[ntp:questions] YEA! My Sure Electronics GPS just arrived.
Ron Frazier (NTP)
timekeepingntplist at c3energy.com
Tue Mar 27 05:44:11 UTC 2012
Hi David,
See below.
On 3/24/2012 12:11 PM, David J Taylor wrote:
> "Ron Frazier (NTP)" <timekeepingntplist at c3energy.com> wrote in message
> news:4F6DDA72.30407 at c3energy.com...
> []
>> Hi David,
>>
>> You appear to be up early. I'm curious to know what time this email
>> says it arrived. If it says it arrived at about 1030, then that's my
>> time. If it says it arrived at about 14:30, then that's your time.
>
> I am on UTC here, and the posting was made in the (very) early hours.
>
>> Since I wrote that, it seems to have centered itself around zero. I
>> now have a very nice + 1.2 ms / - 1.2 ms offset pattern. Since I've
>> been struggling to get anything under 50 ms with other technology,
>> this looks really sweet to me.
>>
>> http://dl.dropbox.com/u/9879631/Sure%20board%20first%20night%20pt1.jpg
>> http://dl.dropbox.com/u/9879631/Sure%20board%20first%20night%20pt2.jpg
>>
>> Conversion of these images to jpeg reduced the clarity a bit, but you
>> can still see what's happening.
>
> I vaguely recall that USB has a polling interval of ~1 millisecond.
> Additionally, unless you use interpolation, Windows timestamping
> introduces a further 1 millisecond quantisation in its timestamps of
> the USB data (that 0.977 ms jitter is the signature of plain Windows
> timestamps), so your +/- 2 milliseconds max seems to be of the correct
> order
>
>> MICROSECONDS, did you say? I'm nowhere near that territory with
>> everything going through a serial - USB converter. However, I'm
>> quite happy with 1.2 ms under the circumstances.
>
> That millisecond polling is the limiting factor, go for a hardware
> serial port and the kernel-mode timestamping and you're an order or
> two better again.
>
>> I am NOW assuming that my clock is more accurate than the internet
>> clocks. I am NOW hoping that neither will appear to be drifting away
>> and that nothing in the system will be having routine heart attacks.
>
> Fingers crossed. For the reasons mentioned above you could be up to a
> couple of milliseconds out in absolute terms.
>
>> I notice there is a difference between my clock and the average
>> internet clock reading. Hypothetically, even though mine is probably
>> closer to UTC than those readings, if I wanted to shift my offset to
>> match them, so NTP won't clockhop, how as long as the GPS is working,
>> how would I do that?
>>
>> Here are my config lines:
>>
>> # COM5 57600 windows lines for testing gps selected as main source -
>> gpgga 57600 baud
>> server 127.127.22.5 minpoll 3 maxpoll 3 # PPS
>> fudge 127.127.22.5 flag2 0 refid PPS # PPS standard polarity
>> server 127.127.20.5 prefer minpoll 3 maxpoll 3 mode
>> 66 # NMEA
>> fudge 127.127.20.5 time2 0.0000 refid GPS1 # use WITH PPS
>>
>> Also, why doesn't the PPS show up in my status screen anywhere? I
>> know it's working, based on the graphs.
>>
>> Sincerely,
>>
>> Ron
>
> Check the driver configuration.
>
> PPS requires the kernel-mode timestamping of the DCD line going
> active, and that's only available in Dave Hart's serial-pps
> driver/DLL. For the same PPS timesamps in other drivers would require
> e.g. USB providers to update their drivers as well, which isn't at all
> likely to happen. If you knew that the average delay between true
> start-of-second and your PC timestamping the USB/serial packet was 1.5
> milliseconds, you could probably use something like:
>
> fudge 127.127.20.5 time2 0.0015 refid GPS1
>
> Looking at your plot of peer offset, though, that might bring the best
> Internet server /nearer/ to zero offset.
>
> BTW: you may find that PNG is a better format for saving graphics -
> except for the lines which are very "noisy" and would increase the
> file size. PNG is lossless, and can produce quite small files of
> plots. That's one reason my program saves data in that format.
>
> I'm delighted with your results so far.
>
> Cheers,
> David
>
That's a great tip about PNG files. I never knew anything about them.
At this point, I don't know if I'll even try to sync the GPS with the
internet servers, since I'm getting more accurate time from the GPS than
I can from the internet. I may change my mind if NTPD ends up clock
hopping too much once I release the internet servers to run as a backup.
I'm looking forward to playing with the real serial port on my other
machine a bit. I'm not quite finished testing the USB port on this
laptop though.
Sincerely,
Ron
--
(PS - If you email me and don't get a quick response, don't be concerned.
I get about 300 emails per day from alternate energy mailing lists and
such. I don't always see new messages very quickly. If you need a
reply and have not heard from me in 1 - 2 weeks, send your message again.)
Ron Frazier
timekeepingdude AT c3energy.com
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