Special Rapporteur Group (readable ascii version)

From: Markus Kuhn <Markus.Kuhn_at_cl.cam.ac.uk>
Date: Thu, 24 May 2001 16:40:50 +0100

And here comes a readable ASCII version of the Word Document that
Demetrios Matsakis <matsakis.demetrios_at_USNO.NAVY.MIL> circulated
yesterday, with reasonable line breaking (thanks to exmh). By the way,
has my UTS proposal been seriously discussed anywhere already?



Markus G. Kuhn, Computer Laboratory, University of Cambridge, UK
Email: mkuhn at acm.org,  WWW: <http://www.cl.cam.ac.uk/~mgk25/>
|[pic]        |INTERNATIONAL TELECOMMUNICATION UNION  |                        |
|             |RADIOCOMMUNICATION                     |Revision 2 to           |
|             |STUDY GROUPS                           |Document 7A/TEMP/14-E   |
|             |                                       |9 May 2001              |
|             |                                       |English only            |
Chairman, Special Rapporteur Group 7A
UTC Time Scale
3-4 May 2001 Meeting Report
This is the activities report of the Special Rapporteur Group 7A (SRG 7A)
on the future of the UTC Time Scale, through May 2001.  This meeting
concluded the preliminary plan of action on this question developed and
agreed to by ITU-R Working Party 7A (WP 7A) at their meeting in October
1 Background
A new question was generated by WP 7A concerning the future definition and
use of Coordinated Universal Time (UTC) in the ITU-R Recommendations as a
result of issues raised by sector members and a letter from the Director of
the Bureau International des Poids et Mesures (BIPM) to the Secretary
General of the ITU.  The Question is ITU-R 236/7 (2000), "The Future of the
UTC Time Scale".  Determination and maintenance of the UTC time scale is
conducted by the BIPM in conjunction with the International Earth Rotation
Service (IERS).  But the question was raised to the ITU-R due to its
responsibility in the definition and use of time scales for radio and
telecommunications purposes.
The implication of changes to the recommended UTC time scale in the current
recommendations could have significant impact on scientific, governmental,
commercial and regulatory interests.  Accordingly, WP 7A on Standard
Frequency and Time Signal Services established this Special Rapporteur
Group (SRG) to address this significant question on the use of the UTC.
SRG Members
Mr. Ronald Beard (USA), Chairman, SRG
      Tel: +1 202-404-7054
      Fax: +1 202-767-2845
      Email: beard_at_juno.nrl.navy.mil
Mr. William Klepczynski (USA - IAU)
            Secretary, SRG
      Tel: +1 703-8412669
      Fax: +1 703-465-9158
      Email:      bill.ctr.klepczynski_at_faa.gov
Mr. Jacques Azoubib (BIPM)
      Tel: +33 145 077 062
      Fax: +33 145 077 059
      Email: jazoubib_at_bipm.org
Mr. Thomas Bartholomew (USA)
      Tel: +1 301 483 6000 ext.2019
      Fax: +1 301 604 0500
      Email: bartholo_at_boulder.nist.gov
Mrs. Francoise Baumont (France)
      Tel: +33  4 93405338
      Fax: +33 4 93405333
      Email: baumont_at_obs-azur.fr
Mr. Michel Brunet (France)
      Tel: +33 561 273 345
      Fax: +33 561 282 613
      Email: michel.brunet_at_cnes.fr
Mr. Yury Domnin (Russia)
      Tel:  +7 095 5359151
      Fax: +7 095 5340609
      Email: ydomnin_at_imvp.aspnet.ru
Mr. Donald Hanson (USA)
      Tel: +1 303 497 5233
      Fax: +1 303 497 3228
      Email: hanson_at_boulder.nist.gov
Activities and Results (October 2000 - May 2001)
The objectives and activities of the SRG were originally determined at the
formation of the group.  The plan and results shown in italics are
discussed below.
Identify Participating Organizations Points of Contact for coordination of
            The draft letter of the Director, ITU-R, sent to announce the
       formation of the SRG on the Future of UTC included a number of
       suggested participating organizations.  The draft letter and
       clarifying information was submitted through SG7 to the secretariat
       for approval and release.
