomplete geographical coverage, since ground stations at higher than roughly 60 degrees latitude have difficulty reliably receiving signals at low elevations. Satellite dish
Satellite dish
A satellite dish is a dish-shaped type of parabolic antenna designed to receive microwaves from communications satellites, which transmit data transmissions or broadcasts, such as satellite television.-Principle of operation:...
es at such high latitudes would need to be pointed almost directly towards the horizon. The signals would have to pass through the largest amount of atmosphere, and could even be blocked by land topography, vegetation or buildings. In the USSR
Soviet Union
The Soviet Union , officially the Union of Soviet Socialist Republics , was a constitutionally socialist state that existed in Eurasia between 1922 and 1991....
, a practical solution was developed for this problem with the creation of special Molniya
Molniya
Molniya may refer to:* The Molniya , the Soviet military communications satellite.* Molniya orbit, distinctive orbit associated with the satellite....
/ Orbita
Orbita
Orbita is a Soviet-Russian system of broadcasting and delivering TV signals via satellites. It is considered to be the first national network of satellite television....
inclined path
satellite networks with elliptical
Ellipse
In geometry, an ellipse is a plane curve that results from the intersection of a cone by a plane in a way that produces a closed curve. Circles are special cases of ellipses, obtained when the cutting plane is orthogonal to the cone's axis...
orbits. Similar elliptical orbits are used for the Sirius Radio satellites.
History
The concept was first proposed by Herman PotočnikHerman Potocnik
Herman Potočnik was an Austro-Hungarian rocket engineer and pioneer of cosmonautics . He is chiefly remembered for his work addressing the long-term human habitation of space.- Early life :Potočnik was born in Pola, southern Istria, Austria-Hungary...
in 1928 and popularised by the science fiction author Arthur C. Clarke
Arthur C. Clarke
Sir Arthur Charles Clarke, CBE, FRAS was a British science fiction author, inventor, and futurist, famous for his short stories and novels, among them 2001: A Space Odyssey, and as a host and commentator in the British television series Mysterious World. For many years, Robert A. Heinlein,...
in a paper in Wireless World in 1945. Working prior to the advent of solid-state electronics, Clarke envisioned a trio of large, manned space stations arranged in a triangle around the planet. Modern satellites are numerous, unmanned, and often no larger than an automobile.
Widely known as the "father of the geosynchronous satellite", Harold Rosen, an engineer at Hughes Aircraft Company, invented the first operational geosynchronous satellite, Syncom 2. It was launched on a Delta rocket
Delta rocket
Delta is a versatile family of expendable launch systems that has provided space launch capability in the United States since 1960. There have been more than 300 Delta rockets launched, with a 95 percent success rate. Two Delta launch systems – Delta II and Delta IV – are in active use...
B booster from Cape Canaveral
Cape Canaveral
Cape Canaveral, from the Spanish Cabo Cañaveral, is a headland in Brevard County, Florida, United States, near the center of the state's Atlantic coast. Known as Cape Kennedy from 1963 to 1973, it lies east of Merritt Island, separated from it by the Banana River.It is part of a region known as the...
July 26, 1963. A few months later Syncom 2 was used for the world's first satellite-relayed telephone
Telephone
The telephone , colloquially referred to as a phone, is a telecommunications device that transmits and receives sounds, usually the human voice. Telephones are a point-to-point communication system whose most basic function is to allow two people separated by large distances to talk to each other...
call. It took place between United States President John F. Kennedy
John F. Kennedy
John Fitzgerald "Jack" Kennedy , often referred to by his initials JFK, was the 35th President of the United States, serving from 1961 until his assassination in 1963....
and Nigeria
Nigeria
Nigeria , officially the Federal Republic of Nigeria, is a federal constitutional republic comprising 36 states and its Federal Capital Territory, Abuja. The country is located in West Africa and shares land borders with the Republic of Benin in the west, Chad and Cameroon in the east, and Niger in...
n Prime minister Abubakar Tafawa Balewa
Abubakar Tafawa Balewa
Alhaji Sir Abubakar Tafawa Balewa, KBE was a Nigerian politician, and the only prime minister of an independent Nigeria. Originally a trained teacher, he became a vocal leader for Northern interest as one of the few educated Nigerians of his time...
.
The first geostationary communication satellite was Syncom 3, launched on August 19, 1964 with a Delta D launch vehicle from Cape Canaveral. The satellite, in orbit near the International Date Line
International Date Line
The International Date Line is a generally north-south imaginary line on the surface of the Earth, passing through the middle of the Pacific Ocean, that designates the place where each calendar day begins...
, was used to telecast the 1964 Summer Olympics
1964 Summer Olympics
The 1964 Summer Olympics, officially known as the Games of the XVIII Olympiad, was an international multi-sport event held in Tokyo, Japan in 1964. Tokyo had been awarded with the organization of the 1940 Summer Olympics, but this honor was subsequently passed to Helsinki because of Japan's...
in Tokyo to the United States. It was the first television program to cross the Pacific ocean
Pacific Ocean
The Pacific Ocean is the largest of the Earth's oceanic divisions. It extends from the Arctic in the north to the Southern Ocean in the south, bounded by Asia and Australia in the west, and the Americas in the east.At 165.2 million square kilometres in area, this largest division of the World...
.
See also
- Geosynchronous orbitGeosynchronous orbitA geosynchronous orbit is an orbit around the Earth with an orbital period that matches the Earth's sidereal rotation period...
- Geostationary orbitGeostationary orbitA geostationary orbit is a geosynchronous orbit directly above the Earth's equator , with a period equal to the Earth's rotational period and an orbital eccentricity of approximately zero. An object in a geostationary orbit appears motionless, at a fixed position in the sky, to ground observers...
- Graveyard orbitGraveyard orbitA graveyard orbit, also called a supersynchronous orbit, junk orbit or disposal orbit, is an orbit significantly above synchronous orbit, where spacecraft are intentionally placed at the end of their operational life...
- List of orbits
- List of satellites in geosynchronous orbit
- Molniya orbitMolniya orbitMolniya orbit is a type of highly elliptical orbit with an inclination of 63.4 degrees, an argument of perigee of -90 degree and an orbital period of one half of a sidereal day...
- Polar mountPolar mountA polar mount is a movable mount for satellite dishes that allows the dish to be pointed at many geostationary satellites by slewing around one axis...
- Mount for aiming a satellite dishe at geosynchronous satellites - Satellite televisionSatellite televisionSatellite television is television programming delivered by the means of communications satellite and received by an outdoor antenna, usually a parabolic mirror generally referred to as a satellite dish, and as far as household usage is concerned, a satellite receiver either in the form of an...
External links
- NASA's software for satellite tracking shows clearly the position of satellites in geosynchronous orbit.
- Lyngsat list of satellites in geostationary orbit