Re: Comments on Civil Time decision tree
Rob Seaman wrote: Perhaps I might expand on some of Bill Thompson's statements in the context of the great convenience factor of using the current UTC standard. ... Each instrument team commands its own instrument directly. Time-tagging is done via UTC. UTC - or any future civil time scale - provides a common clock to tie together both scientific and logistical requirements between a multiplicity of teams and team members. It is often the case that different instruments on a spacecraft are operated by different teams. Those teams have to address scientific concerns - they also have to interoperate with FedEx and with each other. One other thing I should have mentioned is that we also coordinate our spacecraft observations with other missions and ground-based observatories. UTC is a convenient interoperability standard for the scientists, even if it gets translated by the commanding software into TAI or SCLK. ... For both missions, one has to deal with a significant light travel time, much larger than the required time accuracy. Much of the discussion to date has implicitly assumed that civil time here on Earth can simply be transported across the Solar System or the Galaxy as needed - that it is trivial to correct for distant locales. On the scale of tenths of seconds, this is likely true for spacecraft bound to the Sun. One questions if this is achievable - or desirable - at the level of nanoseconds, or perhaps even of microseconds or milliseconds, for very many scientific or utility purposes for Solar System travel. There's definitely a limit to which the accuracy of a spacecraft clock can be known, whether or not that knowledge is folded back to the spacecraft. Currently, I believe that limit is mainly technological, based on how well one can calibrate the innate delays within the electronics. My recollection of seeing presentations of the time drift data for SOHO is that the measurements are good to the millisecond level. The process of taking this into account is essentially the same, no matter where you are in the solar system, and no matter whether you feed the results back to the spacecraft, or simply take it into account on the ground. And more to the point - why should such projects have to be responsive to such alien requirements? Presumably there were any number of factors involved in determining the timekeeping choices of these space projects as a response to various use cases. Is there any particular reason that they should be forced to put their system clock on the ground rather than on the spacecraft simply to meet the one-size-fits-all expectations of others? One thing I don't want to leave is the impression that I was criticizing the earlier post from Randy Kaelber. I merely wanted to share a slightly different real-world use-case. Bill Thompson -- William Thompson NASA Goddard Space Flight Center Code 612.1 Greenbelt, MD 20771 USA 301-286-2040 [EMAIL PROTECTED]
Re: Comments on Civil Time decision tree
Perhaps I might expand on some of Bill Thompson's statements in the context of the great convenience factor of using the current UTC standard.The accuracy requirement for the delivery of UTC to the instruments is +/- 0.410 seconds.High quality, cutting edge science doesn't always require nanosecond precision. In particular, when you are talking about an "experimental apparatus" such as the Solar System, in which light takes 16 minutes to cross the orbit of the Earth, and many hours to reach the outer planets, the few tenths of a second provided by raw UTC may well be exactly what is required. UTC provides simplicity in handling while retaining the option of later converting observed timestamps to TAI and thus to some dynamical timescale.Each instrument team commands its own instrument directly. Time-tagging is done via UTC.UTC - or any future civil time scale - provides a common clock to tie together both scientific and logistical requirements between a multiplicity of teams and team members. It is often the case that different instruments on a spacecraft are operated by different teams. Those teams have to address scientific concerns - they also have to interoperate with FedEx and with each other.I don't have the number in front of me, but I believe that the timing requirements for the SOHO spacecraft is even more stringent, on the order of 0.1 seconds. This is to support the helioseismology instruments.I worked on an asteroseismology experiment several years back in which 40,000 spectra of Procyon had to be acquired on an even barycentric time grid. Cadencing the exposures required converting Earth time (UTC) to the time as the wavefronts passed the Sun and triggering on the predicted clock tick. Sure, I could have referenced the barycentric corrections to something other than UTC, but I challenge anybody to engender confidence in such a conversion from an obscure, moving, Earth clock to a remote location a hundred million miles away. I can handle one or the other, but not both at the same time, with anything like an intuitive feel.There's a one-second period of ambiguity on the STEREO spacecraft whenever a leap second is inserted, and timecritical operations will be avoided during that