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Robust airborne collision avoidance through dynamic programming

Published in:
MIT Lincoln Laboratory Report ATC-371

Summary

The Traffic Alert and Collision Avoidance System (TCAS) uses an on-board beacon radar to monitor the local air traffic and logic to determine when to alert pilots to potential conflict. The current TCAS logic was the result of many years of development and involved the careful engineering of many heuristic rules specified in pseudocode. Unfortunately, due to the complexity of the logic, it is difficult to revise the pseudocode to accommodate the evolution of the airspace and the introduction of new technologies and procedures. This report summarizes recent advances in computational techniques for automatically deriving the optimal logic with respect to a probabilistic model and a set of performance metrics. Simulations demonstrate how this new approach results in logic that significantly outperforms TCAS according to the standard safety and operational performance metrics.
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Summary

The Traffic Alert and Collision Avoidance System (TCAS) uses an on-board beacon radar to monitor the local air traffic and logic to determine when to alert pilots to potential conflict. The current TCAS logic was the result of many years of development and involved the careful engineering of many heuristic...

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MIT Lincoln Laboratory TCAS surveillance performance

Published in:
MIT Lincoln Laboratory Report ATC-370

Summary

The Traffic Alert and Collision Avoidance System (TCAS) Version 7 surveillance requirements were developed in the mid-1990s with the use of limited radar data. Recently, a more comprehensive radar data source has become available, enabling a thorough analysis of TCAS surveillance performance throughouth the National Airspace System (NAS). This paper characterizes six high traffic terminal environments over three months. A busy one hour period was selected from each location for density and equipage measurements. This paper then describes the use of a high fidelity simulation to characterize TCAS surveillance performance in the isx locations. Transponder utilization due to TCAS and TCAS surveillance range are compared with the design requirements, including interference limiting specifications. The effect of TCAS surveillance activity on Air Traffic Control (ATC) ground radar performance is also investigated. Results indicate that the surveillance algorithms perform as intended and that TCAS has a minimal impact on ground radar. Areas of concern are noted for future investigation.
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Summary

The Traffic Alert and Collision Avoidance System (TCAS) Version 7 surveillance requirements were developed in the mid-1990s with the use of limited radar data. Recently, a more comprehensive radar data source has become available, enabling a thorough analysis of TCAS surveillance performance throughouth the National Airspace System (NAS). This paper...

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Robustness of optimized collision avoidance logic to modeling errors

Published in:
29th Digital Avionics System Conf., 3-7 October 2010.

Summary

Collision avoidance systems, whether for manned or unmanned aircraft, must reliably prevent collision while minimizing alerts. Deciding what action to execute at a particular instant may be framed as a multiple-objective optimization problem that can be solved offline by computers. Prior work has explored methods of efficiently computing the optimal collision avoidance logic from a probabilistic model of aircraft behavior and a cost function. One potential concern with using a probabilistic model to construct the logic is that the model may not adequately represent the real world. Inaccuracies in the model could lead to vulnerabilities in the system when deployed. This paper evaluates the robustness of collision avoidance optimization to modeling errors.
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Summary

Collision avoidance systems, whether for manned or unmanned aircraft, must reliably prevent collision while minimizing alerts. Deciding what action to execute at a particular instant may be framed as a multiple-objective optimization problem that can be solved offline by computers. Prior work has explored methods of efficiently computing the optimal...

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A decision-theoretic approach to developing robust collision avoidance logic

Published in:
2010 13th Int. IEEE Annual Conf. on Intelligent Transportation Systems, 19-22 September 2010, pp. 1837-1842.

