What is a medium earth orbit satellite?
A medium earth orbit (MEO) satellite orbits the earth at an altitude above that of a low earth orbit (LEO) satellite and below that of a geostationary earth orbit (GEO) satellite. MEO, which is sometimes also called intermediate circular orbit (Ico), provides a vast range of options to those deploying satellites and strikes a balance between the costs of higher altitude constellations and the coverage of low orbit satellites. MEO satellites operate at altitudes between 1,000 miles and 22,000 miles and orbit the earth at least twice a day. Some have perfectly circular orbits while others track elliptically, but all track the same orbit continuously once it has been established.
How did MEO satellites come about?
They are not a new concept in the satellite world. The first communications satellite, Telstar, which was launched in 1962 was a medium earth orbit satellite designed to facilitate high-speed telephone signals. Following its launch, it rapidly became apparent that having a single MEO satellite in space was inadequate because it only provided transatlantic telephone signals for 20 minutes of each of its approximately 2.5 hour orbits. The concept of satellite constellations composed of multiple MEO satellites was born.
What are the advantages of MEO satellites?
MEO satellites are considered to be a happy medium between the LEO and GEO types of satellite. LEO satellites typically orbit in a circular pattern around the equator and approximately two dozen are required to provide continuous coverage.
MEO satellites orbit the earth at higher altitudes and therefore provide a greater coverage area to the extent that a company with 24 MEO satellites in position will have four covering any given spot on the earth at any time during the day. That’s important because multiple satellites are used by service providers to eliminate communications outages caused by inclement weather or obstructive objects. LEO satellites’ main advantage is that they can provide much clearer surveillance images and require far less power to transmit their data to earth.
GEO satellites, on the other hand, operate at greatly higher altitudes which assure wide areas of coverage on earth. A satellite geostationary orbit circles the earth in exactly one day and is placed above the equator. At this altitude, the orbit is 24 hours long, so the satellite appears to be in a fixed position in relation to the Earth. This is common for general-purpose communications satellites and is perfect for predictable communications relays. In principle, three satellites, 120 degrees apart, could carry out a mission that requires continuous coverage of the Earth. However, orbital position – at least for satellites that operate on the same radio frequency – could become a limited resource since only so many satellites can safely occupy the orbit without some danger of interference or even collision.
There are further drawbacks to GEO satellites. Getting a satellite into that orbit, of 22,300 miles or 35,800km, takes a large rocket and considerable onboard fuel. The general practice for reaching such orbit is indirect: the satellite initially goes into geosynchronous transfer orbit then, at a precise time, it fires rockets to boost it to the higher altitude.
MEO satellites therefore are a compromise between the advantages of LEO and GEO (or even higher orbit satellites) satellites and their costs and drawbacks. MEO satellites are longer in time than LEO satellites, but, to an observer on the planet, still seem to move.
A constellation is needed for continuous coverage but the constellation will need fewer satellites than if they were in LEO, but many more than GEO thereby offering a middle road when it comes to cost of construction and deployment. In addition, because they are lower than GEO satellites, latency – the lag between when a signal is sent and when it is received – is less.
Further benefits of MEO satellites include that, at the lower altitudes, they can capture weaker signals than in GEO. Their transmitter power and antenna size also represent a compromise between the modest requirements of LEO and the substantial requirements of GEO.
So what are the applications of MEO satellites?
Aside from defence industry applications, MEO satellites are ideally suited to deployment for communications purposes. They are most frequently used in GPS tracking and mobile telephone communications although their successful deployment is making them more frequently discussed as a potential solution for the expanding needs of asynchronous transfer mode (ATM) and other broadband communications networks.
Navigation systems remain the most common use of MEO satellites. Current deployments include the Global Positioning System and Russia’s Glonass. A proposed MEO navigation for the European Union called Galileo is expected to begin operations in 2013.
