Geostationary orbit

A geostationary orbit, often referred to as a geosynchronous equatorial orbit[1] (GEO), is a circular geosynchronous orbit 35,786 km (22,236 mi) above Earth's equator and following the direction of Earth's rotation. An object in such an orbit appears motionless, at a fixed position in the sky, to ground observers. Communications satellites and weather satellites are often placed in geostationary orbits, so that the satellite antennae (located on Earth) that communicate with them do not have to rotate to track them, but can be pointed permanently at the position in the sky where the satellites are located. Using this characteristic, ocean-color monitoring satellites with visible and near-infrared light sensors (e.g. GOCI) can also be operated in geostationary orbit in order to monitor sensitive changes of ocean environments.

A geostationary orbit is a particular type of geosynchronous orbit, which has an orbital period equal to Earth's rotational period, or one sidereal day (23 hours, 56 minutes, 4 seconds). Thus, the distinction is that, while an object in geosynchronous orbit returns to the same point in the sky at the same time each day, an object in geostationary orbit never leaves that position. Geosynchronous orbits move around relative to a point on Earth's surface because, while geostationary orbits have an inclination of 0° with respect to the Equator, geosynchronous orbits have varying inclinations and eccentricities.

Geostationaryjava3D
Two geostationary satellites in the same orbit
Geosats compilation
A 5° × 6° view of a part of the geostationary belt, showing several geostationary satellites. Those with inclination 0° form a diagonal belt across the image; a few objects with small inclinations to the Equator are visible above this line. The satellites are pinpoint, while stars have created small trails due to Earth's rotation.

History

The first appearance of a geostationary orbit in popular literature was in the first Venus Equilateral story by George O. Smith,[2] but Smith did not go into details. British science fiction author Arthur C. Clarke disseminated the idea widely, with more details on how it would work, in a 1945 paper entitled "Extra-Terrestrial Relays — Can Rocket Stations Give Worldwide Radio Coverage?", published in Wireless World magazine. Clarke acknowledged the connection in his introduction to The Complete Venus Equilateral.[3] The orbit, which Clarke first described as useful for broadcast and relay communications satellites,[4] is sometimes called the Clarke Orbit.[5] Similarly, the Clarke Belt is the part of space about 35,786 km (22,236 mi) above sea level, in the plane of the equator, where near-geostationary orbits may be implemented. The Clarke Orbit is about 265,000 km (165,000 mi) in circumference.

Practical uses

Most commercial communications satellites, broadcast satellites and SBAS satellites operate in geostationary orbits. A geostationary transfer orbit is used to move a satellite from low Earth orbit (LEO) into a geostationary orbit. The first satellite placed into a geostationary orbit was the Syncom-3, launched by a Delta D rocket in 1964.

A worldwide network of operational geostationary meteorological satellites is used to provide visible and infrared images of Earth's surface and atmosphere. These satellite systems include:

A statite, a hypothetical satellite that uses a solar sail to modify its orbit, could theoretically hold itself in a geostationary "orbit" with different altitude and/or inclination from the "traditional" equatorial geostationary orbit.[6]

Communications

Satellites in geostationary orbits are far enough away from Earth that communication latency becomes significant — about a quarter of a second for a trip from one ground-based transmitter to the satellite and back to another ground-based transmitter; close to half a second for a round-trip communication from one Earth station to another and then back to the first.

For example, for ground stations at latitudes of φ = ±45° on the same meridian as the satellite, the time taken for a signal to pass from Earth to the satellite and back again can be computed using the cosine rule, given the geostationary orbital radius r (derived below), the Earth's radius R and the speed of light c, as

(Note that r is the orbital radius, the distance from the centre of the Earth, not the height above the equator.)

This delay presents problems for latency-sensitive applications such as voice communication.[7]

Geostationary satellites are directly overhead at the equator and appear lower in the sky to an observer nearer the poles. As the observer's latitude increases, communication becomes more difficult due to factors such as atmospheric refraction, Earth's thermal emission, line-of-sight obstructions, and signal reflections from the ground or nearby structures. At latitudes above about 81°, geostationary satellites are below the horizon and cannot be seen at all.[8] Because of this, some Russian communication satellites have used elliptical Molniya and Tundra orbits, which have excellent visibility at high latitudes.

