[LEAPSECS] D.H. Sadler in 1954

[Brooks Harris]

Hi John,
On 2018-03-19 01:12 AM, John Sauter wrote:
> On Sat, 2018-03-17 at 22:52 +0000, Michael.Deckers via LEAPSECS wrote:
>
>>      So, the likely future is that the limit on |UT1 - UTC| will be
>> dropped,
>>      leap seconds will no longer be applied, and UTC will become a
>> fixed
>>      translate of TAI (so that dissemination of TAI - UTC becomes
>> unnecessary).
> I think you are reading too much into the recommendation.  There is no
> mention made of letting UT1 - UTC become unbounded, but only to
> "consider the present limitation on the maximum magnitude of UT1 - UTC"
> and to "improve further the accuracy of the prediction of UT1 - UTC".
>
> Allowing UT1 - UTC to increase from plus or minus 0.9 seconds to plus
> or minus 1.9 seconds would require changes in the protocols used to
> disseminate UT1 - UTC, but it is definitely possible.  Improving the
> accuracy of the prediction of UT1 - UTC is a good idea but may not be
> possible, since predicting the rotation of the Earth is like predicting
> the weather.
>
> In my opinion, the intended future is that the frequency of Bulletin C
> is decreased from twice a year to once a year, or once every two years.
>      John Sauter (John_Sauter at systemeyescomputerstore.com)
I'm afraid that phrase "consider the present limitation on the maximum 
magnitude of UT1 - UTC" is a subtle rhetorical loophole to accommodate 
those with the idea of eliminating Leap Seconds without actually using 
those words. But I don't believe there is any consensus or plan to 
change UTC, Leap Seconds, or the contents of Bulletins A, B, C, or D, at 
least not yet.

There would be many consequences to allowing UT1-UTC to grow. It clearly 
*breaks* the radio time broadcast protocols because their DUT1 variable 
size won't accommodate larger values and there is little or no room in 
the protocol to extend its range. Radio broadcasts have been operating 
for decades and there is a large number of applications that rely on it, 
including the so-called "atomic clock" industry, and its used as an 
alternate time source as backup for GPS and NTP. It seems unlikely to me 
a disruption of those services would be seen as acceptable.

Another consequence of eliminating Leap Seconds or allowing UT1-UTC to 
grow would be its impact on the procedures used by BIPM and IERS in 
maintaining TAI and UTC. There is a very large and complex 
infrastructure of cooperating organizations, observation technologies, 
and established data processing procedures that leads to UTC and DUT1, 
amongst other things. The procedures for maintaining the reference 
clocks and the EOP are complex and sophisticated, relying on software 
development that stretches back decades, much of it written in Fortran. 
These are vast and complex software packages, with each institution 
running systems appropriate to their technology and responsibility to 
each other and the system. Any substantial change to UTC, DUT1, or 
UT1-UTC could have unknown impact on any part of those systems and their 
interaction with each other.

While there have been many opinions advanced why Leap Seconds should be 
retained I don't believe any formal analysis of the impact of a change 
on the procedures and software used by BIPM, IERS, and contributing 
organizations has been undertaken. Its not clear to me such an analysis 
is even feasible but it seems pretty clear it would be a difficult. How 
would this be funded? If a substantial change were made, how long would 
it take to make appropriate modifications to the software and 
procedures? Any change would not only effect the internal calculations 
but also the database schemas and interchange formats and protocols used 
to exchange information amongst the many contributing systems and 
organizations. How long would it take to make these changes and verify 
the results? Any bug anywhere could upset the whole system. I'm not a 
expert in this but it appears to me the scale and complexity is very 
significant and a major change would be very expansive and take years to 
implement.

While the idea of eliminating Leap Seconds seems simple on the surface 
it has important ramifications that could upset any number of systems 
including the BIPM and IERS themselves. SDO's are inherently 
conservative because reverse compatibility and continuity with existing 
practice must always be honored. It is very difficult to make changes to 
any widely adopted standard, and UTC must be one of the most widely used 
standards of all time, except maybe the Gregorian calendar itself.

My opinion about Leap Seconds has changed from "what a neat solution!" 
to "its probably impossible to change it because its just way too 
expensive and dangerous". I think we're going to have to find a 
different solution to the UTC (with Leap Seconds) v.s. Gregorian 
calendar/compute timekeeping dilemma.

-Brooks