Knowledge of thermosphere mass density and wind is essential for awide range of applications, including the development of thermosphere models and advancing the understanding of thermosphere–ionosphere coupling and solar–terrestrial physics. It is also widely used in space operations, such as mission planning, fuel budget estimation, reentry prediction, and collision risk assessment. Thermosphere mass density and wind can be obtained in situ from accelerometer measurements onboard Low Earth Orbit (LEO) satellites combined with precise GNSS positioning. Since the beginning of the 21st century, numerous LEO satellites equipped with accelerometers have been launched, providing several invaluable mass density and wind datasets. This dissertation focuses on two accelerometer-carrying LEO missions: the Gravity Field and Steady-StateOcean Circulation Explorer (GOCE), which was part of ESA’s Living Planet Program, and the Gravity Recovery and Climate Experiment Follow-On (GRACE-FO), a joint initiative between NASA and DLR.

The accuracy of the accelerometer-derived thermosphere mass density and wind datasets is coupled with uncertainties in the aerodynamic and radiation pressure modelling, where the latter plays a major role, especially during the periods of low solar activity. This dissertation aims to advance the radiation pressure models for GRACE-FO and GOCE. This is achieved by using satellites’ high-fidelity geometries supplemented by the thermo-optical properties of the surface materials. These thermo-optical properties are first redefined and fine-tuned using numerical optimisation, satellite photos and synergy with other missions. Finally, the augmented satellite models are analysed using the ray-tracing technique, which additionally accounts for self-shadowing and multiple reflections, to derive the force coefficients.

For Earth-orbiting satellites, the thermal radiation pressure accounts for one-fifth of the total cross-track radiation pressure acceleration. This research utilises the thermal model based on the concept of thermal inertia, in which the satellite heats up by absorbing incoming radiation and cools down by emitting radiation. This process was implemented using thermal model control parameters such as the internal heat generation from batteries and onboard electronics, heat capacity of the panels, conductance towards the satellite’s inner parts, and efficiency of the solar panels. Moreover, this research leverages in-situ measurements from onboard thermistors, which provide additional insights for selecting realistic thermal model control parameters.

The goal of this dissertation was to improve the accelerometer-derived thermosphere mass density and wind datasets of the GRACE-FO and GOCE satellites by advancing the modelling of radiation pressure and satellite thermal emission. The newly produced datasets were then compared with the previously available products and models. Additionally, the impact of introducing various modelling approaches was assessed and quantified.

Current accelerometer-derived thermosphere mass density and wind data are provided without comprehensive uncertainty information. This information is particularly important for data assimilation and for comparing thermosphere products obtained by different measurement techniques. This dissertation builds on the recently developed thermosphere density error propagation method and extends it to propagate errors in wind data. In this research, a sensitivity analysis was performed to assess the impact of uncertainties arising from measurement noise, radiation pressure, relative velocity, and aerodynamics on the GRACE-B satellite thermospheremass density and wind data. The objective of this study was to explore the potential of the propagation tool to augment the existing density and crosswind datasets with uncertainty information.
... Read more

