Space Sustainability Report
Insight | Space explained: How do satellite orbits work?
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On Earth, we are never more than around 100km from space. This is the height of the Kármán Line, the generally accepted international standard for where Earth ends and space begins. This theoretical line is the point that scientist Theodore von Kármán proposed an aircraft could reach before the surrounding air became too thin for it to fly. However, it is mainly today used as a reference point. Most commercial airlines generally fly up to 11km altitude, while weather balloons can reach up to 50km.
The Kármán Line falls within the thermosphere, the fourth highest of the five sections of the Earth’s atmosphere. The thermosphere is the thickest section rising to 690km above the Earth. Here, you will find the Hubble Telescope and the International Space Station in what is called ‘low Earth orbit’.
Low Earth orbit, or LEO, begins around 160km above the Earth’s surface and rises to around 2,000km, however typically you will find most satellites orbiting at around 400km. Here, satellites, stations or other objects must move extremely quickly to stay in orbit and avoid the downward pull of gravity, as the Earth’s gravitational pull is stronger the closer an object is to the surface. The International Space Station, for example, orbits in LEO (around 408km altitude) and travels faster than 28,000kph. This means it completes a cycle of the Earth every 90 minutes.
Satellites in LEO do not need to be continuously powered to keep up these speeds. Due to lack of atmosphere and wind resistance, the initial speed provided by a satellite’s launch rocket is enough to keep the object orbiting for several years, however satellites will degrade faster the closer they are to Earth, meaning they must be replaced to provide continuous service.
The primary benefit of LEO orbit is, predictably, that it is physically closer. This makes a space station easier to reach and means signals sent from LEO satellites can reach Earth more quickly (which is known as providing low latency).
However, because satellites in LEO move so quickly, many more satellites are required to provide wide coverage. For example, if you are trying to connect to a LEO satellite in London, UK, its signal may only be available for a few minutes before the satellite has flown overhead and started across the Atlantic Ocean.
Because many satellites are required to provide global coverage and the space industry is undergoing rapid growth with new LEO players entering the market, the orbit is now becoming congested. So called ‘mega-constellations’ made up of thousands of small satellites are being launched to provide space-based services, like the internet for those in rural areas. Such a dramatic increase in the number of satellites will drastically increase the likelihood of collisions and potential disasters, which in turn may add to the problem of space debris.
Medium Earth Orbit begins between 2,000km and rises to around 36,000km above Earth. You will find most MEO satellites above 5,000km. There are several satellite constellations in MEO that provide important global navigation services, including the United States’ Global Positioning System (GPS), the European Space Agency’s Galileo, and China’s BeiDou. Inmarsat previously worked on a project to provide a new UK Space-Based Augmentation System, enhancing the USA’s GPS constellation. This was required because the UK no longer has access to Galileo or any similar EU service since leaving the European Union.
Satellites in MEO do not move as quickly as in LEO because the Earth’s gravitation pull is not as strong. This means its signal is available for longer over a fixed position on Earth and its beam can transmit over a much wider coverage area, due to its distance from Earth. This makes them far more efficient for providing global navigation services.
A geostationary orbit path is around 36,000km above the Earth. Here, satellites move at the same speed as the Earth’s rotation which means they always stay above the same point on Earth, this is also referred to as geosynchronisation. This provides two main benefits: antennas on Earth do not need to move to receive signals and each satellite can provide coverage for a third of the Earth.
Geostationary orbit is also used by a worldwide network of meteorological services to observe the Earth’s surface, weather conditions and atmosphere. Many global Navigation Satellite Systems (GNSS) also operate in GEO to service different areas of the world.
GEO is where you will find Inmarsat’s satellites, which provide global, mobile connectivity coverage. Our fleet of in-service geostationary satellites deliver award-winning operational, safety and mission-critical connectivity services to organisations, governments, and individuals around the world, providing global coverage through a variety of networks, including ELERA and Global Xpress. In February we launched the second of our new Inmarsat-6 (I-6) satellites, the largest and most sophisticated commercial communications satellites ever built, from Cape Canaveral, Florida.
A satellite in a highly elliptical orbit (also known as HEO or a Molinya orbit) is one which orbits the Earth in an oval or elliptical pattern. This means a satellite will move more quickly when it is closest to Earth - known as its perigee - but more slowly when it is furthest away – known as its apogee.
What makes this orbit path beneficial in satellite communications that it can provide coverage around the Earth’s high latitude areas – those around the North and South Poles – for longer.
Inmarsat’s GX10A and 10B satellite payloads will operate in HEO. The world’s first commercial satellites dedicated to the Arctic region, they will integrate seamlessly into the existing and planned Global Xpress network and will be fully compatible with current and future Global Xpress terminals. Our Arctic expansion is being delivered in a partnership with Space Norway and its subsidiary Space Norway HEOSAT, as part of the Arctic Satellite Broadband Mission.
Inmarsat’s ORCHESTRA network will be a unique multi-dimensional network, combining our current geostationary ELERA and Global Xpress, HEO Global XPress satellites, with a terrestrial 5G network and a focused constellation ofLEO satellites. ORCHESTRA anticipates future demand and ensures hotspots, such as ports and busy shipping lanes and air routes receive the best possible connectivity via the most appropriate element of the network, depending upon its intended use. This is all seamlessly delivered to the customer, who receives the service they need, when they need it.
While there are other distinctions and classification of Earth orbit patterns, this article has explained some of the key concepts and definitions. Different sections provide various technological benefits for our Earth and enable expansive global coverage for industries all over the world.
Jerome Soumagne is Inmarsat’s VP of Systems Engineering and an experienced leader in telecommunication technologies.
He has worked at Inmarsat for the past 11 years in a variety of technical engineering roles, including as Senior Director in Services and Applications and Vice President of Service and Network Engineering. Jerome has led the development of Inmarsat’s Global Xpress (GX) satellite network, which powers the world’s first and only globally available high-speed broadband network.
Jerome has worked in the telecommunications industry for more than 30 years, in several technical roles across the world, including U.S.A., Switzerland and his native country France.
