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 may remain still or trace 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 circular geosynchronous orbit is 35,786 km (22,236 mi) above 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.
The first appearance of a geosynchronous orbit in popular literature was in October 1942, in the first Venus Equilateral story by George O. Smith, 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. The orbit, which Clarke first described as useful for broadcast and relay communications satellites, is sometimes called the Clarke Orbit. Similarly, the collection of artificial satellites in this orbit is known as the Clarke Belt.
The first geosynchronous satellite was designed by Harold Rosen while he was working at Hughes Aircraft in 1959. Inspired by Sputnik 1, he wanted to use a geostationary (geosynchronous equatorial) satellite to globalise communications. Telecommunications between the US and Europe was then possible between just 136 people at a time, and reliant on high frequency radios and an undersea cable.
Conventional wisdom at the time was that it would require too much rocket power to place a satellite in a geosynchronous orbit and it would not survive long enough to justify the expense, so early efforts were put towards constellations of satellites in low or medium Earth orbit. The first of these were the passive Echo balloon satellites in 1960, followed by Telstar 1 in 1962. Although these projects had difficulties with signal strength and tracking, that could be solved through geosynchronous satellites, the concept was seen as impractical, so Hughes often withheld funds and support.
By 1961, Rosen and 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 with a dipole antenna producing a pancake shaped waveform. In August 1961, they were contracted to begin building the real satellite. 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 John F. Kennedy to phone Nigerian prime minister Abubakar Tafawa Balewa from a ship on August 23, 1963.
Although most populated land locations on the planet now have terrestrial communications facilities (microwave, fiber-optic), which often have latency and bandwidth advantages, and telephone access covering 96% of the population and internet access 90%, some rural and remote areas in developed countries are still reliant on satellite communications.
A geostationary equatorial orbit (GEO) is a circular geosynchronous orbit in the plane of the Earth's equator with a radius of approximately 42,164 km (26,199 mi) (measured from the center of the Earth).:156 A satellite in such an orbit is at an altitude of approximately 35,786 km (22,236 mi) above mean sea level. It maintains the same position relative to the Earth's surface. If one could see a satellite in geostationary orbit, it would appear to hover at the same point in the sky, i.e., not exhibit diurnal motion, while the Sun, Moon, and stars would traverse the skies behind it. Such orbits are useful for telecommunications satellites.
A perfectly stable geostationary orbit is an ideal that can only be approximated. In practice the satellite drifts out of this orbit because of perturbations such as the solar wind, radiation pressure, variations in the Earth's gravitational field, and the gravitational effect of the Moon and Sun, and thrusters are used to maintain the orbit in a process known as station-keeping.:156
Elliptical geosynchronous orbit
Elliptical geosynchronous orbits are used in communications satellites to keep the satellite in view of its assigned ground stations and receivers. A satellite in an elliptical geosynchronous orbit appears to oscillate in the sky from the viewpoint of a ground station, tracing an analemma in the sky. Satellites in highly elliptical orbits must be tracked by steerable ground stations.
Inclined geosynchronous orbit
An inclined geosynchronous orbit (IGSO) is a geosynchronous orbit with a non zero inclination. It traces an analemma (figure 8) across the sky.
Due to their inherent instability, geostationary orbits will eventually become inclined if they are not corrected using thrusters. At the end of the satellite's lifetime, when fuel approaches depletion, satellite operators may decide to omit these expensive manoeuvres to correct inclination and only control eccentricity. This prolongs the life-time of the satellite as it consumes less fuel over time, but the satellite can then only be used by ground antennas capable of following the north-south movement, satellite-tracking Earth stations.
NAVIC is a regional — i.e. non-global — Indian navigation system currently operating with 7 satellites, of which 3 are in geostationary orbit and 4 in geosynchronous orbit.
The Chinese BeiDou GNSS constellation uses five satellites in a geosynchronous orbit.
The communications satellite AMC-14 was sold to the US government and placed in a 13° inclined geosynchronous orbit following a rocket malfunction.
The Quasi-Zenith Satellite System (QZSS) is a three-satellite regional time transfer system and enhancement for GPS, covering Japan at high elevation. Each satellite dwells over Japan, allowing signals to reach receivers in urban canyons then passes quickly over Australia.
The Tundra orbit is an eccentric Russian geosynchronous orbit, which allows the satellite to spend most of its time over one location. It sits at an inclination of 63.4°, which is a frozen orbit, which reduces the need for stationkeeping. It is used by the Sirius XM Satellite Radio to improve signal strength in northern US and Canada.
Other related orbit types are:
- Supersynchronous orbit: a disposal / storage orbit above GSO/GEO. Satellites drift in a westerly direction.
- Subsynchronous orbit: a drift orbit close to but below GSO/GEO. Used for satellites undergoing station changes in an eastern direction.
- Graveyard orbit: a supersynchronous orbit where geostationary satellites are intentionally placed at the ends of their operational lives.
It would hold its location over the dark side of the Earth at a latitude of approximately 30 degrees. It would return to the same spot in the sky every 24 hours from an Earth-based viewer's perspective, so be functionally similar to a geosynchronous orbit.
A further form of geosynchronous orbit is the theoretical space elevator. When one end in attached to the ground, for altitudes below the geostationary belt the elevator maintains a shorter orbital period than by gravity alone.
Other synchronous orbits
Synchronous orbits can only exist for bodies that have a fixed surface (e.g. moons, rocky planets). Without such a surface (e.g. gas giants, black holes) there is no fixed point an orbit can be said to synchronise with. No synchronous orbit exists if the body rotates so slowly that the orbit would be outside its Hill sphere, or so quickly that it would be inside the body. Large bodies held together by gravity cannot rotate that quickly, since they would fly apart, so the last condition only applies to small bodies held together by other forces, e.g. smaller asteroids. Most inner moons of planets have synchronous rotation, so their synchronous orbits are, in practice, limited to their leading and trailing (L4 and L5) Lagrange points, as well as the L1 and L2 Lagrange points, assuming they do not fall within the body of the moon. Objects with chaotic rotations, such as exhibited by Hyperion, are also problematic, as their synchronous orbits change unpredictably.
A typical geosynchronous orbit has the following properties:
- Period: 1436 minutes (one sidereal day):121
- Semi-major axis: 42,164 km
- Inclinations: 0° (Geostationary), 63.4° (Tundra)
All geosynchronous orbits have an orbital period equal to exactly one sidereal day. This means that the satellite will return to the same point above the Earth's surface every (sidereal) day, regardless of other orbital properties.:121 This orbital period, T, is directly related to the semi-major axis of the orbit through the formula:
- a is the length of the orbit's semi-major axis
- is the standard gravitational parameter of the central body:137
An inclination of zero is used for geostationary satellites, ensuring that the orbit remains over the equator at all times, making it stationary with respect to latitude from the point of view of a ground observer (and in the ECEF reference frame).:122
Other popular inclinations include 63.4° for a Tundra orbit, which ensures that the orbit's argument of perigee doesn't change over time.
In the special case of a geostationary orbit, the ground track of a satellite is a single point on the equator. In the general case of a geosynchronous orbit with a non-zero inclination or eccentricity, the ground track is a more or less distorted figure-eight, returning to the same places once per sidereal day.
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