second.So, presumably this is an example where handling a rare - but necessary - phenomenon "properly" was deemed to be not cost effective. However, they simply redefined the problem such that the "outage" was reduced to its minimum duration of one-second. In six hundred years, one might expect many affected parties to do the same - but of course, the minimum outage then will be 3600 seconds. On the other hand, we've heard apocryphal horror tales of GLONASS falling from the sky and other Ghostbuster scale apocalyptic events resulting from a leap second. Even if such projects chose not to handle leap seconds transparently, why should their system outages persist for more than the second achieved here by another space mission?For both missions, one has to deal with a significant light travel time, much larger than the required time accuracy.Much of the discussion to date has implicitly assumed that civil time here on Earth can simply be transported across the Solar System or the Galaxy as needed - that it is trivial to correct for distant locales. On the scale of tenths of seconds, this is likely true for spacecraft bound to the Sun. One questions if this is achievable - or desirable - at the level of nanoseconds, or perhaps even of microseconds or milliseconds, for very many scientific or utility purposes for Solar System travel.The process of taking this into account is essentially the same, no matter where you are in the solar system, and no matter whether you feed the results back to the spacecraft, or simply take it into account on the ground.And more to the point - why should such projects have to be responsive to such alien requirements? Presumably there were any number of factors involved in determining the timekeeping choices of these space projects as a response to various use cases. Is there any particular reason that they should be forced to put their system clock on the ground rather than on the spacecraft simply to meet the one-size-fits-all expectations of others?We've heard many times in many ways that atomic clocks are orders of magnitude more regular than our wobbling Earth. But this extends to other physical phenomena. We all know that atomic clocks are sufficiently regular time pieces that detection of relativistic effects is trivial. Think about that - what used to require exquisitely sensitive measurements during exceedingly rare opportunities such as a transit of Venus can now be carried out by a high school student (albeit a relatively well-heeled student) on a weekend trip to the mountains. General Relativity for everyman - Albert in a box.How nice! Isn't that support for ditching that dirty old UTC for 21st century technology? Well - no. A single atomic clock is a loose cann
Re: Comments on Civil Time decision tree
Randy Kaelber wrote: On Tue, Sep 27, 2005 at 03:56:07PM -0400, William Thompson wrote: The spacecraft that I've had experience with coordinate the spacecraft clocks with Earth-based time standards. The Solar and Heliospheric Observatory (SOHO) spacecraft (which at a distance of 0.01 A.U. can be considered to be on the edge of interplanetary space) synchronizes its onboard clock to TAI time, expressed as the number of TAI seconds since 1 January 1958. The spacecraft operators keep track of the clock drift, taking into account the approximately 5 second light travel time, and periodically uploads new clock frequency parameters to keep the onboard clock in sync with TAI to within a specified requirement. I suppose I should've prefaced with an "in my experience". :-) The upcoming Solar Terrestrial Relations Observatories (STEREO) go in orbit around the Sun, and are thus definitely interplanetary. They also use the JPL SPICE system, and thus spacecraft SCLK files, like other interplanetary missions. The STEREO clocks will be synchronized to UTC, including an adjustment for leap seconds. Ugh. Is there a compelling science or operations reason to try to synch this clock with terrestrial times on-board that I'm just ignorant about? It sounds like a more work and more things that can break, versus just profiling the on-board clock and making SCLK kernels to map back to terrestrial time frames. Maybe the exact times aren't so important? With Odyssey and THEMIS, if we're off by a second, it's a 30 pixel along-track offset error in our images, so we're pretty obsessed with knowing exactly when we start and stop imaging. On the contrary, exact times are extremely important in the STEREO project. Images from the two spacecraft are supposed to be synchronized to each other to better than a second. (The images from one spacecraft are actually delayed relative to the other to take into account the different solar distances of the two spacecraft--the amount of delay is periodically uploaded from the ground.) The accuracy requirement for the delivery of UTC to the instruments is +/- 0.410 seconds. Each instrument team commands its own instrument directly. Time-tagging is done via UTC. I sincerely doubt that the individual instrument commanding workstations know anything about SCLK files. The orbit and attitude files are provided in SPICE format, which is probably a first for these teams, and I've been leading the effort to learn how to use the