ITU-R Fact Sheet

+-----------------------------------+-----------------------------------+
| **[ITU-R Study                    | **[Document No.:]{.underline}**   |
| Group:]{.underline}** USWP-7A     | USWP-7A/1                         |
|                                   |                                   |
| **[Reference:]{.underline}**      | **[Date:]{.underline}** 14 July   |
|                                   | 2004                              |
| ITU-R Question 236/7              |                                   |
+-----------------------------------+-----------------------------------+

Document Title:

+-----------------------------------------------------------------------+
| Proposed Revised Recommendation                                       |
|                                                                       |
| ITU-R TF.460-6                                                        |
|                                                                       |
| Standard-frequency and time-signal emissions                          |
+-----------------------------------------------------------------------+

  --------------------------- -------------------------- ------------------------ ---------------------------
  **[Author:]{.underline}**   **[Phone:]{.underline}**   **[Fax:]{.underline}**   **[e-mail:]{.underline}**

  Wayne Hanson                303-497-5233               303-497-3228             hanson@boulder.nist.gov
  --------------------------- -------------------------- ------------------------ ---------------------------

**[Purpose/Objective:]{.underline}** Change the definition of
Coordinated Universal Time (UTC) effectively resulting in a uniform time
scale.

**[Abstract:]{.underline}** Recommendation ITU-R TF.460-6 allows for the
insertion of leap seconds into the UTC time scale keeping it within 0.9
seconds of mean solar time or the UT1 time scale. These insertions of
leap seconds result in steps or discontinuities in UTC that increasingly
cause problems for navigation systems, communication networks and time
distribution performance. In order to reduce the frequency of
introducing discontinuities and still keep UTC coordinated with UT1,
leap seconds will be dropped and shifts of one hour, theoretically first
occurring around the year 2600, will be substituted. These "leap hour"
shifts would effectively make UTC a uniform time scale for the
foreseeable future without noticeable environmental or psychological
effects as the general public has already become accustomed, in the use
of daylight savings time, to one hour shifts in UTC, twice a year,
relative to mean solar time.

The change in definition is proposed to occur December 21, 2007 giving
adequate time for possible affected systems to prepare. Until that time
the Recommendation will basically remain unchanged.

  ----------------------------------- -----------------------------------
  DOCUMENTS                           Document USWP-7A/1

  RADIOCOMMUNICATIONS                 14 JULY 2004

  STUDY GROUPS                        ORIGINAL: ENGLISH

                                      
  ----------------------------------- -----------------------------------

UNITED STATES OF AMERICA

PROPOSED REVISED RECOMMENDATION ITU-R TF.460-6[^1]

Standard-frequency and time-signal emissions

(Question ITU-R 102/7)

(1970-1974-1978-1982-1986-1997-2002)

The ITU Radiocommunication Assembly,

considering

a\) that the World Administrative Radio Conference, Geneva, 1979,
allocated the frequencies 20 kHz  0.05 kHz, 2.5 MHz  5 kHz (2.5 MHz
 2 kHz in Region 1), 5 MHz  5 kHz, 10 MHz  5 kHz, 15 MHz  10 kHz, 20 MHz
 10 kHz and 25 MHz  10 kHz to the standard‑frequency and time-signal
service;

b\) that additional standard frequencies and time signals are emitted in
other frequency bands;

c\) the provisions of Article 26 of the Radio Regulations;

d\) the continuing need for close cooperation between Radiocommunication
Study Group 7 and the International Maritime Organization (IMO), the
International Civil Aviation Organization (ICAO), the General Conference
of Weights and Measures (CGPM), the Bureau International des Poids et
Mesures (BIPM), the International Earth rotation and Reference system
Service (IERS) and the concerned Unions of the International Council of
Scientific Unions (ICSU);

e\) the desirability of maintaining worldwide coordination of
standard-frequency and time‑signal emissions;