Generate clarification or additional material on Question ITU-R 236/7 for
distribution and comment.
         Additional information in the form of presentation materials was
         prepared for use at two conferences where these issues would be
         discussed.  Some additional clarification was necessary.  In
         general, the presentations were focused on issues and
         participation desired from other scientific and operational
Circulate a letter from Director, ITU-R, to the interested organizations by
1 November 2000.
          The draft letter initiated by WP 7A was finalized and released
          from the ITU-R on 8 Jan 2001.  The letter was distributed to the
          WMO.  As a result of the ITU-R letter three additional
          participants joined the SRG.  These participants are Mr. Sigfriedo
          Leschuttia (CCTF), Mr. Dennis McCarthy (IAU), and Mr. Daniel
          Gambis (IUGG).
Introduce Question ITU-R 236/7 (2000) and ITU-R intentions to the timing
community at the Annual Precise Time and Time Interval (PTTI) Meeting,
28 November 2000.
         A special panel discussion session was held at the PTTI Dec 2000
         meeting concerning the Leap Second.  Short presentations were made
         by Mr. Dennis McCarthy (USNO), Mr. Steve Malys (NIMA), and Mr.
         Ronald Beard (SRG).  The presentations and subsequent discussion
         will be published in the PTTI Proceedings.  During the discussion
         period, Mr. Demetrios Matsakis (USNO) mentioned an URSI Study
         group that he chaired that was formed to also address the Leap
         Second issue.  He provided the results of a survey they conducted.
          The survey report is attached.
Collect preliminary statements/comments from participating organizations as
basis for the initial meeting to coordinate issues and ascertain the extent
of required actions that may be necessary.
         No preliminary statements or comments were received.
Conduct coordination meeting in conjunction with European Frequency and
Time Forum, March 2001, Neuchatel, Switzerland.
         Another special meeting was arranged in conjunction with the EFTF
         in lieu of a coordination meeting.  In light of little response
         from organizations other than the IAU, a summary of the IAU
         considerations was presented and Mrs. Felicitas Arias, Head of the
         BIPM Time Section, was invited to present information on this
         issue.  She gave a brief history of the formation of UT1 and UTC
         and the current procedures used for the formation of UTC and the
         Leap Second.  She mentioned that systems, such as GLONASS and
         those using Network Time Protocol (NTP) have had difficulties with
         Leap Seconds.  She also pointed out that other international
         organizations are starting to discuss the issue. She concluded
         that it might take some time for their deliberations to come to a
         definitive conclusion.
Based on the actions defined in steps 1) - 6), determine if additional
efforts are necessary to evaluate potential changes to ITU-R
Recommendations. If determined necessary, formulate final plan of action
and time required.
         Based on the results of the activities discussed above and the
         insufficient information available, it was not yetpossible to come
         to any conclusions or formulate any recommendations.
Meeting of the Special Rapporteur Group 3-4 May 2001.
         The results of this SRG meeting are reported below.  An additional
         phase to the effort was determined to be necessary.  The course of
         action developed is described below and is believed to be
         sufficient to study the question and provide definite suggestions
         on revision of the recommendations.
