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

Charles Elliott elliott.ch at verizon.net
Sat Sep 7 13:43:31 UTC 2013



> -----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 unruh
> Sent: Friday, September 6, 2013 12:23 PM
> To: questions at lists.ntp.org
> Subject: Re: [ntp:questions] GPS/PPS and "enable calibrate"
> 
> On 2013-09-06, Charles Elliott <elliott.ch at verizon.net> wrote:
> > 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
> 
> Why? Why not just do it the average offset?

Many inexpensive GPS devices exhibit a saw tooth pattern of their offsets
relative to
a known good time source, where the period is less than constant, and the
amplitude 
often is plus or minus 60 ms relative to 0 offset.  According to Dave Hart,
this
saw tooth pattern is inherent in the GPS device itself (the time between fix
and 
time output "wanders") and not caused by NTPD processing.  Regardless of its

cause, until one understands this saw tooth pattern it appears like no time2
adjustment works for any length of time, hence the attempt to sneak up on
it.

One could avoid the ineffective sneaking by recording and plotting the GPS
time offset for at least a week.  Then set time2 to be the average of the
top
and bottom of the saw-toothed wave.  Then GPS time, w/o PPS assist, will be
accurate about twice a day.

I suppose faced with a saw toothed offset with non-constant period and
uncertain 
amplitude relative to known good time, programmers and pundits have paused
in
their efforts to devise a calibrate algorithm that would do much good.


Charles Elliott
> 
> > 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.
> 
> It should be within a usec of UTC. If the receiver emits a pulse more
> than 1 us off it is seriously broken. And the interrupt processing
> should not take more than a us or two unless the machine is really
> overloaded.
> 
> 
> >
> > 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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> >> questions at lists.ntp.org
> >> http://lists.ntp.org/listinfo/questions
> 
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