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Why GPS/GLONASS/Galileo satellites are on low earth orbit?

Why geostationary orbit is so bad? Sattelites might be placed there 'statically' and more precise...

The only problem I can see is navigation close to poles, but they have this problem anyway.

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Actually, they're on medium earth orbit. They're closer to the geosynchronous orbits than they are to low earth orbits. –  Carl Brannen Feb 15 '11 at 0:13

4 Answers 4

up vote 7 down vote accepted

This question always interested me. I found this recently -

"Another issue we wrestled with is which satellite orbits to use. We did not want to be in geostationary or geosynchronous orbits. The reason was these alternatives would force us to deploy ground stations on the other side of the globe, whereas, by putting them in some orbit that periodically passed across the United States, you could update the knowledge of where they were and what time it was on the satellite, then store that information in the satellite and continue to broadcast as it went around the Earth. That is the fundamental way we ended up with twelve-hour orbits. We also wanted to be reasonably high because we didn’t want the orbits significantly disturbed by the atmospheric drag. At the same time, by going high you had more visibility, more coverage on the Earth. So with an Earth coverage antenna and suitable power densities on the Earth, you ended up with the ability of twenty-four satellites to provide very solid, total Earth coverage."

Brad Parkinson

See: http://www.ieeeghn.org/wiki/index.php/Oral-History:Brad_Parkinson#Parkinson.27s_educational_background_and_the_origins_of_GPS.5D

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Why they're not on the geostationary orbit?

It's because the geostationary orbit - and indeed, there's just one such orbit, a one-dimensional curve in space - only exists above the equator while the GPS has to cover the whole planetary surface, including the points closer to the poles.

However, it's untrue that the GPS satellites are located at the LEO, either. Low Earth Orbit is defined as 160-2000 kilometers of altitude. However, the GPS satellite constellation is located roughly 20,200 km above the surface - over one half of the geostationary radius - in such a way that the position of each satellite returns to the same place twice per 24 hours. This is very convenient for synchronization and planning.

The Galileo satellites will be at altitude 23,222 km. It's also an intermediate circular orbit, much like for the 19,100 altitude of the GLONASS whose orbital period is about 11 hours.

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Well, even if they are in 1 plane - it is still possible to determine the location - the only problem is determining which half of the earth you are - but this is also solvable. If we have let's say 15 satellites on GEO - at nearly any point of earth we'll see at least 5 of them... Even at the pole - the'll be all around :-D –  BarsMonster Jan 14 '11 at 17:49
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@BarsMonster: yes, but if you're far to the north or south, you'd start to run into problems with the satellite visibility, and you'd also increase error by having the satellites be farther away. If you have at least one that is close to overhead at all times, you increase the accuracy of the system. –  Jerry Schirmer Jan 14 '11 at 22:32
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@JerrySchirmer: That's not quite right. The usual goal is to locate yourself on a map, i.e., horizontally, and for that you get the best accuracy when the satellites are close to the horizon. –  Ben Crowell Oct 22 at 21:27

The Chinese satellite navigation system is in geostationary orbit. From the Wikipedia article:

Unlike the American GPS, Russian GLONASS, and European Galileo systems, which use medium Earth orbit(MEO) satellites, BeiDou-1 uses satellites in geostationary orbit(GEO). This means that the system does not require a large constellation of satellites, but it also limits the coverage to areas on Earth where the satellites are visible. The area that can be serviced is from Logitude 70°E to 140°E, and from Latitude 5°N to 55°N.

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Interesting. I knew that China's Beidou has a limited coverage region centered around China, but it's only after reading your comment that I finally understood why it has a well-defined coverage region at all. –  felix Dec 5 '11 at 8:15
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I believe the article is imprecise. BeiDou-1 is geosynchronous, not geostationary. astronautix.com/craft/beidou.htm –  BowlOfRed Oct 22 at 21:52

This question is slightly faulty... Part of the US GPS system is geostationary (the WAAS component). It's used in conjunction with the non-geostationary birds for higher precision fixes. While primarily used for aircraft instrument approaches, there are off the shelf USB GPS computer peripherals that use WAAS.

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