Results of the Meeting of Special Rapporteur Group, 3-4 May 2001
The agenda of the meeting that was held at the ITU-R in Geneva follows:
           I.   Introduction and Purpose
           II.  Recognition of Participants and Representatives
                 Reports from Representatives
           III. Preliminary Review of uses of UTC
                 National and International
           IV.  Time Scale Considerations
                 Proposed changes to UTC
                 TAI and Alternatives
                 Comments from SRG Members
           V.  Discussion of Views by the Group
           VI. Study Plan Discussion
           VII. Summary & Action Items
      Mr. Ronald Beard (USA), Chairman
      Mr. William Klepczynski (USA - IAU), Secretary
      Mr. Jacques Azoubib (BIPM)
      Mrs. Francoise Baumont (France)
      Mr. Michel Brunet (France)
      Mr. Daniel Gambis (IUGG)
      Mr. Donald Hanson (USA)
      Mr. Sigfriedo Leschiutta (CCTF)
      Mr. Dennis McCarthy (IAU)
After introduction of new members and general discussion of the objectives
of the SRG, comments by representatives were invited and they took the form
of short presentations.  Mr. Gambis summarized the role of the IERS and its
contribution to Earth rotation monitoring that determine Leap Seconds for
maintenance of the international UTC time scale.  After his comments, there
was a discussion of the areas that use UTC in national and international
systems.  There were five areas discussed.  These areas were satellite
navigation systems, telecommunications systems, computer networking,
broadcast services, and scientific uses.  The dominance of GPS systems in
these areas was mentioned.  Since GPS disseminates UTC (USNO), there is
considerable investment in GPS equipment serving these systems and this may
create a considerable reluctance to change.  This factor should be
considered in the SRG deliberations.  The SRG needs more information on use
and investment in these areas.  Procedures to effectively gather data in
these areas were discussed extensively by the members.  Conclusions on new
procedures were incorporated into the final course of action described
Before discussing time scale considerations and the presentation by Mr.
McCarthy on IAU studies, Question ITU-R 236/7 was reviewed in greater
detail and the three study areas mentioned in the Question were discussed.
>From this discussion the SRG concluded that clarification of these areas
would be necessary in the information provided by the contributing groups.
The additional factors to be considered by the SRG are outlined below.
Point 1.  Clarify requirements by identifying the following additional
Accuracy of timing information;
Stability of references;
Basis for the second used in the time scale;
Uniformity and accessibility of references;
Relation to legal time;
Coverage needed.
      A special query should be directed to the international timing centers
      to clarify the uses of UTC in the area of civil and legal timekeeping.
Point 2.  The definition of future requirements on tolerance limits between
UTC and UT1 may be very difficult for users to foresee.  Information on the
sensitivity of changing the tolerance on operations may be a better
Point 3.  Possible alternative procedures and specific changes being
considered should be clarified for the user.  As an example, better
explanation of current procedures detailed in a handbook might satisfy user
needs rather than making major system changes.
The relationship of internal system time references to the requirements
that determined their design and operation needs to be better understood by
the SRG.  In other words, why did a system not adopt a standard time scale
as its reference.
The presentation by Mr. McCarthy discussed the options and changes under
considerations by the IAU working group on this issue.  The various options
were discussed by the SRG and were categorized into three major areas of
options.  These options for the UTC time scale are discussed below.
Option 1:  Maintain the Status Quo
In order to maintain the system and operations as they are currently, it
was apparent to the SRG that additional information is necessary for users
to effectively use current time scale information.  UTC would continue as
the recommended time scale and its relation to other time scales, such as
TAI, should be better explained for system designers and operators.  Design
for use of time scale information should be explained and understood by
system users.  Implementation of more advanced notice of Leap Seconds that
is more widely available, could significantly aid current users in their
operations.  It may be necessary within this option to consider the
creation of new, lower accuracy time scales to meet the needs of users who
do not require high accuracy, e.g. celestial navigation needs.
Option 2: Modify Leap Second Procedures or Occurrence
To lessen the impact of Leap Seconds on systems requiring continuity in
their operation, the tolerance between UTC and UT1 could be modified.
Various modifications were discussed.  However, specific values would be
dependent upon the information gathered from users.  Alternatives to
modifying |UTC-UT1| tolerance could be to vary or fix the interval of
occurrence.  In this case, multiple Leap Seconds would be necessary and
could be applied at fixed intervals.  Another alternative discussed along
this line was to increase the prediction interval based on a longer-term
deceleration model of the earth rotation rate for time scale correction at
fixed prediction intervals.  The difficulty is the un-predictability of the
deceleration rate over long periods of time.  The adoption of this
alternative would need to be carefully considered.
