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Meeting	ACGS Committee Meeting 95 - Salt Lake City - March 2005
Agenda Location	4 GENERAL COMMITTEE TECHNICAL SESSION 4.2 Research Institutions, Industry and University Reports 4.2.1 Research Institutes and Companies 4.2.1.9 Saab
Title	Saab
Presenter	Roger Burton
Available Downloads*	presentation
	Downloads are available to members who are logged in and either Active* or attended this meeting.
Abstract	The system consists of a data link for communication between the aircraft, the algorithm described below and the flight control system (FCS), which is used for executing the avoidance maneuver. If the aircraft is already equipped with an appropriate data link no additional hardware is needed in which case the Auto-ACAS system can be implemented by software changes only. Claim space method This Auto-ACAS algorithm does not try to identify collisions based on predicted probable trajectories of the aircraft. Instead it claims space along a computed escape trajectory (time tagged positions where the aircraft will be after an avoidance is activated) which the aircraft will use in the case an avoidance maneuver is necessary. The major benefit of using an escape trajectory is that it can be predicted much more accurate than the probable trajectory which the aircraft will follow if no avoidance is executed. This is because the escape trajectory is executed in a predetermined way by the Auto-ACAS algorithm using the FCS, whereas the probable trajectory is affected by the change in pilot commands. The size of the claimed space is computed using knowledge of the wingspan, navigation uncertainty and accuracy of the predicted trajectory compared to the one the FCS will make the aircraft follow if the escape command is given. Each aircraft sends its predicted escape maneuver and the size of the claimed space along this track to other aircraft, using the data link. All aircraft will use the escape maneuvers from the different aircraft to detect a future lack of escape, see Figure 1. If the distance between the escape trajectories is greater than the safety distance, the track is stored as the one to use in case of avoidance. Else the avoidance is executed using the FCS to make the aircraft follow the stored trajectory. Figure 1. Collision detection using predicted escape maneuvers The escape maneuver directions are chosen to maximize the minimum distance between all aircraft. In this way the avoidance will be executed at the last possible instant and the system will thus guarantee a very low nuisance level. Failures affecting the algorithm Data dropouts, due to errors identified through parity check of the link data, “shadowing” or misalignment of the antennas etc., causes the established data communication between two algorithms to disappear. To allow dropouts, even close to an activation, and still supply protection against collision, the change of escape direction is limited as a function of actual distance and estimated time to activation. This limitation of change is balanced by the requirement that the escape maneuver shall be optimal and thus have the ability to change fast. At data dropouts the claimed space for the aircraft which the communication is lost for is also expanded in the own aircraft to handle unknown maneuvering and change of escape direction of the other aircraft. Navigation degradation, due to loss/degradation of GPS, air data sensors, inertial navigation system or terrain navigation etc. is inherently handled by the algorithm. As the size of the claimed space is computed using the current navigation uncertainty a degradation of navigation performance only expands the claimed space according to the new uncertainty. Failures in other sensor data, used in the computation of the predicted escape trajectory, is handled dependent of how imminent the activation is. Close to an activation (collision) the latest computed own predicted escape trajectory is dead reckoned and the size of the claimed space is increased correspondingly for up to 4 seconds. After this time of normal collision detection the system goes to failed state. When no activation is imminent the system goes directly to failed state. At failed state Auto-ACAS stops transmitting own messages over the link. Formation flying logic To enable aircraft equipped with Auto-ACAS to rejoin and fly in formation, the algorithm contains logic which inhibits the activation of Auto-ACAS against aircraft who fulfill the condition in the inhibit region in Figure 2. (The condition also contains a hysteresis to be less sensitive to noise in the transition phase). Figure 2. Inhibit condition in Formation Flying Logic If the distance between the aircraft becomes less than the claimed spaces at the first point along the escape trajectory, Auto-ACAS is inhibited for all aircraft. This is done to ensure that Auto-ACAS does not activate a maneuver, which could cause a collision. An activation of a maneuver when the algorithm is not sure of the relative position of the aircraft (i.e. they are inside each others position uncertainties) might turn the aircraft into each other. When Auto-ACAS is totally inhibited in an aircraft fulfilling this last condition, the algorithm in all other aircraft is set to yield to this formation. This includes boosting their claimed space and re-computing/predicting the trajectory of the formation to be along the velocity vector of the formation. This makes aircraft not flying in formation do all of the maneuvering in case of an activation.