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

Joseph Gwinn JoeGwinn at comcast.net
Fri Jun 17 21:21:58 UTC 2005


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.   

On reading the NTP logs, you could just feel the rising frustration in 
the system administrator as he killed and restarted the NTP daemon, to 
no avail - the year was still wrong.  Eventually, he just shut NTP off 
and set the computer clock by hand.  This worked well enough for a 
while, but after two or three months the computer clock had drifted 
enough to cause communications to be lost (because message timestamps 
were so far off that the receiver threw the messages away).  (My role 
was to figure out why.)

The solution was to update the maintenance manuals to ensure that the 
1344 information was both sent (by the GPS receiver) and received (by 
the NTP time server).

Joe Gwinn



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