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Why do gps satellites rotate over the Earth? Some says because of it there are at least 4 gps satellites at any location of Earth. But I can't understand it.

What would happen if gps satellites stays at where they are like tv broadcast satellites? Like this? There are still at least 4 gps satellites at any location of Earth.

gps

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The only class of orbit in which a satellite always remains over the same point on the earth's surface is a geostationary orbit, which can only be directly above the equator. Putting the GPS satellites in such orbits would have several disadvantages:

  • Poor satellite visibility for users at high latitudes - the satellites would always be low on the southern horizon (for a northern hemisphere user) and often blocked by buildings or terrain. At high enough latitudes they would not be visible at all.

  • Greater signal attenuation - the average distance from a user to the satellites would be around 40000 km rather than approx 20000 km for the constellation that's actually in use. This would result in a 6 dB attenuation in signal power (a 4x reduction) on a signal that is already extremely weak. The satellites would have to have higher power transmitters and larger solar arrays to compensate.

  • Poor solution geometry - GPS navigation works best when the satellites are in diverse directions. In addition to poor DOP, if the satellites are all in one plane as they would be in the case of geostationary orbits, there's an ambiguity - the distances to the satellites would be the same for a user at the same longitude and either sign of latitude, so the receiver wouldn't be able to tell whether it was in the northern or southern hemisphere.

  • Greater launch cost - geostationary orbits are higher energy than MEO; not all launch vehicles can get there and those that can would have a lower payload capacity.

For completeness, there would be some minor advantages to placing GNSS spacecraft in geostationary orbits, though these are outweighed by the disadvantages given above:

  • Simpler acquisition for receivers - because a satellite in GEO has ~zero velocity with respect to the Earth's surface, a stationary or slowly-moving receiver will see very little Doppler effect on the ranging signal. This can make the search phase of the signal acquisition process faster and simplify the receiver's circuitry. When GPS receivers were first developed this would have been a significant factor but these days the search is very fast anyway and silicon is cheap.

  • More benign radiation environment for the spacecraft - the MEO orbits used by GPS and most other GNSS pass through some of the stronger portions of the Van Allen radiation belts, which requires expensive (and in some senses antiquated) radiation-hardened electronics. GEO is no picnic either but the radiation dose is significantly less than that in LEO.

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  • $\begingroup$ 1- So as I understand, satellites can't stay fixed anywhere different than directly above the equator? 2- GPS satellites orbiting the earth twice a day. That's like 14,000 km/hour speed. So this energy comes from solar wings. Is this kind of power lower than the required power for transmitting more powerful signal? $\endgroup$ – digiogi Sep 23 '14 at 21:05
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    $\begingroup$ 1. Yes. 2. Your question is unclear. If I understand correctly, you're suggesting that maintaining orbital velocity requires electrical energy. That is not the case - see Newton's 1st law. $\endgroup$ – pericynthion Sep 23 '14 at 22:17
  • $\begingroup$ Oh. I didn't know that. At 20.000 altitude, there are still some atmospheric gas and maybe some gravity? And gps satellites rotating at the same orbit for many years. So no power is required for orbiting or at least for correcting the it's way? $\endgroup$ – digiogi Sep 23 '14 at 22:39
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    $\begingroup$ I think it's worth mentioning that aside from the listed drawbacks of geostationary orbits for GPS satellites, a geostationary orbit confers no advantage i.e. it's not a "pros-vs-cons" thing, it's all "cons". $\endgroup$ – Anthony X Nov 24 at 23:08
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    $\begingroup$ @AnthonyX there are a couple of very minor advantages, I'll note them $\endgroup$ – pericynthion Nov 25 at 5:28

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