[time-nuts] Delay through GPS antenna splitter/amplifier -- an answer, and a question
Tom Van Baak
tvb at LeapSecond.com
Mon Mar 12 13:31:51 EDT 2007
> In short, yes. If you want true traceability to NIST, you need to take
> into account UTC(GPS) versus UTC(NIST).
And it gets uglier yet. If you want UTC you have to take
into account the UTC - UTC(NIST) delta, which was about
16 ns in January. See the full 2006 record:
The point is GPS, NIST, USNO often vary up to tens of
ns over the span of days and months. It highlights the
problem of knowing what the "true time" is.
Also note that, strictly speaking, there is no such thing
as "UTC(GPS)" and BIPM frowns on the phrase UTC(k)
where k isn't one of the national time labs (e.g., NIST,
USNO, NPL, PTB).
What we call "GPS time" is in fact a good approximation
to UTC(USNO) but since the official UTC(USNO) clock
goes through so many levels of distribution, Kalman filters,
uplinks, downlinks, 24+ individual SV clocks, and PLL'd
disciplined oscillators before it reaches your 1 PPS BNC
output, it is no longer "UTC(USNO)".
For ns-level work the key is post-processing, where many
of the systematic and variable offsets can be estimated
days or months later and mathematically backed out from
your archived physical time interval measurements.
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