Subject: [IGSMAIL-1558] Explanatory Supplement 1997
From: IERS / CB
Date: Tue, 4 Mar 1997 1:36:18 PST

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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:  [email-masked])
		- 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:  [email-masked])
  		- 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:  [email-masked])
  		- 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             
[email-masked]			[email-masked]		        

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.

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