The number of space objects has largely increased in the recent years, becoming a considerable threat to the present and future of satellite operations and human spaceflight. This introduces a need to accurately compute the risk of collision between satellites and space debris. Current methodologies are limited by the type of encounter and the vehicle shape, usually simplified by a sphere. These assumptions rely mainly on linearization of the dynamics and simplification of the uncertainty distribution as a Gaussian function. Developing a collision probability calculation method that overcomes these limitations and provides high-accuracy results is the topic of this work. The methodology developed follows a hybrid combination of trajectory propagation and statistical methods for uncertainty propagation. With the propagation method covariance information of the satellite position and velocity will be available. Finally, the method is adapted to any satellite geometry by developing a numerical quadrature technique based on a combination of spheres to model the real body shape. The combination of these methodologies to compute the collision probability has been verified with a set of test cases by means of a Monte Carlo simulation and compared to alternative algorithms. It is found that the method improves the accuracy in calculating the collision probability by more than 70% with respect to conventional alternatives. Regarding real-life scenarios, the Cosmos-2251/Iridium-33 and the Chang Zheng-4C/Cosmos-2004 encounters are simulated. In both cases, the time of encounter and collision risk have been correctly estimated with the developed method. Finally, in view of the recent destruction of satellite Cosmos-1408 as a result of an anti-satellite missile test, a screening of its resulting debris threatening the International Space Station (ISS) is performed. During a two-week period, ten close encounters were detected, one of which posed a significant risk to the ISS. When applying the method considering the real shape of the ISS, the collision probability decreases by two orders of magnitude, proving the significance of this approach to correctly estimate the risk and guide space operations.

Autonomously landing a spacecraft on the surface of a planetary body with a degree of precision in the order of meters is highly challenging. Over the course of time, the landing ellipse, defined as the region with a 99% likelihood of where a space vehicle will land, has improved steadily but currently still has dimensions in the order of kilometers. The first and single Martian spacecraft that has performed a guided atmospheric entry and utilized precision landing technologies is the Mars Science Laboratory (MSL). The MSL probe and its focal point, the Curiosity rover, has thus been the most advanced mission yet to have flown to Mars. Nonetheless, space missions to the Red Planet have thus-far never landed following fuel optimal paths. Similarly, dispersions for landing on Titan with current technologies expand to hundreds of kilometers. The only reference mission to Titan is the Huygens probe, which has not utilized precision landing technologies nor optimal path planning. Besides, the goal of the Cassini-Huygens mission was to maximize descent time to augment scientific data retrieval of Titan's atmosphere. As part of the NASA Space Exploration Technology Directorate, a parafoil is proposed for landing on Titan due to its cost effectiveness, ease of deployment, relatively low mass compared to the prospective payload and capabilities of precise autonomous delivery. While considering all phases of Entry, Descent and Landing (EDL) and all elements of Guidance, Navigation and Control (GNC), the central focus of the research was put on the (powered and parafoil) terminal descent phase. The research core concerns a convex optimization programming approach to guarantee soft-landing. The algorithm has been verified based on the extensive Mars powered descent guidance literature. As part of the research conducted at NASA/JPL/Caltech, the algorithm has been extended to become compatible with landing a parafoil on Saturn's moon Titan. Throughout the discussion a distinction is made between lossless and successive convexification for optimal guidance. Both types have been simulated to either compute fuel optimal paths for powered Mars landing or pull-power optimal paths for parafoil Titan landing. The soft-landing is guaranteed while adhering to imposed mission constraints. By using the full capability of the spacecraft unprecedented precision may be achieved. This will enable engineers and scientists to reach the most alluring places on planetary bodies, thereby providing humanity a deeper understanding of the Universe.

Uncertainty in ice sheet modelling affects centennial and longer time-scale projections of the Greenland Ice Sheet's (GrIS) sea level contribution. One source of this uncertainty is the interaction between the ice sheet’s evolution and the Earth’s deformation in response to changes in the applied surface loading as the ice sheet waxes and wanes. Such deformation, Glacial Isostatic Adjustment (GIA), causes a vertical displacement of the ice sheet’s surface which affects temperature dependent precipitation and ice ablation. The significance of this feedback mechanism depends on the magnitude of GIA which is dependent on the mantle’s viscosity. Studies find that the sub-Greenland mantle viscosity varies in three dimensions which leads to a spatially varying rate of GIA. Results obtained with an ice sheet model coupled with an Earth model including 3D variations in sub-Greenland viscosity illustrate its significant effect on the GrIS’ modelled evolution by affecting both elevation-melt and elevation-precipitation feedback mechanisms.

