[time-nuts] question about multi-way measurement

[Achim Gratz]

Charles Wyble wrote:
> I built a dedicated server room in my house, with it's own air
> conditioner. I've been working on overall instrumentation , especially
> temperature.

If the rasPi is a dedicated system and does not serve extra tasks, just 
record its CPU temperature, no extra sensor needed.  The absolute 
temperature will be an almost constant offset from the ambient, so the 
changes are well preserved (except for short periods, e.g. when a cron 
job runs).

Any temperature transient will show up in the timing error, however 
small.  The FLL/PLL code in NTP will need to chase the changing crystal 
frequency and then eliminate the already accrued timing error.  The 
faster the transient, the larger the error.  So on-off aircon with 
forced convection is pretty close to worst case.

> Could you share the snippets of the
> PPS logging? I'm not 100% sure what you mean by the PPS timestamps.

Just reading from /dev/pps0 (system time and capture timestamp in my 
case), really; additionally system load and CPU temperature and logging 
the everything into a file.

> Interesting. My ultimate application of this high precision timing is
> driving TDMA wifi links as low cost as possible.
I'm not familiar with the requirements for that.  I suspect that the 
absolute timing between stations is actually pretty unimportant.  So the 
only thing that matters is that the relative timing error between 
beacons or syncs must keep below some threshold, which means the 
frequency offset of the system must be kept within (probably not too 
tight) bounds.  The latter is much easier to achieve and maintain.

Over wired Ethernet you can expect to synchronize a bunch of systems to 
within a ~200µs envelope of absolute time and maybe a factor of 2x-3x 
lower if you can control certain things more tightly than usual if you 
run those system on a single hop switched LAN that have a GPS-based 
stratum-1 NTP server.  For anything better than that you'd need PTP, 
which unfortunately the rasPi is incapable of.  Or do you plan to use 
the rasPi itself as the RX/TX?  The few  papers I've just looked up all 
use Atheros 9k hardware.


-- 
Achim.

(on the road :-)