Geostationary orbit

[EvilotionCR2]

What the heck is geostationary orbit?

[Baconluver25]

Geostationary orbit is when your ground speed is zero, so your orbital "falling" speed is 1000 m/s at the equator.

[Vasili]

A geostationary orbit is where you're orbiting at the same speed as the rotation of the planet thus always over one spot on the planet.

It's useful for communications satellites as it only takes 3 to have a communication network all around a planet, like this:

Edited October 9, 2012 by Vasili

[Rascal Nag]

I'd just like to clarify that it's rotation period (amount of time to complete a rotation), not speed that must be the same to get a geostationary orbit. Orbiting at the same speed would not work because the radius of the orbit is larger than that of the planet, meaning that the circumference is greater than that of the planet's, and so you would fall behind the rotation since you might be going speed A over distance 2D while the planet rotates at speed A over distance D (just a random scenario, not meant to refer to any real orbit around a real body). Obviously, the planet will complete its rotation in this case quicker than you can complete your orbit and so you'll fall behind.

[phoenix_ca]

Look, I don't mean to be snarky, but...really...

[EvilotionCR2]

Look, I don't mean to be snarky, but...really...

I don't know these astronomical things...

[flightmaster]

I don't know these astronomical things...

And that's fair enough, astronomy is often seen as scary and hard to understand but it's really not if you have the motivation to learn. It's good that you asked and it's people like phoenix_ca who make people who don't understand something not want to ask about it.

[Camacha]

It's useful for communications satellites as it only takes 3 to have a communication network all around a planet, like this:

Correct me if I'm wrong, but wouldn't that leave the poles uncovered? I think you'd need an additional three satellites rotating over the pole(s) to be able to communicate from whatever point to any other point.

[Rune]

Yup, but since there are very few people at the poles, we say that is global coverage. Actually, that is also the reason Molniya orbits (highly elliptical polar orbits with a lot of dwell time over one pole) were developed, and are still used to communicate in the high northern latitudes and Antartica.

Rune. Always nice to read someone with curiosity.

[tomf]

Any 3 satellites in the same orbit over at a radius of 2 * the planet are going to be enough for coverage.

The most useful thing about a geostationary orbit it that because it moves at the same speed as the surface to someone standing on the surface the satellite doesn't seem to move. So you signal receiving dish can always point in the same direction and doesn't have to pivot to track the target.

[Nibb31]

You don't use geostationary satellites if you're looking for global coverage. Geostationary satellites are designed specifically for one-way regional coverage over a specific country, which is what makes them perfect for TV broadcasting. They are not suitable for two-way communication because of the distance, which causes latency. Each signal has to travel 72000km. Because of their power requirements, they are also big, heavy, and expensive to launch.

If you want global coverage, you are better off with a constellation of LEO sats, like GPS, Iridium, or TDRSS systems. They will be smaller, cheaper to build and launch, and have a much lower latency.

Molniya orbit sats are suitable for regional coverage over high inclination locations, which is why they were developed mainly for Russia. You still need 3 satellites, but you don't need directional dishes on the ground, because there will always be one satellite within the azimut range of your dish.