[IGSMAIL-1558] Explanatory Supplement 1997
IERS
IERS
Tue Mar 4 01:36:18 PST 1997
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IGS Electronic Mail Tue Mar 4 1:36:18 PST 1997 Message Number 1558
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Author: IERS / CB
Subject: Explanatory Supplement 1997
INTERNATIONAL EARTH ROTATION SERVICE (IERS)
SERVICE INTERNATIONAL DE LA ROTATION TERRESTRE
MARCH 1997
EXPLANATORY SUPPLEMENT TO IERS BULLETINS A AND B
There are significant changes in the 1997 Explanatory
Supplement which are advertized by the sign " NEW ". In addition,
users of the publications are informed that the Web and anonymous
adresses of the IERS Central Bureau are changing on March 15, 1997.
(see the end of this publication).
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 semi-weekly; 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
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 at the end).
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
uncertainty of the latter (+/- 80 milliarcseconds). The ICRS is
realized by estimates of the coordinates of a set of quasars: the
International Celestial Reference Frame (ICRF), see part II-2,
1995 IERS Annual Report. 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 milliarcseconds. 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).
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 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 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 related to the Greenwich mean sidereal time (GMST) by a
conventional relationship (Aoki et al., 1982). It gives access to
the direction of the 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. 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, which is broadcast with the time signals and announced in
Bulletin D. The changes in DUT1 are decided by the IERS.
UT2 can be derived 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 being the second and t being 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.
============================== NEW ===============================
IERS Earth orientation data are produced at daily intervals and
do not include the effects of semidiurnal and diurnal variations.
Users who require high accuracy information may want to interpolate
the published data and include the semidiurnal/diurnal variations.
Ray's model, adopted in the 1996 IERS Conventions has been coded. The
corresponding numerical program is available on request. More details
can be found in the IERS Gazette # 13.
===================================================================
3. dPsi and dEpsilon are the offsets in longitude and obliquity of
the celestial pole with respect to its position defined by the
conventional IAU precession/nutation theory. An a priori correction
model is available in the IERS Conventions (1996), (McCarthy, 1996).
============================== NEW ===============================
Starting from January 1, 1997, EOP series are expressed in the new
1997 system. A correction has been added to the EOP (x, y, UT1 and
dPsi) in order to make the Earth orientation data consistent with
the ITRF and ICRF (see IERS Gazette #8).
This correction is made in the various IERS solutions: Bulletin A,
Bulletin B, EOP(IERS) C01, C02, C03 and C04.
===================================================================
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.
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 (part II), 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 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 to three days 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
normal values at five-day intervals and smoothed solutions at
one-day and five-day intervals. This involves the application of
the 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 series is computed weekly
(Gambis, 1996a).
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) and Luzum et al. (1997) 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 on 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 0.0007" in terrestrial pole,
0.00012s in UT1, and 0.0005" in celestial pole.
-------------------------------------------------------------
Solutions ! terr.pole UT celest.pole
! 0.001" 0.0001s 0.001"
--------------------------!----------------------------------
Bulletin A daily (1) ! 0.4 1.2 0.3
prediction (2) 10d ! 3.9 15. 0.3
40d ! 12. 67. 0.3
90d ! 18. 21. 0.3
!
Bulletin B !
smoothed (1)1-d, 5-d ! 0.3 0.3 0.5
raw (1) 5-d ! 0.3 0.3 0.4
prediction (1) 10d ! 4.0 20.0 0.4
30d ! 9.0 40.0 0.9
Notes.
(1) Based on 1995-96 data.
(2) Based on data since 1993.
CONTENTS OF BULLETINS A AND B.
BULLETIN A (semi-weekly)
General information including key definitions and the most
recently adopted values of DUT1 and TAI-UTC.
Plots of recent variations in UT1-UTC and polar motion.
Observed values of EOP contributed by participants in the IERS.
This includes the most recent VLBI, SLR, LLR and GPS data received
by the Sub-bureau for Rapid Service and Predictions.
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 to three days
between the date of publication and the last available date with
estimated EOP.
Predictions of x, y, UT1-UTC daily up to ninety days following the
last day of data in Section 4 and predictions at monthly intervals
up to a year in advance.
Observations of celestial pole offsets, smoothed daily values, and
predictions 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 2d 3d 5d starting with B97
----------------------------------------------
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 1996, a mixed series of
UT1-UTC based on GPS and calibrated by VLBI solutions is routinely
computed and integrated in the combination (Gambis, 1996b). This
series is also used for near real time determination from the last
currently available 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 1997. 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.
