It has its own timescale, but that still traces back to NIST.
In particular, the atomic clocks on board the GPS satellites are not sufficient to maintain a time standard because of relativistic variations and Doppler effects, both of which can be corrected, but only if the exact orbit is known to within exceeding tight tolerances. Those orbital elements are created by reference to NIST. Essentially, the satellite motions are computed using inverse GPS and then we use normal GPS based on those values.
> It has its own timescale, but that still traces back to NIST.
GPS gets its time from the US Naval Observatory:
> Former USNO director Gernot M. R. Winkler initiated the "Master clock" service that the USNO still operates,[29][30] and which provides precise time to the GPS satellite constellation run by the United States Space Force. The alternate Master Clock time service continues to operate at Schriever Space Force Base in Colorado.
> As a matter of policy, the U.S. Naval Observatory timescale, UTC(USNO), is kept within a close but unspecified tolerance of the international atomic timescale published by the Bureau International des Poids et Mesures (International Bureau of Weights and Measures [BIPM]) in Sevres, France. The world's timing centers, including USNO, submit their clock measurements to BIPM, which then uses them to compute a free-running (unsteered) mean timescale (Echelle Atomique Libre [EAL]). BIPM then applies frequency corrections ("steers") to EAL, based on measurements from primary frequency standards and intended to keep the International System's basic unit of time, the second, constant. The result of these corrections is another timescale, TAI (Temps Atomique International or International Atomic Time). The addition of leap seconds to TAI produces UTC. The world's timing centers have agreed to keep their real-time timescales closely synchronized ("coordinated") with UTC. Hence, all these atomic timescales are called Coordinated Universal Time (UTC), of which USNO's version is UTC(USNO).
The two organizations do seem to keep an eye on each other:
> The United States Naval Observatory (USNO) and the National Institute of Standards and Technology (NIST) make regular comparisons of their respective time scales. These comparisons are made using GPS common-view measurements from up to approximately 10 GPS satellites. The table below lists recent differences between the two time scales.
In particular, the atomic clocks on board the GPS satellites are not sufficient to maintain a time standard because of relativistic variations and Doppler effects, both of which can be corrected, but only if the exact orbit is known to within exceeding tight tolerances. Those orbital elements are created by reference to NIST. Essentially, the satellite motions are computed using inverse GPS and then we use normal GPS based on those values.