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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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  • $\begingroup$ It's useful to think of why tv broadcast like direct tv would favor geosynchronous or stationary orbits. Your audience is the US, which is 2 percent of the globe by area. Much more sense to not have sats that will spend 98 percent of the time not where you want them (obviously more complicated but that's the ballpark). Also video is high data and you save a lot on ground equipment that doesn't need highly accurate tracking capabilities. Meanwhile internet and phone sats are typically lower orbit, because like GPS they need to cover the whole globe. $\endgroup$
    – eps
    Mar 7, 2022 at 17:52

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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.

  • 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 MEO.

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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. $\endgroup$ Sep 23, 2014 at 22:17
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    $\begingroup$ 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, 2014 at 0:43
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    $\begingroup$ (in reality, they're overbuilt — the Block IIs had a design life of 7.5 years, and the last one was retired in 2007 after 16.5 years of service.) $\endgroup$
    – hobbs
    Sep 24, 2014 at 0:46
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    $\begingroup$ @Everyone Yes. That's the point. The only positioning satellites in GEO are SBAS such as WAAS, which augment GPS rather than being an alternative to it. $\endgroup$ Sep 24, 2014 at 18:49
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    $\begingroup$ @AnthonyX there are a couple of very minor advantages, I'll note them $\endgroup$ Nov 25, 2019 at 5:28
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The position of each GPS satellite should be known very precisely. Special ground stations are needed to measure the position of the satellites. Three stations are needed to measure a 3D position of a satellite if each ground station is equipped with a precise atomic clock.

If the satellites are distributed in orbits as shown in the question, too many ground stations are needed so that each satellite is in view of at least three ground stations.

The 12 hour orbits of the GPS satellites are determined to guarantee that each satellite is in view of only three ground stations for at least one time in a 24 hour day.

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