Ashok Verma, doctorant à l’Institut UTINAM, a soutenu sa thèse :
"Improvement of planetary ephemerides using spacecraft navigation data and its application to fundamental physics"
le jeudi 19 septembre à 10 h, en salle de conférences de
l’Observatoire de Besançon.
Résumé :
The planetary ephemerides play a crucial role for spacecraft navigation, mission planning, reduction and analysis of the most precise astronomical observations. The construction of such ephemerides is highly constrained by the tracking observations, in particular range, of the space probes collected by the tracking stations on the Earth. The present planetary ephemerides (DE, INPOP, EPM) are mainly based on such observations. However, the data used by the planetary ephemerides are not the direct raw tracking data, but measurements deduced after the analysis
of raw data made by the space agencies and the access to such processed measurements remains difficult in terms of availability.
The goal of the thesis is to use archives of past and present space missions independently from the space agencies, and to provide data analysis tools for the improvement of the planetary ephemerides INPOP, as well as to use improved ephemerides to perform tests of physics such as general relativity, solar corona studies, etc.
The first part of the study deals with the analysis of the Mars Global Surveyor (MGS) tracking data as an academic case for understanding. The CNES orbit determination software GINS was used for such analysis. The tracking observations containing one-, two-, and threeway Doppler and two-way range are then used to reconstruct MGS orbit precisely and obtained results are consistent with those published in the literature. As a supplementary exploitation of MGS, we derived the solar corona model and estimated the average electron density along the line of sight separately for slow and fast wind regions. Estimated electron densities are
comparable with the one found in the literature. Fitting the planetary ephemerides including additional data that were corrected for the solar corona perturbations, noticeably improves the extrapolation capability of the planetary ephemerides and the estimation of the asteroid masses
(Verma et al., 2013a).
The second part of the thesis deals with the complete analysis of the MESSENGER tracking data. This analysis improved the Mercury ephemeris upto two order of magnitude compared to any latest ephemerides. Such high precision ephemerides, INPOP13a, are then used to perform general relativity tests of PPN-formalism. Our estimations of PPN parameters (β and γ) are the most stringent than previous results (Verma et al., 2013b).