As climate change continues to drive an increase in extreme weather events, the need for reliable predictions and observational data has never been more critical. Oceans, playing a vital role in regulating the Earth’s climate, are central to understanding these changes. Specifically, accurately modeling ocean wave dynamics, i.e., how waves are generated, evolve, and interact with oceanic processes such as currents, helps track ocean circulation and predict future variations. Thanks to Earth Observation satellites, continuous global observations have been available over the past few decades. Satellite altimeters, active sensors utilizing radar’s ranging capabilities, have emerged as pioneers in space oceanography. These instruments measure critical geophysical parameters such as sea surface height, significant wave height and near-surface wind speed along satellite tracks. Recognizing the immense value of these measurements for climate studies and operational activities, continuous technological innovations are essential for optimizing the performance and use of these instruments. One major breakthrough in satellite altimetrywas the incorporation of Synthetic Aperture Radar (SAR) technology in 2010, which enhanced spatial resolution from around 7 km, provided by Low-ResolutionMode sensors, to about 300m. In 2017, the fully-focused coherent processing of pulse echoes was implemented, a concept widely used in SAR imaging, enabling meter-scale resolution. This improvement led to significant benefits in near-coast applications, improving the quality of geophysical parameters by reducing signal contamination from surrounding land features. Additionally, for the first time, offnadir signals, previously considered a nuisance, were exploited to map narrow inland water bodies and detect sea-ice leads and floes. Recognizing this imaging potential led to investigating its capabilities over open oceans. Existing challenges in conventional, or unfocused, SAR altimetry relate to the accuracy of significant wave height estimates, especially when long waves, known as swells, dominate the sea surface. Swell waves, with wavelengths exceeding 150 meters, are often too long to be fully captured within the SAR altimeter’s footprint, leading to noisy, multi-peaked waveforms. Recognizing the interference of swell signals in SAR responses, combined with the high-resolution data provided by fully-focused processing, this dissertation first investigated the feasibility of transforming what was previously considered a nuisance into valuable information about the sea surface. To achieve this, the identification of swell-induced power variations in off-nadir altimeter’s signals, representing the so-called trailing edge of the returned echo, was first confirmed. These patterns were analyzed to compute a fully-focused modulation spectrumderived from altimetry. The modulation spectrumis a commonly used Level-2 product provided by satellites designed to measure the wave field, such as Sentinel-1 and CFOSAT, and allows for the estimation of swell characteristics, including wavelength, direction and wave height. The proposed method involved normalizing the signal intensity and re-projecting the range bins to cross-track ground locations, followed by spectral analysis akin to side-looking SAR systems. The analysis revealed that fully-focused altimetrymodulation spectra display power in all four quadrants due to the inherent 180-degree SAR ambiguity, plus two additional ambiguities caused by inseparable signals received from both sides of the radar footprint. The study also identified the main modulation mechanisms, using as reference numerical and analytical models. Range bunching was found to be a dominant mechanism alongside velocity bunching, with their relative strength highly dependent on the wave propagation angle. Fully-focused altimetry modulation spectra, derived from Cryosat-2, were evaluated through comparisons with buoy-derived directional wave spectra, showing good agreement. Furthermore, applying the proposed technique to Sentinel-6A data demonstrated that exploiting its full-beamfootprint, which is partially truncated onboard for data volume efficiency, improves swell retrieval. This is particularly true for waves propagating in or near the cross-track direction, due to its extended observational window and higher resolution compared to the operational truncated data. Yet, the development of a method to invert modulation-derived spectra to real ocean wave spectra is necessary to reliably use these instruments as a new source for providing operational global swell observations. The dissertation further explored the limitations of SAR altimeters in ocean wave imaging, focusing on resolution loss. This was addressed by estimating the azimuth cutoff wavelength, which serves as a proxy for the shortest detectable waves across different sea states and wave directions. The method used to estimate this parameter involved a SAR imaging technique applied in the spatial domain, minimizing residuals between the along-track autocorrelation function of fully-focused SAR radargrams, representing successive waveforms, and a fitted Gaussian function. Sentinel-6A data were then used to evaluate the method’s performance through comparisons with model-derived values globally. The analysis revealed that the method performs well under the majority of sea states but tends to underestimate values in extreme wind wave conditions. Furthermore, sensitivity to swell presence was observed, leading to pronounced over estimations, with the magnitude of these errors influenced by the swell direction. To mitigate these errors, an alternative approach was developed in the wave number domain. Results revealed an improvement in the correlation between azimuth cutoff estimates and model-derived values by 10%. Given the strong relationship between resolution loss and sea state conditions, the azimuth cutoff was further used to derive a new sea-state parameter: the variance of wave orbital velocities. Wave orbital velocity statistics offer valuable insights into wave climate by isolating wave components associated with developing seas. Comparisons between modeled and estimated wave orbital velocity variances showed similar sensitivities to swell presence and high sea states, suggesting further refinement of the proposed methods. Despite these challenges, the ability to extract these two additional parameters from the radar signal is valuable for identifying sensors capabilities and providing a new geophysical parameter for oceanographic studies. Lastly, the dissertation assessed the impact of wave-current interactions on wave products derived from both models and satellites, focusing on the Agulhas Current region, one of the most dynamic ocean environments. In situ wave measurements, collected during the One Ocean Expedition in 2023, in which the author participated, served as a reference for this study. The study first examined ocean current products. A clear underestimation of surface current velocities exceeding 0.5 m/s was found for both the Mercator operational model and the altimetry-derived Globcurrent product, with Mercator showing greater variability. Next, wave products, both with and without these ocean current products included in their modeling, were validated. The ECMWF reanalysis v5, known as ERA5, consistently underestimated wave heights above 2.5 m, whereas the MFWAM, which is the Global Ocean Wave Analysis and Forecast system from Meteo-France, showed good agreement with in situ data. This discrepancy was attributed to the lack of ocean current forcing in ERA5, underscoring the need for refinement in areas dominated by currents. Customized MFWAM simulations, including and excluding current data, further supported this finding. MFWAM forced with Globcurrent aligned most closely with drifter measurements, outperforming the operational product that uses Mercator currents. Comparisons between satellite altimeter observations and drifters also showed good agreement in significant wave height, with clear evidence of current-induced wave height variations along satellite tracks. Additionally, a multi-mission analysis of swellinduced modulation spectra from Sentinel-1, CFOSAT and SAR altimeters demonstrated alignment with in situ data and between them, highlighting the potential for synergistic use of these instruments in operational oceanography and climate studies. ... Read more

