Dottorato in INGEGNERIA AERONAUTICA E SPAZIALE

Titolo della tesi: Analysis and Implementation of Nano-Satellite Navigation Systems

The increasing number of small satellites being launched imposes the need to develop new, reliable navigation systems for nano-satellite platforms, i.e. satellites below 10 kg of mass. A multiplicity of innovative techniques can be used to support the nano-satellite navigation in different orbital regimes, providing reliable but at an affordable cost that make their deployment possible by all operators, including Universities and small companies. Radio-frequency multilateration can support navigation for nano-satellites in a wide range of orbits provided that the transmission signals can be monitored. The Time-Difference of Arrival (TDOA) can be exploited for ensuring navigation if receivers are appropriately placed to provide the geometric diversity that is required for accurate orbit determination. The TDOA is based on a network of receiving radio-frequency sensors at ground receiving a beacon from the nano-satellite, without the need to decipher information being transmitted by the satellite. Every receiver assigns a precise timestamp to each sample received from the satellite. The triangulation of the times of arrival of each signal leads to the estimation of the trajectory. TDOA can provide positioning to satellite systems throughout their mission, from deployment to re-entry, and it is able to support Near-Space vehicles such as suborbital or stratospheric aircraft. In order to test the effectiveness of these tracking techniques, the STRAINS (Stratospheric Tracking Innovative Systems) Experiment being developed at Sapienza University of Rome is planned for launched in 2021 on a high altitude balloon from Esrange Space Center in Kiruna (Sweden). The experiment conceived in late 2018 for the HEMERA H2020 Balloon Launch infrastructure will provide a free launch opportunity from the programme, and it is supported by ASI (Italian Space Agency) and INAF (Italian National Institute for Astrophysics). This thesis reports the design for TDOA sensors network, with particular regards to the future application to the STRAINS Experiment test. Additionally, this work proposes the derivation of a performance parameter, the Minimum DOP (Minimum Dilution of Precision) able to provide conservative estimates of the TDOA sensors networks positioning accuracy for satellite tracking. The MDOP is used to present the positioning accuracy trend with the network baseline and number of stations and with the target altitude, before providing simulation examples of potential future networks in Italy and Europe. Tracking of Low Earth Orbit (LEO) satellite navigation can benefit from the implementation of LED (Light Emitting Diode) -based payloads through persistent satellite illumination. By optimizing the flashing patterns and by coordinating with a ground-based network of optical observatories and telescope stations, it is possible to estimate the satellite orbit and attitude by means of the optical data alone. While the persistent illumination systems can extend the observing interval to include the whole eclipse time, the flashing patterns optimization can significantly improve the quality and precision of the data collected from ground and of the resulting estimate of the satellite dynamic state. For this reason, the LEDSAT 1U CubeSat mission, currently under development at Sapienza University of Rome, is testing the effectiveness of a LED-based payload on-board a CubeSat for optical tracking and navigation. The satellite is participating in the ESA (European Space Agency) Fly Your Satellite! Programme and in the ASI IKUNS Programme. The mission was conceived by Sapienza University of Rome together with the University of Michigan. In the proposed research, the satellite design, the planned operations, the optimization of the patterns, the predictions for the system performances are included. The derivation of the flashing patterns for achieving both orbit and attitude determination is presented, along with the performance simulations on flashing pattern detection and tumbling rate measurement. LEDSAT will be launched in 2021, profiting from a free launch opportunity offered by the Fly Your Satellite! Programme, managed by ESA. Finally, the rising number of Near-Earth, Lunar and Deep Space nano-satellite missions such as the recent announcement of opportunities for Lunar secondary nano-satellite payloads on-board the new Artemis missions by NASA, or the first demonstration of a 6U CubeSat on a Deep Space mission (with the MarCo CubeSat data relay from Mars), might also benefit from affordable navigation systems and be able to serve a great number of similar missions. Furthermore, these navigation systems would not require traditional highly costly data and communication infrastructures managed by Space Agencies (such as the Deep Space Network), and would enable the involvement of smaller institutions, such as independent small companies or Universities, into these classes of missions. For this reason, the development of an autonomous radio-frequency navigation system for nano-satellites and CubeSats is of key importance for the future of space usage. The proposed system is based on the spaceborne implementation of a miniaturized size atomic clock which enables retrieval of precise measurement timing from the signal reception times, and on the reception of samples from ground stations located all over the Earth. While the transmission of the precise time can lead to an immediate estimation of the distance, and to straight forward trajectory estimation, the time difference estimation from multiple signals can lead to the achievement of spacecraft positioning when interfaced to Doppler data or to optical data (e.g. relative angles with the Sun, the Moon, the Earth). A Lunar CubeSat mission designed for deployment from the NASA demonstration missions to Moon orbit is an example use case for this type of tracking technique. An orbit determination filter for trajectory estimation will be proposed that utilizes various combinations of these measurement types, and it will be used to develop the concept of operations (tasking requirements) and to examine the expected performance for the range of different tracking geometries.