Re: what time is it, legally?

2006-12-12 Thread Tom Van Baak
But this needs a clarification. Standard time replaced local apparent solar time in several steps. First, clock (mean) time replaced apparent time for civil purposes. As you can see from the proliferation of railroad standards, these were both still local to one place or another. Later,

Re: trading amplitude for scheduling

2006-08-04 Thread Tom Van Baak
In fact, leap seconds are simply due to the earth being slow. How it got to be slow and whether it is slowing are another issue. Let me see if I have this right: 1) We have leap seconds because the Earth rotates more slowly than once every 86,400 SI seconds. Yes. (and I know what you

Re: trading amplitude for scheduling

2006-08-04 Thread Tom Van Baak
If the SI second were properly tuned to the mean solar day, and the secular slowing were eliminated, there would be no need to mess about with the civil time scale, because the random accelerations and decelerations would cancel out in the long run. Of course, we'd have to tolerate larger

Re: building consensus

2006-06-01 Thread Tom Van Baak
UT1 et al are not really measures of time, but of angle (of Terran rotation). To some degree yes, but don't they also include minor corrections (polar motion, longitude, etc.) and so at one level they already depart from raw angle measurement and instead are trying to act like clocks? /tvb

Re: Precision vs. resolution

2006-06-01 Thread Tom Van Baak
I should perhaps explain that I was interested in an internal representation for durations, which I am now representing as a triple of months, minutes, and seconds (the number of minutes in a month is not predictable, nor the number of seconds in a minute given leap seconds, but all other

Re: Precision vs. resolution

2006-05-30 Thread Tom Van Baak
Can someone lay out for me exactly what the difference is between clock precision and clock resolution? I've read the NTP FAQ and several other pages but am more confused than ever. (I do understand the distinction between precision and accuracy: 3.1429493 is \pi precise to 8 significant

Analog clocks and leap seconds

2006-01-13 Thread Tom Van Baak
Michael Sokolov writes: I cannot make the beautiful analog clock on the tower show 23:59:60. But it's trivial to make it occasionally take 1.1 SI seconds instead of 1 SI second to turn its hands by 1 civil second. Yes, this is one of the awkward features of a leap second (positive leap second,

Re: Monsters from the id

2006-01-13 Thread Tom Van Baak
It should be clear that the gaps and repeats are fictitious, especially if you think of AEST and AEDT as existing beyond the times when they are in legal use. Putting it in practical terms, suppose I have a traffic accident at 0230 on 2006/04/02, what time will the police officer write in

Re: The opportunity of leap seconds

2006-01-08 Thread Tom Van Baak
Research-quality telescopes, in particular all the ones built in the last few decades on alt-azimuth mounts, do of course use UT1; a 0.9s error would be a complex ~10 arcsec error in both axes and give a quite useless pointing performance. However, UTC is often used as a UT1 delivery system;

Re: interoperability

2006-01-08 Thread Tom Van Baak
Without further debating the meaning of civil time, consider the implications of this two stage system. The first stage conveys TAI or something related to it by a constant offset. The second stage at any location (correct me if I misunderstand you) would be a secondary clock disseminated

Re: interoperability

2006-01-08 Thread Tom Van Baak
You cannot divide timekeeping, time dissemination, into neat stages. In the 1960s if ten labs were told to offset their phase or frequency it affected only a handful of people or systems. Today when IERS announces a leap second, millions of machines, systems, and people are affected.

Re: Diagram of CHU Leap-Second Recording and Atomic Clock

2006-01-04 Thread Tom Van Baak
It is correct that DUT1 changes by +1.0 across a positive leap second; going from a negative value (e.g., -0.6) to a positive value (e.g., +0.4). You would see the inverse in the case of a negative leap second (DUT1 will, by definition, be positive before the negative leap second and go negative

Re: Diagram of CHU Leap-Second Recording and Atomic Clock

2006-01-04 Thread Tom Van Baak
The majority of such clocks only run the receiver for some part of the day to save power. One particular kind I examined ran the receiver until it had sync, then powered the receiver down for 23 hours and repeated the cycle. Yes, but the LS bit stays lit for the entire month (at least for

Re: Longer leap second notice

2006-01-04 Thread Tom Van Baak
A more apt comparison would be to the leap year rules that we have. We know the rules going forward a thousand years or so. Apt indeed. Leap seconds are scheduled at least six months in advance. That's about one part in 15 million. A thousand year horizon for scheduling leap days is

Double 59; 60; or double 00?

