Home > RECOMMENDATION ITU-R TF.460-6* - Standard-frequency and time-signal emissions
Rec. ITU-R TF.460-6
Rec. ITU-R TF.460-6
ITU-R Fact
Sheet
|
ITU-R Study Group:
USWP-7A Reference: ITU-R Question 236/7 |
Document
No.: USWP-7A/1 Date:
14 July 2004 |
Document Title:
|
Proposed Revised Recommendation ITU-R TF.460-6 Standard-frequency and time-signal emissions |
| Author: | Phone: | Fax: | e-mail: |
| Wayne Hanson | 303-497-5233 | 303-497-3228 | hanson@boulder.nist.gov |
Purpose/Objective:
Change the definition of Coordinated Universal Time (UTC) effectively
resulting in a uniform time scale.
Abstract:
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*
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
Rotation 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);the continuing need to make universal time (UT) immediately
available to an uncertainty of one-tenth of a second,
h) that
the complexity of the variability of the Eartht��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
(IERS) 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 see Annex 1); that the transmission
transition of the
time signals should not deviate from UTC by more than 1 microsecond
ms; that the standard frequencies should not deviate
by more than 1 part in 101110, 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 fo UT1-UTC so that users have access to UT1;standard-frequency
and time-signal emissions, and other time-signal emissions intended
for scientific applications (with the possible exception of those dedicated
to special systems) should contain information on UT1 UTC
and TAI UTC (see Annex 1);.
3 that, if necessary, adjustments in the epoch of UTC be made following the guidance given in the Annex.
ANNEX
1
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).
Universal
time (UT) is the general designation of time scales based on the rotation
of the Earth.In applications in which an imprecision of a few hundredths
of a second cannot be tolerated, it is necessary to specify the form
of UT which should be used:UT0 is the mean solar time of the prime meridian
obtained from direct astronomical observation;UT1 is UT0 corrected for
the effects of small movements of the Earth relative to the axis of
rotation (polar variation); UT2 is UT1 corrected for the effects of a
small seasonal fluctuation in the rate of rotation of the Earth;
UT1 is used in this Recommendation, since it corresponds directly with
the angular position of the Earth around its axis of diurnal rotation.Concise
definitions of the above terms and the concepts involved are available
in the publications of the IERS (Paris, France).
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).
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.
The following
operational rules apply:
1
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.
2.1 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 3).
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.
3.1 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 2.
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.
E DTAIThe value of the difference TAI – UTC,
as disseminated with time signals, shall be denoted DTAI. DTAI TAI UTC
may be regarded as a correction to be added to UTC to obtain TAI.The
TAI UTC values are published in the
BIPM Circular T. The IERS should announce the value of DTAI in integer
multiples of one second in the same announcement as the introduction
of a leap-second (see �� D.2).
ANNEX
2
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:
ANNEX
3
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:
Operational Rules
(after 0000 UTC December 21, 2007
The difference of UT1 from UTC should not exceed 1h.
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.
* 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).
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