DFG Research Unit:
Space-Time Reference Systems for Monitoring Global Change and for Precise Navigation in Space

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Barycentric Ephemeris

Enrico Mai, Jürgen Müller
Institut für Erdmessung
Leibniz Universität Hannover

Motivation

Barycentric ephemerides represent dynamical realizations of the Barycentric Celestial Reference System (BCRS). The BCRS is fundamental not only for the problem of interplanetary spacecraft navigation but through its intimate relation to the Geocentric Celestial Reference System (GCRS) also for any high precision Geocentric Reference System and therefore modelling of global geodynamics. Within the DFG research unit FOR1503 on reference systems, this project will provide access to a new ephemeris.

Objective

We want to lay the foundation of a new (German) ephemeris of the Solar System, to overcome structural as well as general deficits of other institutions' modern ephemeris. The focus will be on the organization of observational data and their storage, the improvement of the force model, respective software coding, and finally parameter estimation.

Regarding the force model, we focus on the lunar libration as well as the perturbation effects due to asteroids. In view of future processing and analysis, a data pool will be created to store all necessary data in a unique format, in order to realize a Barycentric Dynamical Reference System (BDRS). Eventually, improved estimates on fundamental parameters like planetary masses, initial states of the planets, or solar flattening, shall become available.

Methods

Theoretically one starts with the metric of the BCRS. For its materialisation, however, post-Newtonian equations of motion for solar-system ephemerides are used. As far as the gravitational interaction between mass-monopoles (point-masses) is concerned the usual relativistic (post-Newtonian) Einstein-Infeld-Hoffmann (EIH) equations are used. We add several higher order perturbation effects, e.g. figure-figure interactions. The resulting equations of motions of the multi-body problem are numerically integrated simultaneously. In an iterative manner, the initial conditions and selected parameters of the system are adjusted to planetary observations of various kind. The combined planetary and lunar ephemeris will be provided to the collaborative research unit in requested format(s).