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

David Lord snews at lordynet.org
Tue Mar 20 17:00:54 UTC 2012

Ron Frazier (NTP) wrote:
> 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?

Easy, configure it that way.

> 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 

NMEA gives me around +/- 10ms mean, 20-30ms rms and
40-80ms maximum

PPS gives me around 0.000ms mean, 0.004ms rms and
0.015-0.035ms maximum

Attempting to compare time vs NMEA offset doesn't get
you anywhere.

There are more productive ways of spending your time
such as sorting out your wireless network or having a
wired link to one pc to confirm if the delays you're
seeing are due to the wireless network or your provider.


> 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

More information about the questions mailing list