Thesis title: Orbit Determination and Gravity Field Mapping for VERITAS mission to Venus
Among the planets of the Solar System, Venus most closely resembles Earth in terms of mass, size, and bulk density, which is why is often referred to as Earth’s “twin sister”. Despite these similarities, however, Venus and Earth have followed markedly different evolutionary paths, particularly with respect to surface temperatures, atmospheric composition and pressure, and rotational dynamics. In the coming decade, often described as the “decade of Venus”, a series of missions will investigate key questions about Venus’s atmospheric composition and evolution, the presence of past or present water, and the history of its volcanic and tectonic activity.
The Venus Emissivity, Radio Science, InSAR, Topography and Spectroscopy (VERITAS) mission, selected in 2021 within NASA Discovery program, is designed to address fundamental questions about Venus’s evolution, internal structure, and geological processes. Among its instruments, VERITAS carries a gravity science experiment that exploits two-way coherent radio links in X- and Ka- bands to enable precise orbit determination of the spacecraft, thereby allowing for accurate estimation of Venus’s gravity field and VERITAS’s trajectory.
The dissertation focuses on the analysis and simulation of VERITAS’s gravity science experiment with the goal of assessing the achievable accuracy in recovering Venus’s gravity field and key physical parameters. Different mission configurations, dynamical models and noise assumptions are explored to evaluate their impact on the recovered solution. High-resolution mapping of Venus’s gravity field will provide essential contributions to meeting the science objectives of the mission and to furthering our understanding of Venus. Thanks to VERITAS’s near-polar, near-circular, low altitude orbit (~220 km) and the state-of-the-art quality of Doppler tracking data, the mission will achieve a gravity map with substantially higher and more uniform spatial resolution with respect to Magellan. Numerical simulations show that the VERITAS gravity science experiment can robustly fulfill the scientific requirements, attaining a gravity field spatial resolution ranging from 85 to 120 km, with more than 90% of the planet mapped with a spatial resolution better than 106 km. In addition, VERITAS will retrieve crucial parameters related to Venus’s tidal response and rotational state, thereby improving our understanding of the planet’s interior structure.
The robustness of these results has also been confirmed under the presence of colored (i.e., correlated) noise, validating the assumption of white noise for VERITAS. A whitening procedure was developed to transform correlated Doppler noise into an equivalent white noise representation, ensuring unbiased parameter estimation within standard orbit determination filter. This approach is general and particularly valuable for future missions where colored noise sources, such as plasma induced red noise, dominate the Doppler error budget.