[ntp:questions] GPS/PPS and "enable calibrate"

Charles Elliott elliott.ch at verizon.net
Fri Sep 6 15:22:52 UTC 2013


Calibrate has not worked for several years, although it is possible it is
fixed now.  Someone on this list investigated and reported that there was
just pro forma or "suggestive" code in ntpd as to how it could work.

I plotted the data on Windows with the Meinberg "NTP Tome Server Monitor,"
but it can be done with NTP Plotter (which Google) or a spreadsheet.  I
plotted Offset v. time and computed the average.  Then I adjusted time2 and
after a day ot two, plotted again and again computed the average.  Each time
I adjusted time2 I did it 1/2 the average offset or 1/2 the average time
distance between the consensus time of several (9) external time servers and
the GPS device time, set to noselect mode.  I was very careful of the
algebraic signs of the adjustment; it is not obvious.  Fortunately, time2
has always been positive for me, probably because the GPS device takes time
to compute and emit the time after a fix, it is needs a several hundred
millisecond boost to bring its time up to the present.  I am not sure if PPS
needs a time2 adjustment.

Charles Elliott

> -----Original Message-----
> From: questions-bounces+elliott.ch=verizon.net at lists.ntp.org
> [mailto:questions-bounces+elliott.ch=verizon.net at lists.ntp.org] On
> Behalf Of Horvath Bob-BHORVAT1
> Sent: Friday, September 6, 2013 9:20 AM
> To: david-taylor at blueyonder.co.uk; questions at lists.ntp.org
> Subject: Re: [ntp:questions] GPS/PPS and "enable calibrate"
> 
> Sorry for top posting, but Outlook is quoting things correctly.
> 
> I did check out your site and based my original setup on it.  Lots of
> impressive stuff there!
> 
> The part I couldn't figure out the fudge time2 values.  Using the one
> that was pointed out to me by Wolfgang is working very nicely with the
> other image.
> 
> I am curious how that value is determined.  I am assuming someone that
> understood it used the calibrate capability to come up with the 0.496
> number that works for rpi-gpio cases.
> 
> -----Original Message-----
> From: questions-bounces+bob.horvath=motorolasolutions.com at lists.ntp.org
> [mailto:questions-
> bounces+bob.horvath=motorolasolutions.com at lists.ntp.org] On Behalf Of
> David Taylor
> Sent: Friday, September 06, 2013 6:10 AM
> To: questions at lists.ntp.org
> Subject: Re: [ntp:questions] GPS/PPS and "enable calibrate"
> 
> On 05/09/2013 21:00, Horvath Bob-BHORVAT1 wrote:
> []
> > Hi,
> > this runs perfectly for me:
> > <http://open.konspyre.org/blog/2012/10/18/raspberry-pi-time-server/>.
> >
> > OK, maybe I'll have to try that image.
> >
> > In general though, when it comes to this line...
> >
> >    fudge 127.127.20.0 flag1 1 time2 0.496
> >
> > ... how do you know what value to set for time2?
> >
> > Did you got through the instructions here?
> >
> > http://www.eecis.udel.edu/~mills/ntp/html/drivers/driver20.html
> >
> > ... or are you using 0.496 - what is shown on the
> http://open.konspyre.org/blog/2012/10/18/raspberry-pi-time-server/ ?
> >
> >
> >
> > Bob
> 
> No need to change the Kernel,Bob.  There's a program which does /not/
> require you to change the OS - it runs the PPS collection in user-mode
> rather than kernel-mode, and the performance is only a shade worse than
> the kernel-mode version:
> 
> Program:
>    http://vanheusden.com/time/rpi_gpio_ntp/
> 
> Using:
>    http://www.satsignal.eu/ntp/Raspberry-Pi-NTP.html#user-mode
> 
> Performance:
>    http://www.satsignal.eu/mrtg/performance_raspi-4.php
> 
> All my Raspberry Pi experiments with different NTP versions and
> different GPS receivers are here:
> 
>    http://www.satsignal.eu/ntp/Raspberry-Pi-NTP.html
> 
> so I hope you find something of interest there.
> 
> 
> 
> 
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