[ntp:questions] ARRGH!!! I woke up to a 50 SECOND clock error.

Ron Frazier (NTP) timekeepingntplist at c3energy.com
Wed Mar 14 00:05:55 UTC 2012

On 3/13/2012 7:24 PM, Chuck Swiger wrote:
> On Mar 13, 2012, at 4:08 PM, Ron Frazier (NTP) wrote:
>> I was speculating that perhaps my only selectable clock, the GPS, failed, and that something went nuts, and that's why I found the clock 50 seconds off this morning.  However, I don't have any evidence of a GPS failure.  In any case, I just figured the local option might prevent any major clock changes if all other sources are not available.  I only wanted the local option to kick in if there were no other sources.  I did not have that option in the ntp.conf file when I started this thread.  When I started this thread, my GPS was the only selectable clock, and there was no local option.
> One problem is that the local clock will always claim to be perfectly sync'ed, regardless of how close or far from "real time" it actually happens to be.  The fact that ntpd's stats will look wonderful doesn't mean anything in this case.
> If you configured a reasonable # of internet NTP servers, and maybe your GPS in noselect mode while you try to debug whatever the issue is with it, you'd likely avoid a number of the issues you've been reporting.
>> For now, I'm mainly wanting to compare the GPS to one primary other source because I've been experiencing a slow drift in NMEA time with a variation of about 120 ms and an oscillation period of several days.  I'm trying to isolate the source, either the GPS, or the subsystem that's getting time from the internet servers.  I'm assuming all the internet servers are not drifting, but my instrumentation, ie the Meinberg time server monitor, or whatever drives it, could be off.  That's why only have one source selectable, either the GPS, or one NIST server.
> Well, it's hardly unexpected that the NMEA sentences would show variable timing; they are only supposed to be correct to the nearest second, so 120 ms is well within specs.  It's the PPS signal which is expected to provide sub-millisecond precision.
> [ Hasn't this already been discussed? ]
> Regards,

I don't have PPS on this computer.  It has no serial port.  When I get 
my Sure board, assuming I can solder it without killing it, I'm going to 
try bringing the PPS signal through a Prolific chipset based serial - 
USB converter which is supposed to pass handshaking signals.  I don't 
know if that will work.

Of course, this thread was started because of a mysterious 50 second 
jump in my system clock.  That's still a mystery.

Regarding the NMEA though.  If I can get those data sentences to my PC 
with a jitter of 3 ms through USB, which I can, I don't see why the 
start and end time of the sentence should vary by 120 ms over 2-4 days.  
I'm trying to figure out where that variance is coming from.  If I can 
consistently keep time with this device, or some other USB device, to 
within + / - 6 ms or even 10 ms of "true time", that's plenty good 
enough for my purposes.  If I can only keep time to within + / - 70 ms, 
then the device is more or less worthless, no better than the internet 
servers.  So, I'm trying to figure out whether it's good enough, or 




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Ron Frazier
timekeepingdude AT c3energy.com

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