[ntp:questions] question about logs of PPS

Richard B. Gilbert rgilbert88 at comcast.net
Wed May 16 14:32:07 UTC 2007

Christian Pinedo Zamalloa wrote:
> On 2007-05-15, Richard B. Gilbert <rgilbert88 at comcast.net> wrote:
>>The Motorola M12+T is specified to deliver its Pulse Per Second output 
>>with the leading edge within 50 nanoseconds of the top of the second. 
>>Getting that into your computer is where the problem lies.  Things like 
>>cable length can be compensated for.  Interrupt latency is more 
>>difficult because it's difficult to determine exactly what value you 
>>need to compensate for.  Depending on interrupt priorities and what else 
>>is going on, the latency may not even be constant.
>>I'd say that you are probably within one microsecond of the correct time 
>>if you are using the Motorola M12+T as a reference clock.
> So my offset is very bad, i have peaks of milliseconds. I have done
> several graphics with gnuplot and i have put them available at:
> http://bipt106.bi.ehu.es/~jtbpizac/ntp/loopstats.20070515.png
> http://bipt106.bi.ehu.es/~jtbpizac/ntp/loopstats.20070516.png
> At 15 i restarted several times the NTP daemon but today (16) i haven't
> touch the server.
> I think that the graphics are very bad and this could be by various
> reasons:
> 1. The GPS aerial is in the roof inside a plastic water bottle to protect from
> rain. I have seen that the bottle has too much adhesive tape that could
> difficult the GPS to find satellites. I will remove the excess.
> 2. I don't know how my compains obtained the GPS coordinates. I will
> run WinOncore during a week to check the latitude, longitude and height.
> Could a bad value of these parameters produce a unstable GPS clock??

No.  Incorrect values of latitude, longitude, and height can cause the 
clock to be inaccurate but will not affect stability.  As long as the 
error in position is small, the error in time will be small.  By running 
a 10,000 sample site survey, you should get a position correct to within 
                    about three hundred feet.

> 3. The Motorola M12 board is near other electronic components: wifi
> access point, printer, PC, ... Could these components perturb the clock
> quality??

I don't believe so.  Mine is within three feet of: two monitors, four 
computers, and a Cisco 1548M switch.  It seems to be working just fine.

More information about the questions mailing list