Since Galileo Galilei’s first discovery of natural satellites orbiting around other planets, observing and reconstructing their dynamics has been at the core of our efforts to understand and characterise these distant worlds. Far from following perfect, frozen in time Keplerian orbits, the dynamics of these satellites keep evolving, with tides as driving mechanism. The dissipation of energy in natural bodies due to their visco-elastic response to tidal forcing both heats up the moons’ interiors and causes their orbits to expand or shrink, as well as to become more circular or elliptical. Refining the moons’ ephemerides (i.e., tabulated solutions of their motion as a function of time) is thus key to studying not only their present-day dynamics, but also the long-termthermal-orbital evolution of planetary systems.... ... Read more

The polarization of light has become a powerful tool for scientists in recent years. Astronomy, climatology, chemistry, and medicine are just a few sectors that are turning to this characteristic of light to produce a finer description of the environment they study.
However, the complete measurement of polarization at different wavelengths remains difficult. The difficulties are even more significant when the polarization measurement occurs in space. Current technologies are bulky, mainly featuring rotating components that can bring additional risk to the space mission. In this context, the present research discusses the development of a new method for measuring light polarization compatible with use in space.
Starting from ideas already presented in the specialized literature, we have refined here a new way to access the polarization of light that promises the construction of compact, robust, and highly accurate instruments. The present research provides a detailed theoretical description of this new method's operating principle and a practical demonstration. The results confirmed the ability to translate this method into high-performing instruments capable of accessing any polarization.
In addition, this research also highlights the versatility of the new method for measuring light polarization. It can be translated into instruments intended for use in space or for other applications; it can be adapted to determine only certain types of polarization, or it can be the basis for building imaging instruments. It opens a new horizon of development in polarimetry and spectropolarimetry. ... Read more

The dynamics of rendezvous of spacecraft in circular orbits is a problem that is well-understood and that is regularly taught in orbital mechanics majors, as are the techniques of linearization and the applications of the state transition matrix. The design of strategies for rendezvous and formation flying often makes use of standard building blocks in the form of specific manoeuvres and trajectories. Typical rendezvous manoeuvres include the Hohmann transfer and the radial hop, a manoeuvre that can change the along-track separation but that does not change the semi-major axis. Typical rendezvous and formation flying trajectories include drift orbits, safe orbits and hold points on V-bar. In this study, the Hohmann transfer, the radial hop and the useful relative trajectories were generalized to elliptical orbits, and this has enabled the application of the insights gained from circular orbit rendezvous and formation flying to elliptical orbits. The insights gained from the theoretical developments have been applied to the mission analysis for Proba-3, a formation flying mission in a highly elliptical orbit. ... Read more

