[ntp:questions] Re: Some anomalies

mike michael.no.spam.cook at wanadoo.fr
Sat Dec 4 07:37:18 UTC 2004

You say your platform is a laptop. Have you disabled the power saving
features??? These cause clock frequencies to be changed, which throws 
ntp. I think it's a FAQ.

Richard B. Gilbert wrote:
> Vaidotas J. wrote:
>> Hello,
>> recently my office decided to create some QoS monitoring system and 
>> the main goal of it's correct results on some tests is precise time 
>> synchronization. So we bought few Trimble Acutime 2000 SyncKits and 
>> I've conected them at first to my WS and then to project notebook. 
>> It;s a little bit pitty, that (as far as i know), that Trimbles 
>> Palisade ref clock driver doesn't use ports B output also no PPS usage 
>> too ;(
>> But anyway, the synchronization is received and so on... After some 
>> time, i wrote this veeeery small c program to get the offset value 
>> from ntp_adjtime() function for one hour (calling it every 1 second). 
>> Here it is:
>> #include <sys/timex.h>
>> #include <sys/time.h>
>> #include <stdio.h>
>> int main(){
>> int i;
>> struct timeval ts;
>> struct timex *adjtm; //ntp_adjtime pointeris
>> adjtm = (struct timex *)malloc(sizeof(struct timex));
>> for (i=0;i<=3600;i++) {
>> ntp_adjtime(adjtm);
>> printf("%i\t%i\n",i, (unsigned int) adjtm->offset);
>> sleep(1);
>> } //foras
>> }
>> And i graphed some results enabling/disabling some kernel functions:
>> http://orka.litnet.lt/~dtroit/
>> (gnuplot was used as a ploting engine, with just simple input x,y from 
>> file, generated by this smaaaal c program of mine:)
>> So in one graph we see that for sometime offset was 0, in some of the 
>> graphs (that are wildely absurd) the offset is taken just right after 
>> the ntpd is started and other graphs are commented.
>> The last graph I think is quite good and stable (that last jump is a 
>> mystery to me), but such offset with direct startum0 connected to PC 
>> isn't satisfieing. So maybe any ideas/responses to these graphs or 
>> some experience using Trimble GPS Acutime? What's the offset with Your 
>> Trimble Acutime and the settings of the system that runs it and other 
>> things. So if any ideas please help me to get the smallest ofset :) 
>> Because rigth now i've no more ideas :)
>> PS> The notebook is:
>> model name      : Intel(R) Pentium(R) 4 Mobile CPU 1.70GHz
>> System parameters:
>> Linux ippm 2.6.9 #4 Fri Nov 26 16:39:29 EET 2004 i686 GNU/Linux
>> Debian 3.1 distr.
>> System has no other processes than ntpd, apache, sshd and the GPS is 
>> connected via UST-RS232 converter (no comm port on this notebook).
>> Sincerely,
>> Vaidotas J.
>> LitNET,
>> Network engineer,
>> http://www.litnet.lt
>> dtroit at ktu.lt
> My first guess is that you are having problems receiving the satellite 
> signals.  Further, I'd say that the times when you show a long run of 
> zero offset, you are getting a solid signal from four satellites.  Four 
> satellites are necessary and sufficient to solve for latitude, 
> longitude, height, and time.
> I'm not familiar with your Trimble hardware or the driver for it but; in 
> principle, if you know your position accurately you need only one 
> satellite to get the time.
> Things to check:
> 1.  Does your antenna have a clear view of the entire sky?  If not, find 
> a better location for it.
> 2.  Use nptq -p and check the "reach" field for your reference clock.  
> If it is not 377 you have a problem!
> 2.  Does your driver allow you to ignore satellites close to the 
> horizon?  If so, it may help to enable this feature.
> Another possibility is that both Linux and Windows have a reputation for 
> losing interrupts!  Losing clock ticks will do horrible things to your 
> timekeeping!!!!

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