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

David Lord snews at lordynet.org
Tue Mar 20 21:19:59 UTC 2012

Ron Frazier (NTP) wrote:
> Hi David L,
> See below.
> On 3/20/2012 1:00 PM, David Lord wrote:
>> Ron Frazier (NTP) wrote:


>>> 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.

"ntpd" is not designed to be restarted often, either by
scripts or by reboot to another operating system. If that
is your requirement use ntpdate to get you perhaps within
a second.

Ntp.conf provides options to limit responses from ntpd.

That begs the question, was your ntpd answering requests
when your server was 50 seconds out?

>>> 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 

Your perception of GPS time was understandably wrong.

> 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.

My NMEA time data wanders.

It is not broken, the NMEA spec is for the time data to be
accurate to within one second.

If you require millisecond offset from GPS you need to use
PPS which will give timing within a few microseconds.

Otherwise if you still want to use NMEA rather than internet
servers you need to configure ntpd to accept the larger
variations in offset without dumping the source as


> 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 
> inconclusive.
> Do all NMEA outputs on all GPS's everywhere wander?  I don't know about 
> that.
> 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 
> intermittent.
> 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.
> Sincerely,
> Ron
>>> 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

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