THE air traffic management (ATM) system is designed for air traffic surveillance, air transportation resource scheduling, and aircraft coordination to enhance air traffic efficiency, fuel economy, passenger comfort, and other related factors. Because of the complexity of the current air transportation system, many scientific and technological advances have been made to improve the performance of the current ATM system. However, as clearly stated in [1], the current air transportation system is under significant stress. In addition, the demand in air traffic will grow by two to three times in the next 25 years [2, 3]. To meet this increasing air traffic demand, the Next Generation Air Transportation System (NextGen) has been proposed and studied widely [2]. Up to now, some concepts of NextGen have been implemented, and various research is still going on. One core difference between NextGen and the current ATM system is that NextGen replaces the current pyramidlike ATM framework with a new flat netcentric structure [4]. In the current system, the airspace is divided into many sectors, and at least one controller provides the aircraft in the sector with proper guidance and commands. In NextGen, information sharing between aircraft will be greatly enhanced so that each aircraft receives and transmits the cooperative surveillance information, and thus aircraft can take over a certain amount of ATM tasks from ground air traffic controllers. These tasks include flight plan changes, trajectory prediction, and conflict detection. In this way, NextGen can alleviate the workloads of ground air traffic controllers [5, 6]. Because of the decentralized nature of NextGen, safety is a crucial element in the system. One of the major safety critical situations that needs to be considered is midair conflict between aircraft. A conflict occurs when the separation standard between aircraft is violated. By the current Federal Aviation Administration (FAA) standard, the protected zone of an aircraft is based on the following rule: the minimum allowed horizontal separation of en route airspace is 5 nm, and the vertical separation requirement is 2000 ft above the altitude of 29,000 ft and 1000 ft below that altitude [6]. Currently, air traffic controllers monitor the trajectories of aircraft and alert the aircraft if a potential separation violation standard is identified. In NextGen, aircraft are assumed to receive flight information about the nearby aircraft via a data link called the Automatic Dependent Surveillance-Broadcast (ADS-B), which contains the aircraft, s position, speed, heading, flight intent, etc. The onboard equipment will be able to make a real-time decision about whether there are some potential conflicts within a look-ahead time horizon [4]. There are two important and interconnected procedures to predict a midair conflict: aircraft trajectory prediction and conflict detection. Aircraft trajectory prediction involves estimating an aircraft, s future trajectory or, more strictly, future flight state by using the measurements up to the current time. Based on the trajectory prediction results, the corresponding conflict detection algorithm makes a decision (or calculates the probabilities) about whether there will be a potential conflict in the predicted time span [7]. Typically, methods to solve these two problems can be categorized into three cases: nominal, worst case, and probabilistic [8]. The nominal trajectory prediction method projects the current aircraft position into the future along a single trajectory, the worst-case method considers the scenario under which the aircraft can do any maneuvers within its capability and declares a conflict if there is a maneuver that can cause a conflict, and the probabilistic method predicts the …