Option 3: Use, or transition to another time scale
The sole use of the TAI time scale in current systems would need a
carefully developed transition plan.  Immediate use would introduce
considerable operational problems.  As part of a transition plan the
availability and maintenance of TAI for general use would need to be
improved.  A handbook explaining its use and transition details would be
necessary prior to implementation.  As discussed above in Option 1, a lower
accuracy time scale for general-purpose use should be considered in this
option as well.  The formation of a totally new time scale based on a
redefinition or modification of the SI second was considered to be
possible.  However, redefinition of the SI second would be highly dependent
upon technological advancement and many other complex issues beyond time
scale considerations.  Such a change is considered by the SRG to have a
very low probability of occurring in the near future.
Plan of Action
Having discussed the results of the SRG activities to this point and
identifying the possible alternatives, a plan of action was developed.  To
remain within the original scope of effort in producing definitive
suggested recommendations on the future of the UTC time scale for
consideration by Working Party 7A, a plan, for completion by their 2002
meeting, was developed.
The approach followed in the preliminary plan of action in contacting
organizations for participation and obtaining necessary information on time
scale usage has not been very productive.  An alternative plan was
developed to produce an increased awareness within the ITU and other
scientific and technical organizations.  It is felt that this plan will
develop the sources of information necessary to base a recommendation for
WP 7A.
The two aspects of this plan are to increase the awareness within other
organizations of the existence of the SRG and to the Question of the future
of the UTC time scale, through:
            (1)  Release of a general circular letter to both sectors of
           the ITU announcing the SRG and it's objectives; and
           (2)   Publication of articles and notices in the newsletters and
           general information journals of scientific and technical
This new approach should increase awareness and participation in this
effort.  In addition, the SRG will directly follow up and contact the
sector members who received the original letter from the Director, ITU-R.
Additional correspondence and contacts with other potentially interested
organizations will also be pursued.
E-mail will be the primary means for coordination and exchange between SRG
members between meetings.  The consideration of an ITU e-mail mirror site,
as was discussed in earlier sessions, was held in abeyance pending the
results of contacts and response of organizations involved.
Two SRG meetings were scheduled for the coming year.  They are currently
planned as one-day sessions concurrent with the PTTI meeting during the
last week of November 2001, and the EFTF meeting in mid March 2002.
With these planned efforts and meetings it is anticipated that a final
report with suggestions on the recommendations, will be available for WP7A
meeting in 2002 and the mission of the SRG will be completed.
Attachment A
Report of the URSI Commission J Working Group on the Leap Second
2 Date: July 2, 2000
2.1 Abstract and Conclusions
An e-mail survey to find possible adverse effects of a redefinition of UTC
has identified some possibly expensive or unsolvable problems involving
software rewriting or checking, which are listed below.  Although it was
not possible to quantify the financial scale of resolving the software
problems, the largest expenses appear to be for satellite systems, of which
one estimate of several hundred thousand dollars was supplied.  The
quantity and quality of the responses opposed to a change indicate that
those who favor any change must be prepared to make a very convincing
argument to people and groups who initially will disagree with them.
To further discuss this issue and inform the community of any developments,
an archived electronic listserv has been set up.  Anyone wishing to join
can do so using http://clockdev.usno.navy.mil/archives/leapsecs.html.
2.2 Introduction
UTC (Coordinated Universal Time), which the public commonly confuses with
Greenwich Mean Time, is computed by occasionally adding leap seconds to
International Atomic Time (TAI).  Since 1972, these leap seconds have been
added on December 31 or June 30, at the rate of about one every 18 months,
and serve to keep atomic time in step with the Earth's rotation. Although
it is recommended that users use only TAI or UTC, as their needs indicate,
many major navigation system have used times offset from TAI by fixed
amounts.  The most important of these is GPS, which is offset by 19 seconds
from TAI.
A segment of the international timing community has proposed a revision of
the definition of UTC to avoid the discontinuities due to intermittent leap
seconds.  A discussion of the motivations for a change and of possible
solutions has been published by McCarthy and Klepczynski in the Innovations
Section of the November, 1999 issue of GPS World.   The authors consider
the most significant reason for a change to be keeping spread-spectrum
communication systems and satellite navigation systems compatible with each
other and with civil times.  Another reason is the emerging need in the
financial community to keep all computer time-stamps synchronized.
In order to survey the effects of any action, an URSI Commission J Working
Group (WG) was formed, whose purpose was to prepare this report and propose
further actions.
