[ntp:questions] About use of PPS in NTP sync

William Unruh unruh at invalid.ca
Wed Sep 3 14:30:06 UTC 2014


On 2014-09-03, Harlan Stenn <stenn at ntp.org> wrote:
> "Charles Elliott" writes:
>> It is possible to distribute more accurate and stable time using a local GPS
>> device, either home built (e.g.,

If you are using an operating system like linux or bsd  and the GPS puts
out
a PPS signal then yes. If
windows it is far less clear that it is more accurate and stable. 

The PPS itself has an accuracy of 100s of nanoseconds. The interrupt
processing hadware and software has a delay and fluctuation of the order
of microseconds (on Linux/By longer on Windows). The clock has a
precision of nsec (on Linux/bsd, much less on windows) 
Stability depends more on the quality of the hardware, but the GPS
discipline can detect and correct drifts to the microsecond level.

Without PPS, the GPS has an accuracy (delay and fluctuation) 
in the tens of milliseconds range

>> http://www.satsignal.eu/raspberry-pi/index.html) or purchased (e.g.,
>> http://www.meinberg-usa.com/products/network-time-server.htm), but the
>> original cost is greater and/or can take a long time to get working
>> satisfactorily.  
>
> Yes.  There are many types of GPS pucks (for example) that will do well,
> especially if you can get a PPS signal out of it.

If you cannot they will not do very well. 
>
> H



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