452 research outputs found
A simplicial algorithm for the nonlinear stationary point problem on an unbounded polyhedron
No full textOrbital Lifetime Predictions: An assessment of model-based ballistic coefficient estimations and adjustment for temporal drag coefficient variations
No full textThe ballistic coefficient β of a satellite indicates the influence atmospheric drag has on the orbital decay of said satellite. The method developed in this study estimates the ballistic coefficients of 1U CubeSats by analysing both its historical TLE data and the density it experienced along its trajectory according to the empirical density model NRLMSISE-00. In parallel the same is done for a spherical satellite with an already known ballistic coefficient, given the object is experiencing near-similar atmospheric conditions as that of the CubeSat. Any density error associated with NRLMSISE-00 along that trajectory is consecutively largely mitigated, resulting in an improved model-dependent estimated ballistic coefficient for the CubeSat. This procedure is iterated upon with a batch of 60+ CubeSats, leading to a single procedure capable of estimating the ballistic coefficient of a large group of objects. The estimated β are consecutively adjusted for the epoch- and altitude-dependent drag coefficient, performed analytically as a function of ambient gas-composition and spacecraft geometry. Additionally, an investigation in the computation of the energy accommodation coefficient α, an essential variable for the CD computation, is performed, assessing whether a constant α = 0.8 can be assumed or if Langmuir’s adsorption model based on the ambient presence of atomic oxygen wouldn’t be more suited. When subjecting the estimated and adjusted β values to a propagator build in TUDAT, based on Runge-Kutta-Fehlberg 7(8), and comparing them to their actual orbits, prediction improvements of up to 40% are observed compared to regular non-adjusted β values for CubeSats re-entering the atmosphere. For CubeSats orbiting Earth above 500 km, improvements between 5% and 15% are observed. The research furthermore highlighted the sensitivity of the iterative process and the orbital element selection criteria through which satellite pairs are assumed to have near-similar atmospheric conditions – the tighter the selection criteria the better the quality of the β estimations were, though at the cost of fewer β estimations to occur. Overall, it can be concluded that the β estimation method can be applied on a scale covering multiple satellites, potentially growing to a global ballistic coefficient estimation model which could also include space debris. Improvements to orbital lifetime predictions are primarily seen in the lower parts of LEO. Furthermore, the adjustment for the variable drag coefficient proved most useful for the CubeSats above 500 km, given Langmuir’s adsorption model was used. For the lower orbits, Langmuir’s adsorption model lost its accuracy and an adaptation of Langmuir’s model which considers an object’s historical orbit data is suggested.Aerospace EngineeringSpace Engineerin
Determination of the attitude state of space debris objects based on Satellite Laser Ranging, using Envisat as a test case
No full textThe attitude state of the passive Envisat satellite (ESA) has been estimated before using various techniques like Satellite Laser Ranging, radar and using light-curves. This research focusses on the use of Satellite Laser Ranging. Due to the relatively large (meter scale) offset between the center-of-mass of the satellite and the reflector where the laser signal is reflected back to the transmitting and receiving ground station, large oscillations in the range residuals are visible. These oscillations show the rotating behaviour of Envisat, and can be translated to its rotational state by re-producing this signal using a corresponding attitudemodel and the offset between the reflector and the center-of-mass. First a purely theoretical case was considered where a known simulated orbit and attitude where estimated for various cases in order to validate the use of the estimation scheme. Afterwords, the real Satellite Laser Ranging data of Envisat was used for the time period 2013-2015.Aerospace EngineeringAstrodynamics and Space Mission
Body-wave seismic interferometry applied to earthquake- and storm-induced wavefield
