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I'm not really good at geometry, and I was discussing with a friend about GPS. Let's for fun say that they are geostationary (we ignore the poles), then how many sattelites do we need to cover the entire earth, such that at any given point on earth at least 3 satelites are visible.

If we use this info then how is it possible to calculate how many satelites are needed, and how far apart they should be? My guess would be to divide the 360 degress with the 75 degress that each satelite can cover, and then multiply by 3.

360/(75)*3 = 14.4 ~ 15 satelites

Is that statement true?

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The page you link to says a GEO satellite can cover $75$ degrees either direction from its location, for a total span of $150$ degrees. That is the basis for the famous statement from Arthur C. Clarke that you only need three GEO satellites to cover the earth. Your calculation would suggest you need $7.2$ satellites for triple coverage, rounding up to $8$. You can show $8$ is sufficient by suggesting they be positioned every $45$ degrees and noting that an observer at $0$ degrees can see the ones at $315,0,45$ degrees. As the observer moves toward higher angles, the $315$ sets when the observer hits $30$ degrees but the $90$ degree one has already risen, so you have the desired triple coverage.

The real GPS satellites are in a lower orbit and planes inclined to the equator. The constellation nominally has $24$ in operation and now has $31$ in operation so the coverage is much better. One can see at least six satellites most of the time.

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