17 research outputs found
Analysis of star camera errors in GRACE data and their impact on monthly gravity field models
No full textStar cameras (SCs) on board the GRACE satellites provide information about the attitudes of the spacecrafts. This information is needed to reduce the K-band ranging data to the centre of mass of the satellites. In this paper, we analyse GRACE SC errors using two months of real data of the primary and secondary SCs. We show that the errors consist of a harmonic component, which is highly correlated with the satellite’s true anomaly, and a stochastic component. We built models of both error components, and use these models for error propagation studies. Firstly, we analyse the propagation of SC errors into inter-satellite accelerations. A spectral analysis reveals that the stochastic component exceeds the harmonic component, except in the 3–10 mHz frequency band. In this band, which contains most of the geophysically relevant signal, the harmonic error component is larger than the random component. Secondly, we propagate SC errors into optimally filtered monthly mass anomaly maps and compare them with the total error. We found that SC errors account for about 18 % of the total error. Moreover, gaps in the SC data series amplify the effect of SC errors by a factor of 5 . Finally, an analysis of inter-satellite pointing angles for GRACE data between 2003 and 2010 reveals that inter-satellite ranging errors were exceptionally large during the period February 2003 till May 2003. During these months, SC noise is amplified by a factor of 3 and is a considerable source of errors in monthly GRACE mass anomaly maps. In the context of future satellite gravity missions, the noise models developed in this paper may be valuable for mission performance studies.Geoscience & Remote SensingCivil Engineering and Geoscience
Helium and fission gas behaviour in magnesium aluminate spinel and zirconia for actinide transmutation
No full textIn order to reduce the long-term radiotoxicity of spent nuclear fuel, many studies are performed on partitioning and transmutation of actinides. In such a scenario, the long-lived radio-isotopes (mostly actinides) are partitioned from the nuclear waste, and subsequently transmuted or fissioned in a neutron flux in shorter-lived or stable isotopes. In order to place the actinides in a neutron flux, they must be embedded material that behaves inert under neutron irradiation, a so-called inert matrix material. As most actinides decay by emitting a a-particle, large quantities of helium will be build up in the inert matrix material, before, during and after neutron irradiation. The behaviour of the helium in the material gives information whether or not a material could be used as an inert matrix. In the thesis, the behaviour of helium in two candidate inert matrix materials has been studied: magnesium aluminate spinel (MgAl2O4) and zirconia (ZrO2) fully stabilised with yttria (Y2O3). In most cases, a known quantity of helium has been implanted in the material and various investigation techniques were applied to see at which temperatures helium is released from the specimen and how the damage, introduced by the ion-implantation recovers. Investigations have also been performed on the behaviour of helium and fission gases in a spinel matrix irradiated with neutrons in a reactor. This inert matrix had an initial weight fraction of 241Am of 11.2%, of which after irradiation 96% had been transmuted or fissioned. The work described in the thesis has been supported with a Marie Curie fellowship. The major part of the work has been performed in the laboratories of the Institute for Transuranium Elements of the European Commission at Karlsruhe, together with the department of Defects in Materials of the Interfaculty Reactor Institute at Delft.Interfaculty Reactor Institut
A Sensitivity Study into Strapdown Airborne Gravimetry
No full textAirborne gravimetry is an important tool for the geodesy and geophysics communities. Able to provide medium to high-resolution measurements over large areas, it is the link between the low-resolution satellite measurements and expensive terrestrial campaigns, especially in remote areas. To explore the potential of airborne gravimetry, the Gravimetry using Airborne Inertial Navigation (GAIN) project was recently established at the faculty of Aerospace Engineering at TU-Delft, and is currently building and testing an in-house strapdown airborne gravimetry system with the objective of providing low-cost, high-accuracy gravity data for use in a wide range of applications in geodesy and geophysics. Within this thesis, the inertial sensors that will be used within the GAIN strapdown IMU are calibrated and modeled with a simulator to predict the accuracy of the airborne system when completed. A sensitivity study of several campaign parameters is done to understand which parts of the hardware and operating conditions are critical to the performance of