The first mission proposals to visit the Alpha Centauri system use photon-sail acceleration as a mode of propulsion to reach this stellar system closest to our own Solar System. To prepare for a future mission, the photon-sail dynamics in the system is investigated. Planar Lyapunov orbits around the colinear classical Lagrange points are designed to explore the Alpha Centauri system. This has been done before in other systems like the Earth-moon and Sun-Earth systems, but not yet in an elliptical binary star system. Starting with an initial guess in the circular restricted three-body problem without photon-sail acceleration, a Multiple Shooting Differential Correction (MSDC) algorithm changes the trajectory to a periodic orbit. A continuation method increases the eccentricity to match e = 0.5208, which is the eccentricity of the inner binary system of Alpha Centauri. The lightness number of the photon sail is increased to add photon-sail acceleration to the model up to a defined maximum of ?ZNe = 2. A set of five constant steering laws is chosen to investigate its effect. Next to that, the moment at which the periodic orbit starts in terms of the true anomaly is varied as well. This results in a set of 40 families of periodic orbits with increasing lightness numbers. Depending on the orientation, the augmented Lyapunov orbit either shrinks into smaller orbits or expands into larger orbits when increasing the lightness number. If the orbit shrinks, it can either converge into an artificial equilibrium point or the photon-radiation pressure on the sail can become minimal. In that case, the Lyapunov orbit becomes (almost) independent on the lightness number and reaches ?ZNe = 2. If the orbit expands, the maximum velocity will eventually go to infinity. At this vertical asymptote, the maximum lightness number is found. The initial true anomaly of Alpha Centauri 0 has a great effect on the Lyapunov orbits around L2 and L3 in the classical ER3BP. For 0 = 0, the orbit either converges to an AEP or the maximum velocity goes to infinity. For 0 = , a few orientations can reach ?ZNe = 2. To further explore Alpha Centauri, an adaptive differential evolution algorithm is used to design trajectories between the Lyapunov orbits. The performance of the algorithm is expressed as the Euclidean difference between the states at the end of the departure leg and the beginning of the arrival leg. Three different lightness number of ? = 0.1, 0.5 and 2 are used for these trajectories. With a lightness number of 0.1, the dimensionless Euclidean error is in the range of 1E-1 to 1E-3, depending on the Lyapunov orbits. With this lightness number, the stars are also used as a gravity assist. For larger lightness numbers, the Euclidean error becomes negligible in the range 1E-7. With a lightness number of 2, the time of flight during the trajectory is significantly lower. In future research, this can be further decreased using an MSDC algorithm. ... Read more

To be able to understand and predict space weather better, global in-situ satellite measurements are needed at an altitude of 100-150 km. However, due to the high density, space weather induced winds and the lack of data, it is a challenge to design a satellite mission to that region. Therefore, in this thesis, the influence of horizontal wind on satellite aerodynamics in the lower thermosphere is analyzed. The impact of the choice in satellite geometry design and orbital parameters on the satellite's aerodynamics and wind sensitivity is tested. To model satellite aerodynamics in this high-density region, a method of using the Stochastic PArallel Rarefied-gas Time-accurate Analyzer, SPARTA, is proposed. It is proven that the worst-case horizontal wind can have a negligible influence on the satellite’s drag coefficient depending on its attitude and satellite geometry. Based on the obtained conclusions, recommendations are given to simplify the design process and reduce aerodynamic drag. ... Read more

Many contemporary interplanetary missions use efficient low-thrust engines to reach the far corners of our Solar System. Their trajectories, however, have proven to be complicated to optimise due to the non-impulsive manoeuvres involved in low-thrust spaceflight. Even though shaping methods have been used extensively to reduce the computational burden, multiple-gravity assists and the presence of constraints create significant computational hurdles. Reducing the number of fitness evaluations during optimisation is one way of speeding up the search and can be done by `pruning away' regions of infeasible trajectories. In this research, we approach this by applying clustering, an unsupervised machine learning approach, to single leg trajectory optimisation problems based on a hodographic shaping trajectory model in combination with restricted two-body dynamics. Through clustering, groups of promising trajectories can be isolated so that unwanted regions can be discarded. Earth --- Mars, Earth --- Venus, and Earth --- 9P/Tempel 1 trajectories are used as test cases and are shown to exhibit periodic behaviour (related to the synodic periods of the departure and target bodies), which enables clustering on grid search-generated datasets. Different clustering algorithms were compared using the Silhouette, Davies-Bouldin, and Calinski-Harabasz internal validation indices. However, traditional clustering algorithms such as (H)DBSCAN, OPTICS, KMeans, and Gaussian Mixture Models, failed to robustly provide clusterings that can be used for pruning, because of the oblong shape of the clusters and the absence of data/noise density differences due to the artificial nature of the problem. Instead, a multimodality-based clustering model called SkinnyDip was found to be much more promising for this task. This algorithm comes with the additional advantage of having very few hyperparameters, eliminating the need for extensive parameter tuning. ... Read more