Orbit allocation

Satellites in geostationary orbit must all occupy a single ring above the equator. The requirement to space these satellites apart to avoid harmful radio-frequency interference during operations means that there are a limited number of orbital "slots" available, and thus only a limited number of satellites can be operated in geostationary orbit. This has led to conflict between different countries wishing access to the same orbital slots (countries near the same longitude but differing latitudes) and radio frequencies. These disputes are addressed through the International Telecommunication Union's allocation mechanism.[9][10] In the 1976 Bogotá Declaration, eight countries located on the Earth's equator claimed sovereignty over the geostationary orbits above their territory, but the claims gained no international recognition.[11]

Orbital stability

A geostationary orbit can be achieved only at an altitude very close to 35,786 km (22,236 mi) and directly above the equator. This equates to an orbital velocity of 3.07 km/s (1.91 mi/s) and an orbital period of 1,436 minutes, which equates to almost exactly one sidereal day (23.934461223 hours). This ensures that the satellite will match the Earth's rotational period and has a stationary footprint on the ground. All geostationary satellites have to be located on this ring.

A combination of lunar gravity, solar gravity, and the flattening of the Earth at its poles causes a precession motion of the orbital plane of any geostationary object, with an orbital period of about 53 years and an initial inclination gradient of about 0.85° per year, achieving a maximal inclination of 15° after 26.5 years.[12] To correct for this orbital perturbation, regular orbital stationkeeping maneuvers are necessary, amounting to a delta-v of approximately 50 m/s per year.

A second effect to be taken into account is the longitudinal drift, caused by the asymmetry of the Earth – the equator is slightly elliptical. There are two stable (at 75.3°E and 252°E) and two unstable (at 165.3°E and 14.7°W) equilibrium points. Any geostationary object placed between the equilibrium points would (without any action) be slowly accelerated towards the stable equilibrium position, causing a periodic longitude variation.[12] The correction of this effect requires station-keeping maneuvers with a maximal delta-v of about 2 m/s per year, depending on the desired longitude.

Solar wind and radiation pressure also exert small forces on satellites; over time, these cause them to slowly drift away from their prescribed orbits.

In the absence of servicing missions from the Earth or a renewable propulsion method, the consumption of thruster propellant for station keeping places a limitation on the lifetime of the satellite. Hall-effect thrusters, which are currently in use, have the potential to prolong the service life of a satellite by providing high-efficiency electric propulsion.

Limitations to usable life of geostationary satellites

When they run out of thruster fuel, the satellites are at the end of their service life, as they are no longer able to stay in their allocated orbital position. The transponders and other onboard systems generally outlive the thruster fuel and, by stopping N–S station keeping, some satellites can continue to be used in inclined orbits (where the orbital track appears to follow a figure-eight loop centred on the equator),[13][14] or else be elevated to a "graveyard" disposal orbit.

Derivation of geostationary altitude

Comparison satellite navigation orbits
Comparison of geostationary Earth orbit with GPS, GLONASS, Galileo and Compass (medium Earth orbit) satellite navigation system orbits with the International Space Station, Hubble Space Telescope and Iridium constellation orbits, and the nominal size of the Earth.[a] The Moon's orbit is around 9 times larger (in radius and length) than geostationary orbit.[b]

In any circular orbit, the centripetal force required to maintain the orbit (Fc) is provided by the gravitational force on the satellite (Fg). To calculate the geostationary orbit altitude, one begins with this equivalence:

By Newton's second law of motion,[15] we can replace the forces F with the mass m of the object multiplied by the acceleration felt by the object due to that force:

We note that the mass of the satellite m appears on both sides — geostationary orbit is independent of the mass of the satellite.[c] Calculating the geostationary altitude, therefore, simplifies down to calculating the altitude where the magnitudes of the centripetal acceleration required for orbital motion and the gravitational acceleration provided by Earth's gravity are equal.

The centripetal acceleration's magnitude is:

where ω is the angular speed, and r is the orbital geocentric radius (measured from the Earth's center of mass).

The magnitude of the gravitational acceleration is:

where M is the mass of Earth, 5.9736 × 1024 kg, and G is the gravitational constant, (6.67428 ± 0.00067) × 10−11 m3 kg−1 s−2.

Equating the two accelerations gives:

The product GM is known with much greater precision than either factor alone; it is known as the geocentric gravitational constant μ = 398,600.4418 ± 0.0008 km3 s−2. Hence

The angular speed ω is found by dividing the angle travelled in one revolution (360° = 2π rad) by the orbital period (the time it takes to make one full revolution). In the case of a geostationary orbit, the orbital period is one sidereal day, or 86164.09054 s).[16] This gives

The resulting orbital radius is 42,164 kilometres (26,199 mi). Subtracting the Earth's equatorial radius, 6,378 kilometres (3,963 mi), gives the altitude of 35,786 kilometres (22,236 mi).

