[ntp:questions] Extracting ntpq like information programmatically

Claudio Carbone erupter at libero.it
Fri Mar 29 17:26:30 UTC 2013

On 29/03/13 18:04, David Woolley wrote:
> Single PPS source with equal length cables to each machine.
Machines are moving robots, and we operate indoor.

> If the clocks are already synchronised by NTP, none of these will give 
> you a better time; all they will do is given you an idea of the error 
> bounds of the measurements.  NTP offsets measured at the same time as 
> propagation time measurements for other messages will typically have 
> the same errors, and should be completely ignored.

I measured and plotted the offset for a few minutes and it hovered 
around -2ms for a certain machine with respect to the master.
Isn't this a decent estimate? Why can't I use a moving average of this 
offset to correct the other measurements?

I mean that, when I have certain timings from different machines, if I 
correct them by their average offset from a common source, doesn't this 
augment the precision of the measure?
I'm using a single external source to compare all other clocks.

Since we're talking ms here, I can't put a blind eye to a 2ms offset: it 
may be even bigger than the measure itself.
What I'm talking about is: if I measure the lag between a request and a 
replay between two machines, and this time is in the ms range, the clock 
offset may well represent 50% or more of the whole measure.

I have to find a way to account for that or at least have an estimate.


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