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

unruh unruh at invalid.ca
Fri Sep 6 16:22:47 UTC 2013


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? 

> 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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