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

Ron Frazier (NTP) timekeepingntplist at c3energy.com
Tue Mar 20 19:09:13 UTC 2012

Hi David L,

See below.

On 3/20/2012 1:00 PM, David Lord wrote:
> 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.

I'm not sure how to do that within the confines of ntp.conf.  David T. 
suggested I could run a Perl script every minute to shut down NTP if the 
GPS fails.  But, I'd rather keep NTP running and just not distribute 
time on the LAN when my time server is polling the internet.  Which 
brings up a question.  If my time server on my LAN is attached to the 
GPS, that GPS is considered stratum zero and my time server on the LAN 
appears to be a stratum 1 device to other computers, right?  Then, what 
if the time server stops using the GPS and begins using internet stratum 
2 servers as it's time source?  Does my LAN time server now present 
itself as a stratum 3 device to the other PC's on the LAN?  If so, they 
might automatically stop using it and poll the internet stratum 2 
servers themselves.  That would be fine.

>> 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.
> David

The first thing a timekeeping newbie like me hears when asking about 
accurate PC timekeeping is "go hang a GPS on it".  So, back when I 
started experimenting in January, and without the benefit of 2 months of 
banging my head on this wall and all these discussions, I go to Amazon, 
find a cheap GPS with good reviews, and buy it.  I hook it up, configure 
NTP, and start getting + / - 10 ms of offsets of the PC versus GPS 
time.  Since I think GPS time = UTC time, for most practical purposes, 
I'm happy.  For my particular purposes, + / - 10 ms is fine.  I plan to 
pursue PPS mostly for learning reasons, but I don't have to have 
microsecond level accuracy.  I just want my PC's clocks to be right to 
less than 500 ms but would really prefer less than 10 ms.  I also want 
to be doing better than the + / - 50 ms I'm getting from the internet at 
the moment.  Another goal of having the GPS was possibly to poll the 
internet less often.  Now, only after 2 months of head banging and 
discussions, do I find that the NMEA data is wandering.

So, since I could live with NMEA only if it didn't wander, even though 
it only give + / - 10 ms accuracy, the question on my mind is:

Does only this GPS wander?  Evidence is no.  Others wander.  David T 
mentioned a wandering Garmin and someone on another list mentioned 
another SIRF unit that does it?
Do all SIRF GPS's wander?  Very possible, but evidence that I have is 
Do all NMEA outputs on all GPS's everywhere wander?  I don't know about 

I think it's important to have this discussion, and I think it's 
important that it goes on these public lists.  If it turns out to be 
true that almost no GPS with NMEA only output, or with PPS but that's 
not used, will ever provide more than 100 ms peak - peak accuracy, then 
that's important to know.  Now, if that 100 ms performance is 
consistent, then it may be adequate for some applications.  It would 
even be adequate for mine, if it weren't for the darned heart attacks 
this particular unit seems to have.  However, 100 ms doesn't get me 
anything that I can't get with internet, so why bother?  It could still 
be used in places where internet connectivity is not available or is 

If, on the other hand, there are some GPS's with NMEA only output that 
can consistently provide + / - 20 ms accuracy or so, then I think that's 
important info too.

Keep in mind, many computers don't have serial and never will, other 
than serial - USB converters.

It's good that we get the information out there so other newbies 
researching the issue can zone in on what's possible to do.

Speaking only regarding Windows systems for a moment:

So, let's say a person needs + / - 100 ms performance from UTC.  He can 
use the internet, even with wireless, and multiple routers, like I 
have.  Or he could use even a USB only GPS if there is a reason.

Say he needs + / - 20 ms performance.  Internet may be an option 
depending on the circumstances.  GPS might be a better option depending 
on the GPS.

Say he needs + / - 1 ms performance.  Internet is probably not an 
option, at least in my experience and my location.  USB only GPS with 
PPS through the serial - USB converter might work.  Serial GPS with PPS 
would work.

Say he needs + / - 20 us performance.  Excluding the realm of rubidium 
standards and such, serial GPS with PPS is probably the only option.

So, the more this information is threshed out and clarified, the more I 
can evaluate what options I have for my minimal needs, and the better I 
can point someone in the right direction if I'm asked about it, either 
professionally or personally.  Also, the more other newbies starting to 
explore such things will be able to make the right decision with less 
headaches.  If there were a USB GPS with PPS support through DCD messges 
was already available, one such as myself could jump right to the + / - 
1 ms realm, and forget the rest unless he need to go to + / - 20 us.  
That's what the people on the thumbgps-devel group (Eric Raymond et al) 
are working on.



>> 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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