f\) the need to disseminate standard frequencies and time signals in
conformity with the second as defined by the 13th General Conference of
Weights and Measures (1967);

g\) the desirability of maintaining a relationship between a uniform
time scale (UTC) and the time defined by the rotation of the Earth
(UT1);

h\) that the complexity of the variability of the Earth's rotation
currently limits the accuracy with which the difference between the two
types of time scales can be predicted to a few tenths of a second one
year in advance;

i\) that the International Earth rotation and Reference system Service
provides updated data relating the two time scales daily to users;

recommends

**1** that all standard-frequency and time-signal emissions conform as
closely as possible to coordinated universal time (UTC) (as defined in
the Annex ); that the transmission of time signals should not deviate
from UTC by more than 1 microsecond ; that standard frequencies should
not deviate by more than 1 part in 1011, and that the time signals
emitted from each transmitting station should bear a known relation to
the phase of the carrier;

**2** that the IERS provide convenient access to values of UT1-UTC so
that users have access to UT1;

3 that, if necessary, adjustments in the epoch of UTC be made following
the guidance given in the Annex.

ANNEX

Time scales

# A Universal time (UT1)

# 

UT1 is the time scale determined from astronomical observations of the
rotation of the Earth with respect to the International Celestial
Reference System. A technical description and the concepts involved are
available in the publications of the IERS (Frankfurt am Main, Germany).

# B International atomic time (TAI)

The international reference scale of atomic time (TAI), based on the
second (SI), as realized on the rotating geoid, is formed by the BIPM on
the basis of clock data supplied by cooperating establishments. It is in
the form of a continuous scale, e.g. in days, hours, minutes and seconds
from the origin 1 January 1958 (adopted by the CGPM 1971).

# C Coordinated universal time (UTC)

UTC is the time-scale maintained by the BIPM, with assistance from the
IERS, which forms the basis of a coordinated dissemination of standard
frequencies and time signals. It corresponds exactly in rate with TAI
but differs from it by an integer number of seconds. The UTC time scale
is in approximate agreement with UT1. The value of the difference
UT1-UTC, either observed or predicted, is disseminated by the IERS. It
may be regarded as a correction to be added to UTC to obtain UT1.

Operational Rules

(prior to 0000 UTC, December 21, 2007)

DUT1

The value of the predicted difference UT1 -- UTC, as disseminated with
the time signals is denoted DUT1; thus DUT1  UT1 -- UTC. DUT1 may be
regarded as a correction to be added to UTC to obtain a better
approximation to UT1.

The values of DUT1 are given by the IERS in multiples of 0.1 s.

# 1 Tolerances

**1.1** The magnitude of DUT1 should not exceed 0.8 s.

**1.2** The departure of UTC from UT1 should not exceed  0.9 s (see
Note 1).

**1.3** The deviation of (UTC plus DUT1) should not exceed  0.1 s.

NOTE 1 -- The difference between the maximum value of DUT1 and the
maximum departure of UTC from UT1 represents the allowable deviation of
(UTC  DUT1) from UT1 and is a safeguard for the IERS against
unpredictable changes in the rate of rotation of the Earth.

# 2 Leap-seconds

**2.1** A positive or negative leap-second should be the last second of
a UTC month, but first preference should be given to the end of December
and June, and second preference to the end of March and September.

**2.2** A positive leap-second begins at 23h 59m 60s and ends at 0h 0m
0s of the first day of the following month. In the case of a negative
leap-second, 23h 59m 58s will be followed one second later by 0h 0m 0s
of the first day of the following month (see Annex ).

**2.3** The IERS should decide upon and announce the introduction of a
leap-second, such an announcement to be made at least eight weeks in
advance.

# 3 Value of DUT1

**3.1** The IERS is requested to decide upon the value of DUT1 and its
date of introduction and to circulate this information one month in
advance. In exceptional cases of sudden change in the rate of rotation
of the Earth, the IERS may issue a correction not later than two weeks
in advance of the date of its introduction.

