Geosynchronous vs Geostationary Orbits

Geostationary OrbitIn this morning’s Anniversary Post, I talked about the first geostationary communications satellite, Syncom 3. And I briefly discussed the difference between a geostationary satellite and a geosynchronous satellite. I thought it would be interesting to discuss it, because it is one of those many astronomical concepts that are murky to most people. Even though a geostationary orbit is a special case of a geosynchronous one, it makes most sense to start with the special case.

The main thing you need to understand in all of these orbits is that the satellite moves around the Earth so that it is always at the same longitude. It is thus easiest to think about a satellite that orbits along the equator. You can see this in the animated gif above. What this means is that the angular velocity of the satellite is the same as the rotational rate (also the angular velocity) of the Earth. In this case, the satellite would always be over the same exact spot on the earth. This should all be pretty straightforward — especially with the animation.

But what if you wanted to put a satellite right over San Francisco, which is at 37.8° N latitude and 122.4° W longitude? That’s not possible. Satellites have to orbit around the Earth’s center of mass, so their orbit must trace out a great circle on the earth — like the equator, but rotated in any direction from where it is. It would thus be possible to set up a satellite that orbits right at 122.4° W longitude, but its latitude would change over the course of the day, moving down into the southern hemisphere and then back up to the north, being directly over San Francisco at the same time the following day.

Since a general geosynchronous satellite is not visible at all points along its arc for all inclinations, geostationary satellites are more useful. This is especially true of satellite dishes. If a geosynchronous satellite were used — even from a very low latitude — the dish would need to be constantly adjusted to point to the satellite. This is possible, of course. The last major project I worked on did just that: using a 900 MHz (broad but low bandwidth) signal to fix the location of the object, so that a 2.4 GHz (narrow but high bandwidth) antenna could be pointed at the object. Thus we were able to stream realtime video from an airplane to the ground. But that’s pretty involved, and as you’ve probably noticed: home antennas just point in one direction.

That’s the difference between geosynchronous and geostationary orbits. I think that a lot of the time, people think that anything can be done in space. But that is not true at all. We just put those satellites up there and let gravity work its magic. That’s why the Syncom satellites are still spinning around us even after fifty years.

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