• The precession-nutation of the figure axis in space. It is specified through the conventional precession-nutation model (presently that recommended by the IAU 2000 General Assembly) and corrections determined by the VLBI observations : the celestial pole offsets (dy,de) or (dX,dY) according to the parameterisation which has been adopted. The precession-nutation model and its corrections (up to 1 mas / IAU 2000 model) define a fictitious axis, the Celestial Intermediate Pole or CIP.
  
  N.B.: The CIP was called before 2003 celestial ephemeris pole or CEP. This change of terminology corresponds to the non-ambiguous frequency splitting between diurnal/subdiurnal polar motion and nutation for this axis : by definition the precession-nutation of the CIP does not have spectral components of which the period is smaller than 2 days.
  
  
• The diurnal rotation around the celestial intermediate pole (this one remains close to the instantaneous rotation axis in a tolerance of 20 milliarcseconds) : W^N UT1. It can be decomposed into an angle uniformly varying with TAI, W^N TAI, (with the nominal Earth rotation rate W^N, see useful constants) and corrections showing variations of the Earth angular velocity. These corrections are given by the difference (UT1-UTC) or (UT1-TAI).
  
  
• The polar motion of the celestial intermediate pole with respect to the terrestrial crust. The CIP has for terrestrial coordinate (x,-y,1). As a first approximation, for periods greater than 10 days (x,-y,1) are also the coordinates of the instantaneous rotation axis. It is not possible to confuse both axes at diurnal time scale.
  
  See on the WEB page of Obseratoire Royal de Belgique, simulated movies of these Earth rotation irregularities
  
  The five Earth orientation parameters , derived from the observations, bring corrections to the uniform diurnal rotation and modelled precession-nutation :
  • The celestial pole offsets (dy,de) or (dX,dY) with a maximum temporal resolution of 2 days.
  • UT1-UTC or UT1-TAI (from which we can derive the variations of the length of the mean solar day, dLOD, with respect to its nominal value of 86400 s TAI)
  • The polar motion (x,y)

The Earth orientation is then obtained by inserting those parameters in the coordinate transformation between the Celestial Reference Frame and the Terrestrial Reference Frame :

• for the classical way : [TRF]=PN(dy,de) R₃(UT1-TAI) W(x,y) [CRF]
• for the "non rotating origin" formalism :
  
  [TRF]=PN(dX,dY,s) R₃(UT1-TAI) W(s',x,y) [CRF] (s are s' are practically independent from the EOP)
  

The description of this matrix can be found in the IERS conventions 2003, chapter 5. For more insight on the meaning of the EOP, read the following page.

We propose three kind of EOP series :

• the long term series as determined from the observations of a given astro-geodetic technique VLBI, LLR, SLR, GPS, DORIS) by various organisations all over the world.
• the operational series for a given technique, provided from 1999.
• the combined series by our Service.

• tables modelling the EOP variations (atmospheric, oceanic, lunisolar effects in different frequency band, etc..)
• useful constants of our field.