1,720,974 research outputs found
Development of a support tool to limit workload at a hub for Air Cargo combination Airlines via dynamic loading advices at outstations
No full textThe planning of air cargo across an airlines' network is proven to be a complex problem consisting of multiple subproblems. Currently, all these subproblems are solved sequentially and by hand, resulting in partial solutions instead of an integrated solution. Two of these subproblems are the palletization of items onto ULDs and the scheduling of a workforce at a cargo terminal. This research proposes an implementable tool that tries to find a more integrated solution between these two subproblems. The tool provides loading instructions to ground handlers on how to combine cargo at the outstation. It makes use of both a dynamically determined volume threshold for ULDs containing cargo for one connecting dated flight (T-ULD) as a K-means clustering algorithm to combine cargo based on the connection time in the hub. A 1D Bin Packing Problem and Breakdown Scheduling model was created to study the effects of the new loading instructions. The dynamic threshold showed a predictable behaviour in the reduction of workload while achieving a baseline workload reduction of 1.4%. The tool further opens the opportunity to reduce workload even further with 24.9%, but this is associated with a risk and additional costs. The K-means clustering algorithm did not show an improvement to the baseline but it did offer the opportunity to cluster cargo based on multiple properties besides connection time at the hub.Aerospace Engineerin
Impact of Wrong Ambiguity Fixing on GNSS Positioning
No full textGlobal Navigation Satellite System (GNSS) has been developed in recent several decades, which provides a new technique for timing, positioning and navigation. And GNSS positioning is a basic service to us, both in daily life and scientific research. During high-precise positioning, ambiguity resolution is a key factor that has a huge influence on the accuracy of the result. And the wrong fixing is themain source of lose of accuracy, so many methods of test are proposed to validate the fixing result. We are interested in the performance of solutions that have been labeled as wrong fixing, and want to check if those wrong fixings should be excluded or accepted during estimation.At firstwe introduce the basic model and challenges of ambiguity fixing, aswell as the widely usedmethod integer least squares and Z-transformation. Some previous research of distribution of fixed solution also enables us to compute the bounds of baseline residual. Then we focus on an example to do some pre-research, and find out the main research question - how to accept wrong fixing during estimation and the impact under different scenarios or estimation methods. We use a simulation-basedmethod to do the research but themeasurements are generated based on the real ephemeris in certain day.After giving the double difference measurement model based on code and phase with respect to single or dual frequencies, in short baseline scenario, we define some parameters 1-norm, infinity-norm, weighted 2-norm, and good/bad performance of wrong fixings that might be useful for analysis. And some detailed information in chosen epochs are shown with multiple figures and we derive those which are helpful, such as infinity-normratio. Then we develop 4 validation methods, Infinity-normratio detection (RD),Weighted 2-norm detection (WD), 1-norm baseline residual detection (BD1) and Infinity-norm baseline residual detection (BDi), to check if we could recognize wrong fixings with good performance from all wrong fixings.We also compute some statistics, such as the success rate of detection, the rate of misdiagnose, and the rate of participation to see whether our validation methods are effective or not. The histogram of wrong fixing or correct fixing corresponds the existed research of distribution well. And the time series analysis also proves the reliability of our validation methods, although there exist some errors due to the small sample size of simulations.We also apply the multi-epoch least squares and Kalman filter to see the influence of fixing success rate, standard deviation of horizontal residuals, residual bounds and performance of wrong fixings. We find that there are obvious improvements on all of them. An extra experiment is designed to see the impact of atmospheric delays and the results show that it is really different from short baseline scenarios and we need to find more proper threshold to make sure our validation methods work.Finally, we could draw a conclusion that it is possible to find out wrong fixings that could be accepted, but the threshold for each validation method should be adaptive for different scenarios and Kalman filter is very reliable, with which the wrong fixing is always likely to be excluded during the estimation.Additional Thesis AES4011-10Geoscience and Remote Sensin
Interactions between Ice Sheet Dynamics and Glacial Isostatic Adjustment: The development and application of a new method to simulate the Antarctic Ice Sheet over the last glacial cycle
No full textThe 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.Geoscience and Remote Sensin
Volcanism on Io: A Comparison Between the Volcanic Distribution and Tidal Stress
No full textIo is Jupiter's innermost Galilean satellite and the most volcanically active body in the Solar System. Its volcanism is driven by tidal heating generated by its interaction with Jupiter and maintained by other Galilean satellites. The tidal forces acting on Io also cause significant stresses on its interior. It is investigated if tensional stresses play a significant role in the formation of volcanic centers on Io. On a global scale, it is investigated if the locations of maximum tensile stress coincide with the locations of maximum volcanic activity. On a local scale, it is investigated if chains of Io's volcanoes follow the directions of the tensile stresses. The results on a global and local scale are shown to be consistent with the hypothesis that tensional stresses play a significant role in the formation of volcanic centers on Io. However, tensional stresses alone are insufficient for the formation of volcanic centers.Aerospace Engineerin
Untersuchung der Lithosphärenstruktur Antarktikas mittels geophysikalisch–petrologischer Modellierung
Full text linkIn this thesis, satellite gravity data are combined with seismic findings to invert for the Moho depth of Antarctica. The results suggest a significantly lower density contrast between crust and mantle underneath East Antarctica compared to West Antarctica. Accounting for that, combined geophysical-petrological modelling of the lithosphere in 2-D is conducted along a well studied profile to test different Moho depth scenarios. However, even though multiple geophysical observables are considered in an integrated manner, neither of the competing scenarios can be validated or refuted with this approach. This demonstrates the need for comprehensive modelling in 3-D.
