NASA Thesaurus Supplement: A three part cumulative supplement to the 1982 edition of the NASA Thesaurus (supplement 2)

The three part cumulative NASA Thesaurus Supplement to the 1982 edition of the NASA Thesaurus includes: part 1, hierarchical listing; part 2, access vocabulary, and part 3, deletions. The semiannual supplement gives complete hierarchies for new terms and includes new term indications for terms new to this supplement

3-D GPR survey with a modular system: reducing positioning inaccuracies and linear noise

Recently, the use of ground-penetrating radar (GPR) arrays with a large number of antenna elements in a fixed configuration has become more common. The investment needed for these systems is significant. Although gradually expandable modular systems, consisting of antennas which can be used independently, do not match the fast acquisition of detailed datasets by means of multi-channel arrays, they can help finding a compromise between increased acquisition speed and (limited) resources. In modular systems, the separation between transmitter-receiver pairs is often larger than the sampling distance prescribed by the Nyquist theorem. As a consequence, additional profiles have to be recorded in between, which requires a high positioning precision. As a completely identical response for the different antennas in an array is difficult to achieve, stripes can occur in the horizontal slices, especially when ringing occurs. This complicates the interpretation of features in the direction of the survey lines. In this paper, a three-dimensional frequency-wavenumber filter is proposed, consisting in a combination of a circular filter and a fan filter. The application of this filter to GPR data collected at the Roman town Mariana (Corsica, France) showed a reduction of the stripe patterns, allowing a more reliable characterization of subtle archaeological structures

NASA Thesaurus Supplement: A three part cumulative supplement to the 1982 edition of the NASA Thesaurus (supplement 3)

The three part cumulative NASA Thesaurus Supplement to the 1982 edition of the NASA Thesaurus includes Part 1, Hierarchical Listing, Part 2, Access Vocabulary, and Part 3, Deletions. The semiannual supplement gives complete hierarchies for new terms and includes new term indications for entries new to this supplement

Antenna Array Enabled Space/Air/Ground Communications and Networking for 6G

Antenna arrays have a long history of more than 100 years and have evolved closely with the development of electronic and information technologies, playing an indispensable role in wireless communications and radar. With the rapid development of electronic and information technologies, the demand for all-time, all-domain, and full-space network services has exploded, and new communication requirements have been put forward on various space/air/ground platforms. To meet the ever increasing requirements of the future sixth generation (6G) wireless communications, such as high capacity, wide coverage, low latency, and strong robustness, it is promising to employ different types of antenna arrays with various beamforming technologies in space/air/ground communication networks, bringing in advantages such as considerable antenna gains, multiplexing gains, and diversity gains. However, enabling antenna array for space/air/ground communication networks poses specific, distinctive and tricky challenges, which has aroused extensive research attention. This paper aims to overview the field of antenna array enabled space/air/ground communications and networking. The technical potentials and challenges of antenna array enabled space/air/ground communications and networking are presented first. Subsequently, the antenna array structures and designs are discussed. We then discuss various emerging technologies facilitated by antenna arrays to meet the new communication requirements of space/air/ground communication systems. Enabled by these emerging technologies, the distinct characteristics, challenges, and solutions for space communications, airborne communications, and ground communications are reviewed. Finally, we present promising directions for future research in antenna array enabled space/air/ground communications and networking

A Comparative Framework for Maneuverability and Gust Tolerance of Aerial Microsystems

Aerial microsystems have the potential of navigating low-altitude, cluttered environments such as urban corridors and building interiors. Reliable systems require both agility and tolerance to gusts. While many platform designs are under development, no framework currently exists to quantitatively assess these inherent bare airframe characteristics which are independent of closed loop controllers. This research develops a method to quantify the maneuverability and gust tolerance of vehicles using reachability and disturbance sensitivity sets. The method is applied to a stable flybar helicopter and an unstable flybarless helicopter, whose state space models were formed through system identification. Model-based static H-infinity controllers were also implemented on the vehicles and tested in the lab using fan-generated gusts. It is shown that the flybar restricts the bare airframe's ability to maneuver in translational velocity directions. As such, the flybarless helicopter proved more maneuverable and gust tolerant than the flybar helicopter. This approach was specifically applied here to compare stable and unstable helicopter platforms; however, the framework may be used to assess a broad range of aerial microsystems

