collision avoidance system. A goal of this research is to develop a system that provides a lower alert rate than TCAS while enhancing safety.

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Onboard computation. TCAS currently makes decisions once per second, and a future system will likely be required to make decisions at least as frequently. Many of the algorithms suggested in the literature are not yet suitable for real-time use, but the method pursued in this report provides the required performance using current technology. The implementation of the proposed system is capable of making decisions in less than a millisecond on a single processor, which enables fast-time simulation of the system on millions of encounters during development.

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Coordination. If two TCAS-equipped aircraft encounter each other, they will coordinate their advisories over a low bandwidth communication channel. This coordination aims to prevent, for example, both aircraft from climbing into each other. A new system should also coordinate advisories to enhance safety. Although better coordination may be achieved through a higher bandwidth data link, this report assumes only the existing communication channel currently used by TCAS will be available for a new system.

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Unequipped aircraft. TCAS will avoid aircraft that are not TCAS-equipped, so long as they have a transponder. Although TCAS is not able to coordinate with non-TCAS aircraft, it still provides a signi.cant safety bene.t. Similarly, the new system must prevent collision even when the intruder is not equipped with a collision avoidance system.

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Interoperability. The introduction of a new collision avoidance system is likely to be gradual. Hence, any new system should be capable of interoperating with legacy versions of TCAS. Although the design proposed in this report allows interoperability, evaluating this will be the focus of future work.

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Surveillance compatibility. TCAS was designed for a particular surveillance system. Adjusting the existing logic to accommodate new surveillance systems, such as those that leverage global positioning system information, is nontrivial. Since surveillance technology is expected to evolve, the new collision avoidance logic should be su.ciently .exible to accommodate a wide variety of di.erent surveillance systems.

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Aircraft performance constraints. The TCAS collision avoidance logic was intended primarily for large transport aircraft with certain capabilities, but the new system should be compatible with a wider variety of aircraft, including unmanned aircraft and those typically used for general aviation. Not only should the new system only issue advisories that are feasible for the aircraft on which it is installed, it should take into consideration the performance limits of other aircraft when coordinating advisories. For example, if an intruder is known to be unable to climb quickly, then TCAS may issue a descend advisory earlier than it would otherwise. Accommodating aircraft performance limitations is not a focus of this report, but it is a straightforward extension to the proposed method and will be pursued later.

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Extensibility. The airspace will evolve signi.cantly over the next few decades with the intro-duction of next-generation air tra.c control technologies. Any new collision avoidance system

will need to evolve with the airspace. Changing the TCAS pseudocode can be very di.cult due to the complexity of the logic and the ways in which the various rules and parameters interact with each other. This report investigates an approach that allows the logic to be quickly re-optimized in response to changes in the airspace model.

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Validation. Due to its safety-critical nature, a collision avoidance system must be validated to ensure that the system provides the required level of safety with a high degree of con.dence. The system proposed in this report can be validated using encounter models in Monte Carlo simulation, as has been done with TCAS. The nature of the approach taken in this report, however, permits additional, complementary validation techniques to be discussed later.
　
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