What is a geosynchronous orbit? It is an orbit that exactly matches the rotation of the Earth. A satellite in geosynchronous orbit goes around the Earth at the same speed the Earth rotates, completing one orbit every 24 hours. From the ground, the satellite appears to be stationary.
There are thousands of satellites above us and they are in several different orbits. There are six different types of orbits and they are all used for different things. We have Geosynchronous orbit (GEO), Low Earth orbit (LEO), Medium Earth orbit (MEO), Polar orbit and Sun-synchronous orbit (SSO), Transfer orbits and geostationary transfer orbit (GTO), and Lagrange points (L-points). Let’s look at this list in reverse order. A Lagrange point is a place where the orbits of two celestial bodies cancel each other out. For example, there are Lagrange points between the sun and the Earth. If you place a satellite in a Lagrange point, it will remain stationary because the gravities of both bodies act on it equally. The James Webb space telescope is in a Lagrange point.
A geostationary transfer orbit is used to get a satellite from a low orbit to a higher orbit without needing a rocket to carry it there. A rocket carries the satellite to a low earth orbit and releases it at such an angle that it enters into an elliptical orbit. The satellite picks up speed as it orbits round the Earth and when it reaches the height of its target orbit, motors on the satellite fire, sending it out of the elliptical orbit and into a a higher, circular orbit.
A Sun-synchronous orbit is similar to a geosynchronous orbit, but the satellite stays fixed to the position of the sun and not of the same place on Earth. Any satellite in an SSO orbits at one rotation every 24 hours, but because they are following the line of the sun, it is always the same time as they orbit. SSOs are used for satellites that are monitoring things that require it being the same time of day, such as weather patterns, or accumulating data on long-term situations.
Then there are Low Earth orbits and Medium Earth orbits. Low Earth orbit is from about 200 km to 2000 km from Earth. Most satellites and the International Space Station orbit in this band. Satellites in Low Earth orbit have to move very fast, about 7.8 km a second, which means that the orbit can’t be used for telecommunications satellites because it would be too hard to track them. The satellites in this orbit are used for imaging the ground, and tasks like that. The ISS is in this orbit because it makes it easier to get astronauts to and from it. Medium Earth orbit is from 2000 km up to 35,786 km. At this distance, satellites orbit more slowly and are easier to track. Most of the global positioning satellites orbit in this range.
Which leaves us Geosynchronous orbit. Satellites in GEO orbit at 35,786 km and they travel at 3 km per second. They have to be directly over the equator because that is the plane the Earth rotates along. GEO is used mainly for telecommunications satellites because their height and their position make them “visible” to any point on Earth on the same longitude that they are on, from the North Pole to the South Pole. Because they are in GEO, they do not move and rotate with the Earth. That is what a satellite dish for a satellite TV doesn’t have to move to track the satellite. The satellite is always in the same place. There is only a limited amount of space for the GEO satellites because they all have to be over the equator. There is often competition to get a new satellite up there.
Once the satellites are in GEO, they have to be monitored to keep them there. Each satellite has a fuel supply and thrusters so that it can correct its orbit. When this fuel supply has run out, the satellite has to be decommissioned. They have to correct their orbit because the gravity of the moon and the sun pull on the satellites. Also, the Earth is not a perfect sphere and it has slightly different gravity in different places. Depending on where the GEO satellite is located, it could be pulled towards one of these points of higher gravity. On top of this, the solar wind alters the course of the satellite. All of these things have to be countered with the satellite’s thrusters. And that is what I learned today.
Photo by Pixabay: https://www.pexels.com/photo/space-technology-research-science-41006/
Sources
https://www.esa.int/Enabling_Support/Space_Transportation/Types_of_orbits
https://www.esa.int/Education/3._The_geostationary_orbit
https://en.wikipedia.org/wiki/Geostationary_orbit
https://www.itu.int/newsarchive/wtpf96/fact.html
https://science.nasa.gov/solar-system/resources/faq/what-are-lagrange-points/
https://www.space.com/low-earth-orbit
https://www.everythingrf.com/community/what-is-the-medium-earth-orbit