Re: [LEAPSECS] Looking-glass, through
On 01/13/2011 22:19, Tom Van Baak wrote: It would appear that making adjustments every 10 days is not often enough, at least in the US, viz: http://www.nist.gov/pml/div688/grp50/NISTUTC.cfm http://www.nist.gov/pml/div688/grp50/nistusno.cfm Even if we abandon the leap second, we have issues at the nanosecond level. This is what happens any time you have more than one clock and if you have bounds on frequency steering. You'll find that all of the UTC(k) clocks disagree at the nanosecond level and that they all wander around the mean paper clock, UTC. This also is normal and expected. For large ensembles of clocks, you are pretty much guaranteed to have this level of fussiness too, since you can never set the clock to the frequency that you want. You can only ask it to set it to the frequency you want. Clocks usually comply, mostly. There's always some tiny error that gets through the process. It doesn't matter if that's a Hydrogen Maser, a Cesium HP5071A or your wrist watch. You don't notice the error either until it has had a chance to accumulate. The errors in frequency are on the order of 1 in 1e14 or so. Even these tiny errors accumulate to nanoseconds over days... Warner ___ LEAPSECS mailing list LEAPSECS@leapsecond.com http://six.pairlist.net/mailman/listinfo/leapsecs
Re: [LEAPSECS] Looking-glass, through
On 01/14/2011 00:22, Sanjeev Gupta wrote: On Fri, Jan 14, 2011 at 13:47, Tom Van Baak t...@leapsecond.com mailto:t...@leapsecond.com wrote: You really didn't expect 250 diffeent atomic clocks around the world to all agree at the ns level at all times did you? tounge-in-cheek Why not? nano is 10E-9, and I see references to people trying for clocks with 10E-12 on this list. And what good is the atom part of an atomic clock, if it can't even handle nano? /foot-in-mouth Still waiting for the flying cars I was promised ... A good Cesium standard is good to better than 1ns/day. This is already 1e-12 or 1e-13 depending on the model. Hydrogen Masers are also available commercially, and they push this down to 1e-15 or 1e-16, which is good to about 1ns/year in frequency error. Experimental clocks can do even better, at least in the short term. The problem is that Cesium standards are between $5k and $25k to buy. Hydrogen Masers are more like $1M. It is a lot easier to have a bunch of Cesium standards than HMs. The BIPM collects time and frequency data for the different clocks, measured against each other. Each clock then has an error in frequency and time computed. These clocks are then weighted based on assigned values (based on the time scientists best guest about how good the clocks are). This value goes in to producing what's called a 'paper clock' which is a historical look at what the best guess at the actual time for each of these measurements. Based on that, you can know how close your clocks are running, and can steer them, if you wish. When you are running a clock, one thing that might not be obvious is that you can't have 'phase jumps' and keep the users of the clock happy. If you have a phase error of .1ns and want to steer it out, you have to adjust your frequency by 1e-10 / steer-time. The steer time is how long you want the steer to take, and is usually dictated by how much change in frequency the steering systems can do and how much the users of the time signals can tolerate. Warner P.S. I'm not sure if I agree that this will one day be common place. Having helped in a small way to run an ensemble of clocks at a former job, I know there's a lot of fussiness that goes into it. You need to calibrate the cable lengths, you need to adjust for temperature, you need to review the data frequently to make sure that everything is operating normally, etc. You also need to calibrate it to NIST from time to time. It can be quite the undertaking. I'm not sure that the ns level of accuracy and precision will ever make it into many devices. On the other hand, there's a lot of activity on the chip-scale atomic clocks pushing the cost way down, so who knows. ___ LEAPSECS mailing list LEAPSECS@leapsecond.com http://six.pairlist.net/mailman/listinfo/leapsecs
Re: [LEAPSECS] Looking-glass, through
