INTERNATIONAL EARTH ROTATION SERVICE (IERS)
SERVICE INTERNATIONAL DE LA ROTATION TERRESTRE
MARCH 2001
EXPLANATORY SUPPLEMENT TO IERS BULLETINS A AND B
IERS Bulletins A and B provide current information on the Earth's
orientation in the IERS Reference System. This includes Universal Time,
coordinates of the terrestrial pole, and celestial pole offsets. Bulletin A
gives an advanced solution updated twice weekly by e-mail subscription or
daily by anonymous ftp; the standard solution is given monthly in Bulletin B
and updated twice weekly in the (IERS) C04 solution. The Annual Report, issued
six months after the end of each year, contains information on the data used,
the models, the algorithms and the reference frames, as well as revised
solutions for the past years. All solutions are continuous within their
respective uncertainties. Bulletin A is issued by the IERS Rapid
Service/Prediction Centre at the U.S. Naval Observatory, Washington;
Bulletin B and the Annual Report are issued by the IERS Earth Orientation
Centre at the Paris Observatory.
Bulletins A and B are respectively meant for rapid service/prediction and
standard use. For scientific and long-term analyses of the Earth's
orientation, users are advised to request the long-term continuous series
maintained by the Earth Orientation Centre from 1846 (x, y), 1962 (UT), and
1981 (dPsi, dEpsilon) to the current date. All solutions are available
electronically (see below).
THE IERS CONVENTIONS
The IERS uses the following as its conventions
1. The International Celestial and Terrestrial Reference Systems
-------------------------------------------------------------
The International Celestial and Terrestrial Reference Systems (respectively
ICRS, ITRS) are defined by their origins, directions of axes and, in the case
of the ITRS, length unit. The ICRS is described by Arias et al. (1995). Its
origin is at the barycenter of the solar system. The directions of its axes
are fixed with respect to the quasars to better than +/- 20 micro-arcseconds;
they are aligned with those of the FK5 within the consistency of the latter
(+/- 80 milliarcseconds at epoch J1991.25 (van Leeuwen et al., 1997). The
ICRS is realized by estimates of the coordinates of a set of quasars: the
International Celestial Reference Frame (ICRF) (Ma and Feissel, 1997; Ma et
al., 1998). According to Resolution B2 of the IAU 23rd General Assembly
(Kyoto, 1998), after 1 January 1998 the IAU celestial reference system is the
International Celestial Reference System (ICRS) as defined by the International
Earth Rotation Service (IERS) and the corresponding fundamental reference frame
is the ICRF constructed by the IAU Working Group on Reference Frames. The IERS
was asked to monitor the maintenance of the ICRF and its ties to the reference
frames at other wavelengths. In the present IERS structure, two groups share
this task: the International VLBI Service for Geodesy and Astrometry (IVS) and
the IERS ICRS Centre, which is jointly operated by the Paris Observatory and
the U.S. Naval Observatory.
The ITRS origin is at the center of mass of the whole Earth, including the
oceans and the atmosphere. Its length unit is the meter (SI), consistent with
the TCG time coordinate for a geocentric local frame. The orientation of its
axes is consistent with that of the BIH System at 1984.0 within +/- 3 milli-
arcseconds. The International Reference Meridian (IRM) is implicitly defined
through the adoption of the set of coordinates of stations realizing the ITRF.
Its time evolution in orientation is such that it has no residual rotation
relative to the Earth's crust. The ITRS is realized by estimates of the
coordinates and velocities of a set of observing stations, the International
Terrestrial Reference Frame (ITRF). For more details, see Boucher et al.
(1996) and The IERS Conventions (McCarthy, 1996). A new ITRF realization
(ITRF2000) is now available (http://lareg.ensg.ign.fr/ITRF/).
2. IERS constants and models
-------------------------
The IERS Conventions (McCarthy, 1996) are a set of constants and models
used by the IERS Technique and Analysis Centres for Very Long Baseline
Interferometry (VLBI), Global Positioning System (GPS), satellite
radiopositioning (DORIS), Lunar and Satellite Laser Ranging (LLR, SLR), and by
the IERS Product Centres in the combination of results.