By utilizing satellite gravity gradient data in a thermodynamically self-consistent framework, this dissertation establishes a 3-D model of the Antarctic lithosphere and sublithospheric upper mantle. A new Moho depth map of the continent is derived that is in good agreement with independent seismic estimates. It also exhibits detailed variations for so far scarcely explored areas. Total lithospheric thickness values of the presented model confirm the marked contrast between West Antarctica (<100 km) and East Antarctica (up to 260 km).
The inferred deep thermal field is used to estimate 3-D viscosities of the mantle for GIA modelling. Coupled with present-day uplift rates from GPS, the relatively low viscosity values suggest a bulk dry upper mantle rheology. Finally, the crustal part of the 3-D density model is tested against recent airborne gravimetric data. The short-wave-length residual signal is inverted for near-surface density variations within the crust with the aim to refine the continental lithospheric model on a regional scale. These two applications demonstrate the potential of the presented model for further regional and continental-scale studies of Antarctica.In dieser Dissertation werden Satellitenschweredaten zusammen mit seismischen Befunden dazu verwendet, Inversionsrechnungen zur antarktischen Mohotiefe durchzuführen. Die Ergebnisse deuten auf einen signifikant geringeren Dichteunterschied zwischen Kruste und Mantel unterhalb der Ostantarktis gegenüber der Westantarktis hin. Darauf aufbauend werden zwei konträre Szenarien zur Mohotiefe mithilfe integrierter, geophysikalisch-petrologischer 2D-Modellierverfahren entlang eines relativ gut untersuchten Profils analysiert. Trotz der gekoppelten Einbeziehung mehrerer geophysikalischer Größen konnte auf diesem Wege jedoch kein eindeutiges Ergebnis erzielt werden. Stattdessen sind umfangreiche, dreidimensionale Modellierungen erforderlich.
Unter Verwendung von Schweregradientendaten aus Satellitenmessungen wird in der vorliegenden Arbeit ein 3D-Modell der antarktischen Lithosphäre und des sublithosphärischen Mantels erstellt, das in sich thermodynamisch konsistent ist. Eine neue Tiefenkarte der Moho-Diskontinuität wird vorgelegt, die mit unabhängig gewonnenen seismischen Ergebnissen im Einklang steht und zahlreiche Details in bislang kaum erforschten Gebieten der Ostantarktis aufweist. Die modellierte Tiefe der Lithosphärenbasis bestätigt frühere Studien hinsichtlich des starken Kontrasts zwischen Westantarktika (<100 km) und Ostantarktika (bis zu 260 km).
Aus dem modellierten Temperaturfeld werden Mantelviskositäten abgeleitet, die ihrerseits Eingang in ein GIA-Modell finden. Der Vergleich mit GPS-Messungen gegenwärtiger Landhebungsraten lässt auf relativ niedrige Viskositätswerte schließen, die der Rheologie trockenen Mantelmaterials entsprechen. Abschließend wird der Krustenbereich des Dichtemodells mit aerogravimetrischen Messdaten abgeglichen. Aus dem kurzwelligen Signalanteil werden Dichtevariationen innerhalb der Oberkruste invertiert, um das Lithosphärenmodell auf regionaler Skala nachbessern zu können. Anhand dieser Beispiele sollen die Möglichkeiten des vorgelegten Modells als Basis für weitere Studien zum antarktischen Kontinent aufgezeigt werden
Seperation of coseismic and postseismic signals of Sumatra Andaman earthquake and Indian Ocean earthquake
No full textThe gravity field changes associated with the earthquake are analysed using the GRACE (Gravity Recovery and Climate Experiment) data. GRACE data can track the temporal variations in the gravity field and therefore information on mass redistribution can be achieved. There have been many studies already carried out using the GRACE data to analyse the coseismic and postseismic effects of the earthquakes. The previous studies mainly concentrated on the separation of the earthquake signals from various other signals and noises to understand the internal mass redistribution. In this work two recent past earthquakes have been considered. Sumatra-Andaman earthquake that occurred on 26th December 2004 with a magnitude of Mw 9.1. The other major earthquake that has been taken into account is the off coast Northern Sumatra earthquake (also called as Indian Ocean earthquake) which occurred on 11th April 2012 with a magnitude of Mw 8.6. A new initiative has been taken to separate the long term postseismic term (2004 earthquake) from the coseismic term and the effects of the 2012 earthquake (both the postseismic and coseismic effect). This decoupling process was done using the GRACE monthly solutions of spherical harmonics. Gravity disturbances were calculated from GRACE monthly solutions to understand the internal mass redistribution. <br/
Bridging the GRACE gap: Validation of satellite gravity observations via glacial isostatic adjustment
No full textThe 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.Aerospace Engineerin
Ganymede: A Frozen Enigma: Exploring possible internal density distributions through gravitational potential field variations
No full textIn 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.Aerospace Engineering | Space Exploratio
Autonomous Guidance and Control for Precision Landing on Planetary Bodies: Convex Optimization Approach For Mars and Titan Case Studies
No full textAutonomously 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.JPL Visiting Student Research ProgramAerospace Engineerin
Active Thermal Protection System for Reusable Launch Vehicle: A Conceptual Design
No full textThe 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.Aerospace Engineerin
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