Russell A. Paielli

Conflict Probability Estimation for Free Flight

The safety and efficiency of free flight will benefit from automated conflict prediction and resolution advisories. Conflict prediction is based on trajectory prediction and is less certain the farther in advance the prediction, however. An estimate is therefore needed of the probability that a conflict will occur, given a pair of predicted trajectories and their levels of uncertainty. This paper presents a method to estimate that conflict probability. The trajectory prediction errors are modeled as normally distributed, and the two error covariances for an aircraft pair are combined into a single, equivalent covariance of the relative position. A coordinate transformation is then used to derive an analytical solution. Numerical examples and a Monte Carlo validation are presented. (Author)

Tactical Conflict Alerting Aid for Air Traffic Controllers

The Tactical Separation-Assisted Flight Environment (TSAFE) is designed to alert air traffic controllers to imminent conflicts (predicted loss of separation within approximately 3 minutes). It generates constant-velocity (“dead-reckoning”) trajectory predictions similar to those generated by Conflict Alert, the legacy system that currently performs the tactical alerting function in theUnited States. UnlikeConflictAlert, however, it also generates predictions based on pilot intent as specified in the latest flight-plan route and assigned altitude. The intended route is not always known, however, because controllers sometimes neglect to enter route amendments into the host computer. Hence, both the constant-velocity predictions and the flight-plan-based predictions are checked for conflicts. To reduce false alerts, the time horizons of the two predicted routes are a function of the degree of conformance to the current flight-plan route. Alerting performance was tested using archived tracking data for 100 actual operational errors, and TSAFE was found to provide timely warnings significantly more consistently than ConflictAlert.When tested ona sample of general traffic data, TSAFEwas found to produce substantially fewer false alerts than Conflict Alert.

Modeling Maneuver Dynamics in Air Traffic Conflict Resolution

Much of the previous literature on conflict resolution is based on instantaneous maneuver models, in which speed and/or heading change dynamics are unmodeled. The effects of the actual maneuver dynamics on the resulting minimum separation are analyzed, and a simple numerical algorithm is presented to compensate for those effects. The focus is on level flight in the horizontal plane. Speed changes are modeled as periods of constant along-track acceleration or deceleration, and heading changes are modeled as steady turns of constant rate and radius. These simple kinematic (constrained point-mass) models improve on the resolution accuracy that results from modeling speed and heading changes as instantaneous, but they yield much simpler solutions than general point-mass dynamic models. The accuracy improvement is minor for most heading-change maneuvers, but it is substantial for most speed-change maneuvers. An important operational benefit of the algorithm is that it detects immediately if a conflict is too close to be resolved by a particular maneuver. A method is also outlined for determining the optimal combination of speed and heading change to resolve conflicts. With minor adaptation, the algorithms can also make use of an existing conflict probability estimation algorithm to determine maneuvers for strategic conflict probability reduction.

Linearization of attitude-control error dynamics

Some useful properties of direction-cosine and quaternion attitude formulations that include specification of attitude error and the inversion of rigid-body rotational dynamics are reviewed. The inversion procedure is then applied to a measure of attitude error to realize a new model-follower control system that exhibits linear attitude-error dynamics. Error analyses and simulation results for spacecraft attitude-control systems are presented to demonstrate the more robust performance obtainable from an exact linear-error formulation over that obtained from either direction-cosine or quaternion formulations with simple linear feedback control laws. >

Conflict probability estimation for free flight

The safety and e ciency of free ight will bene t from automated con ict prediction and resolution advisories. Con ict prediction is based on trajectory prediction and is less certain the farther in advance the prediction, however. An estimate is therefore needed of the probability that a con ict will occur, given a pair of predicted trajectories and their levels of uncertainty. This paper presents a method to estimate that con ict probability. The trajectory prediction errors are modeled as normally distributed, and the two error covariances for an aircraft pair are combined into a single, equivalent covariance of the relative position. A coordinate transformation is then used to derive an analytical solution. Numerical examples and a Monte Carlo validation are presented.

