[nobody31415926]

GPS onboard clock is corrected for relativity due to it's orbit height and speed so it's clock time is approximately correct

But measuring the simultaneity of an event between two different points with a moving clock is a lot trick.

[bermanoid]

That might have been an issue if they were measuring anything with the satellite clocks.

But they weren't. By my understanding, they were merely using the satellite signals to synchronize the ground clocks in the ground frame, and after that was done, the satellite's were never heard from again.

[nobody31415926]

Synchronizing two clocks in different positions is very difficult since relativity always comes in as you are moving a third clock between them. It's essentially impossible if you don't know the gravity field encountered by the moving clock.