[ntp:questions] Trying to use Dimension 4 time keeper

unruh unruh at invalid.ca
Sat Sep 14 16:56:15 UTC 2013


On 2013-09-14, David Taylor <david-taylor at blueyonder.co.uk.invalid> wrote:
> On 14/09/2013 05:48, W. eWatson wrote:
>> What have I gained?  What does the data mean from ntpq -n -clpe -clas?
>
> Please see:
>
>    http://www.satsignal.eu/ntp/setup.html#check
>
> for a brief guide to checking that NTP is working.
>
> The command "ntpq -p" or "ntpq -pn" will show the delay between the 
> servers and your PC (in milliseconds, expect 20-30 ms for network-locale 
> servers 100+ ms for servers on an inappropriate continent), and the 
> offset column shows how well NTP is doing, the lower the offset the 
> nearer to UTC is your PC running.  The offset value will be highly 

Well, no. The offset tells you what the difference is between the time
as measured from that remote server and the time on your system. If the
path is completely symmetric, then that time time is also the remote
servers best estimate of UTC (and depending what its stratum is, that
could be pretty far off). But if the delay is assymetric then that
estimate is out by half that assymetry. Thus the lower the offset, the
nearer your machine is to its own best estimate is of UTC based on its
measurement. But ntp works hard to make that offset zero. In fact that
is its whole purpose. Thus that offset should fluctuate around zero over
the long term, because the estimate of UTC fluctuates (due to
assymetries, and errors upstream)



> dependant on the OS (i.e. Windows of Linux/FreeBSD), the servers you are 
> using (i.e. Internet or local), and whether or not you have a PPS (pulse 
> per second) feed to your PC.
>
> In general, you will see offsets in the tens of milliseconds range for a 
> purely Internet-synced Windows PC (but may be better with some versions 
> of Windows), and sub-millisecond for FreeBSD/Linux PCs.  For best 
> performance, you may want a local Linux PC synced to the Internet (or 
> with a GPS/PPS source), and sync the Windows PCs to that server (but 
> sync them more tightly than you would to the Internet).  I don't think 
> you're at that stage yet, but it's worth bearing in mind.  The Linux box 
> could be as simple as the Raspberry Pi card PC, and even adding a GPS to 
> that only adds ~US $35 to the cost.

And some hacking-- both rewiring the gps and rewiring the RPi and
installing GPIO input interrupt software.




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