The gravity assist (GA) plays an important role in space missions since itwas first applied by the Luna 3 vehicle in 1959. For preliminary trajectory design, the so-called patchedconics model provides a simple model for a gravity assist. This approach, based on twobody formulations, splits amulti-body probleminto a succession of two-body problems. This model has a fundamental assumption: the trajectory of the spacecraft is driven by one celestial body only. A boundary for switching the driving bodies is defined by the Sphere of Influence (SoI) of the GA body. The patched conics model cannot be used to study low-energy trajectories. Moreover, it fails to describe special dynamics existing in the multi-body regime, such as the invariant manifolds. The three-body formulation is a logical choice to study the dynamics in the multi-body problem. In order to reduce its inherent difficulty, the circular restricted three-body problem (CR3BP) formulation is developed to study the behavior of the motion of a particle influenced by two massive bodies simultaneously. Flybys in the CR3BP have been studied by many researchers, using a numerical or semi-analytical approach, e.g. the Flyby map (FM) and Keplerianmap (KM), respectively. Inspired by these approaches and the idea of artificial intelligence, this thesis focuses on the investigation of flybys froma machine-learning perspective. ... Read more

In the field of radio astronomy the Ultra-Long Wavelength (f < 30MHz) band is unique, it has been studied for nearly a century yet it is still almost completely uncharted. Due to the reflective properties of Earth's ionosphere this band is nearly completely inaccessible for ground-based observatories, which can only receive frequencies down to 8 MHz in ideal conditions. This band may only be explored from space, but the creation of space based observatoriesis problematic due to the large telescope sizes associated with these frequencies. The only viable solution to space-based radio observation is the use of radio interferometry, a process which combines measurements of multipleradio receivers to act as a single instrument with a larger aperture. Plans to establish these constellations have existed for some time, but have always been too expensive. With the rise of micro-satellites the concept of a radiointerferometry constellation has become economically viable. The OLFAR mission is one among the many recent radio interferometry concepts, which stands out among its peers through the application of a swarm design philosophy.The most important instrumental property of a radio interferometer is the number of available instrument pairs (baselines), and the distribution of their relative orientation in uvw space. To facilitate radio interferometry at 10MHz and lower frequencies the OLFAR swarm requires orbits which offer relative velocities below 1 m/s, while also being sufficiently stable to keep the baseline between its members below a maximum of 100 km. A minimal separationof 500 meters between satellites is used for collision safety. Early concepts for the OLFAR mission made use of Lunar orbits, where the Moon would act as a radiation shield against Earth's interference. Previous studies have shown thatsuch orbits expose the swarm to unacceptably large relative velocities, which is why alternative deployment locations are still being studied. This thesis studies the applicability of swarm orbit designs around the fourth Lagrangianpoint, denoted as L4. This point offers very promising orbital properties, but it has not been studied in detail due to the swarm's exposure to interference. As a basis for this work it is assumed that the 9dB of interference mightbe worked around through the dynamic range of hardware and longer integration times, making L4-centric orbits aviable deployment location.Particular attention is paid to developing an accurate numerical model for a perturbed orbital environment, which is necessary to provide long-term orbits of good quality. The inclusion of perturbations in this model is based on theirmaximum demonstrable effect on baselines and overall satellite positions over a year in orbit. After establishing the numerical simulation environment the satellite swarm design problem around L4 is posed as an optimisation problemfor heuristic algorithms. Based on small-scale experiments the efficiency of different algorithms and architectures is evaluated, and the best-suited solution is used to optimise swarm designs for the OLFAR missions. The resultingmethod of choice maximises the potential of multi-threading, using 32 connected differential evolution algorithms with 48 population members to perform as a single algorithm with a population of 1536 individuals.Using this method satellite swarm designs and orbits of up to 35 elements are found, which demonstrably meet all interferometry-related mission requirements for over a year in orbit while only relying on passive formation flight. These swarm designs rely on a process which is described as swarm folding to achieve this result. By distributing the swarm as a column mirrored in the barycentric z direction with uniform velocities, the swarm initially folds overitself. This folding motion is periodically repeated throughout the designed orbit after the initial fold as a result of the swarms natural orbits. The folding motion greatly enhanced long-term cohesion of the swarm, and it creates avery dynamic baseline distribution pattern for radio interferometry. It is demonstrated that these orbit designs have near-ideal baseline distributions, wherein they are only limited by the natural limitations of the Lunar orbital plane.Though the application of swarm folding these designs remain compact for long time periods, even in a perturbed environment. The demonstrated designs have feasible mission lifetimes up to 3 years in-orbit, requiring only a fewmanoeuvres during this time to enforce the 500 meter separation for collision safety. Through the application of active formation control it is expected that this can be extended to 5 years in-orbit with the right swarm orbit design. Itis also expected that using the methods in this thesis larger swarm designs might also be found, potentially ranging into 50 satellites. The largest hindrance to further extending the size of the swarm is the risk of near-collision events,which are inherent to the folding motion. ... Read more

