Improvement of the IAU 2000 precession model

The IAU 2000 precession consists of the IAU 1976 ecliptic precession (Lieske et al. 1977, A& A, 58, 1) and the precession part of the IAU 2000A equator adopted by IAU 2000 Resolution B1.6 ( Mathews et al. 2002, J. Geophys. Res., 107, B4, 10.1029/ 2001JB000390). In this paper we provide a range of new expressions as possible replacements for the IAU 2000 precession. The new expressions are based upon the so-called P03 solution of Capitaine et al. ( 2003b, A& A, 412, 567) for the equator and the ecliptic. In addition an improved model for the precession of the equator is discussed. This improved solution was obtained in exactly the same way as P03 but using a refined model for the contributions of the non-rigid Earth ( Mathews 2004, private communication) and revised integration constants for the precession rates resulting from fits to the most recent VLBI data. The paper reports on the procedure that was used for improving the P03 solution and on the comparisons of this solution with the MHB 2000, IAU 2000 and P03 solutions. It also discusses the choices for the solution to be put forward as a replacement for IAU 2000. We concluded that the existing VLBI data were insufficient to provide convincing evidence that the improved solutions would deliver better accuracy than the existing P03 solution, and we recommend retaining P03 as the replacement for IAU 2000. P03, which unlike the IAU 2000 precession is dynamically consistent, has the advantage of already having been used experimentally by a number of groups; the model is recalled in Tables 3-5. Due to the strong dependence of the precession expressions on the precession rates and of the precession in longitude ( or equivalently the celestial CIP X coordinate) on the J(2) rate model, we also provide a parameterized P04 solution for these quantities as functions of those parameters. The expressions include the quantities to be used in both the equinox-based and CIO-based (i.e. referred to the Celestial Intermediate Origin) transformations.