Of the four Galilean satellites the only one without ice is Io. While it is possible that Io never contained any ice the starting point of this research work is an icy Io. As present-day Io does not contain any ice, icy Io must have lost its ice through some sort of ice loss mechanism. In this work the assumed ice loss mechanism is geysers driven by tidal heating. Tidal dissipation of sufficient magnitude is able to melt part of an icy layer creating a subsurface ocean and is also able to crack the overlying icy shell. The subsurface ocean then acts as a reservoir from which water vapor molecules and icy grains accelerate upwards through the cracks and are erupted from the surface. Erupted material with a sufficient velocity is able to escape from icy Io resulting in ice loss.

During its thirteen-year long mission in the Saturnian system, the Cassini spacecraft routinely used Titan, Saturn’s largest moon, for performing its gravity assists. The problem of determining the gravity field of Titan by performing a precise fit of Cassini’s orbit to the available Doppler tracking data was reviewed in this study. The knowledge of the gravity field of Titan can be used to constrain the moon’s internal structure and composition, providing clues about its formation process and the properties of an internal ocean (Grasset et al., 2000). Previous solutions obtained by Durante et al. (2019) and van Noort (2021) were respectively found to have inconsistencies regarding the reported small uncertainties given the noise level of the Doppler measurements and the estimation of an unfeasible negative value for the second-degree Love number k2. It was demonstrated in the present work that this last parameter should be estimable. Here, the data from the ten Titan flybys considered in those studies was analysed and emulated by using a relativistic model for Doppler measurements in search of ways of improving the accuracy of the gravity field solution. Both the GEODYN (McCarthy et al., 2015) software from NASA GSFC and Tudat libraries (see appendices in Dirkx et al., 2019) developed by the Astrodynamics and Space Missions department at TU Delft were used to investigate the nature of this gravity determination problem and compare the results against the previous solutions. A model mismatch in Doppler in the order of 102 mHz was found between the estimation setups with GEODYN and Tudat. In the search for the source of the mismodelling, it was tested that the empirical accelerations, measurement biases, measurement timing and the spherical harmonics of Saturn have a great influence on the solution and residuals. Thus, they should be reviewed with care for future estimation efforts. It was proven that the flyby geometry is not favourable for resolving the dipole field spherical harmonics coefficients, especially J2, C22 and k2, which reveal information about the tidal interactions from Saturn on Titan. They were found to be highly correlated and, when simulating equivalent noises to those found in the previous research, presented similar uncertainties to those obtained by van Noort (2021), but large in relation to the ones determined in Durante et al. (2019). Nevertheless, there are some prospects of estimating the dipole coefficients without the need of adding constraints on the solution, although the search for well grounded constraints is recommended and necessary when pursuing a more accurate solution. The power of fully simulated gravity field estimations as a means for addressing the quality of the available real data and verifying models and constrains, has been manifested throughout this study.

The GRACE mission has provided unprecedented insights into mass redistribution processes in the Earth system. Following a strong call for continuation of the mass observations, the GRACE-Follow On (GRACE-FO) mission was launched in May 2018, leaving a coverage gap of ca. 1 year between GRACE and GRACE-FO. Geopotential solutions derived from data of the Swarm mission (2013-present) are candidate data to potentially bridge this gap, as well as bridge the minor discontinuities in the current GRACE time series. This study aims to validate the sensitivity of the Swarm measurement system to mass rates, by comparing GRACE and Swarm observations of the gravity trend induced by glacial isostatic adjustment (GIA). Studies inverting GIA observations (e.g., relative sea level change, surface deformation, [time variable] gravity) into mantle viscosity estimates suggest relatively high viscosity in the Hudson Bay area. This suggests that the North American GIA-induced gravity trend is linear on a multi-decadal time scale, which means we can extrapolate the GRACE-derived GIA observations into the Swarm time period for this area. We find that the Swarm-derived GIA observations correspond well in amplitude and spatial distribution to GRACE observations and conclude that both systems have a similar sensitivity to gravity trends at spatial scales to which Swarm is sensitive to (ca. 1500 km). We validate our findings via geopotential solutions of GRACE-FO and provide a brief analysis of the benefits of adding Swarm-derived information to the combined GRACE / GRACE-FO time series.