The number of Moon missions is increasing, but to facilitate these missions, an improved approach for lunar navigation is required. The proposed strategy is to deploy a constellation in Polar or Equatorial Medium Earth Orbits (PECMEO) based on GNSS technology, which can be utilised as universal lunar navigation system: the Lunar Positioning System (LPS). The system will provide navigation for lunar missions on a more autonomous base and without limitations on the number of receivers. However, for the nine LPS navigation satellites to provide accurate positioning for lunar objects, it is crucial that the positions of the satellites themselves are estimated with high accuracy. This is because the ephemeris error of navigation satellites is a dominant factor in the position error of lunar objects. This thesis investigates the achievable position estimation accuracy for the LPS. The estimation is based on observations provided by GNSS constellations, but also by precise inter-satellite ranging within the constellation to enhance the position knowledge of the satellites significantly. This research focuses on the feasible position estimation of the LPS satellites and, specifically, what improvements can be achieved by including ISL. In order to correctly perform the estimation for the ISL systems, modified least-squares algorithms have been developed suitable for observations linked to multiple satellites and the simultaneous position estimation. The average 3-dimensional position error is 7.84 cm if no ISL is used. When ISL is added to the estimation, the average 3-dimensional position error reduces to 1.80 cm. Therefore, it is shown that significant improvements can be achieved for the positioning of the LPS satellites by using ISL. With the achieved accuracy and the prospected improvements in the solution, LPS is shown to be a viable system for lunar navigation. ... Read more

Many low earth satellites use GNSS for orbit determination, both for operational purposes and post-facto scientific orbits. Using GNSS systems for Lunar navigation proves difficult, however, due to the signals being weak and having limited coverage. This thesis investigates, through numerical simulation, the usage of a constellation of Polar or Equatorial Medium Earth Orbit (PECMEO) satellites for Lunar navigation. The system is proposed as alternative for orbit determination using ground-based observations, making navigation of the spacecraft more autonomous, without limitations on the number of receivers. A navigation constellation of 9 satellites at an orbital radius of 14 000 km is designed by minimizing the Geometric Dilution of Precision (GDOP). The resulting Pareto front of optimum solutions consists of two distinct groups of constellations. Firstly, constellation with lower mean GDOP and higher maximum GDOP have two orbits approximately orthogonal to the ecliptic plane, while the third orbit is in the ecliptic plane. Meanwhile, a second group of solutions with slightly higher mean GDOP, but significantly lower maximum GDOP are found for constellation with one plane approximately orthogonal to the ecliptic plane, while the remaining two planes have an inclination of near 45 degrees. One of the latter group of solutions has been chosen for the system for its lower maximum GDOP. The effect of omitting observations passing through the ionosphere is investigated to determine if there is a need to obtain observations at multiple frequencies to allow for ionospheric corrections. Omitting those observations results in a slightly increased GDOP right before and after a satellite passes behind Earth, of up to 56.4%. However, only 13.9% of the samples are effected, making the overall effect rather small. The positioning accuracy of the system is assessed by simulating pseudorange and carrier phase observations, and solving these with point position and kinematic least squares methods. Two main error contributions on the solutions are caused by observation noise and navigation satellite ephemerides errors. A solution for observations without ephemeris error, a mean 3-dimensional position error of 3.3m is obtained. A solution for noiseless observations yields a mean 3-dimensional position error of 26.8m. Finally, the solution obtained from observations with all simulated errors results in a mean 3-dimensional position error of 27.0m, to which the ephemeris errors are thus the main contributors, despite optimistic assumptions on their magnitude. The observed accuracy shows that the system is a viable navigation method for Lunar spacecraft. ... Read more

