We "know" GPS satelites send three signals: a unique ID, a location (where it is over the globe) and a time stamp for each signal it sends. A "receiver" uses signals from three or more GPS satelites to calculate position, based on the "Delta" or time passed between the time stamp in the signal, and the time on their GPS device. From the delta, you can work out how far you are from each satelite, and with three or more, you can determine the point where that delta "holds true" for all satelites. From the location information of each satelite, you can relate that information to a map of the globe, and "know" your position on that globe.
As far as flat earth is concerned, GPS can be "faked" from ground based locations. And it's true: triangulation is possible from ground based antennas. You can use cell phone towers to locate people for example, in exactly the same way as described above (time stamps, deltas, know location of the tower, map)
However, we also "know" GPS satelites move: they are not in geosyncronous orbit. Therefore, for two receivers (Fred and Wilma) measuring their location at different times (a and b), I'm saying
it is impossible for land based signals to "spoof" that informationDiagram above shows what happens with GPS: each satellite continuously sends signals giving it's ID, Location and a Timestamp for that information. We can see two receivers held by "Fred" and "Wilma"... They haven't moved, and they're some distance apart (5km say). Each satelite is moving, and each delta changes as that satelite moves. I've only shown delta from "Sat2". There are two ways this could be "spoofed" or mimicked by Flat Earth conspiracy agents, but neither makes sense:
In the above, the stationary tower "Fixed2" sends an ID, Location, and Timestamp. However, Fred and Wilma are considerable distances apart, so their "Delta" changes quite a lot between each signal. The stationary antenna could "pretend" and give a fake time stamp to Fred the first time (T2a), and then give the real time stamp T2B the second time... but Wilma will receive those same time stamps, and Wilma would think they're much further away with the first signal, and then the correct distance in the second signal? Or the antenna could give the "correct" Time stamp the first time, and Wilma will have the correct delta, and then the antenna could "spoof" a time stamp for the second signal so Wilma still gets the correct delta, but then that would through Fred completely out of whack. It is not possible to spoof for one receiver without confusing the second. It is impossible for both to calculate the "correct" location for both signals.
Option two: with two fixed antennas, the flat earth agents could "pretend" that the GPS has moved, simply by changing the ID part of the signal sent by antennas 1 and 2: in the first signal, Antenna 1 says "I'm Sat2!", and then on the second signal, Antenna 2 says "I'm Sat2!" and us globetards are fooled in to thinking this is a moving GPS. Everything else (the delta, calculated distance etc) holds in exactly the same way as for the first "known" diagram of GPS. However there's a huge problem: that only works to explain the two points shown in my diagram! What about the signal sent half way between those two points? A third antenna? What about half way between that? A fourth antenna? Obviously impossible without "infinity antennas"(!)
This is a little tricky to "conceptualise" with moving GPS vs Fixed Time Stamps vs Deltas etc: I found this hard to turn that in to clear diagrams, and perhaps they are not clear enough I don't know. However what I found completely impossible, is how it can be possible to mimic GPS with fixed antennas. Given they are constantly transmitting DIFFERENT locations and time stamps, it's simply impossible to spoof. You could argue that the GPS receivers themselves are in on the conspiracy, but there's any number of free open source aps for your phone to directly read the GPS data, and a LARGE part of the data is not "the data itself" it's simply a time stamp... you can not "spoof" a time stamp for selective people: time stamps get broadcast to everyone, it's up to the receiver to compare that time stamp to their individual device. It's simply impossible to spoof data for one receiver without throwing out a second receiver.