[ntp:questions] Re: GPS, IEEE 1344, and NTP - a field report

Martin Burnicki martin.burnicki at meinberg.de
Mon Jun 20 10:21:40 UTC 2005

Joseph Gwinn wrote:

> In a system I'm involved with, we had a field failure, when time in the
> computers fed by a NTP time server fell out of bed at the turn of the
> year.
> The problem was traced to a misconfigured NTP time server that was
> ignoring the 1344 data (which contains the Gregorian year) in the IRIG-B
> signal from the GPS receiver.  Vanilla IRIG-B carries only the number of
> the day (0-365) within the current year, but doesn't tell you which year
> it is.

Yes, and if the current year is a leap year, then Feb 29 has the same
day-of-year number as March 1 in non-leap years. That one day offset rests
until the end of the leap year. 

If the routine which converts day-of-year back to date-of-year doesn't know
about or account for a year being a leap year then the conversion returns
faulty results.

The knowledge of the year number can be derived from the IEEE1344 data in
the IRIG code, if that is available, or it could be derived from the date
of the target system, which assumes that the system date has been
initialized properly.

Did the NTP time server have a built-in IRIG decoder card, or was the IRIG
code fed to ntpd using the audio driver? If using the audio driver, which
version of NTP has been running?

Martin Burnicki

Meinberg Funkuhren
Bad Pyrmont

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