On 01/14/2011 03:29, Tony Finch wrote: On Thu, 13 Jan 2011, Steve Allen wrote: Alas, 'tis neither normal nor expected by the APIs and the programmers who are implementing systems that deal with time. One of the core abstractions provided by operating systems is some coherent model of time. And the time labs provide a similar simplified model of time to the general public. Computers are *full* of clocks, including clocks with nanosecond resolution. Unfortunately the nanosecond clocks (the CPU cycle counters) run at different rates according to the CPU's power saving state. So the OS has to provide an abstraction layer on top of them in order to save the sanity of the programmer, and to allow the OS to do things like migrate threads from one CPU to another without affecting their idea of time. Older Intel parts had this problem. Same with some older MIPS designs. Newer designs don't have this issue with the time counters. Of course, there are other reasons for the OS to provide a time abstraction that's apart from this... phk has a good paper on this very topic, since he wrote the basic time counter stuff in FreeBSD :) For more along these lines, see http://www.youtube.com/watch?v=Dj7Y7Rd1Ou0 Tony. ___ LEAPSECS mailing list LEAPSECS@leapsecond.com http://six.pairlist.net/mailman/listinfo/leapsecs
Re: [LEAPSECS] Looking-glass, through
I can't help with the flying cars, but UTC does deliver a frequency that is the most precisely and accurately measured quantity known to humans. Time is the integral of that frequency, and over one leapsecond-less day a frequency error of 1.E-12 corresponds to a time error of 86400*1.E-12 = 86 nanoseconds. The USNO and BIPM web pages give our algorithms, though it takes a bit of clicking. The basic idea is that each clock's systematic errors in time, frequency and/or frequency drift are corrected for and the result goes into a weighted average. -Original Message- From: leapsecs-boun...@leapsecond.com [mailto:leapsecs-boun...@leapsecond.com] On Behalf Of Sanjeev Gupta Sent: Friday, January 14, 2011 2:23 AM To: Leap Second Discussion List Subject: Re: [LEAPSECS] Looking-glass, through On Fri, Jan 14, 2011 at 13:47, Tom Van Baak t...@leapsecond.com wrote: You really didn't expect 250 diffeent atomic clocks around the world to all agree at the ns level at all times did you? tounge-in-cheek Why not? nano is 10E-9, and I see references to people trying for clocks with 10E-12 on this list. And what good is the atom part of an atomic clock, if it can't even handle nano? /foot-in-mouth Still waiting for the flying cars I was promised ... -- Sanjeev Gupta +65 98551208 http://www.linkedin.com/in/ghane ___ LEAPSECS mailing list LEAPSECS@leapsecond.com http://six.pairlist.net/mailman/listinfo/leapsecs
Re: [LEAPSECS] Looking-glass, through
On 2011-01-14 16:26, Warner Losh wrote: The BIPM collects time and frequency data for the different clocks, measured against each other. Each clock then has an error in frequency and time computed. These clocks are then weighted based on assigned values (based on the time scientists best guest about how good the clocks are). This value goes in to producing what's called a 'paper clock' which is a historical look at what the best guess at the actual time for each of these measurements. Based on that, you can know how close your clocks are running, and can steer them, if you wish. The actual process as used by the BIPM (since 1977) is a bit more complex. The weighted mean of atomic clock readings results in an intermediate time scale called EAL (échelle atomique libre); in a second step, TAI is determined as an affine function of EAL so as to approximate the frequencies of the best atomic clocks. See for examle Dennis D McCarthy, P Kenneth Seidelmann: Time -- From Earth Rotation to Atomic Physics. Wiley-VCH. 2009. pages 201..216. The process was even more complex while the rate of TAI was intentionally increased during 1995..1998. Michael Deckers. ___ LEAPSECS mailing list LEAPSECS@leapsecond.com http://six.pairlist.net/mailman/listinfo/leapsecs
Re: [LEAPSECS] Looking-glass, through
Continuously adjusting clocks, even atomic clocks, to keep them within a certain tight tolerance is, in general, not a good pratice. Clocks will keep better time if left running. Rather, the offset of the clock from the standard is measured and used as appropriate. Performance levels of atomic clocks often assume that a linear rate term has been removed. -- Richard Langley On 14-Jan-11, at 12:26 PM, Warner Losh wrote: On 01/14/2011 00:22, Sanjeev Gupta wrote: On Fri, Jan 14, 2011 at 13:47, Tom Van Baak t...