This study investigates the calibration of accelerometer data for the Next Generation Gravity Mission (NGGM), proposed by the European Space Agency. With improved precision, NGGM aims to continue gravity field observations beyond the Gravity Recovery and Climate Experiment Follow-On (GRACE-FO) mission. The mission consists of a satellite pair measuring Earth's gravity field using an onboard laser tracking instrument. To isolate the gravity field signature in these observations, each satellite carries accelerometers to estimate non-gravitational accelerations. This thesis supports the accelerometer calibration process by applying lessons from previous gravity field missions.

A historical review of gravity missions highlights the evolution of scientific and hardware requirements. The study examines accelerometer principles, sources of instrumental imperfections, and existing data calibration techniques. NGGM’s preliminary design includes multiple accelerometers placed away from the satellite’s center of mass, allowing the use of shaking manoeuvres—first introduced in the GOCE mission—for calibration.

A comprehensive model is developed that can generate shaking manoeuvres with varying thrust magnitudes, shaking durations, and shaking frequencies to excite the satellite. This model is used in conjunction with various accelerometer units (two, three, and four accelerometer layouts are considered) and their placement in the satellite's body frame to evaluate the calibration quality against the scientific requirements posed for the mission.


Results indicate that along-track accelerometer placement minimizes non-gravitational acceleration measurement errors due to enhanced centrifugal acceleration from the satellite’s pitch rate during calibration. Furthermore, the along-track placement performs better than radial placement, even though it has the same centrifugal acceleration boost. The suspected cause is the electrode layout of the accelerometer, which boosts the acceleration signal due to the projection of the angular acceleration about the y-axis onto the z component of the linear acceleration. The radial placement of the accelerometers provides no additional signal to the x component of the linear acceleration due to a lack of projection. Lower shaking frequencies improve calibration by accumulating higher angular rates over time. However, due to volume constraints imposed by the laser tracking instrument, cross-track placement may be more favourable. This configuration requires higher thrust levels, as the absence of a pitch rate signal on the cross-track axis worsens the signal-to-noise ratio of the observations, which warrants a revision of the thruster requirements and accelerometer performance. Moreover, more than two accelerometers reduce measurement errors by providing redundancy in the observations. Even with three accelerometers placed in the along-track direction, at least 24 hours of shaking at maximum thrust, as stated by the thruster requirement, is required for effective calibration. Lower thrust or shorter shaking durations would necessitate four accelerometers—two on the x-axis and two on the y-axis. Finally, the accelerometer pair’s arm length is treated as a free variable, as it has minimal impact on calibration performance.

This report provides foundational insight for future gravity missions. Smart accelerometer placement and shaking manoeuvre parameters can improve the measurement quality of the non-gravitational forces and subsequently improve gravity field recovery, which is crucial for tackling the climate crisis.

Given the potential of solar sails for applications around Earth, this paper investigates their performance in executing collision avoidance maneuvers. An extensive set of conjunctions (e.g., varying orbital regimes, sail control authorities, and collision geometries) is built. An "ideal" and a "real" case are examined, with the latter accounting for greater uncertainties (modeled via a covariance-mapping algorithm) and stricter collision probability requirements. The optimal control problem is derived and solved through direct collocation, seeking time optimality while constraining the probability. For all scenarios, maneuvers are completed within the one-day threshold, proving solar sails fit the current operational framework. Key findings are: 1) Better debris state knowledge (and later warnings) reduce maneuver time; 2) Higher altitudes increase the maneuver time, while greater sail control authority decreases it; 3) Eclipses influence results; 4) The Earth-Sun configuration only weakly affects the maneuver time; and 5) In-plane control is always non-zero. The locally optimal steering laws to change the semi-major axis and eccentricity best approximate the optimized maneuvers: especially for longer maneuver times, these straightforward laws comply with collision probability requirements.

Recent years have seen an increased need for a lunar navigation service to support the large volume of missions planned for the coming decade. Supplying lunar navigation from the Earth suffers from limitations deriving from the long distance to the Moon. Thus, it must rely on a constellation of lunar navigation satellites. This thesis addresses the Orbit Determination (OD) of the satellites of a Lunar Navigation System (LNS) using range and Doppler observations from terrestrial stations. It provides a recommendation for the values of the design parameters of the OD system of a LNS to achieve the required OD accuracy and evaluates the impact of non-design factors such as the constellation's geometry and the observational noise. The parameters of the orbit propagation and the length of the estimation arc are identified as drivers of the OD accuracy. The relation between OD errors and navigation performance is also assessed.

Ensuring the sustainability of future space missions and maintaining continuous access to space necessitates a serious and proactive approach to addressing the space debris dilemma. One crucial measure that can be undertaken is actively protecting against space debris by implementing satellite collision avoidance manoeuvres. This thesis focuses on researching robust methods to perform such manoeuvres, with a particular emphasis on the guidance and control systems of manoeuvrable vehicles. The Starlink constellation is used in this research as a reference scenario, considering its popularity and extensive development, although the developed system is applicable to any satellite. The core of achieving successful manoeuvres lies in the creation of a robust guidance system. This system integrates convex optimisation algorithms with reference tracking mechanisms, enabling the autonomous execution of collision avoidance manoeuvres to be both robust and fuel-efficient. The outcomes of this research represent a significant advancement in space debris mitigation, enhancing space sustainability.

Beam-powered thermal propulsion, utilizing an external energy beam to heat a propellant, is proposed as a promising alternative to traditional chemical and electrical propulsion systems. This research aims to contribute to this technology’s numerical and experimental development, offering a comprehensive analysis of the receiver-absorber cavity
(RAC) performance and its optimization.
The work begins with a detailed background study of the principles of beam-powered propulsion. This sets the stage for the subsequent chapters, which investigate the numerical modelling and experimental testing of the thruster.
The numerical part of this thesis focuses on developing accurate prediction tools for the total absorbed beam power by the RAC. This information is added to a previously
developed RAC performance prediction tool and the development of a new tool considering spatial information is started. The optimization process regarding efficiency is
presented with an example case study with optimal values above 80% with a margin to improve.
The experimental component involves the design, manufacturing, and testing of a prototype beam-powered thermal thruster. Leak testing, test bench calibration and a
cold flow test are conducted. Detailed test procedures and setups for these are presented, alongside their results. The experimental findings are compared with the numerical predictions to assess the accuracy and reliability of the models.
The thesis concludes with evaluating the research questions, and discussing the successes and challenges encountered. Recommendations for future work are provided,
aiming to guide further advancements in beam-powered thermal propulsion technology. The combination of numerical and experimental insights gained through this research contributes to the feasibility and optimization of beam-powered thermal propulsion systems.

The escalating problem of space debris necessitates effective solutions, such as active debris removal, to ensure sustainable orbital environments. Solar sails, with their nearly unlimited ΔV budget, present a promising, propellant-free method for targeting and potentially removing debris. However, the unique dynamics of solar sails prevent them from maintaining fixed positions relative to a target. This research explores the use of hold trajectories as alternatives to static hold points during far-range rendezvous operations. Using InTrance, a trajectory optimisation tool based on neuroevolution, the study assesses how operational constraints impact hold trajectories and the viability of preliminary homing trajectories. Results indicate that hold trajectories allow safe, extended solar sail operations close to debris, supporting sustained observation and monitoring efforts for debris management.