PT Barnum was right
In the news.google this week is a press release for a clock that automatically tracks leap seconds. The PR glowingly touts how the clock traceable to NIST, so it is useful for timekeeping in all sorts of processes that need ISO 9000 certification. http://news.thomasnet.com/fullstory/488673 It goes on to say that the clock is always accurate to 1/1 s. The manufacturer has more to say http://www.control3.com/5125p.htm For less than $20 this battery powered clock is certified by an ISO 17025 calibration laboratory accredited by A2LA. But the only output is a liquid crystal display, and liquid crystals have response times around 10 ms. That's 1/100 s, not 1/1 s. 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. Finally, I've been spending a lot of time in the LA region lately. The CBS radio affiliate in the SF Bay area broadcasts the hourly national news spot on, and the time tone is useful for setting a watch. 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. Sometimes system delay is unintentional, sometimes it is intentional. Even before PT Barnum latin had a two word phrase for such products. Somebody tell me again -- why is it thta broadcast civil time signals need atomic accuracy? -- Steve Allen [EMAIL PROTECTED]WGS-84 (GPS) UCO/Lick ObservatoryNatural Sciences II, Room 165Lat +36.99858 University of CaliforniaVoice: +1 831 459 3046 Lng -122.06014 Santa Cruz, CA 95064http://www.ucolick.org/~sla/ Hgt +250 m
Re: PT Barnum was right
Steve Allen wrote:In the news.google this week is a press release for a clock thatautomatically 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 crystalshave response times around 10 ms. That's 1/100 s, not 1/1 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 whichdisplay a time that is off by about 2 seconds. I've never botheredto dig on that, but my impression is that they probably also displaya 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 timecompressing/FFT pitch shifting device on the live national feed. Thetime 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 signalsneed 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.RobNOAO
Re: PT Barnum was right
In message [EMAIL PROTECTED], John Cowan writes: Rob Seaman scripsit: 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? Jetliners travel at about 550 mi/hr, or 246 m/s. Required horizontal separation depends on circumstances, but is rarely less than 3 miles = 4828 m. So if the discrepancy is 2 s, there is a safety factor of about an order of magnitude. Good enough. The ban on hand-held GPS for primary means of navigation is partly because of the worries about the slowness of updates. -- Poul-Henning Kamp | UNIX since Zilog Zeus 3.20 [EMAIL PROTECTED] | TCP/IP since RFC 956 FreeBSD committer | BSD since 4.3-tahoe Never attribute to malice what can adequately be explained by incompetence.
Re: PT Barnum was right
- Original Message - From: Steve Allen [EMAIL PROTECTED] To: LEAPSECS@ROM.USNO.NAVY.MIL Sent: Thursday, July 06, 2006 7:27 AM Subject: [LEAPSECS] PT Barnum was right (gesnippt) Finally, I've been spending a lot of time in the LA region lately. The CBS radio affiliate in the SF Bay area broadcasts the hourly national news spot on, and the time tone is useful for setting a watch. 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. You can thank the U.S.' increasingly draconian anti-obscenity laws for that. In response to an email inquiry to KNX on this very subject (the delay I noticed was about nine seconds), I was told that the station delays their broadcast in order to enable on-the-spot editing of objectionable material. The FCC recently increased fines for violation of anti-obscentity rules, and some television stations were even fined for not censoring all 500 milliseconds of the Janet Jackson wardrobe malfunction, so it's in networks' best interests to do this even if it means setting your watch to their time signal means being 9 seconds late. Somebody tell me again -- why is it thta broadcast civil time signals need atomic accuracy? I rather like the way James Jesperson and Jane Fitz-Randolph put it in their excellent layperson's introduction to time and frequency, _From Sundials to Atomic Clocks_: The 'time' as most of us know it is simply inexpensive crumbs from the tables of the few rich gourmet consumers of time and frequency information. The guy wearing the trendy Swatch with hands pointing at a blank dial neither knows nor cares about any of the esoterica discussed on this list; for him, the time will be whatever They(tm) say it is. If that's what is meant by broadcast civil time signals, then it's true: we're being served chronological junk food and most folks couldn't care less. Bon appetit! Brian Garrett
Re: PT Barnum was right
On Jul 6, 2006, at 12:46 PM, Brian Garrett wrote: I was told that the station delays their broadcast in order to enable on-the-spot editing of objectionable material. Surely the requirement is to permit review of *potentially* objectionable material. A time signal is no such thing and need not be delayed. Proper system design would interpose time signals (and perhaps other safe content such as weather reports or what have you) after the delay line. Am also skeptical that most stations employ a 24 hour censor to monitor and bleep all content before it reaches the air. (And who would want such a horrific job?) it's in networks' best interests to do this even if it means setting your watch to their time signal means being 9 seconds late. That's just one use case, of course. Folks who set their watches using well synchronized time signals, or who consult their cellphones or NTP driven laptops, will be ahead of this particular radio station. Nine seconds is meaningless for many purposes, but a 9s simulcast delay would be intolerable, for instance. Usage issues might also be revealed when switching between stations. My thought when we reach one of these topics is to marvel at the chutzpah of proposing a solution like leap hours without investing the slightest effort in characterizing potential issues. Timekeeping can't simultaneously be the deeply important issue we all must think it is (or else would not be reading this :-) and also be worthy of such abject neglect. The 'time' as most of us know it is simply inexpensive crumbs from the tables of the few rich gourmet consumers of time and frequency information. Astronomers have traditionally been not only among the most demanding gourmets, but have also employed some of the greatest temporal chefs. This is indeed a pretty good analogy, although the word inexpensive is out of place. The point is that a crumb from the table of a gourmet is still a gourmet crumb. A parvenu of time can also aspire to become a gourmet should the need or interest arise. I believe this describes the world we currently inhabit. we're being served chronological junk food and most folks couldn't care less. This is a different analogy. It isn't a question of most folks, it is a question of for most purposes. Even gourmets sometimes appreciate a simple meal. And on the other hand, the temporal hoi polloi are dependent more-and-more on chronological caviar through the offices of various technological agents. Cellphones don't only report high quality timing information, they and their networks require this to operate. But your analogy is quite apt for the world that would follow the adoption of the Absurd Leap Hour Proposal. All time signals would then become junk food. All gourmets would find themselves in the position of dumpster divers. Rob NOAO