Orbital speed is calculated by multiplying the angular speed by the orbital radius:

By the same formula, we can find the geostationary-type orbit of an object in relation to Mars (this type of orbit above is referred to as an areostationary orbit if it is above Mars). The geocentric gravitational constant GM (which is μ) for Mars has the value of 42,828 km3s−2, and the known rotational period (T) of Mars is 88,642.66 seconds. Since ω = 2π/T, using the formula above, the value of ω is found to be approx 7.088218×10−5 s−1. Thus r3 = 8.5243×1012 km3, whose cube root is 20,427 km (the orbital radius); subtracting the equatorial radius of Mars (3396.2 km) gives the orbital altitude of 17,031 km.

Orbital speed of a Mars geostationary orbit can be calculated as for Earth above:

See also

Notes

  1. ^ Orbital periods and speeds are calculated using the relations 4π²R³ = T²GM and V²R = GM, where R = radius of orbit in metres, T = orbital period in seconds, V = orbital speed in m/s, G = gravitational constant ≈ 6.673×1011 Nm²/kg², M = mass of Earth ≈ 5.98×1024 kg.
  2. ^ Approximately 8.6 times when the moon is nearest (363 104 km ÷ 42 164 km) to 9.6 times when the moon is farthest (405,696 km ÷ 42,164 km).
  3. ^ In the small-body approximation, the geostationary orbit is independent of the satellite's mass. For satellites having a mass less than M μerr/μ ≈ 1015 kg, that is, over a billion times that of the ISS, the error due to the approximation is smaller than the error on the universal geocentric gravitational constant (and thus negligible).

References

  1. ^ "Ariane 5 User's Manual Issue 5 Revision 1" (PDF). arianespace. July 2011. Archived from the original (PDF) on 4 October 2013. Retrieved 28 July 2013.
  2. ^ "(Korvus's message is sent) to a small, squat building at the outskirts of Northern Landing. It was hurled at the sky. … It … arrived at the relay station tired and worn, … when it reached a space station only five hundred miles above the city of North Landing." Smith, George O. (1976). The Complete Venus Equilateral. New York: Ballantine Books. pp. 3–4. ISBN 978-0-345-28953-7.
  3. ^ "It is therefore quite possible that these stories influenced me subconsciously when … I worked out the principles of synchronous communications satellites …", op. cit, p. x
  4. ^ "Extra-Terrestrial Relays — Can Rocket Stations Give Worldwide Radio Coverage?" (PDF). Arthur C. Clarke. October 1945. Archived from the original (PDF) on 18 March 2009. Retrieved 4 March 2009.
  5. ^ "Basics of Space Flight Section 1 Part 5, Geostationary Orbits". NASA. Retrieved 21 June 2009.
  6. ^ US patent 5183225, Forward, Robert, "STATITE: SPACECRAFT THAT UTILIZES SIGHT PRESSURE AND METHOD OF USE", published 1993-02-02
  7. ^ The Teledesic Network: Using Low-Earth-Orbit Satellites to Provide Broadband, Wireless, Real-Time Internet Access Worldwide.
  8. ^ p. 123.
  9. ^ [1] Archived March 27, 2009, at the Wayback Machine
  10. ^ ITU Space Services Division.
  11. ^ Oduntan, Gbenga. "The Never Ending Dispute: Legal Theories on the Spatial Demarcation Boundary Plane between Airspace and Outer Space" (PDF). Hertfordshire Law Journal, 1(2), p. 75.
  12. ^ a b OPERATIONAL CONSIDERATIONS OF GEO DEBRIS SYNCHRONIZATION DYNAMICS. Anderson
  13. ^ Shi Hu-Li, Han Yan-Ben, Ma Li-Hua, Pei Jun, Yin Zhi-Qiang and Ji Hai-Fu (2010). Beyond Life-Cycle Utilization of Geostationary Communication Satellites in End-of-Life, Satellite Communications, Nazzareno Diodato (Ed.), ISBN 978-953-307-135-0, InTech, "Beyond Life-Cycle Utilization of Geostationary Communication Satellites in End-of-Life".
  14. ^ "Inclined orbit operation".
  15. ^ "Newton's Second Law". The Physics Classroom.
  16. ^ Edited by P. Kenneth Seidelmann, "Explanatory Supplement to the Astronomical Almanac", University Science Books,1992, p. 700.

 This article incorporates public domain material from the General Services Administration document "Federal Standard 1037C" (in support of MIL-STD-188).

External links

Apstar 6C

Apstar 6C is a Chinese communications satellite which is operated by APT Satellite Holdings as part of the Apstar system.