Many successful landings have been performed on celestial bodies such as Mars, the Moon, Venus and others. All of these had in common that they were designed such that they had to land in regions, which were supposedly free of any hazards or that a certain level of risk was accepted. However, while rocks and other geological features are nightmares of any landing engineer they are the dream targets of scientists. Therefore, currently landing-site selection is a trade-off between the scientists’ wishes and the engineers’ fears. To bring the engineering capabilities closer to what the scientists desire, landing capabilities need to be advanced. Therefore, this work tries to answer the research question: Are autonomous safe landings in hazardous and potentially unknown environments possible? which lead to the following two sub-questions: 1. How can a landing vehicle autonomously assess the safety of a potentially unknown and unmapped landing site? 2. Howcan a landing vehicle ensure a safe touch downavoiding autonomously detected hazards? ... Read more

The Laser Interferometer Space Antenna (LISA) is a European mission for the detection of gravitational waves in space set to be launched in 2034. The mission will see the deployment of 3 spacecraft in heliocentric orbit keeping a triangular formation with side length of 2.5 million km. Laser beams are exchanged between the spacecraft by means of suitably mounted telescopes (2 per spacecraft), with the objective of synthesizing a very-large baseline interferometer. The interferometric measurements are taken between free-floating test-masses placed inside the spacecraft. Due to the nature of the scientific objectives, the mission requirements on spacecraft-spacecraft pointing precision are exceptionally strict. Moreover, the formation needs to operate in almost perfect free-fall, therefore the solar radiation pressure needs to be continuously compensated for by the on-board thrusters. Gravitational wave signals are measured in the frequency bandwidth of 20 μHz to 1 Hz, requiring the vibrations in that domain to also be eliminated both for the attitude and the displacement. The task is made possible by the gravitational reference system, a complex device that keeps the test-masses from touching the walls of the spacecraft by applying on the latter an external force through μNewton thrusters. This mode of operation is called Drag-Free and Attitude Control System (DFACS). In this thesis we attempt to study and design a DFACS for LISA using a technique called Quantitative Feedback Theory (QFT). The design process starts from the definition of the orbits, the goal orientation of the spacecraft, the sizing of the solar radiation pressure induced disturbances and the derivation of the dynamics of the 19 degrees of freedom to be controlled. Using QFT, the design process is carried out on the DFACS using separation of the dynamics. As a result, analytical equations for the calculation of the LISA commands are derived and the methods to design a control system compliant to the scientific requirements imposed on the sensitivity are shown. ... Read more

A detailed modeling of non-gravitational forces is an area that has been left unexplored in the past for orbit modeling of Magellan, primarily due to imperfect knowledge about its attitude and physical properties (optical properties and drag coefficient). As a result, past orbit analyses have restricted the non-gravitational force modeling by implementing a simple cannon-ball model for Magellan. Given that the Venusian surroundings can have significant impact on the spacecraft trajectory, the main objective of this project is to develop a reasonable macro-model for Magellan and investigate its impact on the modeled non-gravitational forces, and consequently on the orbit solution of the spacecraft. Presence of good non-gravitational force models can improve the dynamic orbit modeling of spacecraft, that can eventually enhance the gravity recovery capability of a mission. ... Read more