SPICE kernels. I don't have the number in front of me, but I believe that the timing requirements for the SOHO spacecraft is even more stringent, on the order of 0.1 seconds. This is to support the helioseismology instruments. SOHO doesn't use SPICE, and thus does not have an SCLK file. The telemetry timestamps from both SOHO and STEREO are in TAI and UTC respectively, referenced in both cases to 1-Jan-1958. For the STEREO spacecraft, which uses UTC, one has to interpret this as the number of non-leap seconds, as is done with Unix/NTP time. There's a one-second period of ambiguity on the STEREO spacecraft whenever a leap second is inserted, and time critical operations will be avoided during that second. SOHO uses TAI time onboard the spacecraft, but all ground operations are done in UTC time. I wrote the software that many of the instrument teams use to convert between UTC and TAI in their data analysis software, as well as some of the commanding software. I can see SOHO using a geocentric time. It's relatively close to Earth and holds a more or less constant distance from it. It seems that STEREO is going to have a more complicated relationship with a geocentric coordinate frame. I'm not sure what the extra complication is. For both missions, one has to deal with a significant light travel time, much larger than the required time accuracy. The process of taking this into account is essentially the same, no matter where you are in the solar system, and no matter whether you feed the results back to the spacecraft, or simply take it into account on the ground. Bill Thompson -- William Thompson NASA Goddard Space Flight Center Code 612.1 Greenbelt, MD 20771 USA 301-286-2040 [EMAIL PROTECTED]
Re: Comments on Civil Time decision tree
On Tue, Sep 27, 2005 at 03:56:07PM -0400, William Thompson wrote: > The spacecraft that I've had experience with coordinate the spacecraft clocks > with Earth-based time standards. > > The Solar and Heliospheric Observatory (SOHO) spacecraft (which at a distance > of > 0.01 A.U. can be considered to be on the edge of interplanetary space) > synchronizes its onboard clock to TAI time, expressed as the number of TAI > seconds since 1 January 1958. The spacecraft operators keep track of the > clock > drift, taking into account the approximately 5 second light travel time, and > periodically uploads new clock frequency parameters to keep the onboard clock > in > sync with TAI to within a specified requirement. I suppose I should've prefaced with an "in my experience". :-) > The upcoming Solar Terrestrial Relations Observatories (STEREO) go in orbit > around the Sun, and are thus definitely interplanetary. They also use the JPL > SPICE system, and thus spacecraft SCLK files, like other interplanetary > missions. The STEREO clocks will be synchronized to UTC, including an > adjustment for leap seconds. Ugh. Is there a compelling science or operations reason to try to synch this clock with terrestrial times on-board that I'm just ignorant about? It sounds like a more work and more things that can break, versus just profiling the on-board clock and making SCLK kernels to map back to terrestrial time frames. Maybe the exact times aren't so important? With Odyssey and THEMIS, if we're off by a second, it's a 30 pixel along-track offset error in our images, so we're pretty obsessed with knowing exactly when we start and stop imaging. I can see SOHO using a geocentric time. It's relatively close to Earth and holds a more or less constant distance from it. It seems that STEREO is going to have a more complicated relationship with a geocentric coordinate frame. -- Randy Kaelber[EMAIL PROTECTED] Scientific Software Engineer Mars Space Flight Facility, Department of Geological Sciences Arizona State University, Tempe, Arizona, USA
Re: Comments on Civil Time decision tree
Randy Kaelber wrote: On Mon, Sep 26, 2005 at 02:33:00PM -0400, Daniel R. Tobias wrote: I would suppose that such a space probe would have little need to be synchronized with earthly solar time, and thus might be best off operating on TAI, with any adjustments to UTC for the sake of humans observing it on Earth being done at the Earthly end of things. That's the way we do it for interplanetary stuff now. Data from spacecraft are typically returned in spacecraft clock time (SCLK, which is pronounced "sclock") and then translated to whatever time base you want it in. Right now, the clock on Mars Odyssey (as I type this) should be reading 2/0812228033. Dealing with things like leap seconds, local time conventions, and other time conversions are all handled here on Earth. The spacecraft that I've had experience with coordinate the spacecraft clocks with Earth-based time standards. The Solar and Heliospheric Observatory (SOHO) spacecraft (which at a distance of 0.01 A.U. can be considered to be on the edge of interplanetary space) synchronizes its onboard clock to TAI time, expressed as the number of TAI seconds since 1 January 1958. The spacecraft operators keep track of the clock drift, taking into account the approximately 5 second light travel time, and periodically uploads new clock frequency parameters to keep the onboard clock in sync with TAI to within a specified requirement. The upcoming Solar Terrestrial Relations Observatories (STEREO) go in orbit around the Sun, and are thus definitely interplanetary. They also use the JPL SPICE system, and thus spacecraft SCLK files, like other interplanetary missions. The STEREO clocks will be synchronized to UTC, including an adjustment for leap seconds. For both of these missions, the Earth-based time values, TAI or UTC, are calculated and maintained onboard the spacecraft. Bill Thompson P.S. Please excuse me if you get this message twice. I was having trouble with my subscription setup, and wasn't sure if it really went out the first time. -- William Thompson NASA Goddard Space Flight Center Code 612.1 Greenbelt, MD 20771 USA 301-286-2040 [EMAIL PROTECTED]
