> The field units are mains powered and draw hundreds of Watts each, so
> the added power consumption is not an issue.
>
> As long as the GPS locks onto the time within 5 minutes, I think the
> client will be happy.  They are also fixed position.  Would this help?
>
> Maybe using the GPS to correct an on-board RTC would be a better option?

Off the cuff low practical experience comments:

GPS sounds like a good fit.
A typical GPS (all modern?) maintain  ephemeris (short term) and
almanac (long term) satellite constellation descriptions to allow
location of the satellites. If the system is always on the time signal
is essentially continous. If you turn the system on regularly and
frequently (say hourly to maybe a few hours) the ephemeris information
is usually relevant enough  to allow reacquisition in under about a
minute and sometimes much less. If you turn the system on daily or
weekly it can take minutes to acquire enough satellites to start
producing information. (1 =3D somewhere apparently :-), 2 =3D a torus,  3
=3D where you are or out closer to lunar orbit, 4 =3D where you are, 5+ =3D
better.).

Away from cities you are unlikely to seldom experience catastrophic
loss of enough satellite signals to lose the time signal.

So, if you have a RTC that has enough stability for at least 10
minutes (not exactly hard even on the far side of the black stump on a
hot Australian summer day) then a GPS fix "when required' will keep it
stable enough.
An RTC less stable than that or no RTC would benefit from permanent
GPS operation.


         http://en.wikipedia.org/wiki/GPS_signals


      R
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