Travel time becomes range
Electromagnetic propagation converts a time interval into distance using the speed of light. At approximately 299,792 km/s, a microsecond of timing corresponds to about 300 m of travel. A receiver’s clock error therefore cannot be ignored.
Position plus clock error
Three spatial coordinates and a clock offset give four unknowns in a basic position solution, requiring at least four suitably placed satellite measurements. Extra satellites improve the geometry and allow checks. Atmospheric delay and multipath affect individual ranges.
An orbit can be observed independently
Predicted passes can be compared with a moving point of light, timed radio reception and Doppler shift. A receiver displays satellite identifiers and signal measurements. Track agreement over several passes; distinguish a visible pass from a claim that one sighting alone determines every property of Earth.