Re: Comments on Civil Time decision tree
Randy Kaelber writes ... > That's the way we do it for interplanetary stuff now. Data from > spacecraft are typically returned in spacecraft clock time (SCLK, which is > pronounced "sclock") and then translated to whatever time base you want it > in. Right now, the clock on Mars Odyssey (as I type this) should be > reading 2/0812228033. Dealing with things like leap seconds, local time > conventions, and other time conversions are all handled here on Earth. The spacecraft that I've had experience with coordinate the spacecraft clocks with Earth-based time standards. The Solar and Heliospheric Observatory (SOHO) spacecraft (which at a distance of 0.01 A.U. can be considered to be on the edge of interplanetary space) synchronizes its onboard clock to TAI time, expressed as the number of TAI seconds since 1 January 1958. The spacecraft operators keep track of the clock drift, taking into account the approximately 5 second light travel time, and periodically uploads new clock frequency parameters to keep the onboard clock in sync with TAI to within a specified requirement. The upcoming Solar Terrestrial Relations Observatories (STEREO) go in orbit around the Sun, and are thus definitely interplanetary. They also use the JPL SPICE system, and thus spacecraft SCLK files, like other interplanetary missions. The STEREO clocks will be synchronized to UTC, including an adjustment for leap seconds. For both of these missions, the Earth-based time values, TAI or UTC, are calculated and maintained onboard the spacecraft. Bill Thompson
Re: Comments on Civil Time decision tree
Poul-Henning Kamp scripsit: > You mean decent restaurants are open at 4am on Mars ? :-) Sure. It's the Planet That Never Sleeps. -- There is / One art John Cowan <[EMAIL PROTECTED]> No more / No less http://www.reutershealth.com To do / All things http://www.ccil.org/~cowan With art- / Lessness -- Piet Hein
Re: Comments on Civil Time decision tree
On Tue, Sep 27, 2005 at 01:13:18AM +0200, Poul-Henning Kamp wrote: > In message <[EMAIL PROTECTED]>, Randy Kaelber writes: > > >As an aside, most of the people who were/are on Mars Rover teams that I > >talked to really liked the extra 30+ minutes a day. The only thing they > >didn't like was when mundane earthbound things conflicted (It's 4 am, but > >I don't want to eat at Denny's and I really need to get to the bank.) with > >their Martian schedule. Astronauts on Mars would not have that problem. > >:-) > > You mean decent restaurants are open at 4am on Mars ? :-) Yes. Great food, but very little atmosphere (*rimshot*). -- Randy Kaelber[EMAIL PROTECTED] Scientific Software Engineer Mars Space Flight Facility, Department of Geological Sciences Arizona State University, Tempe, Arizona, USA
Re: Comments on Civil Time decision tree
In message <[EMAIL PROTECTED]>, Randy Kaelber writes: >As an aside, most of the people who were/are on Mars Rover teams that I >talked to really liked the extra 30+ minutes a day. The only thing they >didn't like was when mundane earthbound things conflicted (It's 4 am, but >I don't want to eat at Denny's and I really need to get to the bank.) with >their Martian schedule. Astronauts on Mars would not have that problem. >:-) You mean decent restaurants are open at 4am on Mars ? :-) -- 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: Comments on Civil Time decision tree
On Mon, Sep 26, 2005 at 10:35:15PM +0200, Poul-Henning Kamp wrote: > In message <[EMAIL PROTECTED]>, I wrote: > >That's the way we do it for interplanetary stuff now. Data from > >spacecraft are typically returned in spacecraft clock time (SCLK, which is > >pronounced "sclock") and then translated to whatever time base you want it > >in. Right now, the clock on Mars Odyssey (as I type this) should be > >reading 2/0812228033. Dealing with things like leap seconds, local time > >conventions, and other time conversions are all handled here on Earth. > > But that strategy breaks down for human space flight ? I don't really see why it would. Humans on Mars would probably hold to a Martian solar day to do work much in the same way the mission ops did with the rovers. The science data would likely all still be mapped to a clock associated with the spacecraft. Most manned Mars mission plans I've seen involve multiple spacecraft (landers, orbiters, rovers) and I imagine each would have its own onboard clock. Some of those would involve coordination, I'm sure, but we do coordinated observations between different missions all the time anyway, and I believe that experience directly translates. When all is said and done, resolving small time differences ranks relatively low on the list of challenges of interplanetary manned space flight. As an aside, most of the people who were/are on Mars Rover teams that I talked to really liked the extra 30+ minutes a day. The only thing they didn't like was when mundane earthbound things conflicted (It's 4 am, but I don't want to eat at Denny's and I really need to get to the bank.) with their Martian schedule. Astronauts on Mars would not have that problem. :-) -- Randy Kaelber[EMAIL PROTECTED] Scientific Software Engineer Mars Space Flight Facility, Department of Geological Sciences Arizona State University, Tempe, Arizona, USA
