There follows an ascii version of the Word Document I circulated yesterday.
Please keep in mind that line breaks, spacings, etc. may be out of order.
**************************************************************************
INTERNATIONAL TELECOMMUNICATION UNION
RADIOCOMMUNICATION
STUDY GROUPS
Revision 2 to
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
2000.
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
wklepczy_at_aol.com
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.
1) Identify Participating Organizations Points of Contact for
coordination of materials.
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.
2) 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 organizations.
3) 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 BIPM, CCTF, COSPAR, IAU,
ICAO,
ICSU, IMO, IUGG, IUPAP, URSI, and 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).
4) 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.
5) 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.
6) 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.
7) 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.
8) 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
Attendees
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 later.
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
information.
a. Accuracy of timing information;
b. Stability of references;
c. Basis for the second used in the time scale;
d. Uniformity and accessibility of references;
e. Reliability;
f. Relation to legal time;
g. 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 indicator.
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 organizations.
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
Date: July 2, 2000
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.
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.
Methodology
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 nonmembers.
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 contexts.
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
point.
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" time).
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.
__________
____________________
- 10 -
7A/TEMP/14(Rev.2)-E
M:\BRSGD\TEXT2001\SG07\WP7A\DT\014R2E.DOC 09.05.01 09.05.01
M:\BRSGD\TEXT2001\SG07\WP7A\DT\014R2E.DOC 09.05.01 09.05.01
Received on Thu May 24 2001 - 08:22:36 PDT