Reconstruction of Rosetta's relative orbit around comet Churyumov-Gerasimenko using radiometric measurements

The Rosetta mission escorted the comet 67P Churyumov-Gerasimenko while it passed close to the Sun, to study the behaviour of these objects. The dynamical environment of 67P is complex and dictated by the comet itself. Indeed, the main force felt by the probe is the gravity of the small body, which is highly irregular. Another phenomenon the S/C has to go through is the coma drag, i.e. the acceleration induced on the probe by the interaction between its surface and the atmosphere generated by the comet’s outgassing, which becomes more active as it gets closer to the Sun. Radiometric measurements of the mission have been used to configure a model aimed at performing radioscience analyses with the data collected by the Rosetta mission. After processing the mission’s raw measurements, an observational model was created and tested using a passthrough process. Such procedure allowed us to understand the major sources of errors in the generation of computed observables, namely the steerable high gain antenna, the on-board transponder bias and the ground station biases. Consequently, the dynamical model has been built modelling the gravity sources involved, the solar radiation pressure and the manoeuvres. The contribution of the coma drag has been accounted for by using stochastic accelerations, defined as 2-hour batches in which they assume constant value. Finally, the setup has been tested by performing estimations on Rosetta’s state, its orbital and desaturation manoeuvres and the location of the phase centre of its high gain antenna. The results obtained by these estimations have been compared with the ones provided by Rosetta’s navigation team. A mission arc going from September 22 to 28, 2014 was selected, in which Rosetta is already bound to the comet, but at the same time, it does not get too close to the celestial body. The estimations proved the reliability of the implemented models while highlighting the limits of using only the radiometric measurements.