Geostationary orbit

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Two geostationary satellites in the same orbit
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.

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 has an orbital period equal to the Earth's rotational period, one sidereal day, so to ground observers it appears motionless, at a fixed position in the sky. The concept of a geostationary orbit was popularised by Arthur C. Clarke in the 1940s as a way to revolutionise telecommunications, and the first satellite to be placed in this orbit was launched in 1963.

Communications satellites are often placed in a geostationary orbit so that Earth based satellite antennas (located on Earth) do not have to rotate to track them, but can be pointed permanently at the position in the sky where the satellites are located. Weather satellites are also placed in this orbit for real time monitoring and data collection, and Navigation satellites to provide a known calibration point and enhance GPS accuracy.

History

The first appearance of a geostationary orbit in popular literature was in October, 1942, 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 popularised and expanded the concept 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 collection of artificial satellites in this orbit is known as the Clarke Belt.[6]

The first geostationary satellite was designed by Harold Rosen while he was working at Hughes Aircraft in 1959. Inspired by Sputnik, he wanted to use a geostationary satellite to globalise communications. At the time, telecommunications between the US and Europe was possible between just 136 people at a time, and reliant on HF radios and an undersea cable.[7]

Conventional wisdom at the time was that it would require too much rocket power to place a satellite in a geostationary orbit and it would not survive long enough to justify the expense, so early communication satellites were placed in a low Earth orbit.[8] The first of these was the passive Echo balloon satellites in 1960, followed by Telstar 1 in 1962. Although these projects had difficulties with signal strength and tracking, the geostationary concept was seen as impractical, so Hughes often withheld funds and support.[9][7]

By 1961 Rosen his team had produced a cylindrical prototype with a diameter of 76 centimetres (30 in), height of 38 centimetres (15 in), weighing 11.3 kilograms (25 lb), light and small enough to be placed into orbit. It was spin stabilised and produced a flattened waveform. In August 1961, they were contracted to began building the real satellite.[7]

They lost Syncom 1 to electronics failure, but Syncom 2 was successfully placed into a geosynchronous orbit in 1963. Although its inclined orbit still required moving antennas it was able to relay TV transmissions, and allowed for US President Kennedy to phone Nigerian PM Balewa from a ship.[9][10]

The first satellite placed in a geostationary orbit was Syncom 3, which was launched by a Delta D rocket in 1963. With its increased bandwidth this satellite was able to transmit live coverage of the Summer Olympics from Japan to America. Geostationary orbits have been in common use ever since, in particular for satellite television.[9]

Today there are hundreds of geostationary satellites providing remote sensing and communications.[7][11]

Although most populated land locations on the planet now have terrestrial communications facilities (microwave, fiber-optic), with telephone access covering 96% of the population and internet access 90%[12] some rural and remote areas are still reliant on satellite communications.[13][14]

Uses

Most commercial communications satellites, broadcast satellites and SBAS satellites operate in geostationary orbits.[citation needed]

Communications

Geostationary communication satellites are useful because of their large coverage, extending 81°, and stationary position in the sky, eliminating the need for movable ground antennas.[15]

However, latency becomes significant — about 250ms for a trip from one ground-based transmitter to the satellite and back to another ground-based transmitter.

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

This delay presents problems for latency-sensitive applications such as voice communication,[16] so geostationary communication satellites are primarily used for unidirectional entertainment and applications where low latency alternatives are not available.[15]

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.[17] Because of this, some Russian communication satellites have used elliptical Molniya and Tundra orbits, which have excellent visibility at high latitudes.[18]

Meteorology

A worldwide network of operational geostationary meteorological satellites is used to provide visible and infrared images of Earth's surface and atmosphere for weather observation, oceanography, and atmospheric tracking. As of 2019 there are 19 satellites in either operation or stand-by.[19] These satellite systems include:

These satellites typically captures images in the visual and infrared spectrum with a spatial resolution between 0.5 and 4 square kilometres. The coverage is typically 70°,[27] and in some cases less.[28]

Geostationary satellite imagery has been used for tracking volcanic ash,[29] measuring cloud top temperature and water vapour, oceanography[30], facilitating cyclone path prediction[26] and providing real time cloud coverage and other tracking data[31] Some information has been incorporated into meteorological prediction models, but geostationary weather satellite images are primarily used for short-term and real-time forecasting.[32][33][34]

Navigation

Service areas of satellite-based augmentation systems (SBAS).

