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

David Taylor david-taylor at blueyonder.co.uk.invalid
Sun Sep 15 06:25:07 UTC 2013

On 15/09/2013 00:06, unruh wrote:
> But even that does not help. If your outgoing message to the ntp server
> goes from California to New York via Kansas, and the return consistantly
> goes via the moon, your computer will always be about 2 seconds behind
> UTC, even if the offset is always 0.
> Over a long time, the fluctuations in the offset will give a good idea
> of what the random fluctuations are, and what the random errors are in
> the time on your machine. But they tell you nothing about the systematic
> errors. And the instantanous offset tells you  little as well-- your
> computer could just have done a huge job which heated it up, and made
> the crystal oscillate faster. Over time that should average out. But
> that is only over time. (and it could be that you carry out hot jobs
> every day at 2PM, and thus every day between two and 5 your computer
> will be faster than and displaced to the future of UTC.

I accept that space-based NTP may require different measures.  In my own 
experience (using mostly LAN-based stratum-1 servers), the offset 
magnitude and jitter are good measures to use should you need to improve 
NTP's performance.

> I was probably refering to the Sure unit which is more difficult.
> I have never looked at the Adafruit module. Also doesn't the Adafruit
> also require an extra antenna to work properly?

Yes, the Sure unit, another low-cost option, does require a little 
modification to extract the PPS signal at RS-232 levels, although based 
on my current experiences likely the more readily available TTL-level 
PPS signal would actually be satisfactory unless very long cable tuns 
were involved.  I have detailed the modification required here:


With the Adafruit unit I'm not using an external antenna in my "lab", on 
the second floor of a two-storey building.  The GPS signal creeping in 
via walls and roof is enough.  The Adafruit module has both a built-in 
patch antenna, and a connector for an external antenna if that's needed. 
  Not tested here.

I've not done a definitive test, but as a guide I would expect that if a 
tablet or mobile phone can get a GPS fix at a particular location, the 
Adafruit (or similar) module would work as well with its built-in 
antenna.  If you are in a basement, or heavily shielded environment 
(even some windows can be RF-opaque), an external antenna will be 
required for any GPS.
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