The values of the constants are adopted from recent analyses. In some
cases they differ from the current IAU and IAG conventional ones. The models
are, in general, the best estimates in the field concerned. VLBI and LLR
observations have shown that there are deficiencies in the IAU 1976 Theory of
Precession and in the IAU 1980 Theory of Nutation. However, these models are
kept as a part of the IERS conventions, and the observed differences with
respect to the conventional celestial pole position defined by the models are
monitored and reported by the IERS in its publications.
New IERS Conventions 2000 will be published in the course of 2001. Two key
aspects concern a new nutation model which will be adopted as of 1 January
2003, and the definition of Celestial Intermediate Pole. (More information at
the IAU Commission 19 web site (http://danof.obspm.fr/iaucom19/).
THE EARTH ORIENTATION PARAMETERS
The IERS Earth Orientation Parameters (EOP) describe the rotation of the
ITRS relative to the ICRS, in conjunction with the conventional Precession-
Nutation model.
1. x and y are the coordinates of the Celestial Ephemeris Pole (CEP) relative
to the International Reference Pole IRP. The CEP differs from the
instantaneous rotation axis by quasi-diurnal terms with amplitudes under 0.01"
(see Seidelmann, 1982). The x-axis is in the direction of the IERS Reference
Meridian (IRM), the y-axis is in the direction 90 degrees West longitude.
2. UT1 is the rotation angle about the pole. It is related to the Greenwich
mean sidereal time (GMST) by a conventional relationship (Aoki et al., 1982).
It gives access to the direction of the International Reference Meridian IRM
in the ICRS, reckoned around the CEP axis. It is expressed as the difference
UT1-TAI or UT1-UTC.
TAI is the atomic time scale calculated by the BIPM. Its unit interval is
exactly one SI second at mean sea level. The origin of TAI is such that
UT1-TAI is approximately 0 on 1958 January 1. The instability of TAI is about
six orders of magnitude smaller than that of UT1.
UTC is defined by the 1986 CCIR Recommendation 460-4 (CCIR, 1986). It
differs from TAI by an integral number of seconds in such a way that UT1-UTC
remains smaller than 0.9s in absolute value. The decision to introduce a leap
second in UTC to meet this condition is the responsibility of the IERS; it is
announced in Bulletin C. According to the CCIR Recommendation, first
preference is given to opportunities at the end of June and December and
second preference to those at the end of March and September. Since the
system was introduced in 1972, only dates in June and December have been used.
A new definition of UTC is being discussed. The current leap second
procedure does not satisfy various communities involved in navigation and
telecommunications. This led to the Resolution B2 after the XXIVth
International Astronomical Union General Assembly held in 2000 in Manchester
and to the creation of different special study groups within the International
Astronomical Union (IAU) and the International Telecommunication Union (ITU).
(See Commission 19 website http://danof.obspm.fr/iaucom19/).
DUT1 is the difference UT1-UTC expressed with a precision of +/- 0.1s; it
is broadcast with the time signals and announced in Bulletin D. The changes
in DUT1 are decided by the IERS.
The difference between the astronomically determined duration of the day
(D) and 86400s of TAI, is called length of day (LOD). Its relationship with
the angular velocity of the Earth, Omega, is:
Omega = 72 921 151.467064 - 0.843994803 D,
where Omega is in picoradians/s and D in ms.
UT1, hence D and Omega, are subject to variations due to zonal tides. The
model which is a part of the IERS Conventions includes 62 periodic components,
with periods ranging from 5.6 days to 18.6 years. UT1R, DR, and OmegaR are the
values of UT1, D, and Omega corrected for the short-term part of the model by
Yoder et al. (1981), i.e., the 41 components with periods under 35 days. In
absolute value UT1R-UT1 is smaller than 2.5 ms, LODR-LOD is smaller than 1 ms.
As it was recommended in the IERS Gazette # 13, IERS Earth orientation data
are produced at daily intervals and do not include the effects of semidiurnal
and diurnal variations; Ray's model has been adopted for interpolation. The
corresponding numerical program is available on request.
