[LEAPSECS] Crunching Bulletin B numbers
igb at batten.eu.org
Sat Feb 19 10:59:03 EST 2011
On 19 Feb 2011, at 15:41, Gerard Ashton wrote:
> On 2/19/2011 10:24 AM, Joe Gwinn wrote, in part:
>> I have not been following the proposal in detail, but a key issue to the POSIX community is that their timescale must be implementable in a totally isolated machine, one having no GPS or internet access.
>> There are other requirements as well. This was discussed at length on the Time Nuts reflector, until Tom kicked the thread over to Leap Secs.
>> Joe Gwinn
>> LEAPSECS mailing list
>> LEAPSECS at leapsecond.com
> I think a description of the totally isolated machine requirement would have to be specified
> to make progress. How long would the machine be expected to keep good time, what is the acceptable
> tolerance for time of day, and what technology would be used to keep time?
With respect to leapseconds, a debate starting from a completely isolated computer is almost meaningless. Leapseconds represent a one part in ~5x10^7 divergence (0.02ppm) from TAI+initial offset. Even if you can find a free-running crystal oscillator of that precision given constant temperature, the environment in a piece of computer hardware varies in terms of temperature sufficient to throw that off by more than 0.02ppm, and (whisper it!) computers have been known to drop clock interrupts at a rate at least comparable to 1 in 5x10^7. Show me a computer that can run its operating system clock, ie the return value from time(), such that it gains or loses significantly less than a second over the course of a year and we can talk about the semantics of Posix time relative to leap seconds in an isolated environment.
Now it's possible that one scenario is a machine which is hooked to a Caesium-Rubidium clock, but has no means of obtaining DUT1 or leap second notifications, but nonetheless needs to track UT1 or UTC to high precision. I think that use-case needs to be articulated before it's used to hold this debate hostage, because I'm struggling to think what it is.
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