In multi-stage launch vehicles, the adjacent stage re-enters the Earth atmosphere. Once the stage is re-entering the atmosphere, the low-melting-point materials melt and the inner parts will be exposed to excessive heat. With heat increasing, the parts will break into debris. Some of the fragments will burn in the atmosphere, others will crash into the Earth’s surface. Since there is a lack of knowledge about the re-entering physics, Demise Observation Capsule (DOC) project is started. The DOC collects information about the re-entry progress of launch vehicle stage, where it is mounted on. Several launchers and stages are considered to investigate the effect of the location on the DOC’s configuration. In an ideal situation, the DOC can be placed on all stages/parts of launch vehicles and at the same time it is optimized for that location. A generalized design concept that fits on several locations and stages, will reduce the cost and effort. Therefore, the thesis objective is to determine whether it is feasible to have a generalized DOC or an optimized DOC per location and load case. Additionally, the current DOC is designed for four locations and the second objective of this thesis is to optimize the current DOC and shift its center of gravity to the nose of the DOC. For this reason, the main research questions are as follows: Is it feasible to generalize the DOC concept design so that it will fit on every ESA launcher and stage and meet the predefined requirements and will withstand environmental loads while general operability of the DOC is guaranteed? How can the center of gravity be shifted on the DOC to its nose under the current designed loads such that the DOC can be made more stable? ... Read more

Even though several orbit visualization tools exist, the ability to compare multiple satellite orbits or manipulate time are not readily available with the current tools. Furthermore, most other visualization tools require additional effort or installations to work properly. To address these issues with other tools an interactive, web-based, near-Earth orbit visualization tool was developed. This was done to answer the question: What are the uses of another interactive, web-based, near-Earth orbit visualization tool? After the development of the tool was completed, several use cases were examined. These use cases included visualizing constellations and space debris, reproducing the Iridium 33 and Cosmos 2251 collision, visualizing special orbits for educational purposes, and the reproduction of an aurora event which involved SWARM and EPOP satellites. The tool provides its users with a wide range of functionality, while keeping it easy to use. The tool can be accessed on orbits.tudelft.nl and the source code can be found on https://gitlab.com/SvenKardol/OrbitVisualization-Tool. ... Read more

This thesis aims to assess how different convex penalty functions can be used in orbit determination methods and to design an algorithm to test them under different conditions.
Most traditional Precise Orbit Determination (POD) algorithms use the Least-Squares (LSQ) method to minimise the misfit between a set of modelled and actual measurements. The approach followed in this research is to investigate other convex penalty functions for this purpose in order to achieve better results than the traditional LSQ method, while maintaining the overall quality and robustness of the former.
To simplify the application of convex optimisation methods, an external toolbox was used to implement the different convex cost functions. The testing environment included a Precise Orbit Propagator (POP), measurement generating and processing functions, the POD algorithm itself, and data processing functionalities for the representation of the results. All the different components integrated in the final algorithm were validated before their application.
Tests to assess different aspects of the implemented penalty functions were run, regarding both computational aspects and solution performance. The traditional method was observed to present suboptimal results when the noise present in the observations included unprocessed outliers. In addition, cases where the observations were highly sparse yielded a suboptimal estimation of the trajectory.
After that, the L1-Norm was implemented as penalty function, alongside with Huber’s penalty function, which represents a combination of both LSQ and L1-Norm.
The use of the L1-Norm in the orbit determination algorithm outperformed the traditional method in the cases where it lacked performance, such as in the presence of unprocessed outliers or sparse observation sets. However, in other tests run, the LSQ algorithm was able to reach higher accuracy levels than the L1-Norm. Huber’s penalty function, conversely, proved to be a great candidate for both purposes, closely resembling the results obtained by the best penalty function for each test and even improving it on occasions, at the cost of a higher computational effort.
Finally, the designed algorithms were applied to a real-world study-case making use of GOCE data provided by TU Delft. These applications demonstrated satisfactory performance for each of the methods that were implemented and provided an important validation of the work.
... Read more