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\write 0 {\noindent INTRODUCTION \noexpand\dotfill
\the\pageno\noexpand\break}
\noindent{\large INTRODUCTION }
\bigskip

     This document is intended to define the standard reference
system to be used by the International Earth Rotation Service
(IERS).  It is based on the Project MERIT Standards (Melbourne {\it
et al.}, 1983) and the IERS Standards (McCarthy, 1989; McCarthy, 1992)
with revisions being made to reflect improvements in models or
constants since the IERS Standards were published.  If contributors
to IERS do not fully comply with these guidelines,
they will carefully identify the exceptions.  In these cases, the
institution is obliged to provide an assessment of the effects of
the departures from the conventions so that its results can be
referred to the IERS Reference System.  Contributors may use models
equivalent to those specified herein.
Different observing methods have varying sensitivity to the adopted
standards and reference systems.  No attempt has been made in this
document to assess the sensitivity of each technique to the adopted
reference systems and standards.

     The recommended system of astronomical constants corresponds
closely to those of the previous IERS Standards with the exception
of the changes listed below.  The units of length, mass, and time
are in the International System of Units (SI) ({\it Le Syst\`eme
International d'Unit\'es (SI)}, 1991) as expressed by the
meter (m), kilogram (kg) and second (s).  The astronomical unit of
time is the day containing 86400 SI seconds.  The Julian century
contains 36525 days.

\write 0 {\indent Differences Between This Document and IERS Technical
Note 13 \noexpand\dotfill\the\pageno\noexpand\break}
\noindent{\bf Differences Between This Document and IERS Technical
Note 13}

\nobreak

     Most chapters of IERS Technical Note 13 have been revised, and
known typographical errors contained in that work have been
corrected in this edition.  There are some major differences
between the current work and the past IERS Standards.  The following
is a brief list of the major modifications by chapter.

\noindent{\bf CHAPTER 2  IERS Dynamical Reference Frame}

\nobreak

     The JPL DE 403 ephemeris (Standish {\it et al.}, 1995) replaces 
the DE 200 model of IERS Technical Note 13.

\noindent{\bf CHAPTER 3  IERS Terrestrial Reference System}

\nobreak

     The NUVEL NNR-1A Model (DeMets {\it et al.}, 1994) for plate
motion has replaced the Nuvel NNR-1 Model of IERS Technical Note
13.

\noindent{\bf CHAPTER 4  Numerical Standards}

\nobreak

     Numerical values are now given only for the most fundamental
constants along with their uncertainties and references.  Constants
which have been changed include the astronomical unit in seconds
and meters, precession, obliquity, equatorial radius, flattening
factor and dynamical form factor of the Earth, constant of
gravitation, geocentric and heliocentric gravitational constant.

\noindent{\bf CHAPTER 5  Transformation Between Celestial and
Terrestrial Reference Systems}

\nobreak

     An empirical model to be used to predict the difference in the
celestial pole coordinates between those published by the IERS and
those given by the IAU model is added.

\noindent{\bf CHAPTER 6  Geopotential}

\nobreak

     The JGM3 model replaces the GEM-T3.  Love Numbers are revised.

\noindent{\bf CHAPTER 7  Local Site Displacements}

\nobreak

     The printed table of the components of site displacement due
to ocean loading is no longer included.  References to machine-
readable files are given.  Love Numbers are revised. Atmospheric 
loading and postglacial rebound are included.

\noindent{\bf CHAPTER 8  Tidal Variations in UT1}

\nobreak

     The subdaily and daily tidal variations in Earth orientation
due to the effect of ocean tides has been added.

\noindent{\bf CHAPTER 12  General Relativistic Models for Propagation}

\nobreak

     The formulation has been modified to be consistent with 
IAU/IUGG resolutions.

