On 2017-10-23 09:58 AM, Rob Seaman wrote:
Multiple timescales exist now for multiple purposes. Multiple timescales
will exist under all scenarios. Debasing Universal Time is not a
solution to any "real world" problem. If you want a new timescale,
define a NEW timescale.
Indeed.
To me, the frustrating thing about the discussion at ITU and elsewhere
is the apparent outright refusal to consider a "second timescale". It is
considered and then dismissed out of hand in:
Document 7A/39-E
United States of America
DRAFT NEW REPORT ITU-R TF.[UTC]
The International Time Scale, Coordinated Universal Time (UTC)
https://www.itu.int/md/meetingdoc.asp?lang=en&parent=R15-WP7A-C&source=United%20States%20of%20America
In the last paragraph before the Conclusion they say
"... Another alternative proposed to ensure backward compatibility with
the current UTC time-scale is to use another international coordinated
continuous time-scale on an equal basis. This was suggested as a
suitable method to provide a choice of time scales that could be applied
for a particular system. The implementation of such an option has not
been determined as either possible or practical, and the possibility of
confusing two international standard time scales makes such a solution
unlikely."
The irreconcilable difficulty arises from UTC being a modification of
the Gregorian calendar algorithm. The world (mostly) uses Gregorian, but
then along comes this unpredictable and irregular Leap Second to upset
the apple cart. No clever algorithm can fit that 86401th second label
(23:59:60) back into the Gregorian 86400-second-day. The Leap Second
must go, and so it does, either by ignoring it or smearing it, thereby
creating many incompatible and inaccurate timescales in the real world.
There are two underlying physical phenomenon; time by atomic science,
and time by astronomical observation. The counting mechanisms between
the two are incommensurate because humans (and astronomers) expect the
time-of-day to indicate the position of the Sun in the sky. This is not
just a matter technical considerations but a matter of *principle*.
Earlier in the same document they say:
".. Maintaining a conceptual relation with the Earth’s Rotation Angle
(represented as UT1) does not appear to be a necessity for the sake of
civil time."
Isn't that a *value judgement*? It seems its this sort of value judgment
that upsets many who feel that solar time is important. At the Science
of Time symposium and elsewhere we've heard many impassioned
presentations about how important solar time is to humans; practically,
culturally, and religiously.
Civilians *want* time to reflect astronomical time in a Gregorian YMDhms
form. UTC with Leap Seconds has served that purpose admirably for
decades, tying the worlds timekeeping systems together, albeit
imperfectly. The one second accuracy compromise of UTC has long since
been accepted as a practical matter, and the system has been in effect
since 1972. Proposals to change it meet with impassioned resistance not
so much on technical grounds but on cultural preference. "Civil time" is
*supposed* to be mean solar time, the way its been for centuries, the
way UTC has been since 1972, and the way the Gregorian calendar
prescribes it.
I think atomic time dissemination by UTC with Leap Seconds is unlikely
to change because its so widely deployed, accepted since 1972, works
great for many applications, and efforts to change it have failed since
at least 2000. But still, somehow the Leap Seconds must be eliminated to
reestablish compatibility with the unmodified Gregorian calendar.
I find it a bit incongruous that while the discussion seems to insist
there be only one "international timescale", in fact there are already
two (or three, if you count TAI separate from UTC, but UTC is the
disseminated form of TAI). ITU Rec 460 defines DUT1 (1/10th second
resolution UTC-UT1), the IERS maintains and announces it (Bulletin D),
and the radio signals broadcast it. This could provide the raw material
on which to define a timescale that is more accurate than, and also
traceable to, UTC.
We have the "smeared timescales" (Google, AWS, Bloomberg, etc). Each
generally varies the frequency in the 12 or 24 hours surrounding the
Leap Second to "hide" it from the receiving systems. This eliminates the
Leap Second from view, reestablishing the Gregorian calendar, and
downstream systems and applications behave more reliably. However, these
"smears" do not match each other so tractability amongst them and to UTC
is compromised, and the frequency shifts are more extreme than might be
necessary.
Use of DUT1 could improve this situation. DUT1 values are announced by
IERS, become effective on a specific date, and typically span several
weeks or months periods. If the DUT1 values were used to specify a (very
slight) frequency shift of the dissemination clock during those
intervals the resulting time-points would essentially "slowly smear
away" the Leap Second during the entire period between announced Leap
Seconds.
Current proposals seek to eliminate the Leap Second, decoupling
timekeeping from solar time, or defer the Leap Second, increasing its
inaccuracy. Rather than reducing the accuracies, this DUT1 driven
timescale idea instead *increases* the accuracies by using higher
resolution than one second, essentially "mini-leaps" by frequency shift.
My back-of-the-envelope calculations suggest the precision with respect
to UTC would be in the microseconds, satisfying most definitions of
"legal time" tolerances.
I think the idea that the "possibility of confusing two international
standard time scales" is not so important. As it is there are many
timescales in use and it is likely they are already confused. A new
internationally sanctioned timescale, in addition to the existing UTC
with Leap Seconds, would make the physical realities of atomic time and
astronomical time explicit and standardized. I think having the
selection between two accurate international timescales would be far
better than a single choice that cannot possibly work. I think DUT1
could provide the raw material for such a timescale and the IERS already
has the information and procedures in place to accomplish it.
-Brooks
_______________________________________________
LEAPSECS mailing list
[email protected]
https://pairlist6.pair.net/mailman/listinfo/leapsecs
