A Review of Conflict Detection and Resolution Modeling Methods <s> A. State Propagation <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> BIB001 </s> A Review of Conflict Detection and Resolution Modeling Methods <s> A. State Propagation <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> BIB002
Because conflict detection and resolution can only be as reliable as the ability of the model to predict the future, the most concrete difference between modeling approaches involves the method by which the current states are projected into the future. Three fundamental extrapolation methods have been identified, termed Nominal, Worst-case, and Probabilistic. The three methods are shown schematically in Figure 3 . Nominal projections, however, do not directly account for the possibility that an aircraft may not behave as expected -a factor that is especially important in longer-term conflict detection. Generally, this uncertainty is managed by introducing a safety buffer, minimum miss distance, or time to closest point of approach threshold at which point a conflict will be detected. Alerts for conflicts which are predicted to occur far in the future using a Nominal trajectory model will need to be inhibited so as to not cause a nuisance to the operator. The other extreme of dynamic modeling is to examine a Worst-case projection. Here, it is assumed that an aircraft will perform any of a range of maneuvers. If any one of these maneuvers could cause a conflict, then a conflict is predicted. The result is a swath of potential trajectories which is monitored to detect conflicts with other aircraft (Figure 3b ). Worst-case approaches are conservative in that they can trigger conflict alerts whenever there is any possibility of a conflict within the definition of the Worst-case trajectory model. If such conflict-inducing maneuvers are unlikely, protecting against them may severely reduce overall traffic capacity due to a high false alarm rate. Accordingly, the Worst-case trajectory must be limited to a certain look-ahead projection time. Still, the Worst-case approach may be appropriate when it is desirable to determine if a conflict is possible, or for air traffic concepts in which aircraft are procedurally constrained to remain within a given maneuvering corridor. Each corridor then becomes the boundary of the Worst-case aircraft trajectories, and conflicts can be predicted based simply on whether corridors intersect at the same point in time. In the Probabilistic method, uncertainties are modeled to describe potential variations in the future trajectory of the aircraft ( Figure 3c ). This is usually done in one of two ways. In URET and CTAS, for example, a position error is added to a nominal trajectory, from which the conflict probability can be derived BIB001 BIB002 . A second approach is to develop a complete set of possible future trajectories, each weighted by a probability of occurring (e.g., using probability density functions). The trajectories are then propagated into the future to determine the probability of conflict.