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

John Ioannidis ntp at tla.org
Fri Nov 2 17:16:14 UTC 2007


Kevin Oberman wrote:
 >> Date: Fri, 02 Nov 2007 12:25:46 -0400
 >> From: John Ioannidis <ntp at tla.org>
 >>
 >> Thanks for the reply, but this is not answering my question. I 
wasn't asking how to *configure* the beast (I've already done this 
successfully), I was asking how to verify that it actually works as 
documented!
 >>
 >> What measurements do I have to take that will show the difference 
between a setup that's actually using the PPS signal from the GPS 
receiver and one that's not (because, for example, the DCD line on the 
motherboard is cracked, to give a stupid example).
 >
 > I guess I was not clear. 'ntpq -p' will provide the output I showed and
 > this will tell you whether the PPS is working. (Note: I don't see that
 > it will as your configuration is not correct.)

By "working" I mean "actually having an effect". Not simply that the 
kernel takes notice, but that I'm actually getting better results. Hence 
my insistence on statistical measurements.  I'm also curious as to how 
much better my accuracy is when I use the PPS (and hence whether it's 
worth the extra trouble for my particular application).

AFAICT, my configuration is correct (modulo the polling intervals). The 
"flag3 1" in the fudge line tells the NMEA driver (notice the .20. in 
the third octet of the fake IP address) to actually use the PPS signal.

For the record, here is the output of ntpq -p

# ntpq -p
      remote           refid      st t when poll reach   delay   offset 
  jitter
==============================================================================
*GPS_NMEA(1)     .PPS.            0 l   53   64  377    0.000    0.019 
0.014

Am I missing something?

Thanks,

/ji



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