Estimation of Venus' atmospheric density through EnVision precise orbit determination

EnVision radio science investigation will deepen our understanding of Venus gravity, interior structure and atmospheric properties. To address these scientific questions, a two-way link communication (configuration X/X/Ka-band) is established with the ESA ESTRACK ground stations enabling precise orbit determination (POD) during the science phase. An accurate modeling of the spacecrafts dynamics, including the atmospheric drag acceleration, is key for retrieving EnVisions trajectory and constraining Venus gravity field, tides and orientation parameters.Dedicated radio occultation campaigns are designed to characterize electron density profiles in the ionosphere and atmospheric density, pressure and temperature in the mesosphere and upper troposphere of Venus. Furthermore, an accurate POD of the spacecraft also provides complementary information on the atmospheric density at the altitudes crossed by the probe, extending the science return of the EnVision mission.The atmospheric drag perturbation strongly affects spacecraft trajectories that are characterized by a pericenter altitude above Venus surface of less than 220 km. By accounting for different Venus atmospheric models, e.g., the Venus Climate Database (VCD) and the Venus Global Reference Atmospheric Model (Venus-GRAM), we investigate the impact of potential errors and uncertainties in the predicted atmospheric properties on the orbit evolution of the spacecraft. We note significant inconsistencies between Venus atmospheric models at the spacecraft altitudes including atmospheric density differences of more than 200%. These discrepancies may be representative of the current knowledge of Venus upper atmosphere and thermosphere. Thus, we carried out a perturbative analysis of the dynamical forces by introducing a mismodeling in the atmospheric density profiles. We assumed the VCD for the simulation of the radio tracking measurements and we included as a priori model in the estimation process the Venus-GRAM. By developing a batch sequential filter that adjusts a set of atmospheric density scale factors, we compensated for the mismodeling and improved the quality of the dynamical model and of the orbit determination. The proposed approach enables an estimation of the atmospheric density at the spacecraft altitudes with an accuracy of 25% and accuracies in the orbit reconstruction of 1-2 m, 30-40 m and 20-30 m in the radial, transverse and normal directions.