What are GEO satellites?
GEO satellites work in a geostationary orbit. It is a circular geosynchronous orbit 36,000 kms above the Earth’s equator (or alternatively 42,000 kms radius from the Earth’s centre) following the direction of the Earth’s rotation and takes 24 hours.
Communications satellites in this orbit are fixed above a single point which means that an antenna on Earth does not have to track or rotate but can be pointed permanently at the known position in the sky, despite the satellite actually travelling at 11,300 kms per hour. Satellites require some station keeping in order to maintain position.
The first satellite placed in geostationary orbit was Syncom 3 launched in 1964 and was used to transmit live coverage of the summer Olympics from Japan to the USA. Uses include weather observations, navigation and a huge variety of communications. Intelsat and Inmarsat are two of the better known satellite operators with GEO constellations.
What are LEO satellites?
LEO satellites work in a low earth orbit. It is an earth-centered orbit close to the planet often specified as an orbital period of 128 minutes making 12 orbits per day. The LEO region is generally accepted to be below 2,000 kms altitude. Amazingly the mean orbital velocity needed to maintain a stable low earth orbit is about 28,000 kms per hour but reduces with increased orbital altitude.
Unlike GEOs a LEO satellite has a small field of view and so can communicate with only a fraction of the Earth at a time which means that a network is required to provide continuous coverage.
Although the first LEO satellite was launched in the 1950s, it wasn’t until the 1980s that engineers began to challenge the effectiveness of GEO satellites and their architecture (that many LEO satellites had adopted too). That’s when the idea for a LEO satellite constellation first occurred.
What are GEO satellites used for?
Due to GEO’s first mover advantage, the vast majority of communications by satellite are undertaken by GEOs. Each large satellite covers up to one third of the earth’s surface. GEOs do not suffer from intersatellite handoff and are ideal for broadcasting, weather forecasting and satellite radio. GEO satellites provide the backbone of global space communications and continue to be built.
With global communication evolving and demands increasing for uninterrupted low latency communication anywhere in the world, the fixed nature of GEO satellites limits its ability to provide that type of service. For example if reception is required in an east/west canyon a single GEO satellite may be blocked by a mountain.
What are LEO satellites used for?
LEO satellites provide true global coverage with low latency typically more than five times faster than GEOs, making the user experience more akin to terrestrial fibre connected devices. For this reason, many critical communications are handled over LEO satellite networks, which allow for faster connectivity without wires or cables.
The look angles from an overhead satellite avoids directional obstruction problems since LEO satellites are always moving. The chances of a long or persistent signal blockage are greatly reduced.
The growing LEO satellite communication industry
LEO terminals have inherently been more complicated and expensive to produce when compared to GEO terminals, as they must be able to passively or actively track fast moving satellites.
That cost is starting to be driven down as demand grows for true global internet connectivity and the potential benefits and uses of low latency/high speed connectivity begins to be realised.
How many LEO satellites cover the earth?
In total there 11,102 LEO satellites covering the earth all providing different connectivity uses for a variety of global industries.
A visualization of the number and complexity of LEO satellites provided by LeoLabs.