The Celestial Intermediate Pole and new Earth orientation parameters

Preliminary proposals

Discussion extracted from Newsletter 5 of the IAU Working group T5

Nicole Capitaine

Observatoire de Paris, 17 March 2000

This document is devoted to the preliminaryconclusions concerning the discussion considered within the IAU T5 WG on the future of the Celestial Ephemeris Pole and new parameters for Earth orientation in the ICRS. It includes the following sections:

1. Conclusions of the discussion concerning a modern definition of the CEP and proposal

A summary of the main points of the comparison between two possible options for a modern definition of the CEP and preliminary proposal is given below.

Option (1)

Mathews's proposal (Journees 1998) was to define the CEP by keeping the symmetry in the frequency band between terrestrial motion and celestial motion in extending the definition outside the current frequency interval; the proposed procedure for its realization is to estimate simultaneously, in processing the observations, the current celestial pole offsets and current polar motion as well as the high frequency signal in a symmetric way in the frequency domain in the CRS and TRS. In fact, the essential content of Mathews's proposal at the Journees 1998 was the method for estimation for the higher frequency components, whether in the CRS or TRS. The suggestion for distribution of the different frequency components between the CRS and TRS has now be modified such as the conventional model would imply that the series proposed for $\delta \psi$ and $\delta \epsilon$ would be modified to include also terms with negative n : $n=-1$ for the prograde diurnal nutations as well as the long period ocean tide terms (assuming that the latter can be reasonably well modelled); $n=-2$ for the prograde semidiurnal nutations and for the prograde diurnal ocean tide terms; $n=-3$ for the prograde semidiurnal ocean tide terms; $n=1$ for the retrograde semidiurnal ocean tide terms. All these are, of course, in addition to the long period nutations comprised under $n=0$, and like the latter, will be fixed be determined from theory.

In any case, I am of the firm opinion that the diurnal and semidiurnal nutations (in space) which are indeed predictable, should be part of the conventional model. It would be illogical not to include them.

Option (2)

The CEP is defined by separating the motion into the retrograde diurnal component of the motion wrt the TRS, which is considered as being long periodic motion in the CRS (periods larger than 2 days mainly due to the effect of the external torque on the Earth) and the motion wrt the TRS consisting of the whole complementary part of the motion; the proposed procedure for the realization of the CEP is to use the best model for precession nutation (periods longer than 2 days) in the estimation of the EOP (estimating the celestial pole offsets if possible) and then to extract the high frequency signal (or rather corrections to an empirical model for this signal) from the pole coordinates only.

Both choices (1)and (2) correspond to a clear concept not dependent on further improvements in the model. Both of them extend the current definition in order that the current definition is an approximation of the new one and has minimal impact on users. Both can be realizable by a model as accurately as possible.

The advantage of (1) is its clear mathematical basis.

The advantage of (2) is its clear relation with a deterministic approach.

The preliminary proposal presented in the Newsletter 3, which appears to be the "preferred" option, corresponds to the choice (2).

Concerning its realization, precession and nutation are specified by the best available model including the components with periods larger than 2 days and the high frequency signal in the TRS is specified by a model including the tidal variations in polar motion and the terms corresponding to the sub-diurnal nutations. The corrections to these models are estimated in two steps : the first step corresponds to the current procedure for estimating the EOP (including when it is possible the celestial pole offsets); the second step is the estimation of the high frequency motions from the currently estimated coordinates of the pole.

The reasons for this choice, which considers the whole high frequency motion in the TRS are that: - the prograde diurnal nutations (ampl in Deltapsi x sin eps and Delateps : 15 microarseconds) appear as long periodic polar motion which cannot be separated from the polar motion itself of much larger amplitude, - the semi-diurnal prograde nutations (ampl in Deltapsi x sin eps and Delateps : 15 microarseconds) appear as prograde diurnal variations in polar motion and cannot be separated from the much larger components (150 microarseconds) of the tidal variations of polar motion.

The proposed realization is such that it can be applied to all the techniques, whereas it is not the case of (1).


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2. Conclusions of the discussion concerning new parameters for Earth orientation and proposal

A comparison of the parameters considered above shows that Euler's angles (described in (iii)) or equivalently the celestial coordinates of the z-axis of the TRS (described in (viii)), which reduce to three the number of EOP, do not use any intermediate pole and consequently include both high frequency and low frequency components of the motion of the z-axis of the TRS wrt the CRS.

The other parameters separate the celestial components from the terrestrial components according to a ``frequency criteria'' using an intermediate pole. Such a procedure facilitates the estimation of the parameters from observations, unless the model for low frequency motion in space is perfect.

The difference between the use of a geometrical origin (as $\Sigma$ or $K$ for examples) and a kinematical origin ($\sigma$) is that, in the first case, the instantaneous rotation of the considered origin is included in the derived value for the Earth's angle rotation, whereas it is clearly separated when using directly $\sigma$ as the origin on the moving equator.


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3. Proposed recommendations for the IAU Colloquium 180

Recommendation concerning the Celestial Ephemeris Pole

IAU Colloquium 180 "Towards Models and Constants for Sub Microsecond Astrometry"
Noting the need for accurate definition of reference systems brought about by unprecedented observational precision, and
Recognizing that it is necessary to abandon the conceptual definition of the Celestial Ephemeris Pole as the pole that has no nearly diurnal motion,
Recommends
1. that the Celestial Ephemeris Pole be the pole of the equator the motion of which with respect to the International Celestial Reference System is produced by external gravitational forces acting on the Earth,
2. that at J2000.0 the direction of the Celestial Ephemeris Pole be coincident with the direction of the International Celestial Reference Pole at J2000,
3. that the direction of the Celestial Ephemeris Pole in the International Celestial Reference System at any epoch be defined in part by a nutation model containing only predictable components of nutation with periods greater than two days,
4. that the motion of the Celestial Ephemeris Pole in the International Celestial Reference System be speci ed by:
a precession/nutation formulation designated as the MHB000 model plus
b additional corrections provided by the International Earth Rotation Service through appropriate astrogeodetic observations plus
c the geodesic nutation and
5. that the motion of the Celestial Ephemeris Pole in the International Terrestrial Reference System be provided by the International Earth Rotation Service through appropriate astrogeodetic observations.

Recommendation concerning the Celestial Ephemeris Origin

IAU Colloquium 180 "Towards Models and Constants for Sub Microsecond Astrometry"
Recognizing the need for reference system definitions suitable for modern observational accuracy,
Recommends
1. that the Celestial Ephemeris Origin of the International Celestial Reference System be the non-rotating origin, and
 2. that the time scale UT1 be defined as being proportional to the Earth Rotation Angle defined by the angle measured along the equator between the direction of the International Terrestrial Reference Origin and the Celestial Ephemeris Origin.

Recommendation concerning the the transformation between Terrestrial and Celestial Reference Systems

IAU Colloquium 180 "Towards Models and Constants for Sub Microsecond Astrometry"
Noting the need for accurate definition of reference systems brought about by unprecedented observational precision,
Recommends
that the transformation between the Terrestrial and Celestial Reference Systems be specied by the position of the Celestial Ephemeris Pole in the Celestial Reference System, the position of the Celestial Ephemeris Pole in the Terrestrial reference System, and the Earth Rotation Angle defined by the angle measured along the equator between the direction of the Terrestrial Reference Origin and the Celestial Ephemeris Origin.

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