[ntp:questions] Re: distance to dcf77 transmitter: how to set the fudge?

Bjorn Gabrielsson bg at lysator.liu.se
Thu Jan 27 22:41:49 UTC 2005


Paul.Croome at softwareag.com (Paul Croome) writes:

> Folkert van Heusden <folkert at vanheusden.com> wrote in message news:<mailman.7.1106757430.88045.questions at lists.ntp.isc.org>...
> 
> > Am I right that when I want to tell ntpd that it has to take care of the 
> > distance to the dcf77 transmitter that I have to do that like this?
> 
> Folkert,
> 
> Yes, in principle you're right. But in my experience, the precision of the
> DCF77 signal together with a low-cost receiver is of the order of a few
> milliseconds. 

Read up on Jonathans & others posts. With his driver I got
jittervalues around 0.5ms. At about 1100km from the transmitter I have
to fudge about 20.5ms delay.

> The nature of the polling (serial port at 110 baud) means
> that there's a sawtooth of about 9ms peak-to-peak amplitude. 

If you are not using Jonathans driver (or radioclkd2), you should try
it out! Where do you get the 110 baud from? Baudrate is irrelevant for
timing the pulse! There is no 9ms sawtooth!

---- snip from radioclkd.c ----
     /* wait till a serial port status change interrupt is generated */
        if (ioctl(fd, TIOCMIWAIT, TIOCM_CD | TIOCM_CTS | TIOCM_DSR)!=0)
                return -1;
        gettimeofday(tv, NULL);
        if (ioctl(fd, TIOCMGET, &arg)!=0)
                return -1;
---------------------------------

A few days ago I collected a million samples from a 100PPS signal
generated by a GPS. Resolution from gettimeofday is 1us. 60-70% of the
samples had jitter at 1us or below. 95% was below 10us, 97% below
100us. Actually you get both edges of the pulse for free, which means
that you can measure if the GPS pulse jitters in length down to the
1us level! This is with a free serial port, Linux and a short C hack.


> but they may be significant at greater distances. So trying to tweak
> microsecond accuracy out of DCF77 is an academic exercise but of questionable
> practical value.

Yes us accuracy from just the amplitude modulation will be hard. But
using a more advanced receiver you could use the phase modulation that
DCF77 has. Doesnt Meinberg have DCF receivers using the phase
modulation?

This source states 10 - 25us accuracy if using phase modulation

        http://www.electronic-engineering.ch/microchip/datasheets/dcf77/dcf77.html

> My advice is to beg, borrow, buy or steal a GPS and calibrate your DCF
> receiver against that. 

Getting a few good public Internet servers might be enough, if you
live in a good net neighborhood.

     remote           refid      st t when poll reach   delay   offset  jitter
==============================================================================
 LOCAL(0)        73.78.73.84      5 l   26   64  377    0.000    0.000   0.002
+ntp2.XX.se      .PPS.            1 u   60  512  377   12.490    0.400   0.573
+ntp1.XX.se      .PPS.            1 u   54  512  377   15.945    0.165   0.368
xntp3.XX.se      .PPS.            1 u  512  512  377   20.889    2.026   0.416
+GPS_PALISADE(0) .GPS.            0 l   15   64  377    0.000    0.001   0.002
*GPS_JUPITER(0)  .GPS.            0 l   60   64  377    0.000    0.000   0.002

Note that you will need get a GPS with a PPS signal to get good
results. A handheld Garmin will not do.

Go see this link for serious timekeeping

        http://phk.freebsd.dk/soekris/pps/

/Björn




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