Hardware, Software, and Low-Level Control Scheme Development for a Real-Time Autonomous Rover

The objective of this research is to develop a low-cost autonomous rover platform for experiments in autonomous navigation. This thesis describes the design, development, and testing of an autonomous rover platform, based on the commercial, off-the-shelf Tamiya TXT-1 radio controlled vehicle. This vehicle is outfitted with an onboard computer based on the Mini-ITX architecture and an array of sensors for localization and obstacle avoidance, and programmed with Matlab/SimulinkRTM Real-Time Workshop (RTW) utilizing the Linux Real-Time Application Interface (RTAI) operating system.;First, a kinematic model is developed and verified for the rover. Then a proportional-integral-derivative (PID) feedback controller is developed for translational and rotational velocity regulation. Finally, a hybrid navigation controller is developed combining a potential field controller and an obstacle avoidance controller for waypoint tracking.;Experiments are performed to verify the functionality of the kinematic model and the PID velocity controller, and to demonstrate the capabilities of the hybrid navigation controller. These experiments prove that the rover is capable of successfully navigating in an unknown indoor environment. Suggestions for future research include the integration of additional sensors for localization and creation of multiple platforms for autonomous coordination experiments

Sensor Array Processing with Manifold Uncertainty

<p>The spatial spectrum, also known as a field directionality map, is a description of the spatial distribution of energy in a wavefield. By sampling the wavefield at discrete locations in space, an estimate of the spatial spectrum can be derived using basic wave propagation models. The observable data space corresponding to physically realizable source locations for a given array configuration is referred to as the array manifold. In this thesis, array manifold ambiguities for linear arrays of omni-directional sensors in non-dispersive fields are considered. </p><p>First, the problem of underwater a hydrophone array towed behind a maneuvering platform is considered. The array consists of many hydrophones mounted to a flexible cable that is pulled behind a ship. The towed cable will bend or distort as the ship performs maneuvers. The motion of the cable through the turn can be used to resolve ambiguities that are inherent to nominally linear arrays. The first significant contribution is a method to estimate the spatial spectrum using a time-varying array shape in a dynamic field and broadband temporal data. Knowledge of the temporal spectral shape is shown to enhance detection performance. The field is approximated as a sum of uncorrelated planewaves located at uniform locations in angle, forming a gridded map on which a maximum likelihood estimate for broadband source power is derived. Uniform linear arrays also suffer from spatial aliasing when the inter-element spacing exceeds a half-wavelength. Broadband temporal knowledge is shown to significantly reduce aliasing and thus, in simulation, enhance target detection in interference dominated environments. </p><p>As an extension, the problem of towed array shape estimation is considered when the number and location of sources are unknown. A maximum likelihood estimate of the array shape using the field directionality map is derived. An acoustic-based array shape estimate that exploits the full 360$^\circ$ field via field directionality mapping is the second significant contribution. Towed hydrophone arrays have heading sensors in order to estimate array shape, but these sensors can malfunction during sharp turns. An array shape model is described that allows the heading sensor data to be statistically fused with heading sensor. The third significant contribution is method to exploit dynamical motion models for sharp turns for a robust array shape estimate that combines acoustic and heading data. The proposed array shape model works well for both acoustic and heading data and is valid for arbitrary continuous array shapes.</p><p>Finally, the problem of array manifold ambiguities for static under-sampled linear arrays is considered. Under-sampled arrays are non-uniformly sampled with average spacing greater than a half-wavelength. While spatial aliasing only occurs in uniformly sampled arrays with spacing greater than a half-wavelength, under-sampled arrays have increased spatial resolution at the cost of high sidelobes compared to half-wavelength sampled arrays with the same number of sensors. Additionally, non-uniformly sampled arrays suffer from rank deficient array manifolds that cause traditional subspace based techniques to fail. A class of fully agumentable arrays, minimally redundant linear arrays, is considered where the received data statistics of a uniformly spaced array of the same length can be reconstructed in wide sense stationary fields at the cost of increased variance. The forth significant contribution is a reduced rank processing method for fully augmentable arrays to reduce the variance from augmentation with limited snapshots. Array gain for reduced rank adaptive processing with diagonal loading for snapshot deficient scenarios is analytically derived using asymptotic results from random matrix theory for a set ratio of sensors to snapshots. Additionally, the problem of near-field sources is considered and a method to reduce the variance from augmentation is proposed. In simulation, these methods result in significant average and median array gains with limited snapshots.</p>Dissertatio

