[ntp:questions] PSYCHO PC clock is advancing at 2 HR per second

Ron Frazier (NTP) timekeepingntplist at c3energy.com
Tue Mar 20 14:29:49 UTC 2012


On 3/20/2012 2:25 AM, David J Taylor wrote:
> "unruh" <unruh at invalid.ca> wrote in message 
> news:JDU9r.22132$_C5.11432 at newsfe09.iad...
> []
>> Of course the question still is why in the world did the system go nuts
>> when it was on Local. That itself should not have happened.
>
> If some software had told the system clock to run fast, it simply 
> stays running fast, even on Local.
>
> Ron is using a single GPS device, over USB, without the backup of a 
> few Internet servers to stop such a thing happening, and the GPS has 
> already shown itself to be problematical.  NTP would normally have 
> simply rejected the errant GPS data and not cause the PC clock to run 
> wild, but without the Internet servers as backup, what is NTP to do?  
> I don't think it has a choice other than to believe the GPS, even if 
> it's incorrect or faulty.
>
> Ron, perhaps in the future you could adopt a similar configuration to 
> one I've mentioned before - add some Internet servers with a long 
> polling interval as a second opinion for NTP:
>
> _________________________________________
> server <ref clock stuff>
>
> server  0.us.pool.ntp.org  minpoll 10 iburst
> server  1.us.pool.ntp.org  minpoll 10 iburst
> server  0.uk.pool.ntp.org  minpoll 10 iburst
> server  1.uk.pool.ntp.org  minpoll 10 iburst
> _________________________________________
>
>
> using servers [network] local to yourself, of course.
>
> Cheers,
> David
>

Hi David T,

Eventually, I do plan to have the server preferences as follows:

Time server machine:

    GPS
    Internet as backup

Hypothetically speaking, what if I don't want it to distribute time if 
it's working in internet mode?

Non time server machines

    GPS (if attached)
    Local time server (if available)
    Internet as backup

However, I only plan to do that after thoroughly testing the GPS by 
itself for a week or two to see if it's stable.  I originally had the 
internet servers on with this unit.  It completely surprised me by 
having this tendency to drift apparently and have periodic heart 
attacks.  Unfortunately, this odd behavior may exist in all SIRF III and 
possibly other SIRF units.  It was only by turning off the internet 
servers that I was able to get some clean graphs of exactly what the GPS 
was doing.  When I had the internet servers enabled, once the GPS 
starting acting odd, which it shouldn't do at all, NTPD would clock hop 
to the internet.  Normally, that would be OK.  However, as discussed 
previously, even my errant GPS is more accurate over the short term than 
the internet for me.  With the internet conection, I get + / - 50 ms 
variations in time over a span of an our.  With the GPS, I get + / - 60 
ms variations over several days, with a few wild corrections during its 
heart attacks.  Those are two bad choices, but I think I'd still rather 
run on the GPS.  The only way I can prevent clock hopping is by 
noselecting the internet servers.  Even if I end up with internet 
servers turned on, which I expect to, I think it's much better to know 
about these GPS anomalies before putting it into long term service.  
Anybody considering using a SIRF III or maybe even any SIRF unit for 
timekeeping should be warned by my experience, test the unit, and make 
sure it's up to the task.  These problems could even affect SIRF units 
with PPS outputs, although I don't know.  I'll probably decommission 
this unit from timekeeping duty and relegate it to navigation duty, 
although I'm not sure how trustworthy it is for that when it's throwing 
a temper tantrum.

I've already committed to getting better (hopefully) equipment.  
(Shipping from Hong Kong or where ever seems to take a LONG time when 
you're waiting on something.)  Hopefully, the Sure board will be much 
more stable and reliable.  I'm planning to do the same extensive testing 
on the Sure for a week or two.  I'll start out just plugging the Sure 
into my serial - USB converter using the same com port as the Globalsat 
unit was running on.  I want to see how it does with NMEA only data for 
a while.  I'm hoping NOT to see substantial drifting from UTC and NOT to 
see any heart attacks every few days.  I expect lots of jitter, since a 
number of variable length sentences are being output.  Then, I plan to 
turn off all but GPGGA and test some more, and maybe tinker with the 
baud rate.  Then, if I can solder the board without killing it, I'll 
engage PPS through the serial - USB port and test that for a while.  
Then, I'll try it with PPS and real serial on my other computer, the 
only one with a serial port.

Hopefully, when I'm done, I'll have a qualified unit running stably and 
accurately for the whole network to use.  I've acquired a case and some 
hardware to mount the device similar to yours.  Once I learned that it 
was only 3" x 3", that made me nervous as far as soldering and all, but 
we'll see what happens.

By the way, do you think I should update to Dave H's latest binaries?  
I'm at 4.2.7p259 on Windows.  Almost all these discussions have been 
about Windows.  Linux is a whole other ballgame.  The NTPD there from 
the repositories is about 2 years old, and I'm reluctant to go outside 
the repositories because of the numerous problems it creates.  One very 
serious Linux user on a local message board said even he doesn't compile 
his own programs because of possible problems.  I tried it once and all 
sorts of scripts and file locations that Ubuntu expects got broken.

Sincerely,

Ron


-- 

(PS - If you email me and don't get a quick response, don't be concerned.
I get about 300 emails per day from alternate energy mailing lists and
such.  I don't always see new messages very quickly.  If you need a
reply and have not heard from me in 1 - 2 weeks, send your message again.)

Ron Frazier
timekeepingdude AT c3energy.com



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