Hermite-Gauss functions in the analysis of a category of semiconductor optical devices

Available from British Library Document Supply Centre-DSC:DXN017041 / BLDSC - British Library Document Supply CentreSIGLEGBUnited Kingdo

Detection and tracking of non-cooperative flying obstacles using low SWaP radar and optical sensors: an experimental analysis

Improvements in low altitude, non-cooperative sense and avoid are of major interest for collision hazard mitigation within the UTM/UAM/U-Space framework. In this regard, the sensing architecture must be carefully designed so that its detection and tracking performance is suitable for timely and reliable conflict assessment, while respecting size, weight, power and costs constraints, which are particularly strict for small aerial vehicles. Within this framework, an experimental assessment of non-cooperative sensing solutions based on a lightweight radar and a visual camera, respectively, is presented in this paper. Visual detections are obtained by using a Deep Learning-based neural network, while raw detections produced by the radar are first filtered based on Doppler information to remove ground clutter, and then clustered by means of a centroiding approach. The resulting detection sets are used to generate tentative and firm tracks using customized Kalman filtering techniques. Following a research plan that foresees data gathering with incremental complexity, ground-to-air tests have been carried out using a small UAV as flying intruder, and Carrier-Phase Differential GNSS to get a reference solution and assess visual-based and radar-based detection and tracking performance. Results achieved by standalone radar and visual sensing solutions clearly highlight the potential of sensor fusion strategies to take advantage of their complementary characteristics

Experimental analysis of Radar/Optical track-to-track fusion for non-cooperative Sense and Avoid

In the framework of non-cooperative Sense and Avoid solutions, major attention is reserved to the design of sensing strategies which can enable fast and reliable identification of possible near collision threats, by exploiting passive or active exteroceptive sensors. To overcome the limits of standalone technological architectures and provide more accurate tracking solutions, which can be used to improve conflict detection and thus better support avoidance strategies, data fusion approaches can be considered. Hence, this work proposes a radar/visual fusion method based on the track-to-track fusion approach. The strategy is tested on data gathered during ground-to-air experimental flight tests involving a small UAV commanded to fly near collision approach geometries with respect to a multi-sensor setup placed on the ground. Results collected analyzing three different encounters show that the fusion solution allows retrieving meter and sub-degree level accuracies in the intruder range and bearing estimation, respectively, while ensuring declaration ranges of about 500 meters

Ground-to-air experimental assessment of low SWaP radar-optical fusion strategies for low altitude Sense and Avoid

The future of UAS operations requires adequate and efficient Sense and Avoid strategies to ensure their safe and secure integration in both controlled and uncontrolled airspace. To make onboard implementation of these strategies feasible, research must focus on the development of sensing and tracking solutions, taking size, weight and power (SWaP) constraints as well as the challenging scenarios characterizing low altitude operations, into account. In this framework, visual cameras and low SWaP radars are among the most popular sensing choices. Hence, exploiting such sensing solutions, this paper proposes both single and multi-sensor detection and tracking approaches and compares their performance. Specifically, the developed techniques are tested on data retrieved during ground-to-air tests which involve a small UAV flying near collision geometries, starting from a range of about 550 m. Different tracking strategies are considered including standalone visual, standalone radar, and fused radar-visual. Concerning intruder detection, several visual-based techniques are investigated based on machine learning, morphological filtering and template matching. Radar detections are filtered and centroided with ad hoc algorithms. While in clear air conditions comparable declaration ranges, larger than 500 m, are provided by all the tested approaches, results show the advantages of using a fused strategy to attain sub-degree angular and angular rate tracking accuracy coupled with the highly accurate range and range rate, around 2 m and 1 m/s, respectively, typical of radars. Conflict detection performance is proven to also benefit from the use of a fused strategy in terms of smaller errors in the estimation of the distance at closest point of approach

Onboard and External Magnetic Bias Estimation for UAS through CDGNSS/Visual Cooperative Navigation

