[ntp:questions] Synchronize distributed PCs with GPS 1 PPS and NTPD for OWD measurements

sandip gangakhedkar sandipfloyd at gmail.com
Mon Sep 21 16:23:55 UTC 2015


Thanks for your detailed solution, which was along the lines of my

Do you know of any off-the-shelf GPS receiver with RS232 and PPS


>> What sort of distance accuracy are you expecting?

Not very stringent..up to 10m will do.

>> What sort of distances will you be operating over?

50m - 1000m

>> How unreliable is your link?

Very. The packet error rate can vary between 0 - 100%.

>> I would ignore NTP and do everything yourself.
>> Do something like ping.  That takes 2 packets, but you don't need to
know the
>> time on the other end.  If both ends need to know the distance, you can
>> a measurement with 3 packets.
>> Or you can send a dozen packets and use the minimum time assuming the
>> had delays in the interrupt handler.  (and use the spread in the times
as an
>> indication of quality)
>> You will probably need to calibrate the response times of the CPUs and
>> delays through the radios so you can subtract it out.  The radio delays
>> may depend on signal strength which varies with distance, but will also
>> change if you go behind mountains or trees or buildings.

Interesting technique, but for One way delay measurements, it will
introduce a lot of measurement error, particularly as the links are
unreliable and asymmetric (uneven delays).


On Mon, Sep 14, 2015 at 1:40 PM, Joachim Fabini <Joachim.Fabini at tuwien.ac.at
> wrote:

> On 14.09.2015 12:43, Hal Murray wrote:
> > Joachim Fabini said:
> >> - Re-compile your kernel for LinuxPPS support, following the
> instructions on
> >> http://linuxpps.org/wiki/index.php/LinuxPPS_installation .
> >
> > That hasn't been necessary for a long long time.
> Most recent distributions have PPS line support enabled by default.
> Still, kernel build is necessary if you have special driver requirements
> or change options -  I recompile measurement kernels for increasing the
> Hz rate. The page contains some outdated information but some of it is
> valuable (in particular the reference to the ppstools repository and
> timepps.h copy that is essential for building ntp).
> > What version of the kernel are you using?
> 3.8 - 3.13
> > Recent kernels need something like:
> >   ldattach 18 /dev/ttyS0
> > which creates /dev/pps0
> Yes, that's what I do. Ideally you can automate it and trigger some
> additional ntp prerequisites (udev-related) as recommended on the ntp
> pps support page (http://linuxpps.org/wiki/index.php/LinuxPPS_NTPD_support
> )
> _______________________________________________
> questions mailing list
> questions at lists.ntp.org
> http://lists.ntp.org/listinfo/questions

More information about the questions mailing list