[ntp:questions] How do I know my GPS-based NTP server is actually working properly?

John Ioannidis ntp at tla.org
Wed Oct 31 17:19:46 UTC 2007

Yes, I know this sounds weird.  I've successfully set up my NTP server: 
an old 850MHz P3 box running FreeBSD 6.2-STABLE (kernel built with 
option PPS_SYNC), and a Garmin GPS-18LVC "hokey puck", powered off the 
USB port, with its PPS wire connected to the serial port's DCD line (pin 
1 on the DB9 connector). /etc/ntp.conf reads:

	server    mode 1 prefer
	fudge    time1 0.000 flag3 1 refid PPS

So far so good.  It seems to be keeping time and synchronizing to GPS. 

What's the proper way of telling whether the PPS signal is actually 
having an effect?  That is, what behavior/measurements would show me 
that the PPS signal is or is not being used? What should I be monitoring 
(my guess would be "offset" and "jitter" from the netq -c rv output), 
and what kind of statistical analysis would tell me whether PPS is or is 
not being used?



PS: the easy way to disable the use of the PPS signal is to set flag3 to 
0, right?  Reloading a kernel and rebooting is a bit of a pain on 
antiquated hardware.

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