The Antarctic ice sheet is a complex system highly influenced by global and local processes and characteristics including a varying bedrock elevation and structure of the solid Earth and a changing climate. Sea level rise has a high impact on society and the improvement of forecasts are vital to generate both adaptation and mitigation strategies. A recent comparison of 15 ice sheet models projected that the Antarctic Ice Sheet could contribute -7.8 to 30 centimeters of sea level rise between 2015 and 2100, meaning that sea level rise could increase a lot although the uncertainty is high. To better predict the future of the AIS, more accurate simulations of the evolution of the AIS are needed. The Antarctic ice sheet consist of three main components: grounded slow-moving ice, fast flowing ice streams or outlet glaciers and floating ice shelves. Over glacial-interglacial cycles, the evolution of an ice sheet is influenced by Glacial Isostatic Adjustment (GIA) via two negative feedback loops. First, vertical bedrock deformation due to a changing ice load alters ice-sheet surface elevation. Second, bedrock deformation will change the location of the grounding line of the ice sheet. GIA is mainly determined by the viscosity of the interior of the solid Earth which is radially and laterally varying. Underneath the Antarctic Ice Sheet (AIS), there are relatively low viscosities in West Antarctica and higher viscosities in East Antarctica, which affect the response time of the above-mentioned feedbacks. However, most ice-dynamic models do not consider lateral variations of viscosity in the upper mantle in GIA feedback loops when simulating the evolution of the AIS. The main research question of this study is: •What is the effect of the interaction between Glacial Isostatic Adjustment and ice sheet dynamics on the Antarctic Ice Sheet growth during the last glacial cycle? This study presents a new method to investigate 3D GIA feedback effects in detail at any chosen period during the last glacial cycle. The method is applied using ANICE and a 3D GIA FEM model. This led to the development of a fully coupled ice dynamic-3D GIA model with coupling timesteps of 1000 and 5000 years. Following the new method, the model computations alternate between the ice-sheet model, ANICE, and a 3D Finite Element Method model until convergence of the ice thickness occurs at each timestep. We simulate the evolution of the AIS from 120 000 years to 115 000 years before present, considering 1D and non-linear 3D rheologies. The results of the coupled model are discussed in detail for the period 120,000 years to 115,000 years before present with a focus on the Siple Coast and the Ross Ice Shelf. The maximum difference between the uncoupled deformation (iteration 1) and the coupled deformation (average between the last two iterations) for the period 120,000 to 115,000 years BP is 3 to 8 mm per year, depending on the viscosity of the upper mantle. The maximum difference in ice thickness at 115,000 years BP is 50 meters close the Ronne Ice Shelf and the Ross Ice Shelf. The grounding line position differs up to 80 meters when applying the coupling method compared to the uncoupled result. The increases in deformation using a 3D wet rheology with a grain size of 10 mm are highest at the Siple Coast, the Ronne Ice Shelf, and several other locations along the grounding line of the AIS. The results of this study emphasize the importance of the 3D GIA feedback effects when simulating the evolution of the AIS during the last glacial cycle. Therefore, the GIA feedback effects should be taken into account in future studies.

We have developed, from scratch, a 3D radiative transfer code based on the Monte Carlo method, that fully takes into account the spherical shape of a planetary atmosphere, multiple scattering, and the polarized nature of light. Accounting for the sphericity of an atmosphere is important for the analysis of observations near the planetary limb and the twilight zone, i.e. the regions on a planet where the parent star and/or the observer are low or even below the local horizon. In such regions, the widely used radiative transfer models that are based on the locally plane-parallel atmosphere approximation can lead to significant errors, especially for planets that have extended atmospheres. With our code, we simulate total flux and polarization signals of light that is reflected by spatially resolved and spatially unresolved planets with extended atmospheres. We discuss the effects due to the atmosphere’s sphericity and compare against computations with a locally plane-parallel code. Our results are relevant both for the interpretation of observations of various Solar System planets and moons (with atmospheres) and for the investigation of total flux and polarization signals of exoplanets at all phase angles, including during transits. Especially hot Jupiters are likely to have extended atmospheres and to be detected in edge-on orbits with transits. During a transit, an extended atmosphere can strongly increase the amount of starlight that is measured, thus leading to a smaller derived planet size. We also specifically simulate the polarimetric signal of Titan, Saturn’s largest moon that is known to have an extended atmosphere. We find that at small phase angles, as observed from Earth, Titan’s limb is strongly affected by the sphericity of its atmosphere and that at large phase angles, as observed by a spacecraft like Cassini, Titan’s brightness increases strongly due to forward scattered light.

