[LEAPSECS] leap second roundup 2017

[Brooks Harris]

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