Olin Lathrop wrote:
> A more modern approach would be to make a frequency counter gated by the
> 1Hz signal from a GPS. GPS satellites have their own atomic clocks, so
> GPS time is also highly accurate. I don't know about short term accuracy
> though as exported by common GPS receivers.  You'd probably want to
> observe the target frequency over some time period with the GPS-gated
> frequency counter.

Generic GPS receiver modules can easily put out a 1 PPS signal that's
accurate to +/- 500 ns relative to absolute time, and +/- 50 ns is readily
available with special operating modes. This means that the short-term
accuracy is better than 1 PPM, and the long-term averaged result can be
much better than this. With care, you can begin to approach the 1e-14
accuracy of the Cesium standards themselves, using a good quartz or
rubidium local oscillator for short-term stability -- and the ability to
ride through any GPS outages.

I once used GPS-disciplined oscillators as a reference for aligning FM
broadcast simulcast systems (connected by T1 telephone lines with variable
latencies) to +/- 1 us overall, and it worked just fine.

As it happens, I'm right now in the middle of building a demonstration
network time server (SNTP, TIME and DAYTIME protocols) based on a Motorola
Oncore UT GPS receiver module and the new W7100 Ethernet chip from WIZnet.
It's a sweet little setup.

-- Dave Tweed
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