No full textSeismology is the study of the vibration of the Earth. Seismologists pay much attention to the main source of Earth vibration: earthquakes. But also other seismic sources, like mining blasts, ocean storms and windmills, are studied. All these sources induce seismic waves, which can eventually be recorded as ground vibrations. These seismic records contain not only information about the sources, but also about the part of the Earth through which the waves have propagated. This thesis focuses on a main subclass of seismology: seismic imaging. With seismic imaging, seismic records are studied with the aim to unravel the structure and composition of the Earth. Seismic imaging finds its main use in extracting information about the solid Earth that is within our reach. With the current state of the art, we can only mine minerals from the upper 0.2 percent. Imaging the deep (i.e., unreachable) Earth finds its main use in hazard assessment. Through imaging the deep Earth, we can better understand the dynamics of our planet, manifested in, amongst others, earthquakes and volcanism. By forecasting how the Earth will reshape, we can sensibly adapt. Seismic characterization of the subsurface also improves our ability to assess the direct impact of earthquakes. A large part of the current deep-Earth images are obtained through tomographic inversion. These images are successful for identifying large-scale structural anomalies, like (remnants of) subducting slabs. However, they lack the resolution to accurately image discontinuities in space. Sharp discontinuities, like the interface between sedimentary and crystalline rock, lead to conversions and reflections of seismic waves. It is those reflections that are most suitable for imaging the discontinuities. The technique developed to do this is called seismic reflection imaging (SRI). Though SRI is considered to be an advanced technique for imaging the subsurface, the dense and wide source (and receiver) distribution requirement thus far limited the use for deep-Earth imaging. Deep-Earth imaging is done primarily with natural sources. Their limited and irregular distribution and their unpredictable occurrence time is inconvenient for SRI. In this thesis we work on a methodology to alleviate the stringent requirement on the source distribution. This methodology is called seismic interferometry (SI). SI is a remapping operation. With SI, the responses from many sources are combined to create the response as if there were a source at a receiver position. The response from this "virtual" source could contain both surface waves and body waves. We apply SI such that especially (reflected) body waves are retrieved, because it are these waves that are used for SRI. When we apply SI to a regularly-spaced array of receivers, we could turn uncontrolled natural sources into a well-organized succession of virtual sources, which is amenable for SRI. The aim of this work is to improve the imaging of the interior of the Earth by the application of body-wave SI to naturally induced seismic data. A prior study focused on the exploration-scale (i.e., the part of the Earth that is being mined). In this thesis, we focus on low-frequency waves (< 2 Hz), which contain especially information about the deep Earth. In Part I we consider applications to earthquake recordings. In Part II we evaluate the retrieval of reflections using microseisms. Part I starts by looking into the global-scale configuration. For this configuration, two seismic stations (or two arrays of stations) may be located anywhere on the globe and reflectors may be located anywhere in depth. We derive and numerically test relations to retrieve the complete response between two stations, using worldwide seismicity. The SI relations are valid for a closed entity for seismic waves, like the Earth is by approximation. The relations do not account for inelastic losses, by which eventually all seismic energy is transformed to heat. Though normally of negligible influence, the SI relations do account for the rotation of the Earth. In practice, the distribution of the larger earthquakes is not wide enough to retrieve complete responses between any two points on the Earth. However, still relevant primary reflections can be retrieved with a limited dense distribution of seismicity, like from the Tohoku (2011) aftershock sequence. Part I proceeds by zooming in to the lithospheric-scale configuration. The lithosphere is the part of the solid Earth where most (seismic) activity takes place. It makes up the crust and the upper part of the mantle. It is this part of the Earth that, broken into plates and fueled by convection in the mantle, undergoes birth (at midoceanic ridges), collisions (resulting in stunning topography) and death (subduction back into the mantle). Inspecting the Earth in depth unveils a part of the history of the lithosphere and gives an indication of the processes still ongoing. Information about the structure of the lithosphere is hidden in the coda of wavefields that arrive due to distant seismicity. We test a few different SI approaches to extract this information. We use synthetic data with similar characteristics as field data recorded during the Laramie broad-band experiment (2000-2001). This was an array of seismic stations in Wyoming, USA, to study an suture zone. We evaluate the requirements for obtaining multi-offset reflection responses. These multi-offset