This research focuses on GNSS users interested in a faster position fix in a situation where the receiver holds limited data. This occurs when turning on a mobile phone or sport watch to use a location-based service, driving out of a parking garage with a navigational device or activating a search & rescue beacon. In these cases, users care more about directly setting a certain action into motion, than waiting until a sub-meter level of accuracy is acquired. For this purpose, we aim to decrease the time to first fix (TTFF) of a receiver at the expense of position accuracy by focusing on its most dominant component: the time to read the first fix data, i.e. the TTFFD. In turn, the TTFFD is decreased by reducing the size of the clock correction and ephemeris data (CED) and integrating this reduced set into the existing Galileo I/NAV message while maintaining backward compatibility. Three reduction strategies are presented to reduce the size of the CED. First, the expression of the CED parameters relative to the matching almanac parameters, which requires downloading the most recent almanac. Second, the absorption of the clock and ephemeris time reference into related parameters by leveraging redundancies in the user algorithm. Third, the truncation of the CED parameters, where the optimal parameter bit allocation per CED size is determined by means of a Monte Carlo simulation. Subsequently, the optimal reduced set is integrated in the existing Galileo I/NAV message, since it is not only size but also the location and repetition rate of the reduced CED in the navigation message that determine the final decrease in TTFFD. A simulation is developed where the three reduction strategies are combined and applied to one year of Galileo navigation messages and almanacs starting from October 2017. The current I/NAV message has a size of 428 bits and an average TTFFD of 25.4 seconds. The relative expression of the CED parameters, the absorption of the clock time reference and ephemeris time reference yield a 76, 15 and 10 bit reduction respectively, without a significant decrease in position accuracy expressed as the signal-in-space ranging error (SISRE). The reduction in size that can be realized by truncation results from a trade-off with the maximum acceptable level of SISRE. When combining all three reduction strategies a 296 bit, i.e. 69%, size reduction can be realized at a SISRE level of 10.04 meters, yielding a 132 bit CED. 66% of the size reduction is in this case achieved by parameter truncation. To maintain backward compatibility, the spare and reserved bits of the I/NAV message are used to integrate the reduced set. This finally results in an average TTFFD of 6 seconds, which is a decrease of almost 20 seconds. With the reduced CED the user gains full independence of out-of-band channels for the computation of a fast position fix. In addition, it decreases the impact of bit errors on the TTFF and increases the chance of receiving an error free set of first fix data. Moreover, the reduced CED is not only of interest to fast fix applications that desire a decrease in waiting time. Positioning based on the reduced set also decreases the time the receiver has to be "on" to read the navigation data and consequently the receiver's power usage. This leads to a second type of applications that benefits from the reduced CED: low-power applications that do not require a high accuracy, such as freight management, wildlife tracking and stolen goods trackers. To conclude, the constructed reduced CED provides multiple advantages for a wide range of GNSS users and should therefore be considered as a purpose for the spare and reserved bits of the Galileo I/NAV message.

It has been shown that glacially induced decompression rates under Vatnajökull cause an increase in mantle melt and enhancement in volcanic activity. Eruptions under ice can form tuyas, that can be used to constrain past ice sheet thickness. Several mountains in the Martian South Polar Region qualify as tuyas, but their exact origin is still unknown. In this study we reconstruct a palaeo ice sheet from the height and spatial distribution of these tuyas and study glacially induced decompression rates within the Martian mantle. In a finite element model we use the reconstructed ice sheet to constrain the surface pressure load and test different lithosphere thicknesses and linear deglaciation periods. We find a general decrease in decompression rates over depth and time. Results show that a decompression rate equal to the one induced by present-day deglaciation of Vatnajökull occurs at a lower depth inside the Martian mantle. Given that the mantle temperature is close to the solidus at this depth and magma ascent velocity is sufficiently high, mantle unloading due to ice melt could have contributed to the formation of the tuyas in the Martian South Polar Region. Our study proposes present-day deglaciation of Vatnajökull as a potential analogue for processes related to deglaciation on Mars.

