MARCH 2000

     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 every week 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 Sub-bureau for Rapid
Service and Predictions at the U.S. Naval Observatory, Washington;  Bulletin B
and the Annual Report are issued by the Central Bureau, 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
Central Bureau from 1846 (x, y), 1962 (UT), and 1981 (dPsi, dEpsilon) to the
current date. All solutions are available electronically (see below).


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 International 
Celestial Reference Frame (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 will share this task:  the VLBI Coordinating Center (VCC) 
and the Celestial System Section of the IERS/CB (CSS). 

 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. Theorientation of its 
axes is consistent with that of the BIH System at 1984.0within +/- 3 
milliarcseconds. 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).

2. IERS constants and models.
    The IERS Conventions (McCarthy, 1996) are a set of constants and models
used by the IERS Analysis Centers for Very Long Baseline Interferometry (VLBI),
Global Positioning System (GPS), satellite radiopositioning (DORIS), Lunar and
Satellite Laser Ranging (LLR, SLR), and by the Central Bureau and Sub-bureau 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.


     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 

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.

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.

UT2 is defined from UT1 by adding the following conventional annual and 
semiannual terms:

     UT2-UT1 = 0.0220sin(2*3.141593*t) - 0.0120cos(2*3.141593*t)
             - 0.0060sin(4*3.141593*t) + 0.0070cos(4*3.141593*t),

the unit is the second and t is the date in Besselian years given by t =
2000.000 + (MJD - 51544.03) / 365.2422.

     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,
i.e., the 41 components with periods under 35 days.  In absolute value UT1R-UT1
is smaller than 2.5ms, 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 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.

2.  Analyses (operational and refined) by the IERS Analysis Centers.  The
operational results are transmitted weekly in parallel to the Sub-bureau for
Rapid Service and Predictions to contribute to Bulletin A and to the Central
Bureau to contribute to Bulletin B.  The refined results are transmitted yearly
to the Central Bureau.

3.  General adjustment of ICRF, ITRF and EOP by the Central Bureau, based on
the refined results.  This adjustment, described in the 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 Annual Report.

4.  Determination of EOP by the Sub-bureau for Rapid Service and Predictions 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 Central Bureau 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 now
computed as soon as new contributions are available from the various analysis

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    !    4.0         14.         0.3
                   40d    !   14.3         69.         0.3
                   90d    !   26.5        169.         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    !    2.0         10.0        0.3
                   10d    !    5.0         20.0        0.3
                   30d    !   11.0         77.0        0.3
(1) Based on data since 1998; applies only to latest epoch in each update.
(2) Based on data since 1995.
(3) Based on data since 1997.


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; Feissel and Lewandowski, 1984). Since December 1995, a
mixed series of UT-UTC based on LOD/UT estimates of GPS and calibrated by VLBI
solutions is routinely computed and used for near real time determination from
the last currently available UT1 VLBI estimate.

Section 3:  Five-day normal values of x, y, UT1-UTC, dPsi, dEpsilon, and their
uncertainties, based on a combination of the series of section 6.

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 1999.  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
                        !                    formal uncertainties
                        !  sampling         based on 1998-99 data
        Series          !   time        terr.pole        UT     LOD  celest.pole
                        !                 0.001"           0.0001s       0.001"
EOP(IAA)     98 R 01    !      7d          0.12        0.05               0.07
EOP(USNO)    98 R 10    !      7d          0.12        0.08     0.23      0.10
EOP(USNO)    98 R 11    !   1-3 d                      0.15
EOP(OPA)     98 R 01    !   1-3 d                      0.17
EOP(SPBU)    99 R 01    !   1-3 d                      0.16
EOP(UTXMO)   96 M 01    ! 0.1-30d                      2.46
EOP(CGS)     97 L 02    !      3d          0.23        0.39 *
EOP(CSR)     95 L 01    !      3d          0.38        0.30 *
EOP(DUT)     98 L 01    !      3d          0.14        0.27 *
EOP(IAA)     98 L 02    !      1d          0.04        0.03 *   0.03 *
EOP(MCC)     97 L 01    !      3d          0.07                 0.11 *
EOP(CODE)    98 P 01    !      1d          0.01        0.03 *   0.02 *
EOP(EMR)     96 P 03    !      1d          0.06        0.22 *   0.10 *
EOP(ESOC)    96 P 01    !      1d          0.05        0.08 *   0.06 *
EOP(GFZ)     96 P 02    !      1d          0.02        0.02 *   0.03 *
EOP(JPL)     96 P 03    !      1d          0.04        0.11 *   0.11 *
EOP(NOAA)    96 P 01    !      1d          0.03        0.06 *   0.13 *
EOP(SIO)     96 P 01    !      1d          0.06        0.37 *   0.18 *
EOP(IGS)     95 P 02    !      1d          0.04        0.15 *   0.10 *
EOP(IGS)     96 P 02    !      1d          0.09        0.34 *   0.15 *
* 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.


Sub-bureau for Rapid Service and Prediction, at U.S. Naval Observatory:


        By 0h UTC of Tuesday and Friday of each week via e-mail distribution:
                - e-mail (contact:
                - World Wide Web (           
                - Anonymous ftp ( or

        By about 17:10h UTC daily via anonymous ftp:
                - World Wide Web (           
                - Anonymous ftp ( or

Central Bureau, at Paris Observatory:

                - e-mail (contact:
                - World Wide Web (
                - Anonymous ftp ( or


        Updated at the beginning of each month
                - World Wide Web        
                - Anonymous ftp (directory iers/bul/bulb)
                - airmail

Permanent EOP series
                - World Wide Web
                - Anonymous ftp (directory iers/eop)

IERS Gazettes
                - World Wide Web
                - Anonymous ftp (directory iers/info)

Jim Ray                          Daniel Gambis
Head, IERS Sub-bureau for        Director
Rapid Service and Prediction     Central Bureau of IERS         


AAM          Atmospheric Angular Momentum
BIH          Bureau International de l'Heure
BIPM         Bureau International des Poids et Mesures
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 
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
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          Paris Observatory
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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