@leapsecond.com wrote: You really didn't expect 250 diffeent atomic clocks around the world to all agree at the ns level at all times did you? tounge-in-cheek Why not? nano is 10E-9, and I see references to people trying for clocks with 10E-12 on this list. And what good is the atom part of an atomic clock, if it can't even handle nano? /foot-in-mouth Still waiting for the flying cars I was promised ... A good Cesium standard is good to better than 1ns/day. This is already 1e-12 or 1e-13 depending on the model. Hydrogen Masers are also available commercially, and they push this down to 1e-15 or 1e-16, which is good to about 1ns/year in frequency error. Experimental clocks can do even better, at least in the short term. The problem is that Cesium standards are between $5k and $25k to buy. Hydrogen Masers are more like $1M. It is a lot easier to have a bunch of Cesium standards than HMs. The BIPM collects time and frequency data for the different clocks, measured against each other. Each clock then has an error in frequency and time computed. These clocks are then weighted based on assigned values (based on the time scientists best guest about how good the clocks are). This value goes in to producing what's called a 'paper clock' which is a historical look at what the best guess at the actual time for each of these measurements. Based on that, you can know how close your clocks are running, and can steer them, if you wish. When you are running a clock, one thing that might not be obvious is that you can't have 'phase jumps' and keep the users of the clock happy. If you have a phase error of .1ns and want to steer it out, you have to adjust your frequency by 1e-10 / steer-time. The steer time is how long you want the steer to take, and is usually dictated by how much change in frequency the steering systems can do and how much the users of the time signals can tolerate. Warner P.S. I'm not sure if I agree that this will one day be common place. Having helped in a small way to run an ensemble of clocks at a former job, I know there's a lot of fussiness that goes into it. You need to calibrate the cable lengths, you need to adjust for temperature, you need to review the data frequently to make sure that everything is operating normally, etc. You also need to calibrate it to NIST from time to time. It can be quite the undertaking. I'm not sure that the ns level of accuracy and precision will ever make it into many devices. On the other hand, there's a lot of activity on the chip-scale atomic clocks pushing the cost way down, so who knows. ___ LEAPSECS mailing list LEAPSECS@leapsecond.com http://six.pairlist.net/mailman/listinfo/leapsecs ___ LEAPSECS mailing list LEAPSECS@leapsecond.com http://six.pairlist.net/mailman/listinfo/leapsecs
Re: [LEAPSECS] Looking-glass, through
On 01/14/2011 09:40, Richard Langley wrote: Continuously adjusting clocks, even atomic clocks, to keep them within a certain tight tolerance is, in general, not a good pratice. Clocks will keep better time if left running. Rather, the offset of the clock from the standard is measured and used as appropriate. Performance levels of atomic clocks often assume that a linear rate term has been removed. Yes. That's why most people I've seen that keep their ensemble in sync do it by steering a DDS or similar device to the paper clock that's computed from the inputs of mulitple atomic clocks. Some Warner -- Richard Langley On 14-Jan-11, at 12:26 PM, Warner Losh wrote: On 01/14/2011 00:22, Sanjeev Gupta wrote: On Fri, Jan 14, 2011 at 13:47, Tom Van Baak t...@leapsecond.com wrote: You really didn't expect 250 diffeent atomic clocks around the world to all agree at the ns level at all times did you? tounge-in-cheek Why not? nano is 10E-9, and I see references to people trying for clocks with 10E-12 on this list. And what good is the atom part of an atomic clock, if it can't even handle nano? /foot-in-mouth Still waiting for the flying cars I was promised ... A good Cesium standard is good to better than 1ns/day. This is already 1e-12 or 1e-13 depending on the model. Hydrogen Masers are also available commercially, and they push this down to 1e-15 or 1e-16, which is good to about 1ns/year in frequency error. Experimental clocks can do even better, at least in the short term. The problem is that Cesium standards are between $5k and $25k to buy. Hydrogen Masers are more like $1M. It is a lot easier to have a bunch of Cesium standards than HMs. The BIPM collects time and frequency data for the different clocks, measured against each other. Each clock then has an error in frequency and time computed. These clocks are then weighted based on assigned values (based on the time scientists best guest about how good the clocks are). This value goes in to producing what's called a 'paper clock' which is a historical look at