**3.2** Administrations and organizations should use the IERS value of
DUT1 for standard-frequency and time-signal emissions, and are requested
to circulate the information as widely as possible in periodicals,
bulletins, etc.

**3.3** Where DUT1 is disseminated by code, the code should be in
accordance with the following principles (except § 3.4 below):

-- the magnitude of DUT1 is specified by the number of emphasized second
markers and the sign of DUT1 is specified by the position of the
emphasized second markers with respect to the minute marker. The absence
of emphasized markers indicates DUT1  0;

-- the coded information should be emitted after each identified minute
if this is compatible with the format of the emission. Alternatively the
coded information should be emitted, as an absolute minimum, after each
of the first five identified minutes in each hour.

Full details of the code are given in the Annex .

**3.4** DUT1 information primarily designed for, and used with,
automatic decoding equipment may follow a different code but should be
emitted after each identified minute if this is compatible with the
format of the emission. Alternatively, the coded information should be
emitted, as an absolute minimum, after each of the first five identified
minutes in each hour.

**3.5** Other information which may be emitted in that part of the
time-signal emission designated in § 3.3 and 3.4 for coded information
on DUT1 should be of a sufficiently different format that it will not be
confused with DUT1.

**3.6** In addition, UT1 -- UTC may be given to the same or higher
precision by other means, for example, by messages associated with
maritime bulletins, weather forecasts, etc.; announcements of
forthcoming leap-seconds may also be made by these methods.

**3.7** The IERS is requested to continue to publish, in arrears,
definitive values of the differences UT1 -- UTC and UT2 -- UTC.

# 

# 

Code for the transmission of DUT1

A positive value of DUT1 will be indicated by emphasizing a number, *n*,
of consecutive second markers following the minute marker from second
marker one to second marker, *n*, inclusive; *n* being an integer from 1
to 8 inclusive.

DUT1  (*n* × 0.1) s

A negative value of DUT1 will be indicated by emphasizing a number, *m*,
of consecutive second markers following the minute marker from second
marker nine to second marker (8 * m*) inclusive, *m* being an integer
from 1 to 8 inclusive.

DUT1  -- (*m* × 0.1) s

A zero value of DUT1 will be indicated by the absence of emphasized
second markers.

The appropriate second markers may be emphasized, for example, by
lengthening, doubling, splitting or tone modulation of the normal second
markers.

*Examples*:

![](media/image1.wmf){width="5.834027777777778in"
height="1.8402777777777777in"}

![](media/image2.wmf){width="5.834027777777778in"
height="1.8104166666666666in"}

Dating of events in the vicinity of a leap-second

The dating of events in the vicinity of a leap-second shall be effected
in the manner indicated in the following Figures:

![](media/image3.wmf){width="5.8069444444444445in"
height="4.420138888888889in"}

**Operational Rules**

**(after 0000 UTC December 21, 2007**

1.  Tolerance

> The difference of UT1 from UTC should not exceed 1h.

2.  Adjustments to UTC

    1.  Adjustments to the UTC time scale should be made as determined
        by the IERS to ensure that the time scale remains within the
        specified tolerances.

    2.  The IERS should announce the introduction of an adjustment to
        the UTC time scale as least five years in advance. At the time
        of the announcement the IERS should provide directions regarding
        the details of the implementation of the adjustment.

    3.  All operational rules and nomenclature prior to 0000 UTC
        December 21, 2007 given above no longer apply.

Notes:

\(1\) The broadcast of DUT1 will be discontinued.

\(2\) Analysis of historical observations of the Earth's rotation
currently indicates that such an adjustment would not be required for at
least 500 years.

[^1]: ^\*^ This Recommendation should be brought to the attention of the
    IMO, the ICAO, the CGPM, the BIPM, the IERS, the International Union
    of Geodesy and Geophysics (IUGG), the International Union of Radio
    Science (URSI) and the International Astronomical Union (IAU).