Though the Global Positioning System is the premiere means of disseminating Universal Time Coordinate (UTC) to the world, the underlying timebase for the system is actually called GPS time. GPS time is derived from an ensemble of Cesium beam atomic clocks maintained at a very safe place in Colorado. The time kept by the GPS clock ensemble is compared to the UTC time scale maintained at the United States Naval Observatory (USNO) in Washington, D.C. Various methods are used to compare GPS with UTC-USNO, including two-way satellite time transfer and GPS common view measurements. These measurement techniques are capable of single nanosecond level accuracy. Using these measurements, the GPS time scale is steered to agree with UTC-USNO over the long term.
In the beginning of the GPS project, it was not known how well the two time scales could be synchronized, so +/- 100 nanoseconds was established as the minimum uncertainty of UTC derivable from GPS time. We say minimum uncertainty because there are other factors that limit the overall time transfer accuracy like ionospheric anomalies and satellite position errors. If all of these errors were zero, and GPS time was therefore transferred perfectly to the user, the uncertainty of UTC time would still be +/- 100 nanoseconds. This arises from the actual way that time is acquired from the GPS System as outlined in the interface control document ICD-GPS-200. This is the document that all GPS receiver manufacturers use to design their data processing algorithms.
The ICD-GPS-200 specifies that the corrections that are transmitted in the data from the satellites that relate UTC-USNO to GPS time are no worse than +/- 100 nanoseconds absolute accuracy. To this day, this specification has never been modified to reflect the fact that the two time scales are routinely maintained to less than +/- 10 nanoseconds. This is in part due to the practical problem of retroactively modifying contractual specifications that the various government subcontractors agreed upon when the system was implemented.
GPS has over-achieved its original goals in almost every way - including its reliability, the life expectancies of the satellites, and its accuracy. Because of this, most GPS timing and frequency equipment manufacturers ignore the ICD-GPS-200 so they can specify their equipment at better than the +/- 100 nanosecond level of UTC accuracy.