1,721,068 research outputs found
Distributed Multi-Robot Exploration under Complex Constraints
Full text linkMobile robots have emerged as a prime alternative to explore physical processes of interest. This is particularly relevant in situations that have a high risk for humans, like e.g. in search and rescue missions, and for applications in which it is desirable to reduce the required time and manpower to gather information, like e.g. for environmental analysis. In such context, exploration tasks can clearly benefit from multi-robot coordination. In particular, distributed multi-robot coordination strategies offer enormous advantages in terms of both system's efficiency and robustness, compared to single-robot systems. However, most state-of-the-art strategies employ discretization of robots' state and action spaces. This makes them computationally intractable for robots with complex dynamics, and limits their generality. Moreover, most strategies cannot handle complex inter-robot constraints like e.g. communication constraints.
The goal of this thesis is to develop a distributed multi-robot exploration algorithm that tackles the two aforementioned issues. To achieve this goal we first propose a single-robot myopic approach, in which we build to develop a non-myopic informative path planner. In a second step, we extend our non-myopic single-robot algorithm to the multi-robot case. Our proposed algorithms build on the following techniques: (i) Gaussian Processes (GPs) to model the spatial dependencies of a physical process of interest; (ii) sampling-based planners to calculate feasible paths; (iii) information metrics to guide robots towards informative locations; and (iv) distributed constraint optimization techniques for multi-robot coordination.
We validated our proposed algorithms in simulations and experiments. Specifically, we carried out the following experiments: mapping of a magnetic field with a ground-based robot, mapping of a terrain profile with two quadcopters equipped with an ultrasound sensor, and exploration of a simulated wind field with three quadcopters. Results demonstrate the effectiveness of our approach to perform exploration tasks under complex constraints
Consensus Based Distributed Sparse Bayesian Learning by Fast Marginal Likelihood Maximization
No full textFor swarm systems, distributed processing is of paramount importance, and Bayesian methods are preferred for their robustness. Existing distributed sparse Bayesian learn- ing (SBL) methods rely on the automatic relevance deter- mination (ARD), which involves a computationally complex reweighted l1-norm optimization, or they use loopy belief propagation, which is not guaranteed to converge. Hence, this paper looks into the fast marginal likelihood maximiza- tion (FMLM) method to develop a faster distributed SBL version. The proposed method has a low communication overhead, and can be distributed by simple consensus meth- ods. The performed simulations indicate a better performance compared with the distributed ARD version, yet the same per- formance as the FMLM.Green Open Access added to TU Delft Institutional Repository ‘You share, we take care!’ – Taverne project https://www.openaccess.nl/en/you-share-we-take-care Otherwise as indicated in the copyright section: the publisher is the copyright holder of this work and the author uses the Dutch legislation to make this work public.Signal Processing System
Asymptotic Behavior of Super-resolution Sparse Bayesian Learning
No full textSparse Bayesian Learning with dictionary refinement (SBL-DR) is a gridless technique for sparse signal reconstruction, focusing on
super-resolution estimation of spectral line locations and their quantity. Its cost function coincides with that of stochastic maximum likelihood (SML), a well-known method in array processing for estimating frequencies of complex exponentials. While SML exhibits consistency and efficiency with growing array snapshots or size, SBL-DR faces inconsistency with only one measurement snapshot. This
study explores SBL-DR asymptotic behavior using a single measurement snapshot and growing sample size using Gamma-convergence theory. It computes upper and lower bounds for the SBL-DR cost function, showing their convergence to a Gamma-limit that is minimized at true signal locations. By leveraging the properties of Gamma-convergence, it is established that the minima of the SBL-DR cost function asymptotically approach those of the Gamma-limit function, thereby achieving consistency for SBL-DR
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
LDACS1-Based Non-Cooperative Surveillance
No full textIn civil aviation, ADS-B is expected to become the primary surveillance technology. To ensure that accurate surveillance services can be provided even during possible ADS-B outages or when the localization of a non-cooperative aircraft is of interest, a backup system is required. Inspired by the recent advancements in passive radar technology, we propose to reuse the infrastructure and signals of the future L-band digital aeronautical communications system type 1 (LDACS1) to establish a non-cooperative backup surveillance application. LDACS1 is a multi-carrier system that uses frequency-division duplexing and is designed to