Though NanoSats are becoming increasingly capable of featuring active payloads, and acquiring inertial data at arcsec level precision; the payload operational duty-cycle largely remains limited by its data downlink capabilities. NanoSat laser communication terminal (LCT) promises 1 Gbps data downlink capabilities. However, LEO to ground free space optical data transmission requires precision attitude knowledge under agile ground target tracking conditions. Under such stringent operational conditions, the calibration of gyro-stellar misalignment and scale factors is a well-established practice for traditional satellites. Nevertheless, the impact of such non-idealities in the context of MEMS gyro-stellar based NanoSat precision pose estimation is unclear. Furthermore, the gyro-stellar calibration performance that can be achieved with NanoSat ADCS system requires further investigation. Under agile operational conditions, slew rate induced star tracker drop-out is common for NanoSats, which limits the operational envelope of agile precision tracking missions. To increase our understanding of attitude knowledge estimation capability of today's NanoSats, this thesis work is extended at Hyperion Technologies. In collaboration with Hyperion Technologies, gyro-stellar sensors are characterised; and CubeCAT LCT is used to laydown the reference payload and mission requirements of an agile precision terrestrial target tracking NanoSat mission. Based on state steady noise PSD properties, and attitude knowledge requirements set by the LCT, gyro-stellar configurations are selected, and characterised. A rigid body attitude simulator is developed, considering the characterised sensor suite, to generate representative nominal and non-ideal sensor outputs under four different mission phases: 1.) Inertial pointing with no angular rate, 2.) Non-harmonic sinusoidal manoeuvres 3.) Agile precision ground target tracking and 4.) Inertial target tracking with non-zero angular rate. Reaction-wheel time delay and torque limits are evaluated to analyse the feasibility of attitude pointing requirements under agile target tracking conditions. Due to favourable properties of robustness to large initialisation errors, fast convergence, and preservation of attitude quaternion unity norm constraint; UnScented QUaternion Estimator (USQUE) variant of UKF based filter is synthesised for attitude estimation. The USQUE filter is further extended to facilitate the calibration of gyro-stellar misalignment and scale factors. It is demonstrated that the presence of gyro-stellar misalignment and scale factor, and star tracker occultation under agile slew rates significantly deteriorates the attitude estimation performance of USQUE based attitude estimator. Star tracker misalignments and dropouts are observed to have a significantly larger impact on the attitude knowledge estimation performance, when compared against MEMS rate-gyro scale factor and misalignment. Degraded MEMS rate gyro sensors are observed to have little impact on the attitude knowledge estimation performance of 6/7 state USQUE filter. However, it has a considerable impact on the convergence performance of star tracker misalignment calibration. More persistent calibration manoeuvres are observed to improve the star tracker misalignment parameter estimation performance. Gyro misalignment and scale factor calibration objectives were not met. A root-cause analysis showed that the signal distortions introduced by such non-idealities are below the gyro-stellar noise floor. Calibration filter synthesised has the potential to improve autonomy, reduce commissioning and ground calibration times, and enhance NanoSat pose estimation performance.

