The 38 μs/day clock correction, read from the field's own value
Operational GPS depends on a relativistic correction: clocks on orbiting satellites run faster than clocks on Earth's surface by 38 microseconds per day, and without correcting for the difference, position errors would grow by roughly ten kilometers per day. The net figure combines two effects — the satellite's height in the field, which speeds its clock up, and its orbital speed, which slows it down. Neither needs curved spacetime: Einstein had the velocity rate in 1905 and the gravitational rate in 1907, both before curvature entered the theory. So the interest here is not in avoiding curvature — nothing about this measurement requires it — but in where the rate comes from. A clock at rest reads the field's value: φ/c² is the fraction of the local energy budget the field claims, and that fraction is the clock rate, read off the model's own field rather than imposed by a metric chosen in advance. The Schwarzschild radius appears along the way, a byproduct of the simplification rather than the foundation of the derivation.
Matches the 38 µs/day correction GPS receivers apply in operational use.