operate in the lower part of the L-band. Its main advantage is that LDACS1 channels are placed in the spectral gaps between the channels of the distance measuring equipment - a legacy radio navigation system currently used in civil aviation. The LDACS1 surveillance functionality is enabled through the reuse of the forward link signals which are continuously transmitted by the LDACS1 ground stations. The emitted signals are reflected by aircraft in all directions toward the ground and can be picked up by dedicated surveillance receivers. An appropriate processing of the received reflections allows for the detection of the aircraft and estimation of its position on the ground. This implementation essentially mimics a passive radar system, where commercial transmitters are used as illuminators of opportunity to detect and estimate airborne targets. The main difference to the proposed surveillance technology is that transmitted signals can be readily made available at the receivers since the same infrastructure is used for both communication and surveillance purposes; in other words, there is no need to reconstruct the original transmitted signal at the receivers. The challenges associated with the LDACS1-based surveillance application arise from the relatively low transmit power and small bandwidth characteristics of the LDACS1 signal. This makes the practical implementation of such a surveillance concept challenging. It is thus of great importance to investigate and assess the suitability and limitations of the LDACS1 communication signals for passive radar detection applications
Cramér–Rao bounds for L-band digital aeronautical communication system type 1 based passive multiple-input multiple-output radar
No full textIn this study, of concern is the achievable estimation accuracy for a non-coherent passive multiple-input
multiple-output (MIMO) radar employing orthogonal frequency-division multiplexing (OFDM) based signals of
opportunity. The motivation behind this investigation is the surveillance aspect of the air trac management
modernisation process currently taking place in civil aviation. To this end, the surveillance capability of the OFDM-based
L-band digital aeronautical communication system type 1 (LDACS1) is analysed. The extension of the LDACS1
communication system towards surveillance is of great importance since it would enable an alternative, non-cooperative
surveillance application, and, together with the already validated navigation extension, establish a fully
integrated LDACS1-based communication, navigation, and surveillance technology. The modifed Cramer-Rao bound on
the joint estimation of target location and velocity for a non-coherent passive MIMO radar, employing the OFDM-based
LDACS1 signals as signals of opportunity, is investigated. The analysis is carried out assuming one target, multiple and
widely separated transmit and receive antennas, orthogonal waveforms, independent refection coefficients, and zero mean
white Gaussian noise. Closed-form expressions for the relevant elements of the modified Fisher information
matrix are obtained. The results can be used to assess the estimation accuracy of a multiple antenna passive radar
system employing any OFDM-based signals of opportunity
LDACS1-Based Passive MIMO Radar
No full textIn civil aviation, ADS-B is expected to become the primary surveillance technology. To ensure that accurate surveillance services can be provided even during possible ADS-B outages or when the localization of a non-cooperative aircraft is of interest, a backup system is required. Motivated by the Alternative Positioning, Navigation, and Timing (APNT) technologies currently developed to serve as a GPS backup for aeronautical users, we propose using LDACS1 as a non-cooperative surveillance alternative and backup. To this end, we reuse the signals of the future L-band digital aeronautical communications system type 1
(LDACS1) as signals of opportunity for target detection and parameter estimation. LDACS1 is a multi-carrier system that uses frequency-division duplexing and is designed to operate in the lower part of the L-band. Its main advantage is that LDACS1 channels are placed in the spectral gaps between the channels of the distance measuring equipment - a legacy radio
navigation system currently used in civil aviation. The LDACS1 surveillance functionality is enabled by the forward link signals which, continuously transmitted by the LDACS1 ground stations, are reflected off the aircraft and towards the dedicated surveillance receivers within the radio horizon of the reflecting aircraft. An appropriate processing of the received reflections allows for the detection of the aircraft and estimation of its position on the ground. This implementation resembles a passive radar system, where commercial transmitters are used as illuminators of opportunity to detect and estimate airborne targets. The main difference
to the proposed surveillance technology is the availability of the transmitted signals at the receivers. While, in general, a passive radar system does not have any access to the transmitted signals, which need to be therefore reconstructed, for the LDACS1-based application
the transmitted signals can be readily made available since the same infrastructure is used for both communication and surveillance purposes. Moreover, the LDACS1 infrastructure enables the implementation of a MIMO radar with widely distributed antennas when using the LDACS1 ground stations as MIMO radar transmit antennas and augmenting
the infrastructure with dedicated surveillance receivers. The challenges associated with the LDACS1-based surveillance application arise from the relatively low transmit power and small bandwidth characteristics of the LDACS1 signal which make the practical implementation of such a surveillance concept challenging
- …