Re: Comments on Civil Time decision tree
In message <[EMAIL PROTECTED]>, Randy Kaelber writes: >On Mon, Sep 26, 2005 at 02:33:00PM -0400, Daniel R. Tobias wrote: >> >> I would suppose that such a space probe would have little need to be >> synchronized with earthly solar time, and thus might be best off >> operating on TAI, with any adjustments to UTC for the sake of humans >> observing it on Earth being done at the Earthly end of things. > >That's the way we do it for interplanetary stuff now. Data from >spacecraft are typically returned in spacecraft clock time (SCLK, which is >pronounced "sclock") and then translated to whatever time base you want it >in. Right now, the clock on Mars Odyssey (as I type this) should be >reading 2/0812228033. Dealing with things like leap seconds, local time >conventions, and other time conversions are all handled here on Earth. But that strategy breaks down for human space flight ? -- 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: Comments on Civil Time decision tree
On Sep 26, 2005, at 11:56 AM, Poul-Henning Kamp wrote:Again: merely trying to point out that the "only one timescale" argument Rob pushes doesn't work.This misrepresents my position. There are clearly many time scales for many purposes. One of those purposes is something that might be referred to as "International Civil Time". It is this civil time scale that is the key issue for any proposed change to the current UTC standard. Personally, I am happy to acknowledge that no such international standard currently exists. Shoehorning UTC into that role is not a very good fit, at least not if it is asserted that we must destroy UTC in order to save it.It is rather clever how UTC manages, through the mechanism of leap seconds, to transport both Universal Time (Mean Solar Time) and Atomic Time in one convenient package. The convenience of this mechanism is being criticized. Either those criticisms are invalid, in which case the ITU proposal should be rejected - or the criticisms are valid, in which case it may well make sense to explicitly separate Atomic Time from Solar Time. Perhaps that is what you mean by your statement above.But, in a world with separate time scales for Atomic and Solar Time, it seems far more likely that any representation of International Civil Time should be based solely on Solar Time, not on Atomic Time. For the vast majority of cases, Civil Time clearly "mimics" - and must continue to mimic - Solar Time. Doesn't it make more sense to simply reconfirm the wisdom of the ages that Civil Time IS Solar Time?E pur si muove!Rob SeamanNational Optical Astronomy Observatory
Re: Comments on Civil Time decision tree
In message <[EMAIL PROTECTED]>, "Daniel R. Tobias" writes : >On 26 Sep 2005 at 16:09, Poul-Henning Kamp wrote: > >> Other more laid back parliaments like the Danish have not been able >> to find time to revisit the issue since 18xx and still use solar >> time at some more or less random coordinate. > >You mean like the U.S. Congress? >http://tycho.usno.navy.mil/260.html > >"...the standard time of the first zone shall be based on the mean >solar time of the sixtieth degree of longitude west from >Greenwich..." (and so on for all the other zones) Well, at least they had the sense to use a longitude divisible by 15. Not so lucky in Denmark: 50°19' >> Imagine for instance that we send a probe out of the solar system >> at seriously high speeds and it manages to get as far as 6 light >> months away: Under the current UTC rules we would be unable to >> upload a leap-second warning and get it there before it is too late. > >I would suppose that such a space probe would have little need to be >synchronized with earthly solar time, and thus might be best off >operating on TAI, with any adjustments to UTC for the sake of humans >observing it on Earth being done at the Earthly end of things. Again: merely trying to point out that the "only one timescale" argument Rob pushes doesn't work. -- 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: Comments on Civil Time decision tree