2006-01-04 Thread Tom Van Baak
With the surge of leap second captures this time around, are there any concerns over the growing(?) use of double :59 second or double :00 second instead of :59:60 for a positive leap second? Although not technically correct, they do seem a practical, perhaps even clever, alternative -- in some

Re: Schreiver AFB warns about leapsec

2005-12-20 Thread Tom Van Baak
Thanks for the link. I see the reference to it is on their main page: https://www.schriever.af.mil/GpsSupportCenter/advisories.htm Also note the leap second photos they used in the power point presentation came from: How to Watch a Leap Second http://www.leapsecond.com/notes/leap-watch.htm

BBC - Leap second talks are postponed

2005-11-15 Thread Tom Van Baak
BBC News, 9 November 2005, 08:36 GMT [sic] http://news.bbc.co.uk/2/hi/science/nature/4420084.stm Consideration of a proposal to redefine everyday timekeeping by scrapping leap seconds - small changes made to clock time - has been postponed. A working party weighing the proposed change to

Re: RAS hits the news

2005-09-26 Thread Tom Van Baak
A cold GPS receiver takes about 20 minutes to get the almanac data from the GPS constellation. It is intrinsic to GPS that this is the case. You cannot get around this. It's easy to solve that if the application requires it. You could get the almanac from an external source; such as another

Re: RAS hits the news

2005-09-26 Thread Tom Van Baak
Warner, These instances of overflow come from remainders of division operations overflowing. They all can be derived from a single base number (say number of seconds since 1970, MJD, etc). However, when you are deriving that single base number, it can be much harder. Yeah, I also prefer to

Re: RAS hits the news

2005-09-26 Thread Tom Van Baak
So, dropping leap seconds from UTC would cause these receivers to, eventually, go back 19 years on cold start? Hardly a major catastrophe but worth noting. Ed, There are no proposals to drop leap seconds as such. The proposal, as I understand it, is/was to hold leap seconds at their current

Re: RAS hits the news

2005-09-26 Thread Tom Van Baak
But a GPS receiver which uses the current leap second offset (UTC against GPS time) to help guess which 1024 week period it is in will _eventually_ not work quite right. I guess that begs the question - which of the hundred GPS receiver manufacturers actually use the LS field in the UTC

Re: GMT - UTC in Australia

2005-02-24 Thread Tom Van Baak
UTC is a useful approximation to GMT. Rob, this will always be true, won't it? Whether you have 100 ms time step adjustments, or 100 x e-10 rate adjustments, leap seconds, or leap hours it seems to me there has been and will always be an honest attempt to coordinate the two scales. The

Re: ITU Meeting last year

2005-01-20 Thread Tom Van Baak
It's not a linear curve, it's quadratic. I found some slides from the torino meeting where this was laid out very well but I didn't save the URL, sorry. Ah, yes, I forgot the quadratic term. Steve Allen has a nice page at: http://www.ucolick.org/~sla/leapsecs/dutc.html And his table shows

Leap Seconds on Mars?

2004-01-15 Thread Tom Van Baak
There have been a number of timing related articles on Mars recently that got me thinking. Here's my leap second related question: Does anyone know if Mars is a better timekeeper than Earth? Earth's liquid core, polar ice caps, large moon, oceans, and climate all play a part in our irregular

Re: GPS will fail EVEN SOONER, not

2004-01-02 Thread Tom Van Baak
The current GPS data format will fail in approximately 2057, 2079, or 2095 for decelerations of 42, 31, or 25.6 s/cy2, respectively. In terms of deployed systems, that's Real Soon Now. Not to worry. It won't fail. The solution is simply to let delta t sub LS in page 18 subframe 4 roll over

Re: trading time

2004-01-01 Thread Tom Van Baak
Or put it another way, can you think of a single application where GPS cannot already deliver the same TAI as Galileo will someday deliver? Golly, Tom, it's on your own web page http://www.leapsecond.com/pages/saoff/ At the whim of the commander in chief, GPS can turn on Selective

Re: GPS will fail EVEN SOONER

2004-01-01 Thread Tom Van Baak
The W1K rollover for GPS was in 1999, and all that year was spent testing various systems to see how they would fail. It would not be at all surprising if the impending doom of the leap second counter was noticed during a review of other deficiencies in the GPS system. Please see: Some

Discovery Channel article

2004-01-01 Thread Tom Van Baak
Must be a slow news year; here's another one... http://dsc.discovery.com/news/briefs/20031229/atomicclock.html At least this article is a lot more accurate than CNN's and a rare popular article that correctly distinguishes between rate and deceleration (it mentions 1.5 milliseconds per century):

Re: trading time

2003-12-23 Thread Tom Van Baak
And Galileo will operate on TAI. Could someone explain why this factoid has any bearing on the issue? I've seen this presented before and it makes no sense to me. GPS also operates on TAI: as I'm sure you all know, by definition, GPS internal time is TAI - 19 s. Or put it another way, can you

Time Signal Adjustment

2003-10-01 Thread Tom Van Baak
Here's another leap second triva that gives a historical context you might find interesting. http://www.leapsecond.com/history/wwvb1966.htm /tvb

Re: power grids and leap seconds

2003-09-30 Thread Tom Van Baak
Steve, Can you elaborate on some reports. I have been asking this question for years and have not yet found a definitive answer. Recently I talked with the power company speaker at the ION GPS conference about synchronization -- who appeared very familiar with the issues of grid phase angle