[time-nuts] Time source for indoor standalone PC

Attila Kinali attila at kinali.ch
Fri May 17 03:57:25 EDT 2013


On Fri, 17 May 2013 14:45:09 +1000
"Grant Waldram" <grant at remobs.com.au> wrote:

> I've not had much need for time synchronisation over the years, but in
> recent years NTP has been able to get me by. Unfortunately I'm now faced
> with a network that needs a moderately correct clock (I'm scared of using
> the word 'acurate' around you folks!) to the order of a few seconds or so,
> but with no possibility of an external internet connection (for a number of
> reasons). At present I'm using one PC running Windows Server as an SNTP
> server to synchronise all of the devices, as it is the only device in a
> physically secure location. 

How "secure" is the location? Is it a secure server room (bank/insurance
style) or is it a military facility with 10m thick concrete walls, 100m
below a mountain (swiss bunker style)? Ie. can you get radio signals in
and if so, how strong and which frequency range?

GSM would be a possibility as you said, but GSM networks itself tend to
have a very weakly synchronized clock. I've seen GSM networks with the
network provided clock being off by minutes. If your requried accuracy
is just a couple of seconds, and you can get GSM (probably even EDGE)
in your location, then i would run an internet connection trough GSM
and use NTP over that. That should give you something better than 1s at
least, probably in the range of 10-100ms.

If this is not good enough, and you can get 70cm signals into the facility,
then one way would be to build your own time signal. Use a cheap GPS
receiver outside, that drives a CC1100 (or a CC430) to create a time
signal, that you receive inside the facility with another CC1100 which
then creates a time pulse. That should get you below 1ms.

As for the device, i would not use an Raspberry Pi. Get a Beaglebone Black
instead. The advantage is that you have most of the peripherials directly
connected to the CPU instead of funneling everything trough a single
USB peripherial with a hub inbetween. (The RPI is really just a GPU with a
small CPU glued to it, and not really ment for that kind of job)

Oh.. and switch your central NTP server to Linux. Windows isn't really
a good choice for that stuff. Not at all.


			Attila Kinali

The trouble with you, Shev, is you don't say anything until you've saved
up a whole truckload of damned heavy brick arguments and then you dump
them all out and never look at the bleeding body mangled beneath the heap
		-- Tirin, The Dispossessed, U. Le Guin

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