That video is bogus, the loop of paper is attached to the rocket, so it will stop the thing from moving. Most of what the guy says is actually correct, until he fall flat on his face with the loop of paper.
As for 100%, certainty. I am 100% certain that I am exactly where the GPS receiver tells me I am. If the satellites were not in orbits where the transmissions tell the receiver they are then the receiver could not fix my location correctly.
Since I know where I am, I know the satellites are where they are supposed to be. They can't transmit false information and still give accurate location fix.
I have a GPS receiver connected to a server being used as a network time reference, When look at the satellite constellation. I can see the current positions of all the GPS satellites the receiver can see.
The column on the left is PRN (pseudo random number for identifying the satellite SS sequence) The Azimuth and Elevation, and SNR ( Signal to Noise Ratio) The Y or N tells me if that satellite was used in the position fix.

Some hours later..

If they are ground based towers, then the towers are moving. If they are balloons or stratellites, then they are 12,000 miles away, if the signals are faked then the thing that's faking the signals is in orbit 12,000 miles away and moving.
Yes I'm 100% certain GPS is satellite based and works, exactly as they say it does.
Flight tracking, Rocketry and Gravity can wait for another day.