A Review of Conflict Detection and Resolution Modeling Methods <s> III. CATEGORIZATION OF MODELING APPROACHES <s> This paper describes preliminary studies towards a navigational Knowledge-based System currently in progress within the School of Engineering and Technology Management at Liverpool Polytechnic <s> BIB001 </s> A Review of Conflict Detection and Resolution Modeling Methods <s> III. CATEGORIZATION OF MODELING APPROACHES <s> We present a mathematical index of colllision risk and propose it as a candidate conflict probe for use in free flight. The index can be used to aid and measure ATC and pilot performance during simulated or real flight. The instantaneous value of the index is a relative (ordinal) measure of global situational risk. When the value of the index is graphed over time, conflicts can be easily identified and their severity estimated. Peak values indicate approximate times and degree of risk associated with the minimum separation of aircraft. <s> BIB002 </s> A Review of Conflict Detection and Resolution Modeling Methods <s> III. CATEGORIZATION OF MODELING APPROACHES <s> An algorithm for detecting and analyzing potential enroute conflicts has been designed and implemented within the Center-TRACON Automation System (CTAS). The design uses the 4D trajectories provided by CTAS to produce a set of probable future conflicts, and assists the enroute sector controller in efficiently resolving these conflicts. Conflicts are detected via comparisons of trajectories at closely spaced time instants, with measures taken to limit the computations required to complete the search. Performance tests indicate more than 800 aircraft can be processed by the conflict detection and analysis algorithm within a search cycle of 10 seconds. This suggests that the search algorithm easily meets the performance requirements for an automated conflict detection and resolution tool in the current air traffic system. The algorithm includes a trial resolution functionality which automatically detects conflicts of proposed resolutions, and gives near instantaneous feedback to controller input. Field evaluation of the Conflict Probe will be conducted at the Denver and Fort Worth Air Route Traffic Control Centers (ARTCC) beginning in September, 1997. <s> BIB003 </s> A Review of Conflict Detection and Resolution Modeling Methods <s> III. CATEGORIZATION OF MODELING APPROACHES <s> In order to respond to rising demands for services from the National Airspace System (NAS), the Federal Aviation Administration (FAA) and The Mitre Corporation's Center for Advanced Aviation System Development (CAASD) have developed and evaluated a set of en route air traffic control (ATC) decision support capabilities based on many of years of earlier work on the Advanced En Route ATC (AERA) program. The component capabilities-embodied in a prototype referred to as the User Request Evaluation Tool (URET)-include a conflict probe that continuously checks current flight plan trajectories for strategic conflicts, and a Trial Planning function that allows a controller to expeditiously evaluate problem resolutions before they are issued as clearances. These capabilities are intended to provide the flexibility and decision support needed to allow more user preferences to be met while continuing to maintain or enhance today's level of safety. The URET prototype was deployed to the Indianapolis Air Route Traffic Control Center (ARTCC) for field trials in January 1996 and later expanded to include more sectors and to span longer periods of time. In addition, the prototype has been deployed and is being evaluated at the Memphis ARTCC. This paper presents key findings from the field trials and from other related activities. <s> BIB004 </s> A Review of Conflict Detection and Resolution Modeling Methods <s> III. CATEGORIZATION OF MODELING APPROACHES <s> Contemporary developments of on-board systems for automatic or semi-automatic driving include car collision avoidance. For this purpose a worst case approach based on pursuit-evasion differential games is investigated. On a freeway a correct driver (evader) is faced with a wrong driver (pursuer) ahead. The correct driver tries to avoid a collision against all possible wrong driver's maneuvers and additionally tries to stay on the freeway. The representation of an optimal collision avoidance strategy along a lot of optimal paths is used to synthesize a collision avoidance strategy globally by means of Taylor series expansions of the value function's gradient. Examples of simulations which proved satisfactory performance of the on-board collision avoidance system against various typical maneuvers of wrong drivers are presented. <s> BIB005 </s> A Review of Conflict Detection and Resolution Modeling Methods <s> III. CATEGORIZATION OF MODELING APPROACHES <s> A novel collision cone approach is proposed as an aid to collision detection and avoidance between irregularly shaped moving objects with unknown trajectories. It is shown that the collision cone can be effectively used to determine whether collision between a robot and an obstacle (both moving in a dynamic environment) is imminent. No restrictions