Re: The Martian Chronicles
In message <[EMAIL PROTECTED]>, Steve Allen writes: >On Mon 2007-01-15T08:53:19 -0700, Rob Seaman hath writ: >> Any comments on the practicality of space-rating such timepieces? > >GPS uses rubidium cells, and Galileo will. Galileo will use Rb and Hydrogen, provided the Hydrogen survices the testphase on Giove B. -- 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: The Martian Chronicles
In message <[EMAIL PROTECTED]>, Rob Seaman writes: >Before somebody else calls me on it, I should point out that NTP >actually uses both: > >"The clock discipline algorithm functions as a nonlinear, hybrid >phase/frequency-lock (NHPFL) feedback loop." > >(see http://www.cis.udel.edu/~mills/database/brief/clock) Let me just note that not all of us in the NTP environment belive that algorithm is optimal or even well understood. -- 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: The Martian Chronicles
On Mon 2007-01-15T08:53:19 -0700, Rob Seaman hath writ: > Any comments on the practicality of space-rating such timepieces? GPS uses rubidium cells, and Galileo will. I've seen ruminations about flying a cesium resonator and an ion trap on ISS with a goal of redefining the SI second by allowing a long term calibration of the "continually probe-able on earth" ion against the "it always falls down on earth" cesium. > Also - what are the actual use cases requiring a common time scale, > rather than establishing a separate Martian civil cesium standard and > simply tracking the deltas? Robert A. Nelson. Look up his numerous presentations on a Mars version of GPS in CGSIC, PTTI, etc., and more recently IAU in Prague http://tycho.usno.navy.mil/IAU31/nelson.ppt (Which, by the way, includes the calculation that deviation between Mars coordinate time and Earth coordinate time is about 25 ms peak to peak.) -- 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: The Martian Chronicles
On Mon, 15 Jan 2007, Rob Seaman wrote: > > I'm wondering whether the clock discipline might better emphasize > only frequency in such circumstances, as opposed to merely tempering > the PLL. I think that if you only correct the frequency of a clock then each time the frequency makes a deviation (e.g. because of temperature changes) you will accumulate a phase error, and if there's a bias in frequency deviations then the phase error will increase over time and become a long-term frequency error. Tony. -- f.a.n.finch <[EMAIL PROTECTED]> http://dotat.at/ ROCKALL MALIN HEBRIDES: WEST OR SOUTHWEST 7 TO SEVERE GALE 9, OCCASIONALLY STORM 10, VEERING NORTHWEST 5 OR 6 LATER. HIGH, OCCASIONALLY VERY HIGH. RAIN THEN SHOWERS. MODERATE OR GOOD, OCCASIONALLY POOR.
Re: The Martian Chronicles
Rob Seaman wrote: > I suspect we're all bemused to >contemplate issue terrestrial leap seconds on Martian bases. It seems about as silly as Antarctic bases observing daylight saving time. Which several do. The Amundsen-Scott base at the south pole maintains New Zealand civil time, including DST. So its clocks are an hour further ahead in the southern summer, when the sun is continuously above the horizon, than they are in the southern winter, when the sun is continuously below the horizon. In both cases the motivation is to stay synchronised with the other end of supply/command chains. A native Antarctic culture probably wouldn't come up with (or copy) the concept of DST, and a native Martian culture certainly wouldn't track the LOD variations of the neighbouring planet. I think these situations are both silly, and that the predominant local timescale should be responsive to local needs. -zefram
Re: The Martian Chronicles
I said: I also wonder whether it might be productive to consider closing the NTP "servo loop" in velocity (frequency) in this case, rather than position (phase). Before somebody else calls me on it, I should point out that NTP actually uses both: "The clock discipline algorithm functions as a nonlinear, hybrid phase/frequency-lock (NHPFL) feedback loop." (see http://www.cis.udel.edu/~mills/database/brief/clock) I'm wondering whether the clock discipline might better emphasize only frequency in such circumstances, as opposed to merely tempering the PLL. Rob
