We’ve been talking a lot about terrestrial communications technology recently. For example, take a look at our articles on the history of mobile technology, or on the applications of 5G.
However, that’s not the only kind of communications technology we work with at Darwin. We’re enabling seamless, reliable connectivity by harnessing transmitters both on Earth and in space. Today, we’re discussing different types of satellite orbit.
Satellites are taken into space by rockets and released at high speed, travelling over the Earth at thousands of kilometres per hour. The pull of Earth’s gravity prevents the satellite from flying off into space, but the speed of the satellite prevents it from being pulled down to the surface, as the curvature of the Earth means the ground is always falling away. This combination means that the satellite falls into orbit, looping constantly around the planet.
LEO stands for ‘low Earth orbit’. LEO satellites orbit the Earth between 160 km and 1,000 km above the planet’s surface, or between 160 km and 2,000 km according to some definitions.
The pull of Earth’s gravity becomes stronger as you approach the planet. Because of this, LEO satellites, being relatively close to Earth, need to move very, very fast to counteract gravity.
For example, the International Space Station (ISS), at the relatively low altitude of 400 km, is moving at roughly 27,600 km per hour and orbits Earth about 16 times per day. Geostationary satellites, at the much higher altitude of 35,786 km, move at less than half that speed: about 11,000 km per hour.
MEO stands for ‘medium Earth orbit’, and refers to satellites orbiting the Earth between the LEO and GSO levels. As there’s disagreement over whether LEO ends at 1,000 or 2,000 km, some satellites may be considered to be in either LEO or MEO, depending on the definition used.
You might hear GSO and GEO satellites mentioned in similar contexts. The terms have some overlap, but they’re not identical.
GSO stands for ‘geosynchronous orbit’, meaning the satellite’s orbit is synchronised with the rotation of the Earth. In other words, it takes a day for the Earth to complete a revolution, and it also takes a day for a GSO satellite to complete one orbit of the Earth.
GEO stands for ‘geostationary equatorial orbit’. This is a type of GSO that follows the equator, travelling in the direction of Earth’s rotation. Satellites in GEO always appear to be in the same place, relative to Earth – so, for example, as the Earth rotates, a GEO satellite above Brazil will keep moving so it constantly remains above Brazil.
All GSO satellites (including GEO satellites) are approximately 35,786 km above the Earth’s surface: the only altitude at which geosynchronous orbit can be maintained.
Knowing that low and medium Earth orbit satellites exist, you might expect there to be a high Earth orbit satellite, or HEO.
The acronym HEO is sometimes used for satellites, but it doesn’t stand for ‘high Earth orbit’. HEO is short for ‘highly elliptical orbit’, and it refers to orbits where, rather than remaining at approximately the same height above the Earth at all times, the satellite is much closer to the planet at some points in its orbit than at others.
Satellites with elliptical orbits spend longer over some parts of the planet than over others, which can be useful for communications.
The altitude of a satellite can affect a number of things. For example:
According to the UCS, there are currently over 3,000 operational satellites in our skies. We’ve come a long way since the Soviet Union launched the first manmade satellite, Sputnik 1, in 1957. In a future post, we’ll look in more detail at what those satellites are actually used for.
Darwin Innovation Group is a UK-based company that provides services related to autonomous vehicles and communications. If you’re interested in working with us, take a look at our careers page. If you’d like to know how we can help your organisation make use of autonomous vehicles, contact us. You can also follow us on LinkedIn or Twitter.