GPS and UTC Time Transfer Specifications
How our GPS instrument timing specifications are stated and what they mean.
GPS Time and UTC
The Global Positioning System (GPS) is the premiere means of disseminating Coordinated Universal
Time (UTC) to the world, but the underlying time base for the system is actually called GPS
time. GPS time is derived from an ensemble of cesium beam and rubidium atomic clocks in the
satellites, ground and control stations, and is steered to the UTC time scale maintained at the United States
Naval Observatory (USNO) in Washington, D.C. USNO also monitors the satellites on a daily basis
with specialized GPS reference receivers that measure the clock offset to UTC(USNO). Using these
measurements, corrections are computed for the GPS time scale, satellite clocks, UTC-GPS time
offset, and then sent to the GPS Master Control Station for system-wide upload. This process
results in a worldwide, traceable, UTC time transfer system with extraordinary accuracy.
UTC Time Transfer Accuracy via GPS
In the beginning of the GPS project, it was not known how well the GPS time scale could be
synchronized to UTC(USNO). Therefore, GPS time was conservatively specified to be within one
microsecond of UTC(USNO) and continues to be today in IS-GPS-200H. The accuracy of the
broadcast correction to GPS time to yield UTC is specified at 97 nanoseconds (1 sigma). Through
various modernization efforts to the space, ground and control segments, the GPS time offset and the
accuracy of the broadcast UTC correction have been significantly improved.
GPS performance standards are in place today, per the 2008 Standard (SPS) and Precision (PPS)
Positioning Service Performance Standards, that complement IS-GPS-200. These standards establish
minimum levels of GPS system performance mandated by U.S. law (10 U.S.C. 2281(b)). Per the
standard, normal GPS system operations must ensure the UTC offset data broadcast by GPS is within
40 nanoseconds 95% (20 nanoseconds 1 sigma) to UTC(USNO).
This performance standard far exceeds the IS-GPS-200H specification but is well within the
capabilities of the GPS system. Between 2005 and 2015, the difference between the UTC(USNO) and
GPS time scales, as measured by USNO, was less than 10 nanoseconds RMS. When the broadcast
UTC offset data was applied, UTC time derived from GPS was impressively within 2 nanoseconds RMS
to UTC(USNO) during this period.
GPS has over-achieved its original goals in almost every way - including reliability, satellite life
expectancies, and PNT performance. Modernization continues to improve performance and time
transfer accuracy. The accuracy of GPS timing receivers and instruments, however, can only be
guaranteed to the levels stated in the SPS Performance Standard.
At EndRun Technologies, we specify the time accuracy of our GPS instruments to UTC(USNO) based
on the USNO measured UTC time transfer accuracy delivered by the GPS system over the last 10 years
(<2 nanoseconds RMS). We believe that by specifying our equipment this way, you can see that the
accuracy to UTC of our GPS time and frequency standards is the best in the industry and allows you
to realize the full performance available during normal operations of the GPS system.