The Martian Chronicles
On Fri 2007-01-12T18:35:55 +, Tony Finch hath writ: According to the slides linked from Dave Mills's "Timekeeping in the Interplanetary Internet" page, they are planning to sync Mars time to UTC. http://www.eecis.udel.edu/~mills/ipin.html There seems to be an unwarranted assumption that a spacecraft always knows its own position. I also wonder whether it might be productive to consider closing the NTP "servo loop" in velocity (frequency) in this case, rather than position (phase). Regarding the choice of UTC - it sounds like this project is ongoing. Dave Mills is nothing if not responsive to NTP issues. We might consider raising the issue with him. On Jan 12, 2007, at 12:26 PM, Steve Allen wrote: From an engineering standpoint a variation of 2 ms in a year on Mars is certainly better than any time scale that could be established there in lieu of landing a cesium chronometer. Any comments on the practicality of space-rating such timepieces? Power source, radiation hardening, hand-off maintenance procedures, etc? Also - what are the actual use cases requiring a common time scale, rather than establishing a separate Martian civil cesium standard and simply tracking the deltas? I suspect we're all bemused to contemplate issue terrestrial leap seconds on Martian bases. How does the LOD vary on Mars? No significant moon - but then, leap seconds are needed on Earth even if the tidal slowing is detrended. Can something as naive as POSIX time_t really serve all such applications, even the ones on earth, for the next 600 years? One would argue that deprecating leap seconds will make the naive POSIX model even less acceptable. Some applications (and perhaps we'll be surprised to learn how many) most certainly do require an approximation to actual UT, not just to "I'm going to lie and call it UTC". With leap seconds, this approximation is maintained within the civil time standard and POSIX doesn't need to worry about it. Without leap seconds, this approximation is maintained through an explicit application of a DUT1 correction - a correction that would start to grow without meaningful bound. POSIX exists to support applications of interest to users. The existence of significant numbers and classes of users requiring knowledge of actual UT (mean solar time) would suggest that POSIX would, in turn, need to start accommodating DUT1. Rather than defining a perceived problem out of existence, one could actually find that a real problem has been defined into existence. Or perhaps I'm wrong. Demonstrate why. Rob Seaman NOAO
NTP on Mars
On Mon, 15 Jan 2007, Tony Finch wrote: > On Mon, 15 Jan 2007, Peter Bunclark wrote: > > > > > http://www.eecis.udel.edu/~mills/ipin.html > > > > That page does not seem to mention UTC... > > Look at the slides. Whoops. In my defense, there has been traffic elsewhere pointing out that authoring in powerpoint is a good way to hide information... And having said that, how utterly disappointing that this project is squandering the opportunity to come up with a time-distribution protocol that is not based on UTC. Once this new regime is "bolted in" to the space program, I guess we're stuck with it until the end of civilisation. Peter.
Re: Introduction of long term scheduling
On Mon, 15 Jan 2007, Peter Bunclark wrote: > > > http://www.eecis.udel.edu/~mills/ipin.html > > That page does not seem to mention UTC... Look at the slides. Tony. -- f.a.n.finch <[EMAIL PROTECTED]> http://dotat.at/ BISCAY FITZROY: VARIABLE 4, BECOMING SOUTHWESTERLY 5 TO 7 IN NORTHWEST FITZROY. MODERATE OR ROUGH, OCCASIONALLY VERY ROUGH. SHOWERS. GOOD.
Re: Introduction of long term scheduling
On Fri, 12 Jan 2007, Tony Finch wrote: > According to the slides linked from Dave Mills's "Timekeeping in the > Interplanetary Internet" page, they are planning to sync Mars time to UTC. > http://www.eecis.udel.edu/~mills/ipin.html > That page does not seem to mention UTC... it does mention running at a constant rate relative to TAI. It's not explicit, but one hopes they are considering not running UTC clocks (and converting to UTC when necessary in userland...). Peter.
