Why are GPS satellites in medium orbit rather than geostationary?

Is there an advantage, other than probably cost, to having the GPS satellites at 12,000 miles rather than in geostationary orbit? Would it work either way?|||Geostationary can only apply to equatorial orbits. With GPS satellites it makes more sense to have them in a polar orbit so that they can cover the entire Earth's surface over time. Also I presume that putting a satellite at the relatively large distance required for a stationary orbit (22,000 miles) would significantly reduce the precision with which the distance could be measured (GPS doesn't use triangulation, it works by measuring the distance between satellite and receiver).|||No, they need to be in a medium orbit.





You want to use GPS even at high latitudes, but geostationary satellites are hard to receive close to the poles. Next, you want to have all satellites in a special constellation or formation. When the satellites are all close together in the sky, for example in a straight line or at the same point, the accuracy of the calculations drops as the signals will get the same disturbances because of the atmosphere. Also, the mathematical calculations inside the receiver will not work well then. Then, you also want this constellation to repeat in a good pattern at regular times. The GPS distance means the constallations repeat every 72 hours, other distances change this. Regular repetition patterns mean, that your receiver will have it easier to find satellites and calculate their position, even when the ground station of the GPS fails to update the orbit information onboard the satellites (The satellites don't know this themselves, you need ground stations for the required accuracy).





Finally, you also have to consider the launch vehicles you can use. Today this is not such a big problem, but for GPS, there had been the Delta II planned, which has tight mass limitations.|||Geostationary satellites have a hard time covering far north and south regions. Once you are 75 degrees north or south, the satellite is too low on the horizon to be useful. Even at 50 degrees north or south, the satellite would be only 25 degrees above the horizon, and would not cover the far side of hills or mountains, or into valleys.





Plus, the power requirements to send a signal 12,000 miles is only 30% compared to geostationary orbit. That makes a BIG difference in weight, cost, and lifetime.|||The previous replies about geostationary orbits pretty much answered your questions.





However, there is also some misinformation about GPS given in the previous answers that I'd like to address:





SiriusB said: With GPS satellites it makes more sense to have them in a polar orbit so that they can cover the entire Earth's surface over time.





Firstly, GPS satellites are NOT in polar orbits. Secondly, a polar orbit doesn't cover the earth any more efficiently than non-polar orbits. What gives GPS satellites the coverage they do (which is 100% of the earth) is the way the 24+ satellites are distributed in their six orbital planes.





SiruisB said: Also I presume that putting a satellite at the relatively large distance required for a stationary orbit (22,000 miles) would significantly reduce the precision with which the distance could be measured





The presumption is incorrect. The control and space segments of GPS would still monitor satellite positions and update the system accordingly.





urwumpe said: The GPS distance means the constallations repeat every 72 hours, other distances change this.





GPS orbital periods are 12 hrs (11hrs, 56min).