The next step in spaceflight, among others, is the development of a reusable launch vehicle (RLV) suitable for manned missions. All existing designs of re-entry vehicles are either ballistic or low L/D vehicles. In the future, the aim is to develop a high L/D vehicle, because such vehicles experience lower g-loads, making the flight comfortable for manned flights. The development of these vehicles can be useful for both, space (for example, space tourism) and military applications (for example, long-range missiles). Thermal protection system (TPS) is deemed critical to the RLV development, as high L/D vehicles are expected to experience higher thermal loads. Existing TPS solutions are not suitable for this purpose. Therefore, arises the need to find reusable TPS solutions that can sustain the desired thermal loads. Flight testing is a crucial step for developing any hypersonic system, this applies to the TPS design as well. This study aims to investigate the influence of TPS on designing the mission and vehicle for a test flight. From the various developed and proposed TPS designs in literature, a TPS solution is selected, to meet the requirements identified for future RLVs. The solution is an active cooling concept (cooled metallic TPS), named Enhanced radiation cooling, that uses water as a coolant. Based on a preliminary design investigation, suitable materials are selected and the thickness of the layer is determined. The performance of the proposed TPS design, i.e., the thermal load-bearing ability, is assessed using a transient thermal analysis tool developed for this purpose. The outcome of this analysis proves that the proposed design does not meet the thermal load requirements identified in this study. Nonetheless, the concept shows substantial improvement in performance, it can sustain almost double the heat flux as compared to an uncooled system. To meet the heat flux requirements, a few modifications in the design are proposed and analysed. These modifications increase the complexity of the system and have other adverse consequences that must be addressed. However, thermal analysis proves that the heat flux requirements, identified at the start of this study, are satisfied. A sensitivity analysis of this modified cooling system shows that the performance of the design is not influenced by material uncertainties. Additionally, the design is found to be robust under varying mission and design parameters, within reasonable bounds. These results can serve as a preliminary input for designing a test vehicle and mission.

Satellite-derived gravity models have been used in modelling of several subduction zones, but not yet for the Sumatra subduction zone. There, gravity models can shed light on the geometry and density of the plate, and whether a slab tear is present under northern Sumatra, as seismic and tomographic studies present contradicting conclusions. Ten different gravity sensitivity tests were carried out. Those tests investigated the sensitivities of the radial gravity (gR) and the second gravity gradient tensor invariant (I1) to different slab parameters (depth, thickness, density), three isostatic scenarios, and the inclusion of subduction-specific characteristics, e.g. a slab tear, and subducting crust. An innovative approach was implemented to determine slab thickness estimates based on oceanic lithospheric age, subduction direction, and a weighted half-space cooling model. Thereafter, a numerical model for the Sumatra subduction zone was developed which approximates the XGM2016 satellite-terrestrial gravity observations and supports a slab tear. The gravity sensitivity test results together with the numerical model for the Sumatra subduction zone provide a valuable and universal foundation for future gravity-based subduction zone research.

Saturn's largest natural satellite, Titan, is an icy moon expected to harbour a liquid sub-surface water ocean capable of habiting extra-terrestrial life. The spherical harmonic expansion of the gravity field and the tidal Love number k2 can be used to estimate densities, depths, and phases of different layers from Titan’s interior structure, granting insight on the sub-surface ocean and its habitability. Recent research has determined the spherical harmonic expansion of the gravity field up to degree five including the estimation of Titan’s tidal Love number k2. This work estimates the spherical harmonic gravity field of Titan up to degree three through a multi-arc batch weighted least squares approach, with fundamental differences from previous research. We include observation biases and empirical accelerations for estimation in our dynamic force model and apply no constraints on the gravity and tidal parameters for estimation. Previous research claims the applied global constraints do not constrain the solution, yet an independent estimation of the gravity and tides of Titan yields significantly different results: high correlation between global parameters results in the inability to determine a realistic k2 Love number. We refrain from applying any global constraints to the gravity and tidal parameters, as there exists no a priori information on these parameters to apply a physical and unbiased constraint.

The Indian Space Research Organisation(ISRO) plans to equip future satellites of its Indian Nano Satellite(INS) with COTS dual frequency receivers in order to let them contribute to various scientific research opportunities. This thesis develops a test framework for Commercial Of The Shelf (COTS) Global Navigation Satellite System (GNSS)receivers on Nano satellites of the Indian Space Research Organisation(ISRO) to be used for scientific applications. This thesis aims to provided answer to the suitability of this data for scientific application. We have analyzed that the current generation Skytraq single frequency receiver, mounted in previous INS-1A to INS-1C missions, has with an accuracy in the meters range insufficient position accuracy to be used for scientific applications. Nevertheless, based on historical mission experiences and design constraints, a receiver-antenna system selection has been executed. Using this system as an example configuration, a test framework has been setup to analyze influential design and system configuration parameters influence on the GNSS receivers for future missions. The test setup consist of a series of static tests that enable to research the hardware configuration, such as the influence of a ground plane, and the data processing options, such as filters for the data and the choice of atmospheric models. It is complemented with a dynamic analysis of the receiver simulated performance in orbit using a GNSS simulator. Multiple data series were obtained for each individual test of the framework. These data series were analyzed with a developed data handling protocol in which multiple data processing software packages were used. In order to obtain comparable results in all test series, average deviations of the position fixes in the test periods were calculated and analyzed. This leads to a number of conclusions: The current antenna mounting on INS-1C is less than optimal and a ground plane would enhance the antenna performance with 20%. The raw data are essential for scientific experiments. However, they can most likely be limited to those obtained from GPS satellites excluding GLONASS and other GNSS constellations. It does pay off to include low viewing angle measurements for optimal results. The ionospheric models are indispensable to achieve sub meter accuracy with single frequency receivers.

