Re: [LEAPSECS] PT Barnum was right

From: Rob Seaman <seaman_at_noao.edu>
Date: Thu, 6 Jul 2006 09:44:13 -0700

Steve Allen wrote:

> In the news.google this week is a press release for a clock that
> automatically tracks leap seconds.

Anybody volunteering to tell these guys that their product is about
to be orphaned? Sounds like a lawsuit in the making. Would think
the ITU lawyers would be interested in their own liability.

> But the only output is a liquid crystal display, and liquid crystals
> have response times around 10 ms. That's 1/100 s, not 1/10000 s.

The key word here is "only". Nothing wrong with including a display,
even if the precision is lessened. The issue with the display isn't
precision, it's accuracy - in that a correction for the display's
response time is unlikely to have been included. The digits will
appear something like 10 ms too late. Worse, the response time may
well depend on the value reported, may vary from digit to digit, and
may change with the age of the unit, etc. Anybody have an opinion on
the correct statistical distribution to use for modeling LCD
transition behavior?

> This seems akin to all the complaints about GPS receivers which
> display a time that is off by about 2 seconds. I've never bothered
> to dig on that, but my impression is that they probably also display
> a position of where they were 2 seconds ago.

Hmmm. Does this apply to the kinds of GPS carried by airplanes,
etc? Or is this purely a problem for consumer grade units snapped up
by the millions for $100 at Walmart? A constant offset will tend to
drop out of the equation (to first order) when any sort of
calibration procedure is followed. Higher order effects will emerge
when the unit is moving at high speeds, or if direction or speed
changes frequently. Most troubling would be if two moving platforms
are depending on GPS units with differing delays, e.g., two airplanes
following neighboring flight paths. How far does an airplane move in
2 seconds? What is the minimum separation required by the FAA?
Again - this will preferentially tend to be a 2 second delay, never 2
seconds early.

> The CBS radio affiliate in the LA area very plainly is using a time
> compressing/FFT pitch shifting device on the live national feed. The
> time tone in LA always happens around 10 to 15 seconds after the hour.

Classic! Send it in to comp.risks. (Search the archives, first.)

As Steve knows, mountaintop observatories are great places to reveal
unintended consequences. One of my favorites was an interaction with
a terminal window page view mode (happened to be Sunview, could
happen with current technology, too). The observer would type a
command line to snap a lengthy sequence of several calibration
exposures while they trotted off to dinner. On their return, they
would discover that the sequence had halted after only a few minutes
and was waiting for a SAK.

The problem isn't only with fixing such issues (such that they stay
fixed), it's with recognizing that a problem exists and with having
the imagination to comprehend contributing factors. A listener might
note that the time signal was delayed, but may be unaware of the
existence of time compression technology. Invisible logistical
details may also be key.

> Sometimes system delay is unintentional, sometimes it is intentional.
> Even before PT Barnum latin had a two word phrase for such products.

"Sucker bait"? "Jumbo junk"? "Electronic egression"?

> Somebody tell me again -- why is it thta broadcast civil time signals
> need atomic accuracy?

I think you meant "atomic precision" here, even if it's less
alliterative. I think we all would like to see an improvement to the
accuracy of civil timing against whatever underlying standard -
accuracy averaged over typical ensembles of clocks. Whether a
particular clock is 15s fast or 15s slow, however, often does not
matter. (And your point is well taken about the policy making
implications of chasing unneeded requirements.)

A simple argument of regression to the mean suggests that clocks
(such as radio time signals) used to set numerous other clocks should
be responsive to a requirement for relatively high average accuracy.
The problem with your LA radio station is not that they are
imprecise, it is that they are consistently wrong *in the same
direction*. This is actually something you might be able to get them
to fix, should you choose. The one thing the FCC appears to care
about is performing station identification. Interviews are
constantly interrupted by the requirement to do so "precisely" at the
top of the hour. Send them a letter and copy the FCC.

Improving accuracy often implies that precision is improved as a
result. The reverse is frequently untrue.

Rob
NOAO
Received on Thu Jul 06 2006 - 09:49:18 PDT