Polar satellite

Why are there no polar geo-synchronous satellites? The advantages to stationing one there seem obvious: you could cover the majority of the world’s English speaking population from that spot, couldn’t you? So, what is it? Lack of centrifugal force? Interference from the aurora borealis? A government conspiracy?

On the unlikely assumption this question isn’t a joke, I will just simply ask the poster to try and describe an orbit around the Earth that manages to stay in one spot anywhere OTHER than directly above the Equator, to say nothing of the supremely silly notion of having a sattelite just hover motionless over a pole.

You have to understand what an orbit is, and what a geosynchronous orbit is.

Begin with orbit. Orbit is when an object travels around the Earth. A visual clue to help figure it out. Wad up a piece of paper into a ball. Throw the ball lightly to the side. It will fall in a curved (parabolic) path. Throw the ball harder, it will travel farther before hitting the ground, with longer but still curved path. Now throw really hard. The ball will fly sideways as fast as it falls down, so it will fly past the edge of the horizon before it drops to ground height. That’s an oversimplification, but a visual help. Orbit is having enough sideways velocity to miss the Earth as it falls.

What is geosynch orbit? Well, satellites have to have a certain velocity in order to orbit. If you take said satellite high enough off the surface, that velocity will directly match the velocity that the Earth turns. That is a geosych orbit. So it can only travel in one direction, and must be centered over the equator. If it is off-aligned from the equator, you get an oscillation about the equator as it orbits. For a polar orbit, you are traveling 90 deg out of alignment with the Earth’s rotation, so the satellite goes one way and the Earth another. No geosynch.

A bit of nitpicking here, there could be a polar geosynchronous satellite, but there can’t be a polar geostationary satellite.


geosynchronous: orbit that takes exactly one day. These can have any inclination.

geostationary: orbit that stays above one point on Earth. These can only be above the equator (zero inclination).

A polar geosynchronous orbit will travel from pole to pole in its orbit. I don’t think there are any satellites in actual polar geosynchronous orbits. There are a number of satellites in near-polar orbits, but I believe they are all at lower orbits than geosynchronous satellites.

Because a good deal of their real estate is so far north that equatorial geostationary satellites are out of range, the Russians have a series of communication satellites that are in a very eccentric polar orbit. It swoops close to earth over Antartica and travels high above the North Pole on the other side. The net result based on one of Kepler’s Laws is that the sats spend most of their time in orbit above the Northern Hemisphere where they are used for communication in the wilds of northern Russia and Siberia.

As far as I know, most American polar orbiting satellites are of the spy variety and as such are not in high enough orbits to be geosynched. Early weather satellites were put into polar orbit but I think modern ones hang out with the communication satellites over the Equator these days.

Any satellite that needs to view most or all of the earth is put in a near-polar orbit. These include not just spy satellites but things like LandSat.

For most of them, the ideal orbit is one that has a constant sun angle. That is, the sun will be shining from the same angle on the patch of earth directly below the satellite. This facilitates comparing the images from one orbit to that of another.

I think that depends on the definition of “most of them,” dtilque. The 70-odd (including spares) of the Iridium constellation were/are probably numerically most numerous, and for them the most important thing was near-constant altitude.

However, they’re all gonna be so much atmospheric dust in a little while, so the balance will probably tip back to the sun-angle dependent birds.

Actually, it has been proposed by Dr. Robert Forward. He calls them “statites,” and uses light pressure to hold them up. I’m sure there are papers on the web somewhere; his patent can be found here.

You folks might enjoy this site: http://liftoff.msfc.nasa.gov/RealTime/JTrack/3d/JTrack3d.html

One further reason: it’s a lot harder to get satellites into orbits closer to the poles than it is closer to the equator. At the equator, the rotation of the earth gives your satellite a big boost at lift-off (this push is less towards the poles)–one reason why those folks were so interested in running that floating launchpad in the Pacific (at the equator).

Plus, it’s usu. easier to have the orbits near your launchpad. Hence, most of NASA’s are near Fla.‘s latitude, and, as someone else pointed out, the Russkies’ are at higher latitudes.