As I recall Newton wasn’t good enough for the GPS System either.
I believe this is in reference to a time adjustment.
That's correct. Einstein's Relativity had to be applied. And not just because of the high rate of speed of the satellites, but also the time-dilation effects of Earth's gravitational field. Time apparently ticks differently at the surface as compared to higher altitudes.
According to the theory, a clock in motion ticks out time more slowly than a clock at rest. Likewise a clock experiencing greater gravitational effects. So, in the case of a fast moving satellite, these two effects compete against each other. ie, the rapid motion slows the satellite clock while the greater surface gravity slows the land-based clock. In the case of GPS, gravity actually wins out over speed, so the net result is that the ground-based clock is a wee bit slower than the clock in the satellite. And the rate of time of the signal to and from the satellite is how distance, and thus position, is determined. Without applying Relativity, position would not nearly be so precise.