Satellite communications: what’s the difference between LEO, MEO and GSO?

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.

How do satellites get into 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.

What does LEO stand for?

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.

What does MEO stand for?

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.

What does GSO stand for, and what’s the difference between GSO and GEO?

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.

Are there HEO satellites?

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.

What are the practical differences between LEO, MEO and GSO?

The altitude of a satellite can affect a number of things. For example:

• Cost of launch. Travelling to higher altitudes requires more powerful, more expensive rockets and larger quantities of fuel, meaning that low-altitude satellites are less expensive to launch. This can pay off multiple times; for example, the low orbit of the ISS makes it less expensive to send up supply craft.
• Cost of satellite. Low-altitude satellites can be smaller and less powerful – and therefore less expensive – because they don’t have to transmit signals as far as high-altitude satellites.
• Function. Some orbits are more useful for particular purposes than others. For example, satellites designed to observe or photograph Earth are often in relatively low orbits.
• Ability to provide consistent or widespread coverage. Because they have to move so quickly, a single LEO communications satellite won’t stay over any one location for long, meaning that you’ll need a lot of them to provide reliable coverage to that location. GEO satellites will stay in position, and their high altitude means that they can cover a large area, but they can only be positioned above the equator. The further from the equator you are, the less useful GEO satellites become.
• Latency. Signals have further to travel to and from high-altitude satellites, which means that a person connecting to a satellite may notice slightly longer delays in retrieving information from satellites at higher altitudes.
• Speed of orbital decay. Satellites in low orbit can encounter atmospheric drag, slowing them down and allowing gravity to draw them closer to the Earth. The lower they are, the greater the drag, and the faster their orbit decays. This means that LEO satellites often need to be either reboosted, using their own engines or another spacecraft to restore speed and altitude, or replaced. Either option is expensive. In 2010, the Ad Astra Rocket Company estimated the annual cost of keeping the ISS in a stable orbit, which requires multiple reboosts per year, at $210 million.
• Maximum number of satellites possible. GEO satellites have a natural limit on how many can exist at a time, as they’re at a specific height (35,786 km) and need to travel a specific route (the equator). In his 2000 paper ‘A Lost Connection’, published in the Berkeley Technology Law Journal, Lawrence D Roberts estimates that the equator can only hold up to 1,800 GEO satellites, and that many of the potential satellite positions wouldn’t be useful. At the moment, there are over 500 active GEO satellites.

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.