the system. Of the list of applications for airborne gravity data, natural resource exploration is one of the more demanding in terms of accuracy and resolution, with a requirement of 0.5-2mGal at 2km resolution. This is beyond the range of current strapdown systems, so in addition to assessing the performance of the current strapdown system, additional tests were made to see what would be needed to achieve this higher level of accuracy. The simulation results suggest that the performance of the GAIN strapdown system, under ideal conditions, would be 1.4mGal at 2km resolution. Furthermore, the performance is limited by the accelerometers whose accuracy must improve by a factor of three before the 0.5mGal level can be achieved; however, other options were identified that could also be used to achieve this.Physical and Space GeodesyDepartment of Remote SensingAerospace Engineerin
De ontwikkeling van een kennissysteem ter ondersteuning van het configureren van digitale telefooncentrales
No full textHet digitale telefoniesysteem 5ESS-PRX kent een zeer modulaire en gedecentraliseerde structuur. Hierdoor zijn met dit systeem telefooncentrales te creëren, die onderling sterk in omvang, functie en hiërarchie kunnen verschillen. De schijnbaar ongelimiteerde mogelijkheden maken echter het configureren van deze telefooncentrales tot een omvangrijk en ingewikkeld proces. Daarom is door APT NEDERLAND BV een informatiesysteem (PRIDE) ontwikkeld, dat configureren van een 5ESS-PRX-centrale verzorgt. Het configuratieproces wordt door PRIDE als een aantal uit te voeren configuratie-stappen gepresenteerd. Desondanks blijft het configureren voor de gebruiker een complexe aangelegenheid die niet eenvoudig in handboeken is op te nemen…Applied SciencesElectrotechnie
Distant Retrograde Orbits: Modeling and Stability
No full textDistant Retrograde Orbits (DROs) are special orbits for third bodies in two-body systems. The third body revolves around the secondary – the smaller of the two primaries – in a retrograde way, meaning the direction is opposite to the direction that the primaries revolve around each other. DROs are not close to either of the primaries, making it difficult to model them as perturbed two-body orbits.There is no analytical solution for the initial conditions of DROs. This thesis presents a novel method of calculating an initial velocity guess which is then fed into a differential corrector that is able to calculate the initial conditions. In contrast to the state-of-the-art, this happens without the method of incremental steps in the initial position, which requires to go through all possible DROs for a specific two-body system first.For the calculation of DROs, numerical integration is done. Optimal integrator settings are determined, which is in this case an eighth-order Runge-Kutta method (RK8). By setting the tolerance to the lowest possible value, the accuracy requirements are satisfied.Furthermore, this thesis explores a different method of modeling DROs that makes use of Fourier series and polynomials, which had already been proposed by Hirani in 2006 for a different set of parameters. By exploiting explicit knowledge about the shape of DROs, this approach is made more efficient in terms of accuracy per Fourier/polynomial parameters needed and thus the computation time is enhanced.The second part of this study addresses the stability of DROs. This is analyzed in order to get an idea of what DROs would be suitable for future missions. For mass ratios of primary and secondary that realistically occur in the Solar System, all DROs that are closer to the secondary than the primary turn out to be stable when disregarding perturbations. Perturbations are modeled as a constant external acceleration with a constant direction, which is only a first step towards modeling the Sun's and other planet's point mass gravity (p.m.g.), the solar radiation pressure (s.r.p.), and other perturbations, as they are usually depending on time and position. With this rough estimate, only the Sun's p.m.g. is identified as a possible source of instability for DROs in the Earth-Moon system, as all other perturbations are too small.Aerospace Engineerin
Virtual Reality Treatment in Acrophobia: A Comparison with Exposure in Vivo
No full textThe aim of the present study was to evaluate the effectiveness of low-budget virtual reality exposure versus exposure in vivo in a within-group design in 10 individuals suffering from acrophobia. Virtual reality exposure was found to be at least as effective as exposure in vivo on anxiety and avoidance as measured with the Acrophobia Questionnaire (AQ), and even more effective on the Attitude Towards Heights Questionnaire (AHQ). The present study shows that virtual reality exposure can be effective with relatively cheap hardware and software on stand-alone computers currently on the market. Further studies are recommended, in which virtual reality exposure is compared with in vivo exposure in a between-group design, thus enabling investigation of the long-term effects of virtual reality treatment.Electrical Engineering, Mathematics and Computer Scienc
