[ntp:questions] Adjusting PPS offset
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.
> +220.127.116.11 18.104.22.168 2 u 245 256 377 60.617 3.392 1.033
> -22.214.171.124 126.96.36.199 2 u 238 256 377 93.888 -0.016 0.849
> +188.8.131.52 184.108.40.206 2 u 168 256 377 81.311 4.502 0.391
> *220.127.116.11 18.104.22.168 2 u 212 256 377 80.594 2.411 0.278
> +22.214.171.124 126.96.36.199 2 u 139 256 377 82.841 2.212 0.535
> +188.8.131.52 184.108.40.206 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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