what the best guess at the actual time for each of these measurements. Based on that, you can know how close your clocks are running, and can steer them, if you wish. When you are running a clock, one thing that might not be obvious is that you can't have 'phase jumps' and keep the users of the clock happy. If you have a phase error of .1ns and want to steer it out, you have to adjust your frequency by 1e-10 / steer-time. The steer time is how long you want the steer to take, and is usually dictated by how much change in frequency the steering systems can do and how much the users of the time signals can tolerate. Warner P.S. I'm not sure if I agree that this will one day be common place. Having helped in a small way to run an ensemble of clocks at a former job, I know there's a lot of fussiness that goes into it. You need to calibrate the cable lengths, you need to adjust for temperature, you need to review the data frequently to make sure that everything is operating normally, etc. You also need to calibrate it to NIST from time to time. It can be quite the undertaking. I'm not sure that the ns level of accuracy and precision will ever make it into many devices. On the other hand, there's a lot of activity on the chip-scale atomic clocks pushing the cost way down, so who knows. ___ LEAPSECS mailing list LEAPSECS@leapsecond.com http://six.pairlist.net/mailman/listinfo/leapsecs ___ LEAPSECS mailing list LEAPSECS@leapsecond.com http://six.pairlist.net/mailman/listinfo/leapsecs ___ LEAPSECS mailing list LEAPSECS@leapsecond.com http://six.pairlist.net/mailman/listinfo/leapsecs
Re: [LEAPSECS] Looking-glass, through
It would appear that making adjustments every 10 days is not often enough, at least in the US, viz: http://www.nist.gov/pml/div688/grp50/NISTUTC.cfm http://www.nist.gov/pml/div688/grp50/nistusno.cfm Even if we abandon the leap second, we have issues at the nanosecond level. This is what happens any time you have more than one clock and if you have bounds on frequency steering. You'll find that all of the UTC(k) clocks disagree at the nanosecond level and that they all wander around the mean paper clock, UTC. This also is normal and expected. /tvb ___ LEAPSECS mailing list LEAPSECS@leapsecond.com http://six.pairlist.net/mailman/listinfo/leapsecs
Re: [LEAPSECS] Looking-glass, through
Alas, 'tis neither normal nor expected by the APIs and the programmers who are implementing systems that deal with time. Let me find some good references for you on how the UTC paper clock actually works. Inter-comparing the clocks from each national laboratory is in itself a fascinating subject (or, Demetrios, do you have some canned papers on this?). You really didn't expect 250 diffeent atomic clocks around the world to all agree at the ns level at all times did you? /tvb ___ LEAPSECS mailing list LEAPSECS@leapsecond.com http://six.pairlist.net/mailman/listinfo/leapsecs
Re: [LEAPSECS] Looking-glass, through
On Fri, Jan 14, 2011 at 13:47, Tom Van Baak t...@leapsecond.com wrote: You really didn't expect 250 diffeent atomic clocks around the world to all agree at the ns level at all times did you? tounge-in-cheek Why not? nano is 10E-9, and I see references to people trying for clocks with 10E-12 on this list. And what good is the atom part of an atomic clock, if it can't even handle nano? /foot-in-mouth Still waiting for the flying cars I was promised ... -- Sanjeev Gupta +65 98551208 http://www.linkedin.com/in/ghane ___ LEAPSECS mailing list LEAPSECS@leapsecond.com http://six.pairlist.net/mailman/listinfo/leapsecs
[LEAPSECS] Looking-glass, through
Apologies for a delayed reply, I'm on travel at a conference. On Sat, 8 Jan 2011, I wrote: I do not believe the unstated magic timezone notion (if indeed that is an idea motivating the authors of the draft in front of the ITU) can work (or rather, I do not believe that this notion corresponds to a solution of the problem). ...and I remain unconvinced. Sloshing the timezones around willy-nilly by every regional government on Earth is not a solution to establishing the underlying common timescale. In fact, the only reason that the willy-nilly timezone system can work now is the existence of a mean solar clock underneath beating at the cadence of the synodic day. Which is to say that working has to be defined. The goal isn't just to identify a procedure for setting clocks. There are many (perhaps infinitely many) internally consistent ways to set a clock. The goal is to match the dominant rhythm of life on Earth. That rhythm is the synodic day. The