Updated Delft mass transport model DMT-2: Computation and validation
No full textGeoscience & Remote SensingCivil Engineering and Geoscience
Do Self-Statements Enhance the Effectiveness of Virtual Reality Exposure Therapy? A Comparative Evaluation in Acrophobia
No full textThere is a clear need for more detailed analysis of the role of cognitive self-statements in virtual reality exposure therapy (VRET). To date, no research on this topic has been done. The primary aim of this study was to investigate whether coping self-statements would enhance the effectiveness of VRET. In a randomized crossover design, 26 patients with acrophobia (DSM-IV diagnosis of specific phobia) were randomly assigned to two sessions of VRET followed by two sessions of VRET plus coping self-statements, or the other way around: first two sessions of VRET plus coping self-statements followed by two sessions of VRET. Results showed that VRET, regardless of addition of coping self-statements, decreased anxiety of heights, decreased avoidance of height situations, and improved attitudes towards heights. However, at 6-month follow-up, most gains during treatment were not fully retained.MediamaticsElectrical Engineering, Mathematics and Computer Scienc
Sense of presence and anxiety during virtual social interactions between a human and virtual humans
No full textVirtual reality exposure therapy (VRET) has been shown to be effective in treatment of anxiety disorders. Yet, there is lack of research on the extent to which interaction between the individual and virtual humans can be successfully implanted to increase levels of anxiety for therapeutic purposes. This proof-of-concept pilot study aimed at examining levels of the sense of presence and anxiety during exposure to virtual environments involving social interaction with virtual humans and using different virtual reality displays. A non-clinical sample of 38 participants was randomly assigned to either a head-mounted display (HMD) with motion tracker and sterescopic view condition or a one-screen projection-based virtual reality display condition. Participants in both conditions engaged in free speech dialogues with virtual humans controlled by research assistants. It was hypothesized that exposure to virtual social interactions will elicit moderate levels of sense of presence and anxiety in both groups. Further it was expected that participants in the HMD condition will report higher scores of sense of presence and anxiety than participants in the one-screen projection-based display condition. Results revealed that in both conditions virtual social interactions were associated with moderate levels of sense of presence and anxiety. Additionally, participants in the HMD condition reported significantly higher levels of presence than those in the one-screen projection-based display condition (p = .001). However, contrary to the expectations neither the average level of anxiety nor the highest level of anxiety during exposure to social virtual environments differed between the groups (p = .97 and p = .75, respectively). The findings suggest that virtual social interactions can be successfully applied in VRET to enhance sense of presence and anxiety. Furthermore, our results indicate that one-screen projection-based displays can successfully activate levels of anxiety in social virtual environments. The outcome can prove helpful in using low-cost projection-based virtual reality environments for treating individuals with social phobia
Attention bias modification: the Emperor's new suit?
No full textAbstract A series of primarily laboratory-based studies found attention bias modification in socially anxious participants to lead to reduced anxiety. It is argued that the failure to replicate the positive results of attention bias modification in the study of Carlbring et al. may be due to reasons other than the application through the Internet. A number of controlled studies failed to replicate the positive effects of attention bias modification in clinically rather than subclinically socially anxious subjects. Given the lack of robust evidence for attention bias modification in clinically socially anxious individuals, the author is inclined to consider attention bias modification as 'the Emperor's new suit'. Results achieved with regular Internet-based treatments for social anxiety disorder based on cognitive therapy and exposure methods are much better than those achieved with attention bias modification procedures delivered 'face to face' in clinically distressed participants. Given the lack of robust evidence for attention bias modification in clinical samples, there is no need yet to investigate the implementation of attention bias modification through the Internet. Please see related article: http://www.biomedcentral.com/1471-244X/12/66</p
