A TCAS-II Resolution Advisory Detection Algorithm

The Traffic Alert and Collision Avoidance System (TCAS) is a family of airborne systems designed to reduce the risk of mid-air collisions between aircraft. TCAS~II, the current generation of TCAS devices, provides resolution advisories that direct pilots to maintain or increase vertical separation when aircraft distance and time parameters are beyond designed system thresholds. This paper presents a mathematical model of the TCAS~II Resolution Advisory (RA) logic that assumes accurate aircraft state information. Based on this model, an algorithm for RA detection is also presented. This algorithm is analogous to a conflict detection algorithm, but instead of predicting loss of separation, it predicts resolution advisories. It has been formally verified that for a kinematic model of aircraft trajectories, this algorithm completely and correctly characterizes all encounter geometries between two aircraft that lead to a resolution advisory within a given lookahead time interval. The RA detection algorithm proposed in this paper is a fundamental component of a NASA sense and avoid concept for the integration of Unmanned Aircraft Systems in civil airspace

PRIVATIZATION IN COSTA RICA: A MULTI-DIMENSIONAL ANALYSIS

This dissertation offers an assessment of the last 25 years of privatization efforts in Costa Rica. The study argues that the example of Central America's most economically developed country illustrates that one-dimensional thinking about privatization of state services is misdirected. That judgment remains true whether the "one dimension" represents indiscriminate acceptance of privatization or its doctrinaire rejection. More specifically, separate case studies of three types of privatization attempted in Costa Rica demonstrate that in some cases privatization indeed represents an effective response to the particular historical circumstances the country faces. In other instances, however, privatization can be inappropriate. This judgment is based on historical investigation, the testimony of recognized authorities, logical analysis of arguments both for and against privatization, and on responses to the program on the parts of key economic sectors. The argument is made in seven chapters. The first defines terms and provides historical perspective on the topic by examining the concept of privatization in general within the context of commercial globalization and of capitalism itself. Chapter II continues in this historical vein, this time reviewing the history of Costa Rica's general political-economy - in order to convey Costa Rica's unique character and context. Chapter III connects Chapters I and II by contextualizing Costa Rican privatization within the international economic crisis of the 1970s and early 1980s. Within that context, the stages of Costa Rica's integration into the process of globalization are delineated. Chapters IV, V and VI then evaluate repeated attempts to privatize many of Costa Rica's nationalized enterprises. The chosen ventures include the Costa Rican Development Corporation (CODESA), the Costa Rican Electricity Institute (ICE) (which includes telecommunications), and the Costa Rican Social Security System (CCSS/INS). The chapters in question review the nature of each concern, its genesis, the reasons advanced for its privatization, and the spectrum of opinion evaluating the privatization process and results. A concluding seventh chapter reviews the dissertation's argument, synthesizes the evaluations provided by the case studies, draws conclusions, and makes recommendations for the future of privatization in Costa Rica

Visualization Tools for Visual Impact Assessments: A Study of Existing Technologies

In the last few decades, digital technology has dramatically altered how visual resource management and visual impact assessments (VIAs) are conducted. The recent emergence of low-cost immersive technology offers a suite of tools that could facilitate the VIA process. However, to date there is limited empirical evidence evaluating how these emerging technologies could influence VIA. The research presented here begins to fill this gap by comparing immersive virtual environments to existing 2D photo-based methods for assessing the visual impacts of development. 23 participants familiar with VIAs rated the visual qualities of different scenes presented as 360° images, Google Earth and 2D images. Results show a high similarity in perceived impacts between 2D images and Google Earth; 360° images were rated consistently lower. Overall, participants indicated that immersive visualization may have a role in VIAs, but it is critical these technologies be evaluated against on-site assessments before being adopted

DAIDALUS: Detect and Avoid Alerting Logic for Unmanned Systems

This paper presents DAIDALUS (Detect and Avoid Alerting Logic for Unmanned Systems), a reference implementation of a detect and avoid concept intended to support the integration of Unmanned Aircraft Systems into civil airspace. DAIDALUS consists of self-separation and alerting algorithms that provide situational awareness to UAS remote pilots. These algorithms have been formally specified in a mathematical notation and verified for correctness in an interactive theorem prover. The software implementation has been verified against the formal models and validated against multiple stressing cases jointly developed by the US Air Force Research Laboratory, MIT Lincoln Laboratory, and NASA. The DAIDALUS reference implementation is currently under consideration for inclusion in the appendices to the Minimum Operational Performance Standards for Unmanned Aircraft Systems presently being developed by RTCA Special Committee 228