Geostationary satellites can be used to augment GNSS systems by relaying clock, ephemeris and ionospheric error corrections (calculated from ground stations of a known position) and providing an additional reference signal.[35][36]

This improves position accuracy from ~5m to ~1m or less.[37]

Past and current navigation systems that use geostationary satellites include:

Implementation

Launch

An example of a transition from GTO to GSO.
  EchoStar XVII ·   Earth.

Geostationary satellites are launched from as close to the equator as possible, to provide the maximum launch boost and to limit the amount of inclination change needed later.[41]

Most launch vehicles place geostationary satellites directly into a geostationary transfer orbit (GTO), an elliptical orbit with an apogee at GEO height and a low perigee. On board satellite propulsion is then used to raise the perigee and reach GEO.[42]

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.[43][44] 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.[45]

Statite proposal

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.[46]

Retired Satellites

When geostationary satellites run out of thruster fuel and are no longer able to stay in their allocated orbital position they are generally retired. The transponders and other onboard systems often 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),[47][48] or else be elevated to a "graveyard" disposal orbit. This process is becoming increasingly regulated and satellites must have a 90% chance of moving over 200km above the getostationary belt at end of life.[49]

Space debris

Earth from space, surrounded by small white dots
A computer-generated image representing space debris as could be seen from high Earth orbit. The two main debris fields are the ring of objects in geosynchronous Earth orbit (GEO) and the cloud of objects in low Earth orbit (LEO).

Space debris at geostationary orbits typically has a lower collision speed than at LEO since orbits are mostly synchronous, however the presence of satellites in eccentric orbits allows for collision at up to 4km/s. Although a collision is comparatively unlikely, GEO satellites have a limited ability to avoid any debris.[50]

Debris less than 10cm in diameter can't be seen from the Earth making it difficult to assess their prevalence.[51]

Despite efforts to reduce risk, spacecraft collisions have occurred. The European Space Agency telecom satellite Olympus-1 was struck by a meteoroid on 11 August 1993 and eventually moved to a graveyard orbit,[52] and in 2006 the Russian Express-AM11 communications satellite was struck by an unknown object and rendered inoperable;[53] although its engineers had enough contact time with the satellite to send it into a graveyard orbit. In 2017 both AMC-9 and Telkom-1 broke apart from an unknown cause.[54][51][55]

Properties

A typical geostationary orbit has the following properties:

  • Inclination: 0°
  • Period: 1436 minutes (one sidereal day)[56]:121
  • Eccentricity: 0
  • Argument of perigee: undefined
  • Semi-major axis: 42,164 km

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, one sidereal day. 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.[57][56]:156 To correct for this orbital perturbation, regular orbital stationkeeping maneuvers are necessary, amounting to a delta-v of approximately 50 m/s per year.[58]

A second effect to be taken into account is the longitudinal drift, caused by the asymmetry of the Earth – the equator is slightly elliptical.[56]:156 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.[57] 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.[58]

Solar wind and radiation pressure also exert small forces on satellites; over time, these cause them to slowly drift away from their prescribed orbits.[59]

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.[58]

Derivation of geostationary altitude

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, 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:

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).[60] 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).

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

External links

  • Orbital Mechanics (Rocket and Space Technology)
  • List of satellites in geostationary orbit
  • Clarke Belt Snapshot Calculator
  • 3D Real Time Satellite Tracking
  • Geostationary satellite orbit overview
  • Daily animation of the Earth, made by geostationary satellite 'Electro L' photos Satellite shoots 48 images of the planet every day.
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