Summary

All large transport aircraft are required to be equipped with a collision avoidance system that instructs pilots how to maneuver to avoid collision with other aircraft. The uncertainty in the behavior of the intruding aircraft makes developing a robust collision avoidance logic challenging. This paper presents an automated approach for optimizing collision avoidance logic based on probabilistic models of aircraft behavior and a performance metric that balances the competing objectives of maximizing safety and minimizing alert rate. The approach involves framing the problem of collision avoidance as a Markov decision process that is solved using dynamic programming. Although this paper focuses on airborne collision avoidance for manned aircraft, the methods may be applied to collision avoidance for other categories of vehicles, both manned and unmanned.
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Summary

All large transport aircraft are required to be equipped with a collision avoidance system that instructs pilots how to maneuver to avoid collision with other aircraft. The uncertainty in the behavior of the intruding aircraft makes developing a robust collision avoidance logic challenging. This paper presents an automated approach for...

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Traffic Management Advisor (TMA) weather integration

Published in:
MIT Lincoln Laboratory Report ATC-364

Summary

TCAS behavior in New England airspace is being monitored and analyzed, making use of an omni-directional 1030/1090 MHz receiver. The receiver system, located in Lexington, MA, and operated by M.I.T. Lincoln Laboratory, is used to record Resolution Advisories (RAs). Omni-directional receptions make it possible to examine the air-to-air messages exchanged between aircraft for coordination of RAs. Omni-directional reception rates are also being studied. THe results indicated the percentage of aircraft that are TCAS equipped and the percentage of received signals that originate from TCAS and other systems. A third aspect of the program evaluates the availablity of 1090 MHz Extended Squitter data for use in collision avoidance systems. Data is recorded continuously, and the busiest periods are selected for focused attention.
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Summary

TCAS behavior in New England airspace is being monitored and analyzed, making use of an omni-directional 1030/1090 MHz receiver. The receiver system, located in Lexington, MA, and operated by M.I.T. Lincoln Laboratory, is used to record Resolution Advisories (RAs). Omni-directional receptions make it possible to examine the air-to-air messages exchanged...

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Collision avoidance for unmanned aircraft using Markov Decision Processes

Published in:
AIAA Guidance, Navigation, and Control Conf., 2-5 August 2010.

Summary

Before unmanned aircraft can fly safely in civil airspace, robust airborne collision avoidance systems must be developed. Instead of hand-crafting a collision avoidance algorithm for every combination of sensor and aircraft configuration, we investigate the automatic generation of collision avoidance algorithms given models of aircraft dynamics, sensor performance, and intruder behavior. By formulating the problem of collision avoidance as a Markov Decision Process (MDP) for sensors that provide precise localization of the intruder aircraft, or a Partially Observable Markov Decision Process (POMDP) for sensors that have positional uncertainty or limited field-of-view constraints, generic MDP/POMDP solvers can be used to generate avoidance strategies that optimize a cost function that balances flight-plan deviation with collision. Experimental results demonstrate the suitability of such an approach using four different sensor modalities and a parametric aircraft performance model.
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Summary

Before unmanned aircraft can fly safely in civil airspace, robust airborne collision avoidance systems must be developed. Instead of hand-crafting a collision avoidance algorithm for every combination of sensor and aircraft configuration, we investigate the automatic generation of collision avoidance algorithms given models of aircraft dynamics, sensor performance, and intruder...

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Improved Monte Carlo sampling for conflict probability estimation

Published in:
51st AIAA/ASME/AHS/ACS Structures, Structural Dynamics, and Materials Conf., 12-15 April 2010.

Summary

Probabilistic alerting systems for airborne collision avoidance often depend upon accurate estimates of the probability of conflict. Analytical, numerical approximation, and Monte Carlo methods have been applied to conflict probability estimation. The advantage of a Monte Carlo approach is the greater flexibility afforded in modeling the stochastic behavior of aircraft encounters, but typically many samples are required to provide an adequate conflict probability estimate. One approach to improve accuracy with fewer samples is to use importance sampling, where trajectories are sampled according to a proposal distribution that is different from the one specified by the model. This paper suggests several different sample proposal distributions and demonstrates how they result in significantly improved estimates.
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Summary

Probabilistic alerting systems for airborne collision avoidance often depend upon accurate estimates of the probability of conflict. Analytical, numerical approximation, and Monte Carlo methods have been applied to conflict probability estimation. The advantage of a Monte Carlo approach is the greater flexibility afforded in modeling the stochastic behavior of aircraft...

