[ntp:questions] NTP server with Garmin GPS 18 LVC

David Lord snews at lordynet.org
Fri Dec 20 00:48:49 UTC 2013


unruh wrote:
> On 2013-12-19, David Lord <snews at lordynet.org> wrote:
>> unruh wrote:
>>> On 2013-12-19, David Lord <snews at lordynet.org> wrote:
>>>> Adrian P wrote:
>>>>
>>>> .....
>>>>
>>>>> Everywhere is written that FreeBSD is the best OS for time keeper,
>>>>> since its kernel supports nanosecond accuracies. However, if GPS 18x
>>>>> LVC PPS signal has only microsecond accuracy, then I suppose there is
>>>>> no point on using FreeBSD, any linux would be okay as well. So what do
>>>>> you think, looking from NTP point of view only, is this nanosecond
>>>>> accuracy the only reason why someone would use FreeBSD, or there are
>>>>> some other reasons as well? I see you have experience with all kind of
>>>>> OSes.
>>>> I think the combination of hardware used is at least as important
>>>> as the OS.
>>>>
>>>> <http://www.febo.com/pages/soekris/> 100ns accuracy
>>> AGreed. But their comparison is strongly cooked. At least on my system,
>>> the serial port interrupt performace is far better than theirs--
>>> something like 2 us, not the 50us they are showing. Certainly 100ns is
>>> better, but more like one order of magnitude, not more than 2. 
>>
>> It might not be very clear but PPS to serial port isn't used
>> other than as a comparison, ntpd has been modified to use the
>> Elan Timer with PPS input to the GPIO.
> 
> I understand that they do not use pps to serial port, but they compare
> to ntpd with serial input to show how much better they are. What I
> object to is that that comparison is cooked. ntpd with pps to serial is
> a lot better than their graph indicates. They are doing something very
> wrongly to get such bad results. And it is unnecessary because they are
> better than ntpd with pps to serial. 

My loop_summary from Sure GPS + NetBSD-6 + ntp-dev-4.2.7p401
over a 7 day period has offset(us) 7 +/-34 to 21 +/- 52 and
rms offset from 3.9 to 6.1.

If ntpd is throwing away 7 out of 8 polls I'd say their results
could be similar to mine.

Also since from that page "ntp gettime Thu, Oct 19 2006 20:10"
there have been many changes to ntpd.


David



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