On Mon, Sep 26, 2005 at 02:33:00PM -0400, Daniel R. Tobias wrote: > > I would suppose that such a space probe would have little need to be > synchronized with earthly solar time, and thus might be best off > operating on TAI, with any adjustments to UTC for the sake of humans > observing it on Earth being done at the Earthly end of things. That's the way we do it for interplanetary stuff now. Data from spacecraft are typically returned in spacecraft clock time (SCLK, which is pronounced "sclock") and then translated to whatever time base you want it in. Right now, the clock on Mars Odyssey (as I type this) should be reading 2/0812228033. Dealing with things like leap seconds, local time conventions, and other time conversions are all handled here on Earth. -- Randy Kaelber[EMAIL PROTECTED] Scientific Software Engineer Mars Space Flight Facility, Department of Geological Sciences Arizona State University, Tempe, Arizona, USA
Re: Comments on Civil Time decision tree
On 26 Sep 2005 at 16:09, Poul-Henning Kamp wrote: > Other more laid back parliaments like the Danish have not been able > to find time to revisit the issue since 18xx and still use solar > time at some more or less random coordinate. You mean like the U.S. Congress? http://tycho.usno.navy.mil/260.html "...the standard time of the first zone shall be based on the mean solar time of the sixtieth degree of longitude west from Greenwich..." (and so on for all the other zones) > Imagine for instance that we send a probe out of the solar system > at seriously high speeds and it manages to get as far as 6 light > months away: Under the current UTC rules we would be unable to > upload a leap-second warning and get it there before it is too late. I would suppose that such a space probe would have little need to be synchronized with earthly solar time, and thus might be best off operating on TAI, with any adjustments to UTC for the sake of humans observing it on Earth being done at the Earthly end of things. -- Dan Dan's Web Tips: http://webtips.dan.info/ Dan's Domain Site: http://domains.dan.info/
Re: Comments on Civil Time decision tree
On Sep 26, 2005, at 7:09 AM, Poul-Henning Kamp wrote: Now, what we you mean by "civil time standard" ? Most countries reserve the definition of "civil time" for their national parliaments (or other some other tacitly assumed legitimate political power). They generally take UTC, apply a timezone and very often a DST ruleset. Point taken that the language still needs to be defined. Of course, some might find that additional support for the position that we are in no way ready to decide on changes to civil time standards if we can't even agree what civil time is. It would probably be wiser to recast this question in terms that do not even hint at usurping sovereignty. Funny - my wife always says the same thing... Whether it is called UTC, GMT or solar time, there is some coherent international time scale concept underlying the individual choices made by individual countries. That is what we are trying to define (some of us, anyway). Isn't a suggestion that we ignore "natural time" (based on whatever natural clock) in favor of "technology time" (based on whatever ensemble of physical clocks we have built or might build in the future) more likely to be viewed as an attack on national sovereignty? After all, the current pastiche of legal time systems successfully interoperate precisely because they are all based on the concept of mean solar time no matter how obscure the intervening standards process. It is the proposal to abandon leap seconds from only SOME of the sovereign national clocks that threatens interoperability. I suspect that interoperability would be one issue we can all agree on. So as long as you include "scientific use" with "civil use", then the answer to this question is "many" no matter which way you go. We already have many examples of the distinction between scientific and civil uses of time. This discussion has never really been about the former. If it was, then the folks pushing the ITU proposal would have simply renamed their new timescale something other than UTC - say, TI, as was decided in Torino. That would allow the astronomers to continue to maintain UTC internal to their community which undoubtedly would be a much less expensive proposition. It is the folks who want to abandon leap seconds who are making an unwarranted and unwise connection between civil time and science time. And that was exactly my point: "civil" and "scientific" timekeeping was two different issues and they have different semantics and needs. Well, other than the fact that you are committing the same error in assuming that there is one single scientific time scale, I agree with you. Most of this argument is still centered around the unarticulated question: "who owns UTC". Wouldn't it be fair if the non-scientific (ie: civil) world told the astronomers (and any other scientists) to bugger off and not impose scientific requirements on civil time ? The flaw here is that you are excluding the scientific users - precisely the folks who know how to address the issues - from helping craft a solution. Scientists live in the civil world, too. Under what circumstances are a few dozen committee members a world unto themselves? It seems rather naive (the word "daft" also comes to mind) to suggest that common sense "scientific" issues such as that civil time obviously mimics solar time - to some level of precision we certainly could discuss - have no place in making decisions about civil time. Is the fact that the Earth rotates purely a scientific question? Or is this something a typical Earthling might be expected to know? After all, scientists have several timescales of their own already, and plenty of means to implement them, whereas UTC is the only agreed upon and widely available civil timescale. I agree - although there certainly is nothing to stop us from laying international civil time upon some other underlying timescale like "TI". The fact, that you appear to now be agreeing with, is that there is one civil time standard. That being the case, we really ought to labor to get it right, not to cut some inane deal with naive corporate entities. I suspect plenty of the more cogent corporate entities would reject the current proposal if anybody had thought to ask them about it. Rob Seaman National Optical Astronomy Observatory
