THE P03 PRECESSION MODEL: PROPERTIES
For more details, see the original P03 paper: Capitaine et al., 2003, A&A 412, 567 .
The P03 precession provides new expressions for precession consistent with the IAU 2000A precession-nutation model.
The precession of the ecliptic has been derived from the analytical theory VSOP87 fitted to the JPL ephemerides DE406
for improving the polynomial terms in the expression for the component with respect to a fixed ecliptic of the Earth-Moon
barycenter (EMB) orbital angular momentum (i.e. the angular momentum of the orbital motion of the EMB around the Sun).
It uses the value for the mean obliquity of the ecliptic at J2000
as derived from a fit of the dynamical theory for the Moon to LLR observations.
The equinox offset in the GCRS has been derived from this fit based on VLBI Earth Orientation Parameters.
The model for the precession of the equator has been obtained by solving the dynamical precession equations based
both on the most recent expressions for the theoretical contributions to precession and on the MHB estimates of
the precession rates. The numerical values for the precession rates have taken proper account of some perturbing
effects on the observed quantities.The P03 paper provides solutions for the classical precession angles and for
the X, Y coordinates of the CIP in the GCRS which include precession, nutation and frame biases and therefore
tie the precession nutation directly to the ICRS by simply providing where the pole is in the sky. Their advantage
is of being close to the parameters that are actually observed by VLBI, which is the best way of determining the
precession-nutation motion and is not sensitive to an ecliptic. Note for example that the precession in longitude
that is derived from VLBI is in fact the projection of the X-coordinate of the CIP on a conventional ecliptic and
changing the obliquity of this conventional ecliptic will change the value in longitude, whereas the corresponding
value for X is independent of the ecliptic .