2006. Such a release of planet and time ephemerides enable us to go towards four-dimensional planetary ephemerides. Section 2 describes the INPOP procedure for the computation of the TT-TDB relation.
Since the first release, INPOP06, of the planetary ephemerides developed at Paris and Besan¸con Observatories, (Fienga et al. 2008, www.imcce.fr/inpop), several improvements have been conducted on the dynamical modeling of the INPOP ephemeris. The observation dataset has also been substantially increased, especially with the addition of ranging data from the ESA space missions Mars Express and Venus Express. The resulting new version of INPOP planetary ephemerides, INPOP08, is presented here with the description of its new features. The INPOP08 ephemeris has also been fitted to all available Lunar Laser Ranging data (Manche et al., 2007). One of the novelties present in INPOP08 is the addition, in the distribution of the ephemeris, of a time scale transformation TT-TDB that is coherent with the ephemeris. The basic idea is to provide to users positions and velocities of Solar System celestial objects, and also, time ephemerides relating the Terrestrial timescale, TT, and the time argument of INPOP, the so-called Barycentric Dynamical Time TDB, based on the definition adopted by the International Astronomical Union in
Section 3 is devoted to a brief account of the new constraints and modeling implemented in INPOP for the asteroid perturbations. As in INPOP06, 300 asteroids are included in the dynamical equations of INPOP08, and the remaining ones are modelized as a ring. With respect to INPOP06, the ring model and the asteroid selection have been improved, and the precise description of these advances are made in (Kuchynka et al, 2009).
An important part of the new observations used for the INPOP08 fit consists in the tracking data provided by the ESA space missions Mars Express and Venus Express (Morley 2006a, 2007a, 2007b). These datasets are the first radio ranging data provided by ESA, and their acquisition and reduction process is described in section 4. VEX observations bring a very important set of informations on Venus orbit. It is especially of interest since this orbit is much less perturbed by asteroids than Mars and therefore has greater potential for precise ephemeris motion, fundamental physics testing and reference frame establishment.
Send offprint requests to: A. Fienga, fienga@imcce.fr The section 5 deals with the INPOP fit obtained by comparisons between the dynamical modeling and the observations.

In this process, 34 asteroid masses were fitted against only 5 in INPOP06.

Comparisons are made between obtained asteroid masses and other published masses.

Values for the fitted Earth-Moon barycenter and Sun oblateness J2 are also given.

We have also fitted the AU and comparisons are provided with the latest determinations of DE414 (Standish 2006; Konopliv et al. 2006) and DE421 (Folkner 2008).

In addition, we have performed as well a second fit, where the AU is given the IERS Conventions 2003 value (IERS03), and the Solar GM is deduced.

On several occasions, for planned high precision observations, we had some enquiries about the real accuracy of the position or velocities given by the planetary ephemerides.

This is actually a difficult question to answer, but we have tried in section 6 to provide some estimates of these uncertainties, by comparison with INPOP06 and DE421.

The last section is devoted to the conclusions and perspectives.
2.

INPOP as a 4-D planetary ephemeris With INPOP08, we aim to produce planetary ephemerides as fully compatible as possible with the relativistic background recently adopted by the astronomical community and summarized by the IAU2000 and IAU2006 conventions (Soffel el al., 2003).

This leads to the production of an ephemeris in TDB, and to the construction of a TTTDB transformation.

The following subsections describe the various steps involved in this process and the impact for the ephemeris users.
2.1.

The TCB-TCG transformation Two reference systems are defined: a global one, the Barycentric Celestial Reference System (BCRS), covering the whole Solar System and a local one, the Geocentric Celestial Reference System (GCRS), which is physically suitable for the modeling of processes in the vicinity of the Earth.

BCRS is particularly useful when one wants to model the light propagation or motion of celestial objects in the Solar System.