Geostationary orbit

Circular orbit above Earth's Equator and following the direction of Earth's rotation

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 star trails due to Earth's rotation.

A geostationary orbit, also referred to as a geosynchronous equatorial orbit[a] (GEO), is a circular geosynchronous orbit 35,786 km (22,236 mi) in altitude above Earth's equator, 42,164 km (26,199 mi) in radius from Earth's center, and following the direction of Earth's rotation.

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

Communications satellites are often placed in a geostationary orbit so that Earth-based satellite antennas 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.

Geostationary satellites are launched via a temporary orbit, and placed in a slot above a particular point on the Earth's surface. The orbit requires some stationkeeping to keep its position, and modern retired satellites are placed in a higher graveyard orbit to avoid collisions.

History

Syncom 2, the first geosynchronous satellite

In 1929, Herman Potočnik described both geosynchronous orbits in general and the special case of the geostationary Earth orbit in particular as useful orbits for space stations.[1] 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][4] 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]

In technical terminology the orbit is referred to as either a geostationary or geosynchronous equatorial orbit, with the terms used somewhat interchangeably.[7]

The first geostationary 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 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.[8]

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,[9] so early efforts were put towards constellations of satellites in low or medium Earth orbit.[10] The first of these were the passive Echo balloon satellites in 1960, followed by Telstar 1 in 1962.[11] Although these projects had difficulties with signal strength and tracking, issues that could be solved using geostationary orbits, the concept was seen as impractical, so Hughes often withheld funds and support.[10][8]

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.[12] In August 1961, they were contracted to begin building the real satellite.[8] 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 in Washington D.C., to phone Nigerian prime minister Abubakar Tafawa Balewa aboard the USNS Kingsport docked in Lagos on August 23, 1963.[10][13]

The first satellite placed in a geostationary orbit was Syncom 3, which was launched by a Delta D rocket in 1964.[14] 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.[10]

Today there are hundreds of geostationary satellites providing remote sensing and communications.[8][15]

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%,[16] some rural and remote areas in developed countries are still reliant on satellite communications.[17][18]

Uses

Most commercial communications satellites, broadcast satellites and SBAS satellites operate in geostationary orbits.[19][20][21]

Communications

Geostationary communication satellites are useful because they are visible from a large area of the earth's surface, extending 81° away in latitude and 77° in longitude.[22] They appear stationary in the sky, which eliminates the need for ground stations to have movable antennas. This means that Earth-based observers can erect small, cheap and stationary antennas that are always directed at the desired satellite.[23]: 537  However, latency becomes significant as it takes about 240 ms for a signal to pass from a ground based transmitter on the equator to the satellite and back again.[23]: 538  This delay presents problems for latency-sensitive applications such as voice communication,[24] so geostationary communication satellites are primarily used for unidirectional entertainment and applications where low latency alternatives are not available.[25]

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.[22] Because of this, some Russian communication satellites have used elliptical Molniya and Tundra orbits, which have excellent visibility at high latitudes.[26]

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.[27] 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.[35] The coverage is typically 70°,[35] and in some cases less.[36]

Geostationary satellite imagery has been used for tracking volcanic ash,[37] measuring cloud top temperatures and water vapour, oceanography,[38] measuring land temperature and vegetation coverage,[39][40] facilitating cyclone path prediction,[34] and providing real time cloud coverage and other tracking data.[41] Some information has been incorporated into meteorological prediction models, but due to their wide field of view, full-time monitoring and lower resolution, geostationary weather satellite images are primarily used for short-term and real-time forecasting.[42][40]

Navigation

Service areas of satellite-based augmentation systems (SBAS).[20]

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.[43] This improves position accuracy from approximately 5m to 1m or less.[44]

Past and current navigation systems that use geostationary satellites include:

Implementation

Launch

  EchoStar XVII ·   Earth.