\noindent{\bf Contributors}
\medskip
\halign{\kern1pc#\hfil&\kern1pc#\hfil&\kern1pc#\hfil&\kern1pc#\hfil\cr
Z. Altamimi     &  S. R. Dickman   &  R. Langley      &  J. R. Ray\cr
B. Archinal     &  R. Eanes        &  B. Luzum        &  R. Ray\cr
E. F. Arias     &  T. M. Eubanks   &  C. Ma           &  J. Ries\cr
S. Bettadpur    &  M. Feissel      &  P. M. Mathews   &  M. Rothacher\cr
K. Borkowski    &  H. Fliegel      &  D. N. Matsakis  &  H.-G. Scherneck\cr
C. Boucher      &  T. Fukushima    &  V. Mendes       &  B. E. Schutz\cr
P. Brosche      &  J. Gipson       &  A. E. Niell     &  P. Schwintzer\cr
M. Bur\v sa     &  R. Gross        &  R. Noomen       &  P. K. Seidelmann\cr
N. Capitaine    &  E. Groten       &  T. Otsubo       &  E. M. Standish\cr
M. S. Carter    &  T. A. Herring   &  S. Pagiatakis   &  J. A. Steppe\cr
P. Cook         &  G. H. Kaplan    &  E. C. Pavlis    &  T. vanDam\cr
V. Dehant       &  J. Kovalevsky   &  W. R. Peltier   &  J. Verheijen\cr}

\noindent{\bf Errata from Technical Note 13}

\noindent p. 9:  GPS Block II $dz = 0.9519 m$ vice $1.0229 m$.\hfil
\break
\noindent p. 18:  Table 3.1, first line, last value should be
$-0.0070$ vice $0.0070$.\hfil\break
\noindent p. 83:  Hobart Q1 EW tangential phase should be $160.4$
vice $-160.4$.\hfil\break
\noindent p. 89:  Malibu M2 EW tangential phase should be $-142.4$
vice $142.4$.\hfil\break
\noindent p. 98:  Pearblossom M2 radial phase should be $-24.14$
vice $24.1$.\hfil\break
\noindent p. 137:  paragraph 3, read \dots of the International
{\bf Astronomical} Union.\hfil\break
\noindent p. 137:  the formula reads
$$ds^2 = -c^2 d\tau^2$$
$$= -{\biggl( 1-{2U \over c^2} \biggr)} (dx^0)^2 + \biggl( 1+{2U
\over c^2} \biggr) [(dx^1)^2 + (dx^2)^2 + (dx^3)^2)].$$
(the minus sign was omitted in the second line)\hfil\break
\noindent p. 140, note 2:  the formulae giving TCB-TCG read
$$TCB-TCG = c^{-2}[\int _{t_0}^t (v_e^2/2 + U_{ext}(x_e)dt +
v_e.(x-x_e)].$$
$$TCB-TCG = L_C \times (JD-2443144.5) \times 86400 + c^{-2} v_e.
(x-x_e) + P.$$
(the ``." in the vector product $v_e.(x-x_e)$ was omitted)\hfil\break

\noindent{\bf References}

\nobreak

\item{}\kern-2pc
 DeMets, C., Gordon, R. G., Argus, D. F., and Stein, S., 1994,
     ``Effect of Recent Revisions to the Geomagnetic Reversal Time
     Scale on Estimates of Current Plate Motions," {\it Geophys. Res.
     Let.}, {\bf 21}, pp. 2191--2194.

\item{}\kern-2pc
Fricke, W., 1982, ``Determination of the Equinox and Equator of the
     FK5," {\it Astron. Astrophys.}, {\bf 107}, pp. L13--16.

\item{}\kern-2pc
{\it Le Syst\`eme International d'Unit\'es (SI)}, 1991, Bureau
     International des Poids et Mesures, S\`evres, France.

\item{}\kern-2pc
Melbourne, W., Anderle, R., Feissel, M., King, R., McCarthy, D.,
     Smith, D., Tapley, B., Vicente, R., 1983, Project MERIT
     Standards, U.S. Naval Observatory Circular No. 167.

\item{}\kern-2pc
McCarthy, D. D., 1989, IERS Standards, IERS Technical Note 3,
     Observatoire de Paris, Paris.

\item{}\kern-2pc
McCarthy, D. D., 1992, IERS Standards, IERS Technical Note 13,
     Observatoire de Paris, Paris.

\item{}\kern-2pc
Standish, E. M., Newhall, X X, Williams, J. G., and Folkner, W. F.,
     1995, ``JPL Planetary and Lunar Ephemerides, DE 403/LE 403,"
     JPL IOM 314.10-127, to be submitted to {\it Astron. Astrophys.}

\vfil\eject
