[ntp:questions] Adjusting PPS offset

A C agcarver+ntp at acarver.net
Sun Sep 4 18:48:17 UTC 2011


On 9/4/2011 10:11, unruh wrote:
> On 2011-09-04, A C<agcarver+ntp at acarver.net>  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?
>
> It is a very unreasonable assumption. If your PPS is off by 2ms, throw
> it away immediately. It is completely useless. In general PPS is off by
> less than 1 us and probably 100ns or so. But the computer cannot run its
> interrupt routine fast enough to make use of better than 1us or so
> without veryspecial hardware.
> And 2ms for a bunch of network servers is bad. It seems that there is
> something along the route out of your system which is delaying the
> packets packets really badly but consistantly.
>
> What is the source of your PPS?


PPS is the PPS signal from my GPS receiver.  It's a timing receiver 
meant originally for a CDMA cellular application.  It does poorly for 
navigation but PPS is very steady.

ntpq -pn:

> ==============================================================================
> +72.14.177.132   209.51.161.238   2 u  245  256  377   60.617    3.392   1.033
> -50.16.201.39    209.51.161.238   2 u  238  256  377   93.888   -0.016   0.849
> +130.207.165.28  130.207.244.240  2 u  168  256  377   81.311    4.502   0.391
> *130.207.160.51  128.4.1.1        2 u  212  256  377   80.594    2.411   0.278
> +131.144.4.10    65.212.71.102    2 u  139  256  377   82.841    2.212   0.535
> +131.144.4.9     65.212.71.102    2 u  218  256  377   84.012    1.802   0.551
> o127.127.22.1    .PPS.            0 l    3   16  377    0.000    0.011   0.061



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