On Fri 2007-01-12T18:35:55 +0000, 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
Received on Mon Jan 15 2007 - 07:53:44 PST