Clive D.W. Feather wrote:
Tony Finch said:
So you think that the millions of existing radio controlled clocks
and watches should stop showing civil time?
They already do.
Case in point: When the local Red Cross center relocated a couple of
years ago, new RC "atomic" clocks appeared over each blood donor
station.
- With an 8 week sampling interval, I can report that for the first
six months or so, the clocks were synchronized to the radio with low
dispersion.
- Around the six month mark (say, around the time DST changed for the
first time), the dispersion kicked upward for several of the clocks
but most remained synchronized.
- Around the one year mark it was about half and half - that is, it
looked like half the clocks were now running open loop. Presumably
batteries were changed, etc.
- At 1.5 years, the Red Cross introduced a new hand held data entry
device for scanning bar codes from the bags, etc. This has a printer
such that much less data is handwritten. The device also appears to
include its own clock (perhaps synchronized across the system). The
wall clocks seem to fill a purely utility purpose now.
- After two years, the clocks are all off the grid with no two
synchronized.
Each system we design and use has unique relative and absolute
timekeeping requirements. It is hard, however, to see any problem for
which the current generation of consumer RC clocks is an appropriate
solution. Ease of setting is a great feature. But setting a clock
also involves checking that you set it correctly (selected the right
combination of buttons on the back). Precisely by advertising such
clocks as whizbang atomic clocks, the user community will relax their
own vigilance (never much to begin with).
Rather, a clock that claims extreme accuracy (through overtly extreme
precision) must ultimately be layered on a well designed system of
traceable time signals. RC time signals are meaningless (no matter
the underlying timescale) if the logistical issues (as described by
Steve, for instance) are ignored. Logistics keys on developing a
coherent model of the system being operated.
It is not enough to simply create a single perfect clock. The perfect
clock demands a matching distribution protocol. The ntpwg mailing
list makes interesting reading if one is tempted to believe this is a
solved problem.
What kind of clock does the Red Cross require? Focus on the use
cases, not the technology.
Rob Seaman
NOAO
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