[ntp:questions] PSYCHO PC clock is advancing at 2 HR per second
Ron Frazier (NTP)
timekeepingntplist at c3energy.com
Wed Mar 21 00:00:57 UTC 2012
On 3/20/2012 5:19 PM, David Lord wrote:
> 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:
>>>> 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?
I don't currently have a server set up. I'm only testing on the one
computer at the moment.
>>>> 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.
>> 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
>> 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
>> 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.
(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.)
timekeepingdude AT c3energy.com
More information about the questions