Steve Allen wrote on 2003-01-29 20:53 UTC:
> On Wed 2003-01-29T15:05:59 -0500, John Cowan hath writ:
> > I was a little too clipped.  If you know all the leap seconds, you can
> > convert a Unix-style timestamp to UTC reliably; if you further know all
> > the timezone changes, you can convert UTC to LCT reliably.
> I remain confused about why this "isolated system" cares whether it
> keeps time as UTC or TAI.  How does its time get set?  How does its
> time stay locked to SI seconds?
> Are you supposing that the system is able to keep SI seconds because
> it has some sort of unshielded PLL which is tracking the carrier
> signal from something like the US Navy's high powered VERDIN VLF
> transmissions for submarines?  (With their 50 baud message that
> basically says "We're still here so don't launch" and if your clock
> stops ticking, nothing really matters much anymore.)

VERDIN phase tracking is perhaps a somewhat pathological case.

Here is a more realistic source of standard frequency that can easily be
tracked and is in practice tracked in lots of low-cost consumer

Most of the European continent (excluding Britain and some East European
Countries) runs a 50.0 Hz continent-wide phase-locked power grid known
as the UCPTE grid (Union for the Coordination of Transmission of
Electricity - the organization responsible for the reliable operation of
the interconnected electricity network in Europe). The UCPTE
specification says that the grid phase vectors have to rotate on
long-term average exactly 50 * 60 * 60 * 24 times per UTC day. That is
you get on average 50 * 86400.0000 oscillations out of each power socket
in Europe every day, even if you consider days that end in a leap second
and are actually 86401 SI seconds long. Near an inserted leap second,
they are actually reducing the power grid frequency in a coordinated way
for a few minutes by up to 50 mHz in order to make sure that all the
many clocks that use this 50 Hz standard frequency as their time
reference remain in sync with UTC. You can observe this nicely with an
oscilloscope if you have a stable reference signal to triger it
independently. Power-grid coupled clocks will go 0.1% slower briefly all
over Europe to resync with UTC after a leap second. Note that the power
frequency deviates sometimes significantly from 50 Hz, but the PLL
controllers contain an integrator and eleminates any long-term error
relative to UTC this way.


    Section S.6.1.

    Section 3.1.1 (5)

Another ubiquitously available high-quality reference frequency in
Germany are the national TV broadcast sync signals, which are derived
from caesium clocks in the basements of the broadcasting houses (at
least ZDF does this). Their TV signals are not frequency adjusted to
follow UTC, they stay at 50.000000 Hz exactly. However this is not a
problem for consumer electronics, because the teletext data in the
vertical bank interval labels each TV frame with an ASCII encoded
hh:mm:ss timestamp that tightly follows UTC. In practice, TV sets with
radio controlled clocks simply evaluate the teletext time stamps when
the receiver is switched on and run freely when it is off. Same for
radio receivers that evaluate RDS time signals. They are not phase
locking clocks to the TV signal, and even if they would, they could
learn about the leap second from the teletext data (with a small delay
as teletext lacks a leap second announcement).


Markus Kuhn, Computer Lab, Univ of Cambridge, GB | __oo_O..O_oo__

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