Re: Comments on Civil Time decision tree
In message <[EMAIL PROTECTED]>, Peter Bunclark writes: >On Mon, 26 Sep 2005, Poul-Henning Kamp wrote: >> >> On the other hand, even if we agree on one standard, or even just >> leave UTC as it is, are the astronomers and geophysiscists going >> to abandon UT1 ? >> >> If so, then this is the first I've heard about it. > >Of course not. And that was exactly my point: "civil" and "scientific" timekeeping was two different issues and they have different semantics and needs. Most of this argument is still centered around the unarticulated question: "who owns UTC". Wouldn't it be fair if the non-scientific (ie: civil) world told the astronomers (and any other scientists) to bugger off and not impose scientific requirements on civil time ? After all, scientists have several timescales of their own already, and plenty of means to implement them, whereas UTC is the only agreed upon and widely available civil timescale. -- 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: Comments on Civil Time decision tree
On Mon, 26 Sep 2005, Poul-Henning Kamp wrote: > > On the other hand, even if we agree on one standard, or even just > leave UTC as it is, are the astronomers and geophysiscists going > to abandon UT1 ? > > If so, then this is the first I've heard about it. Of course not. > > Imagine for instance that we send a probe out of the solar system > at seriously high speeds and it manages to get as far as 6 light > months away: Under the current UTC rules we would be unable to > upload a leap-second warning and get it there before it is too late. Any definition of "civil time" is unlikely to be of use to an interstellar space vehicle. Designers of such a project might use an onboard atomic clock synchronised to TAI before lift off, and make all knowable adjustments to the timestamps in the returned telemetry. Unless the thing loops round Alpha Centauri and comes back, in which case we'd be able to calibrate the clock drift as well. Peter. > > Poul-Henning
Re: Comments on Civil Time decision tree
In message <[EMAIL PROTECTED]>, Rob Seaman writes: >I've appended what I call the Civil Time Decision Tree v0.5. >I) Existence [Adopt an international (or intergalactic) civil time >standard?] Now, what we you mean by "civil time standard" ? Most countries reserve the definition of "civil time" for their national parliaments (or other some other tacitly assumed legitimate political power). They generally take UTC, apply a timezone and very often a DST ruleset. Other more laid back parliaments like the Danish have not been able to find time to revisit the issue since 18xx and still use solar time at some more or less random coordinate. It would probably be wiser to recast this question in terms that do not even hint at usurping sovereignty. >II) Multiplicity [How many standards?] > A) one > B) many Before we can answer this, don't we need to know the parameters of the proposed standard ? Ie: If UTC gets a one hour tolerance to UT1 then the answer will obviously be "many" because the astronomers and geophysiscists will need their own timescale (likely UT1). On the other hand, even if we agree on one standard, or even just leave UTC as it is, are the astronomers and geophysiscists going to abandon UT1 ? If so, then this is the first I've heard about it. So as long as you include "scientific use" with "civil use", then the answer to this question is "many" no matter which way you go. Only if we agree that scientific use is specifically not included, something which is strongly hinted in the use of the word "civil" in the first place, does this question have any meaning. >For each civil time standard: > >III) Locale > A) restricted to Earth [projects or users, not necessarily >hardware] > B) other than Earth [e.g., Martian rovers] > B) Solar system scope > C) truly Universal This question is also in trouble. The crucial question here is not so much where you use it, that is mostly just relativistic corrections to your clock model. The relevant question is if you need to be able to hear from the high priests of timekeeping in Paris on a regular basis or not. Imagine for instance that we send a probe out of the solar system at seriously high speeds and it manages to get as far as 6 light months away: Under the current UTC rules we would be unable to upload a leap-second warning and get it there before it is too late. Poul-Henning -- 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.