3. dPsi and dEpsilon are the offsets in longitude and obliquity of the
celestial pole with respect to its direction defined using the conventional
IAU precession/nutation theory. An a priori correction model is available in
the IERS Conventions (1996), (McCarthy, 1996).
THE DATA ANALYSIS
The data analysis which yields the values of the EOP published in Bulletins A
and B includes several steps which are summarized below.
1. Observations by the VLBI, LLR, SLR, GPS and DORIS networks, coordinated by
the individual Technique Centres.
2. Analyses (operational and refined) by the Analysis Centers of the Technique
Centres. The operational results are transmitted at least weekly in parallel
to the IERS Rapid Service/Prediction Centre to contribute to Bulletin A and to
the IERS Earth Orientation Centre to contribute to Bulletin B. The refined
results are transmitted yearly.
3. General adjustment of ICRF, ITRF and EOP by the IERS Product Centres, based
on the refined annual results. This adjustment, described in the IERS Annual
Report provides the basis for determining the systematic corrections to be
added to the individual series for the following year in order to bring them
into the IERS Reference System. These corrections are used in step 5. The
general results are published in the IERS Annual Report (Gambis, 2000).
4. Determination of EOP by the IERS Rapid Service/Prediction Centre is in the
form of slightly smoothed solutions at one-day intervals. This involves the
application of systematic corrections and statistical weighting. The accuracy
of this solution is given in Table 1. The results are published in Bulletin A
with a delay of about one day between the date of publication and the last
available date with estimated EOP. The details of the procedure are outlined
in McCarthy and Luzum (1991a).
5. Determination of EOP by the IERS Earth Orientation Centre in the form of
combined solutions derived from the individual series. Various solutions are
computed: normal values at five-day intervals and smoothed solutions at one-
day and five-day intervals. In the procedure we apply systematic corrections
determined in step 3 and statistical weighting. The accuracy of these
solutions is given in Table 1. The results are published in Bulletin B with a
delay of thirty days between the date of publication and the last date of the
standard solution. EOP(IERS) C 04 solution, taking into account updated values
of the individual series is computed twice weekly.
6. Prediction of the EOP. Bulletins A and B provide predictions of the EOP.
Details of the procedure used are given in McCarthy and Luzum (1991b) for
Bulletin A and in Feissel et al. (1988) for Bulletin B. The predictions use
similar algorithms, based on seasonal filtering and auto-regressive processing
for x, y, UT1 and an approximate modelled correction for the celestial pole
offsets. Their performances are given in Table 1.
Table 1- Precision of the various solutions. The accuracy which
includes the uncertainty of the tie to the IERS System can be
estimated by adding quadratically 0.0002" in terrestrial pole,
0.00003s in UT1, and 0.0002" in celestial pole.
---------------------------------------------------------------
Solutions ! terr.pole UT celest.pole
! 0.001" 0.0001s 0.001"
--------------------------!------------------------------------
Bulletin A daily (1) ! 0.2 0.5 0.3
prediction (2) 10d ! 3.9 16. 0.3
40d ! 11.2 77. 0.3
90d ! 19.7 178. 0.3
!
Bulletin B !
smoothed (3)1-d, 5-d ! 0.2 0.2 0.3
raw (3) 5-d ! 0.2 0.2 0.3
prediction (3) 5-d ! 1.6 6.0 0.3
10d ! 3.0 8.0 0.3
30d ! 10.0 53.0 0.3
--------------------------!------------------------------------
Notes.
(1) Based on data since 1998; applies only to latest epoch in each update.
(2) Based on data since 1998.
(3) Based on data since 1999.
CONTENTS OF BULLETINS A AND B.
BULLETIN A (semiweekly and daily)
General information including key definitions and the most recently adopted
values of DUT1 and TAI-UTC.
Quick-look daily estimates of the EOP, determined by applying systematic
corrections and smoothing the observed data, with accuracies as shown in Table
1. The characteristics of the transfer function of the smoothing process are
given in Table 2. The results are published with a delay of about one day
between the date of publication and the last available date with estimated EOP.