Astra 1M

Astra 1M is a Luxembourgian geostationary communications satellite which is operated by SES. It is positioned in geostationary orbit at a longitude of 19.2 degrees East, from where it is used to provide direct-to-home broadcasting to Europe.

Azerspace-2

Azerspace-2, also known as Intelsat 38, is Azerbaijan's second telecommunications satellite, built by U.S.-based Space Systems Loral and managed by Azercosmos.

DirecTV-11

DirecTV-11, also known as D11, is a Boeing model 702 satellite built by the Boeing Satellite Development Center.

GSAT-11

GSAT-11 is an Indian geostationary communications satellite. The 5854 kg satellite is based on the new I-6K Bus and carry 40 transponders in the Ku-band and Ka-band frequencies (32 Ka × Ku-Band Forward Link Transponders and 8 Ku × Ka band Return Link Transponders), which are capable of providing up to 16 Gbit/s throughput. GSAT-11 is India's heaviest satellite.

GSAT-31

GSAT-31 is a high-throughput telecommunication satellite developed by the Indian Space Research Organisation (ISRO).

Geosynchronous orbit

A geosynchronous orbit (sometimes abbreviated GSO) is an orbit around Earth of a satellite with an orbital period that matches Earth's rotation on its axis, which takes one sidereal day (about 23 hours, 56 minutes, and 4 seconds). The synchronization of rotation and orbital period means that, for an observer on Earth's surface, an object in geosynchronous orbit returns to exactly the same position in the sky after a period of one sidereal day. Over the course of a day, the object's position in the sky traces out a path, typically in a figure-8 form, whose precise characteristics depend on the orbit's inclination and eccentricity. Satellites are typically launched in an eastward direction. A geosynchronous orbit is 35,786 km (22,236 mi) above the Earth's surface. Those closer to Earth orbit faster than Earth rotates, so from Earth, they appear to move eastward while those that orbit beyond geosynchronous distances appear to move westward.

A special case of geosynchronous orbit is the geostationary orbit, which is a circular geosynchronous orbit in Earth's equatorial plane (that is, directly above the Equator). A satellite in a geostationary orbit appears stationary, always at the same point in the sky, to observers on the surface. Popularly or loosely, the term geosynchronous may be used to mean geostationary. Specifically, geosynchronous Earth orbit (GEO) may be a synonym for geosynchronous equatorial orbit, or geostationary Earth orbit. Communications satellites are often given geostationary or close to geostationary orbits so that the satellite antennas that communicate with them do not have to move, but can be pointed permanently at the fixed location in the sky where the satellite appears.

A semi-synchronous orbit has an orbital period of half a sidereal day (i.e., 11 hours and 58 minutes). Relative to Earth's surface, it has twice this period and hence appears to go around Earth once every day. Examples include the Molniya orbit and the orbits of the satellites in the Global Positioning System.

Intelsat 15

Intelsat 15, also known as IS-15, is a communications satellite owned by Intelsat. Intelsat 15 was built by Orbital Sciences Corporation, on a Star-2.4. It is located at 85° E longitude on the geostationary orbit. It was launched from Baikonur Cosmodrome to a geosynchronous transfer orbit on 30 November 2009 by a Zenit-3SLB launch vehicle. It has 22 active Ku band transponders, plus eight spares. Five of those transponders are owned and operated by SKY Perfect JSAT Group under the name JCSAT-85.

Intelsat 21

Intelsat 21 is a communications satellite manufactured by Boeing Space Systems for the Intelsat Corp, based on the BSS-702MP platform. It was launched in August 2012 from a mobile platform in the equatorial Pacific Ocean. and replaces the Intelsat 9 satellite at 58° West Longitude.

Intelsat 3R

Intelsat 3R (formerly PAS-3R) is a communications satellite owned by Intelsat located at 43° W longitude, serving the Americas market.

Intelsat 604

Intelsat 604, previously named Intelsat VI F-4, was a communications satellite operated by Intelsat. Launched in 1990, it was the third of five Intelsat VI satellites to be launched. The Intelsat VI series was constructed by Hughes Aircraft, based on the HS-389 satellite bus.Intelsat 604 was launched at 11:19 UTC on 23 June 1990, atop a Commercial Titan III carrier rocket, flight number CT-3, with an Orbus-21S upper stage. The launch took place from Launch Complex 40 at Cape Canaveral Air Force Station, and successfully placed Intelsat 604 into a geosynchronous transfer orbit. The satellite raised itself into its final geostationary orbit using two liquid-fuelled R-4D-12 engines, with the satellite arriving in geostationary orbit on 28 June 1990.Intelsat 604 initially operated in a geostationary orbit with a perigee of 35,692 kilometres (22,178 mi), an apogee of 35,887 kilometres (22,299 mi), and 0.3 degrees of inclination, however over time this became more inclined. The satellite carried 38 IEEE C band and ten IEEE Ku band transponders, and had a design life of 13 years and a mass of 4,215 kilograms (9,292 lb).Following its arrival in geostationary orbit, Intelsat 604 was deployed at a longitude of 38 degrees west. It was moved to 27.5 degrees west in January 1991, where it operated until February 1992. From October 1992 to March 2002, it was operated at 60 degrees east. After leaving this position, it was positioned at 157 degrees east from August 2002 to September 2005. Its final deployment was from February to March 2006, at 177.85 degrees. The satellite was decommissioned on 6 April 2006 after it had been moved to a graveyard orbit.

