A geosynchronous orbit is a high Earth orbit that allows satellites to match Earth's rotation. Located at 22,236 miles (35,786 kilometers) above Earth's equator, this position is a valuable spot for monitoring weather, communications and surveillance.
“Because the satellite orbits at the same speed that the Earth is turning, the satellite seems to stay in place over a single longitude, though it may drift north to south,” NASA wrote on its Earth Observatory website.
Satellites are designed to orbit Earth in one of three basic orbits defined by their distance from the planet: low Earth orbit, medium Earth orbit or high Earth orbit. The higher a satellite is above Earth (or any other world for that matter), the slower it moves. This is because of the effect of Earth's gravity; it pulls more strongly at satellites that are closer to its center than satellites that are farther away.
So a satellite at low Earth orbit — such as the International Space Station, at roughly 250 miles (400 km) — will move over the surface, seeing different regions at different times of day. Those at medium Earth orbit (between about 2,000 and 35,780 km, or 1,242 and 22,232 miles) move more slowly, allowing for more detailed studies of a region. At geosynchronous orbit, however, the orbital period of the satellite matches the orbit of the Earth (roughly 24 hours), and the satellite appears virtually still over one spot; it stays at the same longitude, but its orbit may be tilted, or inclined, a few degrees north or south.
A satellite in geosynchronous orbit can see one spot of the planet almost all of the time. For Earth observation, this allows the satellite to look at how much a region changes over months or years. The drawback is the satellite is limited to a small parcel of ground; if a natural disaster happens elsewhere, for example, the satellite won't be able to move there due to fuel requirements.
This is a large benefit for the military. If, for example, the United States is concerned about activities in a certain region of the world — or it wants to see how its troops are doing — a geosynchronous orbit allows constant pictures and other surveillance of one particular region. An example of this is the United States' Wideband Global SATCOM 5, which launched in 2013. Joining a "constellation" of four other WGS satellites, it extends the military's communications system to provide blanket coverage over virtually the entire planet. The network serves troops, ships, drones and civilian leaders and is supposed to provide communications for ground personnel.
Communications for civilians also benefit from geosynchronous orbit. There are numerous companies that provide telephone, Internet, television and other services from satellites in that orbital slot. Because the satellite is constantly hovering over one spot on the ground, communications from that location are reliable as long as the satellite is well connected to the location you want to communicate with.
According to Satellite Signals, there are 402 satellites in geosynchronous orbit. At geosynchronous orbit, the “ring” around Earth can accommodate a number of satellites — 1,800 altogether, according to one analysis by Lawrence Roberts, published in the Berkeley Technology Law Review. However, there are obvious space and technological limitations.
Specifically, satellites must remain in a very confined area and not drift too far from their assigned “slot” above Earth; otherwise they may pose a threat to other satellites. The International Telecommunication Union assigns slots for geosynchronous orbit and settles disputes between countries about slots.
Similarly, it is considered good practice to move almost-dead satellites into a "graveyard" orbit above geosynchronous orbit before they run out of fuel, to clear the way for the next generation.
The satellites must also be located far enough away from each other so their communications don't interfere with each other, which could mean a separation of anything between 1 and 3 degrees. As technology has improved, it's possible to pack more satellites into a smaller spot.