[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                        minpoll 3 maxpoll 3 # PPS
>> fudge flag2 0      refid PPS # PPS standard polarity
>> server                 prefer minpoll 3 maxpoll 3 mode 
>> 66 # NMEA
>> fudge 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 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.




(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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