Predictions of x, y, UT1-UTC daily up to 360 days following the last day of
data in Section 4, smoothed daily values of celestial pole offsets.
Table 2. Frequency filtering characteristic of smoothing for
Bulletins A and B
----------------------------------------------
PERIOD FOR
Epsilon REMAINING AMPLITUDE
5% 50% 95%
----------------------------------------------
IERS Bull A - - 1d 3d
IERS Bull B 1e +2 1.5d 2.8d 4.5d
----------------------------------------------
BULLETIN B (Monthly)
Section 1: Five days sampling of section 2. Final Bulletin B values over one
month and provisional extension over the next three months.
Section 2 : Smoothed values of x, y, UT1-UTC, UT1-UT1R, dPsi, dEpsilon, at
one-day interval based on a combination of the series presented in section 6.
Table 2 gives the characteristics of the transfer function of the smoothing
applied (Vondrak, 1977). A new and more general method of smoothing applied on
both values and rates (Vondrak and Gambis, 2000; Vondrak 2000) is being
implemented in the current algorithms. The general combination procedure is
described in Gambis (2000) and Gambis et al. (2001).
Section 3: Five-day normal values of x, y, UT1-UTC, dPsi, dEpsilon, and their
uncertainties (EOP(IERS) C02), based on a combination of the series of section
6. New class of robust M-Huber estimators have been implemented in the
analysis procedures (Bougeard et al., 2000).
Section 4: Smoothed values of DR and OmegaR, with the same degree of
smoothing as UT1R-UTC (see table 2).
Section 5: Current values of UTC-TAI and DUT1, reproducing IERS Bulletins C
and D. Announcement of the leap seconds.
Section 6: This section gives the average precision of the individual series
contributing to the combination and their agreement with the combination.
Section 7: (available only on the electronic and ftp version): Data of IERS
analysis centers (Table 3).
Table 3- Individual series contributing to IERS Bulletins A and B, January 2001.
The formal uncertainties are those which are reported by the contributors.
They are used in the combinations for Bulletins A and B after being calibrated
by statistical assessment.
-----------------------------------------------------------------------------
! formal uncertainties
! sampling based on 1998-99 data
Series ! time terr.pole UT LOD cel. pole
! 0.001" 0.0001s 0.001"
------------------------!----------------------------------------------------
EOP(GSFC) 00 R 02 ! 7d 0.64 0.29 0.15
EOP(BKG) 00 R 02 ! 7d 0.16 0.09 0.08
EOP(IAA) 98 R 01 ! 7d 0.10 0.04 0.10
EOP(USNO)+ 99 R 01 ! 7d 0.14 0.06 0.20 0.13
EOP(BKG) 00 R 01 ! 7d 0.22
EOP(GSFC) 00 R 01 ! 1-3 d 0.21
EOP(IAA) 00 R 04 ! 1-3 d 0.18
EOP(USNO)+ 99 R 02 ! 1-3 d 0.23
EOP(SPBU) 99 R 01 ! 1-3 d 0.22
EOP(CGS) 97 L 02 ! 3d 0.28 0.25 *
EOP(CSR) 95 L 01 ! 3d 0.39 0.31 *
EOP(DUT) 98 L 01 ! 3d 0.11 0.10 *
EOP(IAA) 98 L 02 ! 1d 0.05 0.04 * 0.04
EOP(MCC) 97 L 01 ! 3d 0.05 0.10
EOP(CODE) 98 P 01 ! 1d 0.05 0.10
EOP(EMR) 96 P 03 ! 1d 0.07 1.05 * 0.13
EOP(ESOC) 96 P 01 ! 1d 0.02 0.03 * 0.03
EOP(GFZ) 96 P 02 ! 1d 0.01 0.01 * 0.01
EOP(JPL) 96 P 03 ! 1d 0.04 0.14 * 0.14
EOP(NOAA) 96 P 01 ! 1d 0.03 0.10 * 0.19
EOP(SIO) 96 P 01 ! 1d 0.07 0.45 * 0.16
EOP(IGS) 95 P 02 ! 1d 0.05 0.17 * 0.08
EOP(IGS) 96 P 02 ! 1d 0.08 0.29 * 0.14
-----------------------------------------------------------------------------
+ Until December 2000
* The satellite techniques provide information on the rate of change of
Universal Time contaminated by effects due to unmodelled orbit node motion.
