[ntp:questions] ARRGH!!! I woke up to a 50 SECOND clock error.

Ron Frazier (NTP) timekeepingntplist at c3energy.com
Wed Mar 14 18:41:26 UTC 2012

On 3/14/2012 11:31 AM, David Lord wrote:
> Ron Frazier (NTP) wrote:
>> On 3/14/2012 7:14 AM, David Lord wrote:
>>> Ron Frazier (NTP) wrote:
> ....
>>>> Based on advice from David Taylor, I just changed my 
>>>> configuration.  Previously, I had only the GPS selectable and the 
>>>> internet servers noselected for testing purposes to try to 
>>>> determine where a slow drifting behavior is coming from.  Now, I 
>>>> have changed that so the GPS is noselected and the New York NIST 
>>>> server is the preferred and only selectable peer.  I am monitoring 
>>>> the GPS and other internet servers for comparison.  So, it will be 
>>>> a couple of days before this configuration accumulates some stats 
>>>> files.
>>>> Even though I have been tinkering with this GPS for a couple of 
>>>> months, I have never seen anything like the 50 second jump I 
>>>> started this thread about.  That problem may not be reproducible 
>>>> for some time, if at all.
>>> Faulty GPS, incorrect configuration, anybodies guess?
>>> Internet servers give me an rms offset about 610us, whilst GPS
>>> with PPS gives about 4us. There are day to day variations and
>>> some rare 'events' when GPS or an internet server goes bad.
>> You must LIVE on the internet backbone and have blazing fast routers 
>> to get performance like that.  My typical performance from internet 
>> servers is offsets of + / - 60 ms.
> Hi Ron
> I don't live on the internet backbone nor do I have  blazingly
> fast anything. I have a slow 2Mbit/s ADSL connection.
> You must have a very broken internet connection to get offsets
> that high. A few times I've been routed by satelite or some
> wireless connection and I also sometimes use mobile broadband
> but still don't see offsets as large as 60ms.
>>> GPS without PPS, Globalsat BR304, wasn't worth using as ntp
>>> source due to large variations in offset from the NMEA sentences
>>> that were tried with RMC being best giving 50% of offsets under
>>> 10ms but maximum offsets being near 100ms.
>> With my BU-353, which is similar, by setting the baud rate to 57,600, 
>> programming for ONLY GPGGA sentence, and polling every 8 seconds, I 
>> can keep my offsets from the GPS's estimate of true time to usually 
>> around + / - 5 ms with spikes to 10 ms and almost never higher.  If I 
>> use ONLY the GPZDA sentence, which is essentially fixed length and 
>> reports only time, I can get that down to + / - 3 ms most of the time 
>> with spikes to 6 ms.  However, David Taylor pointed out that the 
>> GPZDA sentence doesn't have any validity check field, and the GPGGA 
>> sentence does.  We verified that the refclock.c code does check for 
>> this.  So, I'm back to using GPGGA even with a bit more jitter.  That 
>> way, if the GPS fails, ntpd is more likely to react gracefully.
> What are you using as a reference to verify the offsets you
> get are offsets from UTC?

Hi David L,

Allow me to clarify.  As we've discussed, using the internet for time 
keeping from here is awful.  The only relatively precise source I have 
for UTC time is my GPS.  The main point I was trying to make is that I 
can get NMEA data, even without PPS, and even over USB, from my GPS with 
a jitter of around 3 ms.  So, the stats I was quoting are loopstats of + 
/ - 10 ms of the offset of the GPS time relative to my PC's clock 
polling it every 8 seconds and with it as the only selectable clock for 
testing purposes.  So, if GPS time = UTC time, then my reported offsets 
are + / - 10 ms to UTC.  However, this file


that I posted earlier shows the PC locked to GPS time, which is the dark 
jaggy line around zero.  The other band of colored lines are the 
internet servers that I'm monitoring for comparison.  The graph seems to 
indicate that either all the internet servers are slowly wandering, or 
my GPS is slowly wandering (which would have the same appearance), or 
neither is wandering and the internet offsets are being reported wrong 
by whatever drives the peerstats file which I graphed.  The most likely 
candidate seems that the NMEA start time from my GPS is wandering around 
+ / - 60 ms over about 4 days, although I'm not 100 % convinced it's not 
a reporting error.

If the start time of the NMEA sentences is varying + / - 60 ms over 
about 4 days, and since my PC is locked to GPS time, then my PC's time 
will also vary about + / - 60 ms over about 4 days away from UTC.  So, 
all I really know for sure is that I'm locked to within + / - 10 ms of 
the time that my GPS is reporting via NMEA data.

>> The problem with using my BU-353 in this way is that the start times 
>> for the NMEA sentence seem to wander over about 60 ms in either 
>> direction over a period of about 4 days.  So, over a few days, my 
>> computer's time will drift away from true UTC time by that amount, 
>> and back again.  However, over the short term, my pc's clock is much 
>> more stable than if I use internet servers as my primary source.
> Can you give your ntp.conf that result in that level of
> offset from internet servers?

You can look at these if you like.  Posting these via my dropbox is the 
only way I know that's compatible with the mailing list.  If you'd like 
them another way, just let me know.  I'm not worried about getting 
stellar performance from the internet time servers.  I just want to get 
a good performing GPS server running, and then use the internet servers 
only as a backup in case of GPS failure.  Note that these files 
represent a computer that is polling the internet only, and which has no 
GPS as of yet.





> The ntp distribution might still have some advice and
> tools for selection of suitable sources. I just select
> from the list of uk public servers at ntp.org and try
> each one to get rtt and their sources then select the
> ones that are closest with different sources. If you
> don't want to do that you can specify your local ntp
> pool. You should select at least four sources. I have
> about eight different sources split between
> ntp0.lordynet.org and ntp1.lordynet.org.
> David


(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

More information about the questions mailing list