Sintra, Portugal
Sintra, Portugal

Time filter

Source Type

Morgado J.A.P.,Telecommunications Institute of Portugal | Morgado J.A.P.,Portuguese Air Force Academy | Fernandes C.A.F.,Telecommunications Institute of Portugal | Fernandes C.A.F.,University of Lisbon | Boavida J.B.M.,Telecommunications Institute of Portugal
Optics and Laser Technology | Year: 2010

This paper shows that a suitable design of a single phase-shift (1PS) distributed coupling coefficient (DCC) distributed feedback (DFB) laser structure can strongly improve both the normalized mode selectivity S and the flatness of the electrical field profile of DFB lasers-usually quantified by a flatness parameter F -in order to ensure the required criteria for single longitudinal mode operation (S ≥ 0.25 and F ≤ 0.05) through an extended range of biasing current injection (I). It is shown that a symmetric longitudinal laser cavity should be used in order to accomplish the requirements imposed by the modern optical communication systems. Photon and carrier rate equations have been used to evaluate the performance of the proposed laser structure in the above-threshold regime. The variations of the selectivity, the flatness, the lasing-wavelength, the emitted power (P) and the side-mode suppression ratio (SMSR) versus current have been assessed. For I=5 I th, where I th is the laser threshold current, substantial improvements in S (60% higher), in F (40% better), in P (45% higher) and in the SMSR (about 4 dB higher) are achieved in the proposed 1PS-DCC-DFB laser when compared to similar single phase-shifted DFB structures referred elsewhere. The improvements are even better when compared to the standard QWS-DFB laser. © 2010 Elsevier Ltd. All rights reserved.

Rocha B.,University of Lisbon | Silva C.,Portuguese Air Force Academy | Suleman A.,University of Lisbon
Shock and Vibration | Year: 2010

The paper presents a structural health monitoring system based on propagation of tuned Lamb waves and their interference with discontinuities. The dispersion curves are studied to determine the appropriate type and dimension of transducers and to select the optimum scanning frequencies and relevant propagation modes. A piezoelectric sensor network was implemented in an aluminum plate in order to generate and to sense the wave propagation and associated reflections. The algorithm developed for damage detection relies on the comparison of undamaged and damaged responses of the structure. Combinations of filters and statistical methods were applied to detect differences in the sensor signals acquired for the two different states (damaged and undamaged), corresponding to damage reflections. In order to eliminate the false positives due to noise, a probability analysis is performed to obtain the final damage position. The software designed for the current application allows the automatic calculation of dispersion curves, it executes the scans, performs data processing, executes the detection algorithm and presents the probable damages and their positions in a graphical form. Experiments were performed with the introduction of cumulative damages in the plate such as surface and through-the-thickness holes and cuts, ranging from 7 mm to 1 mm in diameter. Additionally, a stringer was attached to the plate by a single rivet line to simulate an aircraft skin structure. Cuts originating from rivet holes and connecting adjacent rivets, as well as loosened rivets were detected by the system. The introduction of the stringer resulted in a loss of precision in the determination of the radial position of the damages near it. Also, the network revealed significant difficulties in the detection of damages beyond the stringer. © 2010 - IOS Press and the authors.

Cruz G.,Portuguese Air Force Academy | Bernardino A.,University of Lisbon
Lecture Notes in Computer Science (including subseries Lecture Notes in Artificial Intelligence and Lecture Notes in Bioinformatics) | Year: 2015

This paper presents a method to detect boats and life rafts on long wave infrared (LWIR) images, captured by an aerial platform. The method applies the concept of image saliency to highlight distinct areas on the images. However saliency algorithms always highlight salient points in the image, even in the absence of targets. We propose a statistical method based on the saliency algorithm output to distinguish frames with or without targets. To evaluate the detection algorithm, we have equipped a fixed wing unmanned aerial vehicle with a LWIR camera and gathered a dataset with more than 44000 frames, containing several boats and a life raft. The proposed detection strategy demonstrates a good performance, specially, a low rate of false positives and low computational complexity. © Springer International Publishing Switzerland 2015.

Silva D.,Portuguese Air Force Academy | Aidos H.,University of Lisbon | Fred A.,University of Lisbon
ICPRAM 2016 - Proceedings of the 5th International Conference on Pattern Recognition Applications and Methods | Year: 2016

The unprecedented collection and storage of data in electronic format has given rise to an interest in automated analysis for generation of knowledge and new insights. Cluster analysis is a good candidate since it makes as few assumptions about the data as possible. A vast body of work on clustering methods exist, yet, typically, no single method is able to respond to the specificities of all kinds of data. Evidence Accumulation Clustering (EAC) is a robust state of the art ensemble algorithm that has shown good results. However, this robustness comes with higher computational cost. Currently, its application is slow or restricted to small datasets. The objective of the present work is to scale EAC, allowing its applicability to big datasets, with technology available at a typical workstation. Three approaches for different parts of EAC are presented: a parallel GPU K-Means implementation, a novel strategy to build a sparse CSR matrix specialized to EAC and Single-Link based on Minimum Spanning Trees using an external memory sorting algorithm. Combining these approaches, the application of EAC to much larger datasets than before was accomplished. © Copyright 2016 by SCITEPRESS-Science and Technology Publications, Lda. All rights reserved.