A questionnaire (Appendix I) was distributed as widely as seemed
appropriate (Appendix II). The goal of the questionnaire was to find and
categorize those operations that would be adversely affected should a
change in UTC's computation be made. The questionnaire focused on the
possibility of simply inserting no new leap seconds, although alternative
solutions were also solicited.  Over 200 responses were received, and no
effort was made to separate the responses of URSI members from those of
The principal object of the questionnaire was to find what systems would be
adversely affected should a change be made in leap second procedures rather
than to convince users of the need for a change or to take a vote.
However, so many queries on these matters were received that a "standard
reply" (Appendix III) was developed and distributed as appropriate.   In
the spirit of full disclosure, the number of responses in each category is
given, but we caution that this was by no means an unbiased sampling of all
who would be affected by the change.  All responses were counted only once,
with preference to the most practical grounds for objection.  About half
the responses that were received were opposed to any change, while one-
fourth were in favor of a change, and one-fourth indifferent.
I.  Responses Opposed to Changes in the Status Quo
A. The expense of rewriting software.
Five responses suggested that contractors would have to be hired to
scrutinize and adapt large amounts of code for operational satellite
systems.  Efforts were made to contact these responders for specific dollar
amounts, and one off-the-cuff estimate of "several $100,000" was received.
The impact on such systems would be lessened if any decision to redefine
UTC were announced several years in advance.
Twenty-six others indicated that software would be a serious problem - a
very few of these were from people who did not seem to understand the
proposal. There were 9 responses involving telescope control; one of these,
from the Keck Observatory, provided a rough estimate of a few programmer-
months.  Others pointed out the problems computing eclipses and
occultations, for telescope pointing by amateurs, or with code they had
themselves written for professional-level projects such as speckle
interferometry.  One observatory indicated its station clock can not
accommodate a large UT1-UTC correction.
Fourteen more indicated that software issues would be a problem, but that
they are probably solvable.  Some of these actually indicated support
for the change.
B. Inherent inability to rewrite software to allow for |UT1-UTC| exceeding
.9 seconds.
Ten responses involved navigational software.  Taking the example of a
software product of the US Naval Observatory (USNO), pilots and sailors are
given the option to input UT1-UTC. However, it is expected that many users
would not understand this and enter 0, leading to noticeable errors within
a decade. These are similar to the telescope-control problems covered
above, except that one could not and should not expect the general public
to ever understand these issues.  Problems amateur astronomers might have
are also included here, and were brought up by many responders in other
One of the problems anticipated is that UT1-UTC could be applied with the
wrong sign, just as the leap second is occasionally applied with the wrong
sign.  An example of "buffer overflow" problem would happen in NIST's WWV,
WWVH and WWWB transmissions, which do not allow enough space for |UT1-UTC|
to exceed .9 sec.  Any users of these broadcasts who might need this
information and who are unaware of the problem, decoding in hardware, or
relying on old software would be adversely affected. Under the best of
future circumstances only the sub-second (tenths) digit would be available,
requiring the user to keep track of the digits to the left of the decimal
Not tabulated is an informal comment seriously made to the Working Group's
Chairman, by a respected and competent scientist from a non-western nation,
that astrologers would be adversely affected.
C. Philosophical Objections
Eight thought this would confuse or antagonize the public, religious
authorities, or even scientists - in essence because solar time is "true
time". (In contrast, one responder in favor of a change thought this would
help educate the public to the fact that Earth rotation is not "true"
Three pointed out that legal complications might occur in countries where
laws are specified in terms of solar time, or GMT (which has not existed
for thirty years).  Although one of these responders feared governments
would not follow the scientist's lead, we find it difficult to believe that
governments would, on their own, choose to add leap seconds.  Others
thought that any legal system flexible enough to handle daylight savings
time and the past abandonment of GMT in favor of UTC would easily
accommodate a seamless change in UTC's computation, especially if no other
time standard were available.  Some of the history of legal issues
concerning past changes in time definition can be found in the book
"Greenwich Time and Longitude" by Derek Howse.
