[ntp:questions] Adjusting PPS offset

David Lord snews at lordynet.org
Sun Sep 4 10:07:51 UTC 2011


A C wrote:
> On 9/3/2011 22:21, unruh wrote:
>> On 2011-09-03, A C<agcarver+ntp at acarver.net>  wrote:
>>> I'm still digging through code for PPS and NMEA plus the routines in the
>>> OS so that part is going to take a long time for me to figure out.  So
>>> for now I'm just going to stick with PPS only and the pool servers.  As
>>> for PPS itself, what is the proper (or at least most reasonable way) to
>>> adjust the time1 offset so that PPS agrees with the tick of UTC?
>>
>> And how would you decide what that offset was? Since the PPS is almost
>> certainly the most accurate time you have already.
>> (actually it is off by about 1us due to the interrupt handling.)
> 
> Well, after leaving ntpd running for about a week straight, six network 
> servers have settled down to report an average 2 ms offset (according to 
> ntpq).  Each of the offsets is within about 0.5ms of each other.  The 
> offset shown on PPS was 0.000 with a small amount of jitter.
> 
> I figured that if the entire collection of servers had settled into a 
> 2ms offset then the tick of PPS was probably off by that amount.
> 
> If that's not a reasonable assumption then what would be?

The network paths between your ntp sources and your servers
are not likely to be the same in each direction. You can only
get an approximate idea of the path length although you can
get a fairly good measure of round trip time.

If you doubt that, you may want to get a collection of timing
GPS units and compare times from those.


David




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