Sats are way up in orbit arent they?
So what does this graphic mean below 100ft?
Probably not counting those that are low toward the horizon, that can be obscured by trees and other objects. Other than that … 
Fort instance - sat at home in the house - my phone is only locked into 16 of the 35 that are technically visible, but many of these are close to the outer ring representing the horizon.
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Which app is that please @OzoneVibe?
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I kinda figured they were all in outer space…
They orbit around the planet - so, like the moon, sometimes each is high in the sky, sometimes near the horizon, and sometimes below the horizon and visible only from the other side of the planet…
There is a large number of the satellites in several complex arrays of orbits that pretty much guarantees there is a minimum effective number high enough in the sky at any one moment to provide reliable accuracy.
A simplified view of one system …
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but that is assuming that the planet is the shape of a football 
It most certainly is not.
One thing can orbit something else of any shape … which is just as well since The Earth isn’t the shape of a football - it’s (closer to being) an ellipsoid.
There’s a website where the guy explains GPS in meticulous detail with interactive diagrams.