“Then please explain the bit about the signal disappearing as you drive south into Mexico.”
I’ve used GPS in Mexico, Bolivia, and Paraguay. As long as my antenna can “see” three sats I’m OK.
The signal can be obscured by clouds and rain but the sig definitely comes from the GPS constellation.
DGPS is a system that uses ground-based transmitters to enhance the 17 meter accuracy of GPS, DGPS is definitely ground-based, but sat GPS coverage is pretty much global except for parts of Antarctica where the birds are too close to the horizon to get three at a time.
http://www.gpsinformation.org/dale/dgps.htm
Valuable elaboration.
For persistent Flat-Earthers:
Yes, I’ve also used my satellite based GPS in very remote moutains, valleys, canyons, south of the border down to Chile and even in Nepal.
GPS is most definitely satellite based, as was the Delorme Iridium satellite phone that helped save my life in a remote mountain ravine a few years ago.
Geez
Even at the poles you can get more than 4 at any time. You can solve for lat/long with 3, but you need 4 or more to solve for lat/long/altitude. You are actually also solving for time in both cases (number of range equations equals number of unknowns, for those that are interested in the math. At least that’s the academic way of looking at it. In practice its done with matrix equations and least squares, for those interested in even more complex math).
Perhaps the info you have is dated to when the constellation was not built out. There are almost always 30 to 32 GPS satellites up and running at any time these days.