Autonomous System for Air Traffic Control in Terminal Airspace

In this article we present recent work towards the development of an autonomous system that performs conflict resolution and arrival scheduling for aircraft in the terminal airspace around an airport. An autonomous air traffic control system is defined as a system that can safely solve the major traffic management problems currently handled by human controllers. It has the potential to handle higher traffic levels and a mix of conventional and unmanned aerial vehicles with reduced dependency on controllers. The main objective of this paper is to describe the fundamental trajectory algorithms that must be incorporated in such a system. These algorithms generate arrival trajectories that are free of conflicts with other traffic, and meet scheduled times of arrival for landing with specified in-trail spacings. The maneuvers the system employs to resolve separation and spacing conflicts include speed control, horizontal maneuvers, and altitude changes. Furthermore, the system can reassign arrival aircraft to a different runway in order to reduce delays. Examples of problems solved and performance statistics from a fast-time simulation using simulated traffic of arrivals and departures at the Dallas/Fort Worth International Airport and Dallas Love Field Airport are also provided.

Verification and validation of air traffic systems: Tactical separation assurance

The expected future increase in air traffic requires the development of innovative algorithms and software systems to automate safety critical functions such as separation assurance - the task of maintaining a safe distance between aircraft at all times. Extensive verification and validation (V&V) of such functions will be crucial for the acceptance of new air traffic management systems. This paper reports on work performed at the NASA Ames Research Center. We discuss how advanced V&V technologies can be used to create robust software prototypes for air traffic control software, and how conformance of production code with such prototypes can be assured. We present preliminary results of V&V efforts for a prototype of the Tactical Separation Assisted Flight Environment system (TSAFE).

Tactical Separation Algorithms and Their Interaction with Collision Avoidance Systems

The nation’s air transportation system is currently unable to support the forecasted demand for air travel. New airborne and ground-based capabilities are being investigated to address the myriad of challenges that pilots, operators, and air navigation service providers will face. One approach being studied considers a layered architecture involving a strategic and a tactical system to provide automated separation assurance. Because the tactical system will operate in a time horizon that may overlap with on-board collision-avoidance systems, it must be designed not to interfere with these systems. This paper presents a new set of vertical conflict resolutions for a conflict aircraft pair. Heuristics are presented which govern the use of the new vertical and recently-developed horizontal conflict resolution algorithms to minimize interference with an on-board collision-avoidance system. To address the expected increase in traffic density, an algorithm for globally resolving conflicts involving multiple aircraft is also presented. Evaluation using real-world encounters in both en route and terminal airspace demonstrates the effectiveness of both the vertical algorithm and the heuristics used to reduce the interference.

Algorithms for control of arrival and departure traffic in terminal airspace

This paper presents a design approach and basic algorithms for a future system that can perform aircraft conflict resolution, arrival scheduling and convective weather avoidance with a high level of autonomy in terminal area airspace. Such a system, located on the ground, is intended to solve autonomously the major problems currently handled manually by human controllers. It has the potential to accomodate higher traffic levels and a mix of conventional and unmanned aerial vehicles with reduced dependency on controllers. The main objective of this paper is to describe the fundamental trajectory and scheduling algorithms that provide the foundation for an autonomous system of the future. These algorithms generate trajectories that are free of conflicts with other traffic, avoid convective weather if present, and provide scheduled times for landing with specified in-trail spacings. The maneuvers the algorithms generate to resolve separation and spacing conflicts include speed, horizontal path, and altitude changes. Furthermore, a method for reassigning arrival aircraft to alternate runways in order to reduce delays is also included. The algorithms generate conflict free trajectories for terminal area traffic, comprised primarily of arrivals and departures to and from multiple airports. Examples of problems solved and performance statistics from a fast-time simulation using simulated traffic of arrivals and departures at the Dallas/Fort Worth International Airport and Dallas Love Field are described.

Performance of an Automated System for Control of Traffic in Terminal Airspace

This paper examines the performance of a system that performs automated conflict resolution and arrival scheduling for aircraft in the terminal airspace around major airports. Such a system has the potential to perform separation assurance and arrival sequencing tasks that are currently handled manually by human controllers. The performance of the system is tested against several simulated traffic scenarios that are characterized by the rate at which air traffic is metered into the terminal airspace. For each traffic scenario, the levels of performance that are examined include: number of conflicts predicted to occur, types of resolution maneuver used to resolve predicted conflicts, and the amount of delay for all flights. The simulation results indicate that the percentage of arrivals that required a maneuver that changes the flights horizontal route ranged between 11% and 15% in all traffic scenarios. That finding has certain implications if this automated system were to be implemented simply as a decision support tool. It is also found that arrival delay due to purely wake vortex separation requirements on final approach constituted only between 29% and 35% of total arrival delay, while the remaining major portion of it is mainly due to delay back propagation effects.