In 2022, ESA plans to launch the JUICE (JUpiter ICy moons Explorer) mission which will spend at least three years making detailed observations of Jupiter and three of its largest moons, Ganymede, Callisto and Europa. These moons are currently a hot topic within the science community as their interiors might include oceans consisting of liquid water. These oceans could provide life, but at the moment little is known about the exact composition and structure of these interiors. Only Earth based observations and a few fly-by’s have been performed to measure the characteristics of these moons. The JUICE mission will provide more detailed information on the moons through fly-by’s. This thesis research will focus on Ganymede as JUICE will be the first human-developed satellite to orbit thismoon. Ganymede stands out as a potential scientific target due to several specific reasons; the most remarkable being it’s intrinsic magnetic field. Only two other solid bodies within the Solar System generate such a magnetic dipole field (Earth andMercury). The complex interactions of this magnetic fieldwith Jupiter’smagnetic field are unique and could provide a lot of new knowledge when studied. Measurements from Galileo and the Hubble space telescope suggest that a subsurface layer of (saline) water is present within the moons interior. Saline water could be a good conductor of electricity, generating the magnetic field. The magnetic field of Ganymede could also point towards a complex core, which is another possibility for the generation of this field. It could be that the core of Ganymede consists of liquid, iron rich elements which generate and maintain this magnetic field. Unfortunately, current models of the gravitational potential field and the interior of Ganymede are still uncertain. A precise gravitationalmodel of Ganymede could provide a lot of information about this interior. An orbiter or in-situ probes are required to achieve high precision gravitational potential field models. JUICE is expected to obtain a model of Ganymede’s gravitational potential field of at least degree and order 15. This thesis will provide insight in how different possible internal density distributions of Ganymede influence the gravitational potential field of the moon. Thisway,when JUICE obtains more information on the gravitational potential field of Ganymede, variations within this field can directly be utilized to determine what interior aspects could cause these variations. From 44 billion 1D homogeneous models considered during this research, only 260 adhered to current known characteristics of Ganymede. Certain elements and water phases are present in all models: a pure iron or iron-sulfide core, a silicon mantle, an ice VI layer together with an liquid ocean and a outer crust consisting of Ice Ih. Dependent on the exact layer thicknesses within a model, intermediate ice phases, ice III and V, can also be present. Layer correlations between the 260 models were analyzed and fourteen models where selected for further research. These models were combined with different boundary and density variations to obtain different 3D heterogeneous models. Gravitational potential simulations for spherical harmonics coefficients up to order/degree 48 were performed. It was found that several relations exist between gravitational potential field data and internal density distributions within Ganymede. If one can effectively correct gravitational potential field signals for measurable components within Ganymede’s interior, several sets of internal structures emerge. Furthermore, taking into account the established limitations and correlations between layers, the gradient of the gravitational signal power over spherical harmonics degree can be directly related to the thickness of an interiors ocean. Several distinguishable models show that the presence of ice III, and to a lesser extent ice V, increase the gravitational signal power of a model. When combined with the correlations found between internal layers during this research, one could even establish an accurate first order approximate of Ganymede’s internal composition. These results, together with measurements performed by JUICE, will provide numerous new insights on Ganymede’s frozen enigma.

