[time-nuts] Timing over low bandwidth channels
Poul-Henning Kamp
phk at phk.freebsd.dk
Thu Dec 9 10:55:16 UTC 2010
In message <20101209105031.6104C80003B at ip-64-139-1-69.sjc.megapath.net>, Hal Murray writes:
>Is there a similar sort of high level picture about sending timing info? I'm
>not even sure what the units are.
Basically with timing you only send one bit: "now"
The most precise way to send that bit is to use a very long PRNG
spreading code, and identify the correlator output peak using
statistical estimation on the slopes up to the peak.
DCF77 sends a 512 bit PRNG every second and in hand-run testes I have
been able to determine the peak of the correlation with precision which
is 100-500 times better than the second to second jitter on the 1200km
propagation.
--
Poul-Henning Kamp | UNIX since Zilog Zeus 3.20
phk at FreeBSD.ORG | TCP/IP since RFC 956
FreeBSD committer | BSD since 4.3-tahoe
Never attribute to malice what can adequately be explained by incompetence.
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