reflections are important for obtaining a velocity model. We estimate this model for the subsurface below the Laramie array and we use it to map the extracted reflections to a reflectivity image of the lithosphere. In the remainder of part I, we focus again on the lithospheric scale. This time, we assume that a velocity model is already available through other means. Consequently, only single-offset reflection responses are required to make an image. We use global phases to obtain zero-offset reflection responses. We show the robustness of the method with data from the Hi-CLIMB experiment (2002-2005). This was a large and well-sampled seismic array, passing the Himalayas and a significant part of the Tibetan Plateau. The successive application of SRI leads to an image of the Indian-Eurasian Plate collision. The rediscovery of body waves in low-frequency noise (<1 Hz) opened up the research for Part II of this thesis. The noise are in fact microseisms: Earth vibrations that are indirectly caused by ocean gravity waves. High-amplitude ocean waves are caused when storms, leading to persistent wind fields, cross an ocean. We retrieve reflection responses from body-wave noise using SI approaches similar to the ones used for earthquake recordings. The main difference between applications to noise and (large) earthquake responses is that the origin of large earthquakes is known, whereas the origin of noise is generally not known. Therefore, the main challenge for noise applications is to unravel the noise illumination. For this purpose, we use a well-sampled areal array in Egypt to study the noise illumination in different frequency bands. We only select the noise records with a favorable body-wave content and process them into separate reflection responses, of both the lithosphere and the upper crust. We further evaluate what basin-scale information can be extracted from the retrieved reflections. The wavelengths of the reflections extracted from low-frequency noise are too long to image a sedimentary basin in detail. However, the sharp boundary between the sediments and the underlying crystalline rocks can be delineated. For the same dataset from Egypt, we compare the information that is extracted with SI with the information that is obtained when using two other passive seismic techniques, namely horizontal-to-vertical spectral ratio and receiver function. In the appendices we discuss a few spin-off developments from the main research. In Appendix A we work out an alternative SI relation that integrates over midpoints between receiver positions, instead of over sources. Using this relation, reflections can already be retrieved using only a single source. However, a well-sampled array of receivers needs to be available and an additional evaluation needs to be performed. In Appendix B we develop a technique to estimate loss factors from plane-wave transmission responses. The losses are estimated from amplitude ratios, obtained before and after applying autocorrelation. In Appendix C we present a method that was inspired by SI. Crosscorrelations of diffractions from the core-mantle boundary are used to swiftly scan anomalies in the crust and mantle. Finally, in Appendix D we discuss an alternative implementation of SI: SI by multidimensional deconvolution. With this technique the outcome of SI may be improved. However, well-sampled arrays of receivers are required and additional processing needs to be implemented. In this thesis, we show that the application of SI to earthquake recordings is sufficiently mature to yield high-resolution images of the lithosphere. Relations for imaging deeper structures are worked out and modus operandi are thought through. However, the reality check is still to be encountered with field-data applications. Furthermore, we show that the applications of SI to microseisms can be used to yield information about a basin depth. Moreover, we show the great promise of microseism applications for unveiling a lithosphere in depth, using only a few days of noise recordings. The more repetitive character of microseism sources with respect to earthquakes, in fact makes them more amenable for monitoring applications, if their yearly variations and radiation characteristics are well understood.Geoscience & EngineeringCivil Engineering and Geoscience
Surface-wave retrieval using Seismic Interferometry by multi-dimensional deconvolution
No full textIn this thesis, seismic interferometry by multi dimensional deconvolution is used to retrieve the surface-wave response from a virtual source to a receiver by using the observations due to, often passive, noise sources located elsewhere. The virtual source response is used to retrieve the phase velocity, a property used for shallow subsurface velocity models.Applied Geophysics and PetrophysicsGeoscience & EngineeringCivil Engineering and Geoscience
