Orbit determination techniques for space missions to small bodies

Small bodies represent one of the most active research fields for planetary science and deep space exploration, as witnessed by the increasing number of missions towards these targets. In this framework, Rosetta represented a crucial step towards a higher understating of small bodies, being the first mission to successfully orbit a cometary target. The work described in this thesis focuses on the ephemeris reconstruction of comet 67P/Churyumov-Gerasimenko for the period between July 2014 and October 2016, through a complete reanalysis of the ranging and ΔDOR measurements collected by Rosetta during its proximity phase with the comet. Using as input the relative orbit trajectory reconstructed by ESOC flight-dynamics, the s/c radiometric observables were mapped to the comet nucleus and used to estimate the comet state and some key physical and observational parameters, most notably the non-gravitational acceleration acting on the comet nucleus due to surface outgassing. The main outcome of this work it thus represented by a continuous and accurate trajectory reconstruction for comet 67P/Churyumov-Gerasimenko, and an estimation of the non-gravitational acceleration acting on the comet during its active periods, which will serve as reference for more complex dynamical models of the evolution of the surface activity. To improve the quality of radiometric measurements, fluctuations of the tropospheric excess path length due to atmospheric components, which represent one of the most relevant contributors to Doppler noise, should be removed through accurate calibrations. A prototype of a Tropospheric Delay Calibration System (TDCS), using a 14 channel KA/V band microwave radiometer, has been developed under ESA contract and installed at the DS3 ESTRACK complex in Malargue on February 2019. An additional section of this work is therefore devoted to the performance characterization of this new instrument through the analysis of 44 tracking passes of the GAIA spacecraft, which was used as a testbed.