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

David Taylor david-taylor at blueyonder.co.uk.invalid
Sat Sep 14 19:49:49 UTC 2013

On 14/09/2013 17:56, unruh wrote:
> 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)

Understand that I meant the lower the magnitude of the offset, not its 
signed value.  I would have hoped that would have been obvious.

>> 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.

Yes, you do need to connect the two units together, but no "rewiring" is 
required.  You can use just four wire jumpers (which you can buy, even 
as part of a ribbon cable if you want a neater looking job) to connect 
the pins on the Adafruit module to the appropriate GPIO pins on the 
Raspberry Pi.  The Adafruit unit includes a pin for the PPS output, and 
has the correct voltage levels for the Raspberry Pi.  I did choose to 
solder the wires at the Adafruit end in this installation:


but I could have used jumpers instead.

As the Pi comes without an OS installed, you need to do that 
installation in any case.  The additional software is easiest if you use 
the "user-mode" PPS software, and just a few commands are then required, 
all detailed here:


This is but a small addition to the time taken for the installation and 
configuration of Linux on the Raspberry Pi.  Performance is shown here:


and may be quite adequate for many purposes, especially as a source of 
local stratum-1 time for a collection of Windows PCs, which was my 
suggestion to the OP.
Web: http://www.satsignal.eu

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