This paper describes a calibration technique aimed at combined estimation of onboard and external magnetic disturbances for small Unmanned Aerial Systems (UAS). In particular, the objective is to estimate the onboard horizontal bias components and the external magnetic declination, thus improving heading estimation accuracy. This result is important to support flight autonomy, even in environments characterized by significant magnetic disturbances. Moreover, in general, more accurate attitude estimates provide benefits for georeferencing and mapping applications. The approach exploits cooperation with one or more “deputy” UAVs and combines drone-to-drone carrier phase differential GNSS and visual measurements to attain magnetic-independent attitude information. Specifically, visual and GNSS information is acquired at different heading angles, and bias estimation is modelled as a non-linear least squares problem solved by means of the Levenberg–Marquardt method. An analytical error budget is derived to predict the achievable accuracy. The method is then demonstrated in flight using two customized quadrotors. A pointing analysis based on ground and airborne control points demonstrates that the calibrated heading estimate allows obtaining an angular error below 1°, thus resulting in a substantial improvement against the use of either the non-calibrated magnetic heading or the multi-sensor-based solution of the DJI onboard navigation filter, which determine angular errors of the order of several degrees

Improved Sensing Strategies for Low Altitude Non Cooperative Sense and Avoid

Very low altitude non-cooperative sense and avoid is considered as an essential component of multi-layered mitigation strategies for collision hazard within the UTM/U-Space framework. However, it currently represents a challenging problem with open issues concerning performance trade-offs and technological options. In fact, low altitude operations emphasize sensing challenges such as ground clutter removal for radar, and below-the-horizon detection for optical sensors. These issues have a significant impact in scenarios involving small UAS, which may be characterized by low detectability though potentially generating relatively fast encounter geometries. As a contribution to this framework, this paper proposes some improved sensing strategies for these scenarios. The first idea is to optimize architectural and algorithmic trade-offs by accounting for the possible closure rates corresponding to near collision scenarios. This approach can be used to develop adaptive sensing strategies, as well as to select different sensors to cover different areas of the aircraft field of regard. The concept is demonstrated in numerical analyses focused on the computation of probability of missed and false conflict detection. Then, with regards to purely visual architectures, the possibility to extract and use shape-based ranging information to improve conflict detection performance is investigated through flight tests with small UAS. Finally, the paper describes first experimental activities conducted with a compact collision avoidance radar

Carbon Structures Grown by Direct Current Microplasma: Diamonds, Single-Wall Nanotubes, and Graphene

Plasma assisted CVD is now an established technique for the growth of a variety of dielectrics and semiconductors. The versatility of an in-house developed direct-current (dc) microplasma deposition system is demonstrated here for the growth of a wide range of carbon-based materials. Diamond, nanodiamond, nanocrystalline graphite, single-wall carbon nanotubes, and few-layer graphene have been deposited using the same dc microplasma deposition system using 0.5% CH4/H2 gas feed, but changing only the substrate temperature (in the range 500−1150 °C) and the total pressure (0.3−200 Torr). The different structures have been characterized by scanning electron microscopy and micro-Raman spectroscopy. The experimental data have been interpreted from a thermodynamic point of view by applying a nonequilibrium nondissipative model. Nonequilibrium phase diagrams are presented and compared to the experimental data to provide a wide-ranging interpretation scenario

Analysis of cathode geometry to minimize cathode erosion in direct current microplasma jet

Microplasma jets are now widely used for deposition, etching, and materials processing. The present study focuses on the investigation of the influence of cathode geometry on deposition quality, for microplasma jet deposition systems in low vacuum. The interest here is understanding the influence of hydrogen on sputtering and/or evaporation of the electrodes. Samples obtained with two cathode geometries with tapered and rectangular cross-sections have been investigated experimentally by scanning electron microscopy and energy dispersion X-ray spectroscopy. Samples obtained with a tapered-geometry cathode present heavy contamination, demonstrating cathode erosion, while samples obtained with a rectangular-cross-section cathode are free from contamination. These experimental characteristics were explained by modelling results showing a larger radial component of the electric field at the cathode inner wall of the tapered cathode. As a result, ion acceleration is larger, explaining the observed cathode erosion in this case. Results from the present investigation also show that the ratio of radial to axial field components is larger for the rectangular geometry case, thus, qualitatively explaining the presence of micro-hollow cathode discharge over a wide range of currents observed in this case. In the light of the above findings, the rectangular cathode geometry is considered to be more effective to achieve cleaner deposition