-----------------------------------------------------------------------------
! sampling formal uncertainties
based on 1995-96 data
Series ! time terr.pole UT celest.pole
! 0.001" 0.0001s 0.001"
------------------------!----------------------------------------------------
EOP(JPL) 96 R 01 ! 0.1-13d 1.1
EOP(IAA)* 96 R 03 ! 7d 0.1 0.05 0.2
EOP(IAA)* 96 R 04 ! 1-3 d 0.2
EOP(USNO) 96 R 09 ! 7d 0.1 0.06 0.2
EOP(USNO) 96 R 10 ! 1-3 d 0.2
EOP(UTXMO) 96 M 01 ! 0.1-30d 2.3
EOP(CSR) 95 L 01 ! 3d 0.3 0.4
EOP(DUT) 93 L 03 ! 3d 0.1 0.1
EOP(IAA) 96 L 02 ! 1d 0.2 0.5
EOP(MCC) 96 L 01 ! 3d 0.1
EOP(CODE) 96 P 03 ! 1d 0.1
EOP(EMR) 96 P 03 ! 1d 0.1
EOP(ESOC) 96 P 01 ! 1d 0.1
EOP(GFZ) 96 P 02 ! 1d 0.1
EOP(JPL) 96 P 03 ! 1d 0.1
EOP(NOAA) 96 P 01 ! 1d 0.1
EOP(SIO) 96 P 01 ! 1d 0.1
* Series interrupted in the end of 1996
DISTRIBUTION OF THE PUBLICATIONS
Sub-bureau for Rapid Service and Prediction, at U.S. Naval Observatory:
----------------------------------------------------------------------
BULLETIN A
By 0h UTC of Tuesday and Friday of each week:
- e-mail (contact: ser7 at maia.usno.navy.mil)
- NEOS Bulletin Board (202 762 1570)
- World Wide Web (http://maia.usno.navy.mil/)
- Anonymous ftp (maia.usno.navy.mil or 192.5.41.22)
Central Bureau, at Paris Observatory:
------------------------------------
Adresses UNTIL March 15, 1997
- e-mail (contact: iers at obspm.fr)
- World Wide Web (ftp://hpvlbi.obspm.fr/iers/ierscb.html)
- Anonymous ftp (hpvlbi.obspm.fr or 145.238.100.7)
NEW adresses from March 15, 1997
- e-mail (contact: iers at obspm.fr)
- World Wide Web (http://hpiers.obspm.fr/)
- 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
IERS Gazettes
- World Wide Web
- Anonymous ftp (directory iers/info)
Permanent EOP series
- World Wide Web
- Anonymous ftp (directory iers/eop)
Dennis D. McCarthy Daniel Gambis
Head, IERS Sub-bureau for Head, Earth Rotation Section
Rapid Service and Prediction Central Bureau of IERS
dmc at maia.usno.navy.mil gambis at obspm.fr
GLOSSARY
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
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
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
NEOS National Earth Orientation Service
NOAA National Oceanic and Atmospheric Administration
NRCan Natural Resources Canada, formerly EMR
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
REFERENCES
Aoki, S.,Guinot, B., Kaplan, G.H., Kinoshita, H., McCarthy, D.D.,
Seidelmann, P.K., 1982: Astron. Astrophys.,105, 1.
Arias, F., Charlot, P., Feissel, M. and Lestrade, J.-F., 1995:
Astron. Astrophys., 303, 604.
Boucher, C., Altamimi, Z., Feissel, M., Sillard, P., 1996: IERS T.N. 20,
Observatoire de Paris.
CCIR, 1986: Recommendation and Reports of the CCIR, 16th Plenary Assembly
(Dubrovnik), Vol 7, p 12, International Telecommuniactions Union, Geneva.
Feissel, M. and Lewandowski, W., 1984: Bull. Geod., 58, 464.
Feissel M, Gambis D. and T. Vesperini,1988, The Earth's Rotation and Reference
Frame for Geodesy and Geodynamics, Babcock and Wilkins (eds), Reidel, 269.
Gambis D., 1996a, Proc. coll. IAU 165, Dynamics and astrometry of natural and
artificial celestial bodies Poznan, Poland, July 1996.
Gambis D., 1996b, Proc. International IGS Workshop, Silver Spring, USA, 61.
Luzum, B. J., McCarthy, D. D., and Kosek, W., 1997, to be published.
McCarthy, D.D. (ed.), 1996: IERS Conventions (1996), T.N. 21, Observatoire de Paris.
McCarthy, D.D. and Luzum, B.J., 1991a: Bull. Geod., 65, 22.
McCarthy, D.D. and Luzum, B.J., 1991b: Bull. Geod., 65, 18.
McCarthy, D.D.(ed.) 1996: IERS Conventions, IERS T.N. 21.
Seidelmann, P.K., 1982: Celest. Mech., 27, 79.
Vondrak, J., 1977: Bull. of the Astron. Inst. of Czechoslovakia, 28, 84.
[Mailed From: iers at hpvlbi.obspm.fr]
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