Dottorato in INGEGNERIA AERONAUTICA E SPAZIALE

Titolo della tesi: Orbit Determination and Time Synchronization for a Lunar Radio Navigation System

Interest in lunar exploration has significantly increased in recent years due to the potential for a permanent human presence on the Moon and its value as a testing ground for deep space exploration technologies. In this scenario, the European Space Agency’s Moonlight concept proposes deploying a Lunar Communication and Navigation Service (LCNS) constellation of 4-5 small satellites in Elliptical Lunar Frozen Orbits (ELFOs). This system would offer Position, Navigation, and Timing (PNT) services to cis-lunar platforms and lunar surface users. During phases 0 and A of the Moonlight project, the ATLAS consortium proposed a Lunar Radio Navigation System (LRNS) architecture. This system includes ground support and tracking via a network of small parabolic antennas (approximately 26 cm diameter) operating at K-band (22-27 GHz). The antennas can track multiple satellites simultaneously using Multiple Spacecraft Per Aperture (MSPA) tracking, enabled by Spread Spectrum (SS) modulation with unique codes for each satellite. Onboard transponders establish two-way coherent links to the ground, enabling precise Doppler and ranging measurements with chip rates of 20-25 Mcps. MSPA, combined with SS modulation, facilitates Same Beam Interferometry (SBI), which uses a single ground station to differentiate phase measurements of two satellites, reducing common-mode noise errors and providing accurate relative position measurements to complement Doppler and range. My work focused on orbit determination and time synchronization simulations for the LRNS system to evaluate performance and define navigation message accuracy requirements. I analyzed the constellation’s performance across different scenarios, assessing the effect of orbital maneuvers and the improvements from the additional SBI data compared to Doppler and range measurements alone. Additionally, I considered different media calibration systems at ground stations, such as Global Navigation Satellite System (GNSS) calibration and Water Vapor Radiometers (WVRs) for tropospheric and ionospheric correction, evaluating their effect on the satellite constellation positioning accuracy. A key parameter for the architecture is the Signal-In-Space Error (SISE), primarily related to the ephemerides reconstruction of the satellite constellation and clock desynchronization error. The evolution of SISE as a function of the Age Of Data (AOD) determines navigation message accuracy over time, dictating message validity and imposing constraints on the update frequency. Thus, realistic dynamical mismodeling was included in the orbit determination simulation. The LRNS constellation enables positioning at the Moon’s South Pole, but it can be exploited for navigation of secondary users if the satellites are in view. In the second part of the thesis, I focused on assessing the performance of autonomous orbit determination performed during a lunar transfer orbit using both GNSS and LRNS data. This approach significantly reduces mission costs by eliminating the need for ground station support and alleviates associated time and technical constraints, thus paving the way for autonomous guidance of satellites toward the Moon.