civil timekeeping use cases are diverse, the classes of stakeholders many, the need for clarity and transparency patent. On Jan 9, 2011, at 12:32 PM, Tony Finch wrote: It really depends whether you think easy access to a sub-second accurate realisation of UT1 is part of the problem. No. Words like accuracy and precision have to be defined in context to have any meaning. The focus on technical use cases and technical timescales (especially ones known only after the fact, like UT1) is a distraction from the real discussion, namely, what are the requirements for civil timekeeping? Civil timekeeping is based on the natural (and yes, varying) synodic rotation period of the Earth. This is called a day. It isn't fundamentally a question of the mechanism and schedule for occasionally resetting clocks - the fundamental issue is the clock *rate*. It may be inconvenient that the Earth doesn't rotate at an SI-denumerable rate. It is also the fact. The proposed arrangement of local timezone offsets applied to an atomic timescale only gives you access to local mean solar time accurate to couple of hours or so, which is pretty useless for astronomy or astronavigation but is good enough for civil purposes since that is what the timezone system has given us for the last 50 or 100 years. No. The current timezone system provides access to *Greenwich* mean solar time (under the guise of Universal Time). Civil timekeeping isn't about setting my particular clock to a local standard - it is about marshalling all those local standards into a single coherent global standard. Apparent time is a red herring and local time is a red herring. Also, there is no proposed arrangement of local timezone offsets. What there is, is a draft ITU document that addresses no such issues. Meanwhile, over here in the leap second oubliette there happen to be a few guys promoting a notion that they assert (with no supporting data) fills the gap in the draft document. Alice laughed. 'There's no use trying,' she said: 'one CAN'T believe impossible things.' 'I daresay you haven't had much practice,' said the Queen. 'When I was your age, I always did it for half-an-hour a day. Why, sometimes I've believed as many as six impossible things before breakfast.' A few lines in an email do not correspond to a proposal. If you believe it is trivial for the mechanism of willy-nilly timezone sloshing to resolve all the issues, everywhere, for everybody, then write a proper proposal laying out how this would work. For instance, what authority will historians or lawyers consult to learn the applicable timezone offsets that were in force in some location(s) during some epoch(s) in question? Rob Seaman National Optical Astronomy Observatory ___ LEAPSECS mailing list LEAPSECS@leapsecond.com http://six.pairlist.net/mailman/listinfo/leapsecs
Re: [LEAPSECS] Looking-glass, through
On Wed, 12 Jan 2011, Rob Seaman wrote: Sloshing the timezones around willy-nilly by every regional government on Earth is not a solution to establishing the underlying common timescale. Of course not, that's backwards. The common timescale is the basis of timezones, not the other way round. the fundamental issue is the clock *rate*. Yes, but how accurately do you need clocks to track it? How frequently do you need to make adjustments to correct for the atomic/angular rate error, and what size of adjustment is acceptable? No. The current timezone system provides access to *Greenwich* mean solar time (under the guise of Universal Time). Civil timekeeping isn't about setting my particular clock to a local standard - it is about marshalling all those local standards into a single coherent global standard. That seems completely backwards to me. The common global standard is the basis of the local standards, not the other way round. Civil time (i.e. the legal time in a particular jurisdiction) is established to serve local or regional needs. Jurisdictions base their local time on a round-number offset from the global standard because that is most convenient. What you describe is how the global standard is implemented, not what it is for. For instance, what authority will historians or lawyers consult to learn the applicable timezone offsets that were in force in some location(s) during some epoch(s) in question? That problem exists whether universal time is atomic or angular so it makes no difference to the proposal. Tony. -- f.anthony.n.finch d...@dotat.at http://dotat.at/ HUMBER THAMES DOVER WIGHT PORTLAND: NORTH BACKING WEST OR NORTHWEST, 5 TO 7, DECREASING 4 OR 5, OCCASIONALLY 6 LATER IN HUMBER AND THAMES. MODERATE OR ROUGH. RAIN THEN FAIR. GOOD. ___ LEAPSECS mailing list LEAPSECS@leapsecond.com http://six.pairlist.net/mailman/listinfo/leapsecs
