Timing Accuracy
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GPS and UTC Time Transfer Specifications
Why some of our timing specs are relative to GPS time, rather than UTC.
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GPS and UTC
Though the Global Positioning System (GPS) 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.
GPS Specification States +/- 100 Nanoseconds to UTC
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.
GPS Exceeds Expectations but Spec Remains the Same
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.
Our Philosophy
At EndRun Technologies, we have decided to specify our absolute accuracy to UTC at +/- 100 nanoseconds
so as to be technically correct with the ICD-GPS-200. But we know that this is very conservative,
so for parameters relating to precision and stability, we specify relative to the GPS time scale.
In this way, you can tell how good our equipment really is at transferring GPS time without us having to
bend the GPS System Specification. We believe that by specifying our equipment this way, you can
see that we manufacture the finest single frequency, C/A code GPS time and frequency receivers
in the world.
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