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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.

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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
  • $\begingroup$ There is certainly gravity (necessary for any orbit) but no atmosphere of any significance. Occasional propulsive maneuvers are necessary to correct for perturbations from the moon, solar radiation pressure etc but these do not require electric power. Instead they require chemical propellant. $\endgroup$ – pericynthion Sep 23 '14 at 22:47
  • $\begingroup$ and GPS satellites carry enough propellant to last them at least through their design lifetimes. In the case of the Block IIF birds, they carry 320 pounds of hydrazine and have a design life of 12 years. $\endgroup$ – hobbs Sep 24 '14 at 0:43

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