Oliveira T.,Portuguese Air Force Academy | Encarnacao P.,Catholic University of Portugal
Journal of Intelligent and Robotic Systems: Theory and Applications | Year: 2013

The work here presented contributes to the development of ground target tracking control systems for fixed wing unmanned aerial vehicles (UAVs). The control laws are derived at the kinematic level, relying on a commercial inner loop controller onboard that accepts commands in indicated air speed and bank, and appropriately sets the control surface deflections and thrust in order to follow those references in the presence of unknown wind. Position and velocity of the target on the ground is assumed to be known. The algorithm proposed derives from a path following control law that enables the UAV to converge to a circumference centered at the target and moving with it, thus keeping the UAV in the vicinity of the target even if the target moves at a velocity lower than the UAV stall speed. If the target speed is close to the UAV speed, the control law behaves similar to a controller that tracks a particular point on the circumference centered at the target position. Real flight tests results show the good performance of the control scheme presented. © Springer Science+Business Media Dordrecht 2012.

Gomes A.,Portuguese Air Force Academy | Gomes A.,Instituto Universitario Of Lisbon Iscte Iul | Dias J.G.,Instituto Universitario Of Lisbon Iscte Iul
International Journal of Aviation Psychology | Year: 2015

Latent growth mixture modeling is a statistical approach that models longitudinal data, grouping individuals who share similar longitudinal data patterns into latent classes. We evaluated the application of this method in a sample of ab initio pilot applicants (N = 297), using longitudinal data collected from a military flight-screening program (where the applicants flew seven required flights), resulting in a final pass–fail outcome. Results showed the existence of a two-class solution (Cluster 1 presented an initially higher performance and contained 75% of the Pass candidates) and the psychomotor coordination and general adaptability showed a significant effect. © 2015 Taylor & Francis Group, LLC.

Cruz G.C.S.,Portuguese Air Force Academy | Encarnacao P.M.M.,Catholic University of Portugal
Journal of Intelligent and Robotic Systems: Theory and Applications | Year: 2012

This work is framed within the PITVANT project and aims to contribute to the development of obstacle avoidance techniques for unmanned aerial vehicles (UAVs). The paper describes the design, implementation and experimental evaluation of a potential field obstacle avoidance algorithm based on the fluid mechanics panel methods. Obstacles and the UAV goal position are modeled by harmonic functions thus avoiding the presence of local minima. Adaptations are made to apply the method to the automatic control of a fixed wing aircraft, relying only on a local map of the environment that is updated with information from sensors onboard the aircraft. Hardware-In-Loop simulations show the good performance of the proposed algorithm in the envisioned mission scenarios for the PITVANT vehicles. © 2011 Springer Science+Business Media B.V.

Pereira L.F.,Technical University of Delft | Pereira L.F.,Portuguese Air Force Academy | Weerheijm J.,Technical University of Delft | Weerheijm J.,TNO | Sluys L.J.,Technical University of Delft
International Journal of Impact Engineering | Year: 2016

The development of realistic numerical tools to efficiently model the response of concrete structures subjected to close-in detonations and high velocity impact has been one of the major quests in defense research. Under these loading conditions, quasi-brittle materials undergo a multitude of failure (damage) mechanisms. Dynamic tensile failure (e.g. spalling), characterized by a significant strength increase associated with loading rate, has revealed to be particularly challenging to represent. In this contribution, a rate-dependent stress-based nonlocal damage model has been introduced for the simulation of dynamic tensile failure of quasi-brittle materials. The recently proposed stress-based nonlocal criterion has been updated in order to be consistently combined with a rate-dependent version of the well-known Mazars damage model. The model was implemented in LS-DYNA using a fully explicit computational scheme. Two sets of numerical examples have been presented. First, one-dimensional numerical analyses were conducted to evaluate the model capabilities, applicability and limitations. Second, the model has been validated against experimental results. It has been shown that the proposed model, in addition to correcting spurious mesh sensitivity, also provides a more realistic representation of damage initiation and growth, in particular around discontinuities (notches and free boundaries) and damaged areas. © 2016 Elsevier Ltd. All rights reserved.

Oliveira T.,Portuguese Air Force Academy | Oliveira T.,Science Laboratory | Encarnacao P.,Catholic University of Portugal
AIAA Guidance, Navigation, and Control Conference | Year: 2010

The work here presented is framed in the Research and Technology Project on Unmanned Aerial Vehicles (PITVANT) and contributes to the development of control systems for ground target tracking for unmanned aerial vehicles (UAVs). It is assumed that the UAV has an inner loop controller onboard that accepts commands in indicated air speed and bank, and appropriately sets the control surface deflections and thrust in order to follow those references in the presence of unknown wind. An outer loop is designed so that the UAV can track a moving target on the ground whose position, velocity and acceleration is assumed to be known. The algorithm switches between a tracking control law responsible for taking the UAV to the vertical of the ground target and a path following control law that enables the UAV to follow a circumference centered at the moving target, keeping it in the vicinity of the target even if the target moves at a velocity lower than the UAV stall velocity. Hardware in the loop simulations using one of the PITVANTs vehicles show the performance of the control scheme presented. Copyright © 2010 by Authors.

Boavida J.B.M.,Institute Telecomunicaes | Morgado J.A.P.,Institute Telecomunicaes | Morgado J.A.P.,Portuguese Air Force Academy | Fernandes C.A.F.,Institute Telecomunicaes
Optics and Laser Technology | Year: 2011

Optimized HR-AR-DFB laser structures are proposed foreseeing their use in directly modulated optical communication systems. They present, simultaneously, high single-longitudinal-mode yield, stable single-longitudinal-mode operation, high power efficiency and stable emitted wavelength. Lasers harder to manufacture were previously proposed elsewhere and did not succeed in achieving such high performances. A detailed assessment of the proposed lasers is presented and discussed. © 2010 Elsevier Ltd. All rights reserved.

Loading Portuguese Air Force Academy collaborators
Loading Portuguese Air Force Academy collaborators