Report of the Plasma Physics and Environmental Perturbation Laboratory (PPEPL) working groups. Volume 1: Plasma probes, wakes, and sheaths working group

It is shown in this report that comprehensive in-situ study of all aspects of the entire zone disturbance caused by a body in a flowing plasma resulted in a large number if requirements on the shuttle-PPEPL facility. A large amount of necessary in-situ observation can be obtained by adopting appropriate modes of performing the experiments. Requirements are indicated for worthwhile studies, of some aspects of the problems, which can be carried out effectively while imposing relatively few constraints on the early missions. Considerations for the desired growth and improvement of the PPEPL to facilitate more complete studies in later missions are also discussed. For Vol. 2, see N74-28170; for Vol# 3, see N74-28171

Autonomous Watercraft Simulation and Programming

Automation of various modes of transportation is thought to make travel more safe and efficient. Over the past several decades advances to semi-autonomous and autonomous vehicles have led to advanced autopilot systems on planes and boats and an increasing popularity of self-driving cars. We simulated the motion of an autonomous vehicle using computational models. The simulation models the motion of a small-scale watercraft, which can then be built and programmed using an Arduino Microcontroller. We examined different control methods for a simulated rescue craft to reach a target. We also examined the effects of different factors, such as various biases (which would be analogous to a current of water) and various initial separation distances, on the time it takes the simulated rescue craft to reach the target. The simulations suggested that it is most efficient to continually correct the direction of the simulated rescue craft for movement of the target when the object is moving at random. We predict that these simulations can model not only the small-scale watercraft but also full-size boats. Self-driving technology used here can be applicable in search-and-rescue missions where conditions may be too harsh for human-controlled watercraft and impractical for remote-controlled watercraft. This experiment also raises new questions in methods of control that can utilize machine learning to detect patterns of a moving target

FLY-EAR INSPIRED MINIATURE SENSOR SYSTEM FOR TWO-DIMENSIONAL SOUND SOURCE LOCALIZATION

A micro-scale sound localization sensor is developed and studied in this thesis to address the fundamental challenge of miniaturizing sound localization systems. When miniaturizing a microphone array, there is a critical size limitation at which the array will be unable to localize the sound source in a discernible manner. However, a solution to this dilemma came about when studying the hearing mechanisms of a particular fly, known as Ormia ochracea. Background research into the hearing mechanisms of the fly found that it can accurately locate a sound source even though its eardrums are separated by a distance of only 500 &#956;m. The fly's exceptional directional hearing capability has been linked to a distinct mechanical coupling between its two eardrums, which helps amplify minute directional cues. Inspired by the remarkable hearing capabilities of the fly's micro-scale ear, researchers have sought to develop micro-scale sensors to mimic the fly's ear. One limitation of simply imitating the fly's ear is that the fly is only capable of localizing a sound source in one dimension. In this thesis work, the knowledge gained from understanding the fly ear mechanism is applied to achieve the goal of developing a micro-scale sound localization sensor capable of sound source localization in two dimensions. In this thesis, for the first time, micro-scale fly-ear inspired sensor devices employing three or four coupled membranes have been designed. Reduced-order models have been developed to achieve a fundamental understanding of the performance of each sensor design. Furthermore, a micro-scale sensor device incorporating three mechanically coupled membranes arranged in an equilateral triangular configuration has been successfully developed. Experimental study of the sensor device incorporated with a low coherence fiber optic interferometric detection system has suggested that the micro-scale fly-ear inspired sensor can achieve a much improved performance in terms of phase differences and directional sensitivities when compared to a similar sized microphone array constructed with separate microphones. In addition, localization techniques have been developed to best use the fly-ear inspired sound localization sensors. Future work is suggested to incorporate this sensor system with a fully autonomous robot to improve robot homing and navigation