If you can't get GPS signals 'inside' but you can get GSM coverage, site
screening can't be all that good, so rebroadcasting the data from an
external GPS receiver signal might be the best option. Just pick a transmit
frequency and power combination that also provides coverage 'inside'- with
due regard for local regulatory issues etc. Alternatively, if plus/minus a
few seconds is really all you want/need, any half decent OCXO should provide
a stable enough timebase for a totally isolated 'inside' clock. You haven't
indicated whether or not the 'inside' clock has to have any specific
relationship to time in the real world on the 'outside'. If it does, then
the OCXO option requires a regular 'check and correct' routine at a
frequency that is dependant on the stability of your OCXO. The actual time
xfer should need no more than a digital watch with a seconds display or
something equivalent, given it need take no more than two or three minutes.
(How long does it take to walk through a couple of doors?)
Regardless of the implementation, you should preferably have a separate
master clock on the 'inside' that runs independently from your network-there
are a number of options to then feed its time into the network.
DaveB, NZ
----- Original Message -----
From: "Grant Waldram" <gr...@remobs.com.au>
To: <time-nuts@febo.com>
Sent: Friday, May 17, 2013 4:45 PM
Subject: [time-nuts] Time source for indoor standalone PC
Hi folks. I wouldn't call myself a time nut, so this is really an effort
to
ask for advice from some people who know the field. My first contact with
time synchronisation was looking at the instrumentation clocks for the
Woomera rocket test facility when I went out there for a few (large!)
hobby
rocket launches. I can't even remember the system's name but it used a
series of pulses of various lengths to give a unique time code. But, I
digress...
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. This is inside a security-fob protected room.
I
can't get GPS signals in there, and the Australian radio clock network was
shut down about ten years ago. Our CDMA network was turned off in 2008.
Right now all I can think of is GSM, and while i know it's not terribly
accurate it seems like it might be the easiest. It also might be that I've
got tunnel vision and there's a simpler solution out there.
I would be quite happy with some sort of dedicated GSM/NTP-server box, and
there are Arduino/Raspberry Pi/Linux homebuilts for that out there, but I
have been wondering if one of the fairly common GSM USB sticks could
somehow
be a time source to set the server clock?
Regards,
Grant
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