INTERNATIONAL EARTH ROTATION SERVICE (IERS) SERVICE INTERNATIONAL DE LA ROTATION TERRESTRE 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).EXPLANATORY SUPPLEMENT TO IERS BULLETINS A AND BThe 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 withTHE IERS CONVENTIONS

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 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. 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 EARTH ORIENTATION PARAMETERSThe 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 centers. 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 --------------------------!------------------------------------ Notes. (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.THE DATA ANALYSIS(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 ----------------------------------------------CONTENTS OF BULLETINS A AND B.BULLETIN ABULLETIN 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 assessment. ----------------------------------------------------------------------------- ! 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.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 (maia.usno.navy.mil or 192.5.41.22) By about 17:10h UTC daily via anonymous ftp: - World Wide Web (http://maia.usno.navy.mil/) - Anonymous ftp (maia.usno.navy.mil or 192.5.41.22)DISTRIBUTION OF THE PUBLICATIONSSub-bureau for Rapid Service and Prediction, at U.S. Naval Observatory: BULLETIN ACentral Bureau, at Paris Observatory:- e-mail (contact: iers@obspm.fr) - World Wide Web (http://hpiers.obspm.fr/) - Anonymous ftp (hpiers.obspm.fr or 145.238.100.28)BULLETIN BUpdated at the beginning of each month - World Wide Web - Anonymous ftp (directory iers/bul/bulb) - airmailPermanent 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 jimr@maia.usno.navy.mil daniel.gambis@obspm.frGLOSSARYAAM 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 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 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 InterferometryREFERENCESAoki, 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. Ma, C. and Feissel, M., 1997, Definition and realization of the International Celestial Reference System by VLBI Astrometry of Extragalactic Objects, IERS Technical Note 23, Observatoire de Paris. Ma C., Arias, E.F., Eubanks, T.M., Fey, A.L., Gontier, A.M. , Jacobs, C.S., Sovers, O.J., Archinal, B.A., Charlot, P., 1998, The International Celestial Reference Frame as realized by Very Long Baseline Interferometry, Astron. J., 116, 516. 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. van Leeuwen, F., Lindgren, L., and Mignard, F., 1997: The Hipparcos and Tycho Catalogues, Volume 3, Construction of the Hipparcos Catalogue, ESA Publications Division, Noordwijk, The Netherlands. Vondrak, J., 1977: Bull. of the Astron. Inst. of Czechoslovakia, 28, 84.