The origin of the Martian dichotomy is subject to question and no substantial evidence exists. Some surface and interior features that are not visible in, e.g., topography data, can show up in gravity data. Therefore, this research inverts gravity data to find a crustal and mantle global density model. Previous research performed a one-layer inversion, assuming equal mass in all columns. Also, missions like InSight do not provide global interior information, but only at the landing site. The aim of this research is to provide a global density model of both the Martian crust and upper mantle. The inversion is performed using a weighted, regularized least-squares algorithm. The gravity input consists of the residual between the MRO120F data set and the state-of-the-art gravity field model of the TU Delft. The design matrix is built using Green’s functions, which define the influence of a mass element in all different directions on a measurement point. Using this least-squares algorithm, three different methods for inversion are used. The separate two-layer inversion, the combined independent two-layer inversion and the combined dependent two-layer inversion. All three inversion methods are performed on synthetic planets as well, for verification purposes. By performing all inversions on the synthetic planets, it was found that the combined independent two-layer inversion results in a strong decoupling of short and long wavelength signals, but is not able to attribute gravity signals to different features in the crust and mantle. The combined dependent two-layer inversion does lead to a result that shows decoupling of crust and mantle features. The hypothesis is that adding different gravity components to the combined dependent two-layer inversion will further increase its accuracy. The results of the inversion methods applied to Mars are in agreement with existing research in terms of standard deviations of the crust and mantle density anomalies. The maps were also analysed geologically, where the most important conclusion is the evidence of potential impact basins in the north polar region. These can be evidence to accept the several impact theory for the origination of the Martian dichotomy. Increasing the resolution and refining the third inversion method with multiple gravity components will increase the potential of gravity inversion to define geological features of Mars.

In the Krafla region in North East Iceland, a change in the deformation pattern was observed in 2018. Since the 1975-1984 rifting event, there was subsidence seen in the Krafla caldera and along the fissure swarm. This subsidence was explained by deflation which decreased exponentially along the years. In 2018, this deformation pattern reversed, and for the first time since 1989, uplift was observed in the region. Therefore, this work concentrates on examining the change in deformation pattern in the region that occurred in 2018. Four geodetic techniques, a combination of ground and space geodetic techniques, are used to study this change in deformation pattern noted in the Krafla region. Data from 2016 to 2019 are considered as they span the time of interest when the change took place. In levelling, InSAR and GPS, the uplift observed is located to the North of the Krafla power plant. Similarly, there is an increase in net gravity seen to the North of the power plant. This suggests there is an increase in mass/density. A possible reason could be movement of magma from the deep to the shallow magma chamber. The reason behind this start of the magma movement is not known. On the other hand, there is continued subsidence and an increase in net gravity seen in the fissure swarms present to the south of the power plant. This result agrees with the fact that the uplift is concentrated (a local phenomena) in the northern region and there is still subsidence seen in the south. A simple Mogi model is fitted to InSAR and GPS data to understand the cause for this change in deformation. But, the fitted model provided very few insights in this work. The results from modelling mainly concentrated on the subsidence in the southern region and corresponding model parameters were obtained. The location of the source responsible for the modelled deformation (subsidence) is found to be near the power plant. The depth and volume however are not properly constrained. But since the model tries to constrain at deeper depths, correspondingly larger volume is also seen. This outcome strengthens the hypothesis that the source for the deformation could lie at deeper depths. Based on the overall results, the following recommendations are made to enhance the results: i) Maintaining the continuity in the data ii) To increase the number of benchmarks measured near the deformation area (especially in gravity and levelling) iii) Use of multiple source modelling for the better understanding of the sub-surface processes.

In the last fifty years, the spacemissions Voyager, Galileo, Cassini-Huygens and Juno explored the moons of the outer Solar System and revealed a wide spectrum of worlds. While some of these worlds are barren, others are among the most geologically active of the Solar System. The innermost Jovian moon, Io, showcases spectacular volcanic activity; its three companions, Europa, Ganymede and Callisto, are likely ocean worlds that harbour subsurface oceans beneath their icy crusts. Similarly, the biggest Saturnian moon, Titan, is believed to have a subsurface ocean concealed beneath its icy surface and dense atmosphere. Saturn’s collection of smaller icy bodies feature different levels of activity, Enceladus being the most remarkable. Above its limb, water plumes rise more than hundred kilometers spilling its internal ocean into Saturn’s E-ring. In Neptune, the captured moon Triton orbits in a peculiar retrograde orbit; its barely cratered surface is similar to that of other ocean worlds and shows signs of cryovolcanic activity. The spectrum of geological activity displayed by the moons of the outer Solar System is thought to bemainly the consequence of tides.@en