VLBI-based results have been used to minimize drifts in UT estimates.
DISTRIBUTION OF THE PUBLICATIONS
IERS Rapid Service/Prediction Centre, at U.S. Naval Observatory:
---------------------------------------------------------------
BULLETIN A
By 0h UTC of Tuesday and Friday of each week via e-mail distribution:
- e-mail (contact: ser7@maia.usno.navy.mil)
- World Wide Web (http://maia.usno.navy.mil/)
- Anonymous ftp (ftp://maia.usno.navy.mil/ser7)
By about 17:10h UTC daily via anonymous ftp:
- World Wide Web (http://maia.usno.navy.mil/)
- Anonymous ftp (ftp://maia.usno.navy.mil/ser7)
IERS Earth Orientation Centre, at Paris Observatory:
---------------------------------------------------
- e-mail (contact: iers@obspm.fr)
- World Wide Web (http://hpiers.obspm.fr/eop-pc/)
- Anonymous ftp (hpiers.obspm.fr or 145.238.100.28)
BULLETIN B
Updated at the beginning of each month
- World Wide Web
- Anonymous ftp (directory iers/bul/bulb)
- airmail
C04 series
- Available twice weekly
- World Wide Web
- Anonymous ftp (directory iers/eop/eopc04)
Jim Ray Daniel Gambis
Head, IERS Rapid Service/ Director
Prediction Centre IERS Earth Orientation Centre
jimr@maia.usno.navy.mil daniel.gambis@obspm.fr
GLOSSARY
AAM Atmospheric Angular Momentum
BIH Bureau International de l'Heure
BIPM Bureau International des Poids et Mesures
BKG Bundesamt fuer kartographie und geodaesie
CEP Celestial Ephemeris Pole
CERGA Centre d'Etudes et de Recherches Geodynamiques et Astronomiques
CCIR International Radio Consultative Committee
CIO Conventional International Origin
CODE Center for Orbit Determination in Europe
CGS Space Geodesy Center, Matera
CSR Center for Space Research, University of Texas
DORIS Doppler Orbit determination and Radiopositioning Integrate on
Satellite
DUT Delft University of Technology
ECMWF European Centre for Medium-range Weather Forecasting
EMR See NRCan
EOP Earth Orientation Parameters
ESOC European Space Operations Center
GFZ GeoForschungsZentrum
GMST Greenwich Mean Sidereal Time
GPS Global Positioning System
GSFC Goddard Space Flight Center
IAA Institute of Applied Astronomy
IAG International Association of Geodesy
IAU International Astronomical Union
IERS International Earth Rotation Service
ICRF IERS Celestial Reference Frame
ICRS International Celestial Reference System
IGS International GPS Service for Geodynamics
ITRF IERS Terrestrial Reference Frame
ITRS International Terrestrial Reference System
IRP IERS Reference Pole
IRM IERS Reference Meridian
JPL Jet Propulsion Laboratory
LLR Lunar Laser Ranging
MCC Russian Mission Control Center
MJD Modified Julian Day
NEOS National Earth Orientation Service
NOAA National Oceanic and Atmospheric Administration
NRCan Natural Resources Canada, formerly EMR
OPA Observatoire de Paris
SPBU St Petersburg University
SLR Satellite Laser Ranging
SI Systeme International
SIO Scripps Institution of Oceanography
TAI Temps Atomique International
TCG Geocentric Coordinate Time
TT Terrestrial Time
UKMO U.K. Meteorological Office
USNO United States Naval Observatory
UTC Coordinated Universal Time
UTXMO Dept. of Astronomy. The University of Texas at Austin.
VLBI Very Long Baseline Interferometry
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Observatoire de Paris.
Bougeard M.L, D. Gambis and R. Ray, 2000, Algorithms for box constrained M-
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Vondrak J. and A. Cepek, 2000: Combined smoothing method and its use in
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