Characterizing the Effects of a Vertical Time Threshold for a Class of Well-Clear Definitions

A fundamental requirement for the integration of unmanned aircraft into civil airspace is the capability of aircraft to remain well clear of each other and avoid collisions. This requirement has led to a broad recognition of the need for an unambiguous, formal definition of well clear. It is further recognized that any such definition must be interoperable with existing airborne collision avoidance systems (ACAS). A particular class of well-clear definitions uses logic checks of independent distance thresholds as well as independent time thresholds in the vertical and horizontal dimensions to determine if a well-clear violation is predicted to occur within a given time interval. Existing ACAS systems also use independent distance thresholds, however a common time threshold is used for the vertical and horizontal logic checks. The main contribution of this paper is the characterization of the effects of the decoupled vertical time threshold on a well-clear definition in terms of (1) time to well-clear violation, and (2) interoperability with existing ACAS. The paper provides governing equations for both metrics and includes simulation results to illustrate the relationships. In this paper, interoperability implies that the time of well-clear violation is strictly less than the time a resolution advisory is issued by ACAS. The encounter geometries under consideration in this paper are initially well clear and consist of constant-velocity trajectories resulting in near-mid-air collisions

Analysis of Well-Clear Boundary Models for the Integration of UAS in the NAS

The FAA-sponsored Sense and Avoid Workshop for Unmanned Aircraft Systems (UAS) defnes the concept of sense and avoid for remote pilots as "the capability of a UAS to remain well clear from and avoid collisions with other airborne traffic." Hence, a rigorous definition of well clear is fundamental to any separation assurance concept for the integration of UAS into civil airspace. This paper presents a family of well-clear boundary models based on the TCAS II Resolution Advisory logic. Analytical techniques are used to study the properties and relationships satisfied by the models. Some of these properties are numerically quantifed using statistical methods

A Well-Clear Volume Based on Time to Entry Point

A well-clear volume is a key component of NASA's Separation Assurance concept for the integration of UAS in the NAS. This paper proposes a mathematical definition of the well-clear volume that uses, in addition to distance thresholds, a time threshold based on time to entry point (TEP). The mathematical model that results from this definition is more conservative than other candidate definitions of the wellclear volume that are based on range over closure rate and time to closest point of approach

Flat-panel detectors: how much better are they?

Interventional and fluoroscopic imaging procedures for pediatric patients are becoming more prevalent because of the less-invasive nature of these procedures compared to alternatives such as surgery. Flat-panel X-ray detectors (FPD) for fluoroscopy are a new technology alternative to the image intensifier/TV (II/TV) digital system that has been in use for more than two decades. Two major FPD technologies have been implemented, based on indirect conversion of X-rays to light (using an X-ray scintillator) and then to proportional charge (using a photodiode), or direct conversion of X-rays into charge (using a semiconductor material) for signal acquisition and digitization. These detectors have proved very successful for high-exposure interventional procedures but lack the image quality of the II/TV system at the lowest exposure levels common in fluoroscopy. The benefits for FPD image quality include lack of geometric distortion, little or no veiling glare, a uniform response across the field-of-view, and improved ergonomics with better patient access. Better detective quantum efficiency indicates the possibility of reducing the patient dose in accordance with ALARA principles. However, first-generation FPD devices have been implemented with less than adequate acquisition flexibility (e.g., lack of tableside controls/information, inability to easily change protocols) and the presence of residual signals from previous exposures, and additional cost of equipment and long-term maintenance have been serious impediments to purchase and implementation. Technological advances of second generation and future hybrid FPD systems should solve many current issues. The answer to the question ‘how much better are they?–is ‘significantly better– and they are certainly worth consideration for replacement or new implementation of an imaging suite for pediatric fluoroscopy