1. title and keywords
Title: Coloured Petri Net-based Traffic Collision Avoidance System encounter model for the analysis of potential induced collisions
for Encounter model TCAS Colored Petri Nets evoked potential collision
TCAS (The Traffic Alert and Collision Avoidance System) Traffic warning and collision avoidance system;
Encounter model;
State space;
Potential collision scenario;
Petri net Petri net.
2. Summary
The Traffic Alert and Collision Avoidance System (TCAS) is a world-wide accepted last-resort means of reducing the probability and frequency of mid-air collisions between aircraft. Unfortunately, it is widely known that in congested airspace, the use of the TCAS may actually lead to induced collisions. Therefore, further research regarding TCAS logic is required. In this paper, an encounter model is formalised to identify all of the potential collision scenarios that can be induced by a resolution advisory that was generated previously by the TCAS without considering the downstream consequences in the surrounding traffic. The existing encounter models focus on checking and validating the potential collisions between trajectories of a specific scenario. In contrast, the innovative approach described in this paper concentrates on quantitative analysis of the different induced collision scenarios that could be reached for a given initial trajectory and a rough specification of the surrounding traffic. This approach provides valuable information at the operational level. Furthermore, the proposed encounter model can be used as a test-bed to evaluate future TCAS logic changes to mitigate potential induced collisions in hot spot volumes. In addition, the encounter model is described by means of the coloured Petri net (CPN) formalism. The resulting state space provides a deep understanding of the cause-and-effect relationship that each TCAS action proposed to avoid an actual collision with a potential new collision in the surrounding traffic. Quantitative simulation results are conducted to validate the proposed encounter model, and the resulting collision scenarios are summarised as valuable information for future air traffic management (ATM) systems.
The Traffic Alert and Collision Avoidance System (TCAS) is a worldwide recognized ultimate means of reducing the probability and frequency of air collisions between aircraft. Unfortunately, it is well known that in crowded airspace, the use of TCAS can actually lead to induced collisions. Therefore, further research on the TCAS logic is needed. In this paper, the encounter model is formalized to identify all possible collision scenarios, which may be caused by solutions generated by TCAS that do not consider the downstream consequences of surrounding traffic. Existing encounter models focus on checking and verifying potential collisions between trajectories in specific scenarios. In contrast, the innovative method described in this article focuses on the quantitative analysis of different collision-inducing scenarios that may be generated by a given initial trajectory and rough rules of surrounding traffic. This method provides valuable information at the operational level. In addition, the proposed encounter model can be used as a test platform for evaluating future TCAS logic changes to mitigate potential induced collisions in hot spots. In addition, a colored Petri net (CPN) is used to describe the encounter model. The resulting state space provides a deep understanding of causality, which is recommended for every TCAS action to avoid actual collisions with potential new collisions in surrounding traffic. Quantitative simulation results validate the proposed encounter model and summarize the collision scenarios as valuable information in the future air traffic management (ATM) system.
3. Innovation and academic value
A mathematical model for the TCAS II algorithm was developed to be potentially used in a TCAS-TCAS encounter. TCAS II provides TAs to warn pilots of the encounter with neighbouring traffic and RAs to prevent a collision by offering a suggested resolution manoeuvre to pilots to execute an avoidance manoeuvre in the vertical direction. Using a series of mathematical equations, this CA process has been conceptually described. The process has also been used as the theoretical basis for construction of the encounter model.
(1) Developed a mathematical model of the TCAS II algorithm, which can be used in the TCAS-TCAS encounter. TCAS II provides traffic advisories (TAs) to warn pilots of encounters with neighboring traffic, and provides pilots with resolution advisories (RAs) to implement vertical avoidance strategies to avoid collisions. A series of mathematical equations are used to conceptually describe the CA (collision avoidance) process. This process is also used as the theoretical basis for constructing encounter models.
A novel scenario generation process of potential collisions was proposed. The only input of this model is the aircraft trajectory and the number of intruder aircraft, while the inputs of most other encounter models (Netjasov et al., 2013; Kochenderfer et al., 2010; Billingsley et al.,2009; Tang et al., 2014) are the initial states (e.g., trajectories) of all involved aircraft for the analysis of particular traffic geometries. In contrast, the proposed model presented in this paper aims to generate the potential collision scenarios for a certain number of aircraft, based on the trajectory of just one instance representative aircraft, rather than to test whether a multi-aircraft scenario contains a potential collision or not. The generated encounter scenarios may not be credible within the normal operation of the ATC system, as some of them are designed through the use of different cruising flight levels.
(2) A new generation process of potential collision scenarios is proposed. The only input to this model is the trajectory of the aircraft and the number of invading aircraft, while the input to most other encounter models is the initial state (for example, trajectory) of all relevant aircraft, which is used to analyze specific traffic geometry. In contrast, the model proposed in this paper aims to generate a certain number of potential collision scenarios based on the flight trajectory of a typical aircraft, rather than testing whether a multi-aircraft scenario contains potential collisions. In the normal operation of the ATC system, the generated encounter scenarios may not be credible, because some of these scenarios are designed by using different cruise altitudes.
The encounter model is represented in the CPN formalism. This causal model depicts the procedure that takes an aircraft with a known initial state into various induced collision scenarios containing multiple TCAS-equipped aircraft. With the state space, the model provides a better understanding of the potential collision occurrences for risk assessment by comprehending the cause-effect relationship of each action. The initial states of multiple aircraft that are involved in the scenarios can be generated one by one. Finally, all of the possible situations (state space) can be represented for subsequent analysis to summarise the typical induced collision scenarios.
(3) The encounter model is expressed in CPN form. This causal model describes the process of bringing an aircraft with a known initial state into various induced collision scenarios containing multiple aircraft equipped with TCAS. In the state space, the model can better understand potential collision events by understanding the causality of each action for risk assessment. The initial states of multiple aircraft participating in the scene can be generated one by one. Finally, all possible situations (state space) can be represented for subsequent analysis to summarize typical induced collision scenarios.
A summary is provided of the typical induced collision scenarios based on the simulation results of the causal model. Quantitative measurement experiments were conducted to validate the feasibility and effectiveness of the encounter model. In addition, for scenarios involving three or four aircraft (representative of almost all of the factual situations), the detailed process of a collision and the typical scenarios were illustrated and described in detail.
(4) Based on the simulation results of the causal model, the typical induced collision scenarios were summarized. Perform quantitative measurement experiments to verify the feasibility and effectiveness of the encounter model. In addition, for situations involving three or four aircraft (representing almost all actual situations), the detailed process and typical situations of the collision are explained and described in detail.
4. The understanding of the conclusion and the inspiration for the study work
TCAS aims to directly provide the pilot with the ultimate collision avoidance guide. Long-term practical experience has proved the practicality and efficiency of TCAS. However, even if all the aircraft involved are equipped with TCAS, collisions may occur under special circumstances. When multiple airplanes are involved, TCAS can actually cause a collision that would not have occurred, especially in the dense surrounding traffic. The public domain lacks a causal model to describe surrounding traffic scenes that may induce collisions; such scenes can be used to compare actual traffic scenes to reduce the frequency of collisions. The motivation for developing this causal model is to identify collisions induced by TCAS and to support follow-up research on current and advanced ATM concepts (including TCAS) for safety analysis.
Future research plans:
(1) Develop pattern recognition tools to identify traffic scenarios that may potentially induce collisions;
(2) Use the proposed causal model to promote the improvement of the current TCAS logic, the purpose of which is to solve future busyness by enhancing CA performance And congested traffic.