are placed on the shapes of either the robot or the obstacle, i.e., they can both be of any arbitrary shape. The collision cone concept is developed in a phased manner starting from existing analytical results that enable prediction of collision between two moving point objects. These results are extended to predict collision between a point and a circular object, between a point and an irregularly shaped object, between two circular objects, and finally between two irregularly shaped objects. Using the collision cone approach, several strategies that the robot can follow in order to avoid collision, are presented. A discussion on how the shapes of the robot and obstacles can be approximated in order to reduce computational burden is also presented. A number of examples are given to illustrate both collision prediction and avoidance strategies of the robot. <s> BIB006 </s> A Review of Conflict Detection and Resolution Modeling Methods <s> III. CATEGORIZATION OF MODELING APPROACHES <s> Many of the nation's airspace users desire an increase in efficiency in the air traffic control system. Some feel that an increase in efficiency can be achieved by moving away from a centralized control paradigm towards a distributed control paradigm. In a distributed control paradigm, the cockpit crew would have more freedom in selecting and modifying their routes. This new paradigm is part of the Free Flight concept. Increased autonomy of operations will require increases in cockpit information, pilot responsibility, and avionics capability. One of the key enabling capabilities will be the detection and resolution of air traffic conflicts. CDR a baseline conflict detection algorithm based on instantaneous state vectors; and an analysis of the algorithm's performance for flight data from the ADS-B Operational Evaluation. <s> BIB007
To provide insight into different methods of conflict detection and resolution, a literature review of previous research models and current developmental and operational systems was performed. The overall modeling approaches from a total of 68 methods or systems are discussed here. These models do not represent an exhaustive list, but are believed to encompass the major recent approaches to CD&R problems. Several authors have proposed more than one CD&R method in a single publication; these cases are listed separately where appropriate. Additionally, some effort is underway investigating the human factors issues associated with conflict detection 8 and resolution BIB002  . However, this paper is directed only at numerical or analytical models for evaluating traffic conflict scenarios, not at human-centered issues. Nine of the models that were examined are existing operational systems in use or which have been evaluated in the field: Airborne Information for Lateral Spacing (AILS)  , CTAS BIB003 , Ground Proximity Warning System (GPWS)  and its recent Enhanced version (EGPWS)  , Precision Runway Monitor (PRM)  , TCAS  , Traffic and Collision Alert Device (TCAD)  , URET BIB004 , and a prototype conflict detection system for the Cargo Airline Association BIB007 . The remaining models range from abstract concepts to prototype conflict warning systems being evaluated or used in laboratories. Five of the models were developed for robotic, automobile, or naval applications BIB001   BIB006 BIB005 , but are still applicable to aviation. Based on the framework in Figure 2 , the 68 models were catalogued according to their fundamental approaches to each phase of the CD&R process. To provide a consistent basis upon which to describe the models, each model is classified by the manner in which it is explicitly described in its reference. A model defined here to address only horizontal conflicts, for instance, could potentially be extended to work in three dimensions (and the need for such an extension may have been mentioned in the reference), but such an extension was not specifically described in the reference. As another example, if a model computes aircraft miss distance but does not define an explicit conflict detection threshold, the model is not classified as providing Conflict Detection even though the model could be adapted to perform such a task. The 68 models are organized in terms of six key design factors, discussed in more detail below. Although other differences between the models exist that are not covered here, the six factors that are used represent principal categories by which models differ. An important caveat is that the models have been analyzed, verified, and validated to varying degrees by their developers and the community. This paper focuses only on the specific attributes of each model, not on the depth to which a model has been analyzed, validated, or accepted. Thus, care should be taken to remember that a model that seems to be simple according to our categorization scheme may be 9 significantly more viable than an apparently sophisticated model. Similarly, two models may be identical in terms of the six design factors used here but may be quite different in terms of implementation or maturity. More detail on 33 of the 68 models covered here is available in Kuchar & Yang  .