Re: [LEAPSECS] Looking-glass, through
On 01/12/2011 10:30, Steve Allen wrote: On Wed 2011-01-12T16:36:35 +, Tony Finch hath writ: Yes, but how accurately do you need clocks to track it? How frequently do you need to make adjustments to correct for the atomic/angular rate error, and what size of adjustment is acceptable? It would appear that making adjustments every 10 days is not often enough, at least in the US, viz: http://www.nist.gov/pml/div688/grp50/NISTUTC.cfm http://www.nist.gov/pml/div688/grp50/nistusno.cfm Even if we abandon the leap second, we have issues at the nanosecond level. These are the computed errors between the 'paper clock' that is UTC and the various realizations of UTC. This data shows an error of about .2ns/day (give or take) over the periods listed. This is an error of about 2.5e-15. This likely corresponds to the accuracy of the cesium standards used by NIST to realize time, since high precision 5071A's are good to something less than a nanosecond per day. This is several orders of magnitude smaller than the UT deviation from 86400 SI seconds, which is on the order of 1ms per day with variations on the order of 1ms over time (The 10 day estimates are in the 10's of microsecond range as far as accuracy). For instance, what authority will historians or lawyers consult to learn the applicable timezone offsets that were in force in some location(s) during some epoch(s) in question? That problem exists whether universal time is atomic or angular so it makes no difference to the proposal. When the leap second was invented there were countably few systems which could count every second, so a second was not a problem. Now it is. Right now there are countably few systems which can count every nanosecond. Unless there is some sort of conceptual barrier which prevents a need for nanoseconds, when such systems do become common the problem of historic time zone offset reconciliation will be trivial by comparison to the issues of systems which believe that nanosecond (picosecond) synchronization is possible without table lookups and continuous effort to track the table values. Most phones need to be synchronized to the microsecondish level, so the day is coming when that will be more of an issue. But keep in mind that when you get down to the nano-second level, all those crazy things you learned about in physics start to matter. You have relativistic effects between different frames of reference. You have gravitational effects if you change altitude from sea level, you even have Sagnac effects due to the rotation of the earth, etc. All of these effects can cause changes in time elapsing at the nanosecond level. I think puts a practical limit on how closely clocks can and will be synchronized and syntonized. Of course, reading this now, I may be suffering from the 'Nobody will ever need more than 640k of RAM in their personal computer' argument. Warner Abandoning leap seconds simply sweeps the need for good timekeeping practices under a rug rather than giving ongoing incentive to design systems which match the way chronometers actually work. -- Steve Allens...@ucolick.org WGS-84 (GPS) UCO/Lick ObservatoryNatural Sciences II, Room 165Lat +36.99855 University of CaliforniaVoice: +1 831 459 3046 Lng -122.06015 Santa Cruz, CA 95064http://www.ucolick.org/~sla/ Hgt +250 m ___ LEAPSECS mailing list LEAPSECS@leapsecond.com http://six.pairlist.net/mailman/listinfo/leapsecs ___ LEAPSECS mailing list LEAPSECS@leapsecond.com http://six.pairlist.net/mailman/listinfo/leapsecs
Re: [LEAPSECS] Looking-glass, through
Rob Seaman said: For instance, what authority will historians or lawyers consult to learn the applicable timezone offsets that were in force in some location(s) during some epoch(s) in question? FX: falls about laughing Those of us on the timezone list can't even find out this information for this year for many places. It's almost impossible to determine it for (say) 200 years ago for almost anywhere. This is *nothing* to do with what the underlying time scale is. Tony has it right: you have things completely backwards. -- Clive D.W. Feather | If you lie to the compiler, Email: cl...@davros.org | it will get its revenge. Web: http://www.davros.org | - Henry Spencer Mobile: +44 7973 377646 ___ LEAPSECS mailing list LEAPSECS@leapsecond.com http://six.pairlist.net/mailman/listinfo/leapsecs