Exploration of optimal orbits in the strongly perturbed environment of the 2001 SN263 triple asteroid system
No full textFor the past 20 years, the small bodies of the solar system, such as asteroids and comets, have been increasingly gathering the interest of scientists and space agencies. The latter have been multiplying the number of space missions to study them. Brazil does not want to be left out and has been working on its own mission, ASTER, which has the particularity of having as a target a triple asteroid system. Although adding great scientific interest to the mission, this characteristic considerably complicates the mission design, by making the space probe move in a complex gravitational field and submitting it to very strong perturbations forces. Following the past researches on the ASTER mission, which mostly dealt with the characterisation of the 2001 SN263 asteroid system, this work focuses on the preliminary design of mission orbits suitable for the exploration of the asteroids. Two phases of the mission are considered: the arrival in the system, which requires a parking orbit; and the exploration phase. For the latter, two scenarios are studied: a parallel and a sequential observations of the system. To find the optimal orbits for each of these cases, a computer tool has been designed, which comprises an orbit integrator able to propagate the trajectory of a spacecraft within the asteroid system, and an optimiser which uses evolutionary algorithms to find optima from a 5-dimensional search space in a single- or multi-dimensional objective space, according to objective functions that can be chosen and adapted to match the case considered. The computer tool performs well for all cases, and allows to draw general conclusions on which kind of orbits to consider for the ASTER mission. The results show that the solar radiation pressure is by far the most problematic perturbation and is hence driving the properties of the solutions. Among all cases, many optima are terminator orbits, which are by nature strong against solar radiation perturbations. Moreover, orbits closer to the bodies are more stable, and any trajectory too distant from the bodies will be blown away. This work concludes on the suitability of the optimisation methods selected to the orbit design for this mission, although it is advised to still improve the software to model the dynamics of the system in a more detailed manner, and on recommendations for the ASTER mission. No satisfying parking orbit has been found and the relative strength of the solar radiation pressure implies that there does not exist orbits sufficiently remote from the bodies to serve as parking orbits. It is recommended to investigate other solutions with active orbit maintenance. As for the exploration phase, the sequential observation scheme shows its superiority. Satisfying observation orbits can be found about all three bodies, which is not the case for the parallel observation because of the zones of instability present in-between the bodies.Aerospace EngineeringAstrodynamics and Space Mission
Plays By E.N. Chirikov in the Context of the Russian Drama of the First Third of the XX Century
No full textВпервые исследуется творчество русского писателя Евгения Николаевича Чирикова (1864−1932) в контексте драматургии первых десятилетий XX века; рассматривается специфика его творческого метода и писательской репутации. Предпринята попытка выявить эстетические принципы и определить мировоззренческие, социокультурные и исторические установки, обусловившие восприятие действительности данным автором. Особое внимание уделяется творчеству писателей (А.П. Чехова, М. Горького. С. Найденова и др.)., оказавших влияние на Е.Н. Чирикова. Акцент делается на проблематике его пьес, их месте в литературном процессе указанного периода. = In this article works of the Russian writer Evgeny Nikolaevich Chirikov (1864−1932) in the context of the dramatic art of the first decades of the 20th century are for the first time investigated; the specifics of its creative method and literary reputation are considered. An attempt was made to reveal the aesthetic principles and to de-fine the world outlook, sociocultural and historical attitudes which caused perception of reality by this author. Special attention is paid to the works of writers (A.P. Chekhov, M. Gorky. S. Naidyonova and others.), had an impact on E.N. Chirikov. The emphasis is placed on a set of problems of its plays, their place in the literary process of the specified period
Thermospheric density and wind determination from satellite dynamics
No full textAstrodynamics and Satellite MissionsAerospace Engineerin
Herd behavior, the "Penguin effect", and the suppression of informational diffusion: An analysis of informational externalities and payoff interdependency
No full textTechnology;Information;Econometric Models;Group Behaviour
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