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

Paul Croome Paul.Croome at softwareag.com
Thu Jan 27 08:59:32 UTC 2005


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. The nature of the polling (serial port at 110 baud) means
that there's a sawtooth of about 9ms peak-to-peak amplitude. I'm very close
to Mainflingen (about 40km) so I can't say much about diurnal variations,
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.

My advice is to beg, borrow, buy or steal a GPS and calibrate your DCF
receiver against that. (To calibrate the GPS circuit, you should compensate
for delays in the antenna cable, RS232 level converter, serial cable and
UART, but these are typically orders of magnitude smaller than DCF77 precision
so can often be neglected.)

Paul



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