[ntp:questions] Re: Clock accuracy & auto setting : digital television does a crap job of providing time services...

John Ackermann N8UR jra at febo.com
Mon May 1 18:47:02 UTC 2006


Richard B. Gilbert wrote:
> John Ackermann N8UR wrote:

>> As in timekeeping, GPS has become the method of choice for frequency 
>> calibration, but there are still people using WWVB (and presumably 
>> other standard frequency stations) for their original purpose.  If 
>> nothing else, it provides direct traceability to NBS without needing 
>> to adjust for NIST<-->USNO offsets.

> NIST (National Institue of Standards and Technology) is simply the new 
> name for the former NBS (National Bureau of Standards)!  What difference 
> /offset are you really referring to?

Sorry about using both NBS and NIST; that was a "thinko".

The official United States Frequency Standard is maintained by NIST. 
The GPS and LORAN systems are maintained by USNO.  While the differences 
between them are very small, there is still a frequency offset between 
the USFS and the USNO master clock.  In order to claim traceability to 
NIST, you need to add that correction term to any measurement that's 
based on GPS or LORAN.  The daily phase differences between UTC(NIST) 
and UTC(USNO) are at http://tf.nist.gov/pubs/bulletin/nistusno.htm

You also need to take into account the difference between GPS time and 
USNO time (which is avaiable at 
ftp://tycho.usno.navy.mil/pub/gps/utcgps30.dat).

I wasn't able to readily find frequency offset data; I don't know if 
they still publish that, but it can be derived from the phase 
differences over time.

Again, it's a tiny difference -- the offsets are typically less than 
20ns and have gotten tinier over the years -- but it's important if you 
want to maintain traceability.

John



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