QuetzSat 1

QuetzSat 1 is a Mexican high-power geostationary communications satellite which is operated by the Mexican operator QuetzSat (SES S. A./Grupo MedCom). It is positioned in geostationary orbit, and located at 77° West, from where it provide direct broadcasting services to United States and a part of Mexico for Dish Mexico.QuetzSat 1 was built by Space Systems/Loral, and is based on the LS-1300 satellite bus. It is equipped with 32 Ku band transponder and at launch it had a mass of 5,514 kilograms (12,156 lb). It has a design life of fifteen years. QuetzSat 1 is part of the SES satellite fleet.

SES-11

SES-11 or EchoStar 105 is a geostationary communications satellite operated by SES and designed and manufactured by Airbus Defence and Space. It has a mass of 5,200 kg (11,500 lb) and has a design life of at least 15 years.

SES-14

SES-14 is a geostationary communications satellite operated by SES and designed and manufactured by Airbus Defence and Space. The satellite launched on 25 January 2018 along with the GOLD instrument from NASA. It has a mass of 4,423 kg (9,751 lb) and has a design life of at least 15 years.

SES-16

SES-16 is a geostationary communications satellite operated by SES and designed and manufactured by Orbital ATK. It has a mass of 4,230 kg (9,330 lb) and has a design life of at least 15 years.

SES-6

SES-6 is a commercial geostationary communication satellite owned and operated by SES.

Constructed by EADS Astrium, it was launched on June 3, 2013 and carries 48 Ku-band and 43 C-band transponders.

SES-7

SES-7 (formerly known as Indostar 2 / ProtoStar 2) is a Dutch commercial communication satellite operated by SES World Skies. Originally launched on 16 May 2009 by Boeing for ProtoStar Ltd and later purchased through auction by SES S.A. for SES World Skies unit for $180 million. SES-7 operates in geostationary orbit and intended to be located at a longitude of 108.2° East covering South Asia and Asia-Pacific region. SES-7 is built for optimized HD DTH (direct-to-home) satellite television service and broadband Internet access across the Asia-Pacific region.

The spacecraft was originally built for PanAmSat (now Intelsat) to be used as Galaxy-8iR, but that contract was terminated on November 15, 2002. The satellite was renamed SES-7 in May 2010.

Thaicom 6

THAICOM 6 (Thai: ไทยคม 6) is a Thai satellite of the Thaicom series, operated by Thaicom Public Company Limited, a subsidiary of INTOUCH headquartered in Bangkok, Thailand. THAICOM 6 is colocated with Thaicom 5 at 78.5 degrees East, in geostationary orbit. The total cost for the satellite is US$160 million.

Tianlian I

Tianlian I (Simplified Chinese: 天链一号, Traditional Chinese: 天鏈一號, English: Sky Chain), also known as Tian Lian 1, TL-1, and CTDRS-1 is a Chinese data tracking and relay communications satellite series. Based on the DFH-3 satellite bus, it will provide communication coverage for manned Shenzhou missions, from Shenzhou 7 onwards. Functionally, it is similar to the United States Tracking and Data Relay Satellite System.

The first satellite of the series, Tianlian I-01, was launched on the maiden flight of the Long March 3C carrier rocket, at 15:35 GMT on 25 April 2008, from LC-2 at the Xichang Satellite Launch Centre. Tianlian I-01 is able to cover around half of the spacecraft's trajectory, compared to around 12 percent which had been covered using tracking stations and a fleet of ships. Tianlian I-02 was launched on 11 July 2011, Tianlian I-03 on 25 July 2012 and Tianlian I-04 on 22 November 2016. The satellites are in geostationary orbit.

This page is based on a Wikipedia article written by authors (here).
Text is available under the CC BY-SA 3.0 license; additional terms may apply.
Images, videos and audio are available under their respective licenses.