Three thought we should not adopt a system which will fail in the long run,
even if that is a very distant time in the future.  (It could be pointed
out that all current time systems will eventually fail.  Well before 2050
we could be routinely adding more than one leap second per year, and when
we reach the point where a day is 48 hours long we would have to add a leap
second every second.  Even the Gregorian calendar will eventually need
revision because in a few million years the Earth will rotate less than 365
times per year, and leap days will not be necessary.)
Two thought this would deprive the timing community of free publicity when
leap seconds are inserted.
Thirty-eight expressed opposition, but gave no specific reason. Eleven of
those also indicated that a problem would exist with their system, but did
not specify it.  Some of these pointed out that TAI was readily available,
or indicated that they had seen no justification for a change.  (We had
intentionally provided no justification in the initial questionnaire, but
two responders replied that they still believed there was not enough reason
to change even after having read "standard response".)
Four were against it because they thought problems would happen if some
systems did not use the new system, and because they thought one would have
to separate analyses based upon whether data were recorded with the current
system or the old system.  (We believe these to be based upon a
misunderstanding of the "no new leap seconds" proposal, but entirely
possible if the more drastic measure of re-defining SI, the International
Second, were adopted.)
II.  Alternative Suggestions
Five suggested that it would be better to redefine the second to be longer,
add 1 second every 18 months forever, change on leap years or century-ends,
or change when the number exceeded a fixed amount. These possibilities are
also discussed in the GPS World Article.
A suggestion was received to add enough "negative leap seconds" to bring
UTC in line with GPS.  However, a different responder pointed out that many
of NASA's programs assume UT1-UTC always grows and that problems would
happen with any change that would sometimes lead to a "negative leap
second". (According to Dennis McCarthy of the USNO, this is possible even
with the current system, but unlikely).
It was also pointed out that rubidium atomic fountains or optical standards
could lead to a redefinition of the second using an element  other than
cesium.  If so, it could be redefined using a scale factor of sufficient
magnitude to avoid the short-term need for leap seconds (As noted in the
GPS World article, such a change would alter the values of those physical
quantities that depend upon time.)
One suggested that a redefined UTC should have a new name.
III. Responses in favor of changing the current leap second procedures
Forty-eight responses in favor were received, several from people who
experienced minor problems now in handling leap seconds, such as confusion
due to GPS time being currently 13 seconds offset from UTC and computer
errors at the time of the leap seconds. Along with responses based upon
reasons already covered in the GPS World Article, there were also three
from the highly undersampled group of computer programmers, which pointed
out the growing need to synchronize diverse computers to one second
accuracy and the difficulties of doing so in an when dates of future leap
seconds were unpredictable. Many of the responses indicated that periodic
addition of leap seconds on a scheduled basis would also be acceptable.
One of the responders, who was opposed to a change, suggested that leap
seconds were not a problem because computer users could program an
automatic extraction of the needed information from some publicly available
source; however it must be pointed out that no one can guarantee that a
given file, whether it be for leap seconds or for daily values of UT1-UTC
(see part IV, below), will remain forever available in a given format or
given IP address.
Four responses were from people who indicated that the convenience in not
having to update files every 18 months outweighed the expense and problems
rewriting software.  One of these suggested the use of sufficient negative
leap seconds to bring UTC in line with TAI, and several noted that UT1-
users easily incorporate the offsets due to time zones and daylight time.
IV. Other Responses
Forty-seven responders checked the "indifferent" option.  Most indicated
that there would be no problems with their system, or that the problems
were small enough that they were indifferent to a change.
Seven other responders indicated that the change would be okay if they
could somehow reliably obtain UT1-UTC.  (The USNO, as a sub-bureau of the
IERS, freely provides this information on its web site,
http://maia.usno.navy.mil, and via weekly emailings.  Other organizations
also provide this information.)  Some responses tabulated here were phrased
in the negative because the responders were apparently unaware that the
information was available.  One responder was concerned that the GPS system
itself would be degraded for this reason, however this is not the case
because GPS currently uses UT1-UTC values derived from USNO products.
Received on Thu May 24 2001 - 08:50:52 PDT

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