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Airspace encounter models for estimating collision risk

Published in:
J. Guidance, Control, and Dynamics, Vol. 33, No. 2, March-April 2010, pp. 487-499.

Summary

Airspace encounter models, providing a statistical representation of geometries and aircraft behavior during a close encounter, are required to estimate the safety and robustness of collision avoidance systems. Prior encounter models, developed to certify the Traffic Alert and Collision Avoidance System, have been limited in their ability to capture important characteristics of encounters as revealed by recorded surveillance data, do not capture the current mix of aircraft types or noncooperative aircraft, and do not represent more recent airspace procedures. This paper describes a methodology for encounter model construction based on a Bayesian statistical framework connected to an extensive set of national radar data. In addition, this paper provides examples of using several such high-fidelity models to evaluate the safety of collision avoidance systems for manned and unmanned aircraft.
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Summary

Airspace encounter models, providing a statistical representation of geometries and aircraft behavior during a close encounter, are required to estimate the safety and robustness of collision avoidance systems. Prior encounter models, developed to certify the Traffic Alert and Collision Avoidance System, have been limited in their ability to capture important...

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Model-based optimization of airborne collision avoidance logic

Published in:
MIT Lincoln Laboratory Report ATC-360

Summary

The Traffic Alert and Collision Avoidance System (TCAS) is designed to reduce the risk of mid-air collisions by providing resolution advisories to pilots. The current version of the collision avoidance logic was hand-crafted over the course of many years and contains many parameters that have been tuned to varying extents and heuristic rules whose justification has been lost. Further development of the TCAS system is required to make the system compatible with next generation air traffic control procedures and surveillance systems that will reduce separation between aircraft. This report presents a decision-theoretic approach to optimizing the TCAS logic using probabilistic models of aircraft behavior and a cost metric that balances the cost of alerting with the cost of collision. Such an approach ahs the potential for meeting or exceeding the current safety level while lowering the false alert rate and simplifing the process of re-optimizing the logic in response to changes in the airspace and sensor capabilities.
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Summary

The Traffic Alert and Collision Avoidance System (TCAS) is designed to reduce the risk of mid-air collisions by providing resolution advisories to pilots. The current version of the collision avoidance logic was hand-crafted over the course of many years and contains many parameters that have been tuned to varying extents...

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TCAS multiple threat encounter analysis

Published in:
MIT Lincoln Laboratory Report ATC-359

Summary

The recent development of high-fidelity U.S. airspace encounter models at Lincoln Laboratory has motivated a simulation study of the Traffic Alert and Collision Avoidance System (TCAS) multiple threat logic. We observed from archived radar data that while rarer than single-threat encounters, multiple threat encounters occur more frequently than originally expected. The multithreat logic has not been analyzed in the past using encounter models. To generate multi-threat encounters, this report extends the statistical techniques used to develop pairwise correlated encounters. We generated and simulated a large number of multi-threat encounters using the TCAS logic implemented in Lincoln Laboratory's Collision Avoidance System Safety Assessment Tool. Near mid-air collision (NMAC) count indicates how often close encounters are resolved, unresolved, or induced by TCAS. Change in vertical miss distance shows the effect of the additional threat on the vertical separation between the first two aircraft. Risk ratio measures how the probability of an NMAC changes when an aircraft is equipped with TCAS versus being unequipped. Study results indicate that in multi-threat encounters, the TCAS logic results in a more than twofold increase in unresolved NMACs and approximately five times more induced NMACs than one-on-one encounters. TCAS provides a safety benefit in multi-threat encounters by issuing resolution advisories that result in increased vertical separation between the equipped aircraft and the first intruder.
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Summary

The recent development of high-fidelity U.S. airspace encounter models at Lincoln Laboratory has motivated a simulation study of the Traffic Alert and Collision Avoidance System (TCAS) multiple threat logic. We observed from archived radar data that while rarer than single-threat encounters, multiple threat encounters occur more frequently than originally expected...

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