That is not correct at all. How did you arrive at that conclusion?
GPS has its own independent timescale called GPS Time. GPS Time is generated and maintained by Atomic clocks onboard the GPS satellites (cesium and rubidium).
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.
I think GP might’ve been referring to the part of Jeff’s post that references GPS, which I think may be a slight misunderstanding of the NIST email (saying “people using NIST + GPS for time transfer failed over to other sites” rather than “GPS failed over to another site”).
The GPS satellite clocks are steered to the US Naval Observatory’s UTC as opposed to NIST’s, and GPS fails over to the USNO’s Alternate Master Clock [0] in Colorado.
I find this stuff really interesting, so if anyone's curious, here's a few more tidbits:
GPS system time is currently 18s ahead of UTC since it doesn't take UTC's leap seconds into account [0]
This (old) paper from USNO [1] goes into more detail about how GPS time is related to USNO's realization of UTC, as well as talking a bit about how TAI is determined (in hindsight! - by collecting data from clocks around the world and then processing it).
> If those other applications use their own local GPS clocks, what is the significance of NIST (and the 5μs inaccuracy) in their scenario?
Verification and traceability is one reason: it's all very well to claim you're with-in ±x seconds, but your logs may have to say how close you are to the 'legal reality' that is the official time of NIST.
NIST may also send out time via 'private fibre' for certain purposes:
