TCAS III Bearing Error Evaluation

This document is disseminated under the sponsorship of the Department of Transportation in the interest of information exchange. The United States Government assumes no liability for its contents or use thereof. The Traffic Alert and Collision Avoidance System (TCAS) III seeks to enhance the capabilities of TCAS II by providing additional encounter resolution in the horizontal plane. The addition of horizontal Resolution Advisories (RA) to the array of RA options is enabled through an estimation of the horizontal miss distance, i.e., the predicted point of closest approach in the geometric plane centered at the TCAS aircraft and parallel to the earth's surface. The degree of uncertainty in the horizontal miss distance estimate is directly related to the bearing rate error. Therefore, TCAS III's ability to resolve encounters in the horizontal plane is limited by the accuracy in the estimation of the intruder's bearing rate derived from bearing measurements of the intruder during an encounter. The bearing measurements can contain relatively large errors due to limitations in the antenna subsystem. The bearing measurement errors arc introduced by antenna pattern perturbations, namely those caused by electromagnetic scattering of the airframe and nearby objects, and result in large errors in the miss distance estimation. These systematic errors are correlated in both bearing and elevation and arc highly dependent upon the installed configuration (airframe, nearby objects). In evaluating the performance of the bearing measurement capability, ar'assessment was made of: 1) the expected percentage of horizontal RAs issued, 2) the performance of the Miss Distance Filter (MDF), and 3) the ability to successfully monitor the separation progress during a horizontal RA maneuver. Each of these performance measurements was evaluated over a wide variety of airframe, collision avoidance (CAS) logic, and sensitivity level configurations. For this analysis two CAS logic models were used. The first CAS logic model selects the appropriate RA based on a comparison of the expected separation gains of each valid RA. The second model utilizes a working version of the Minimum Operational Performance Standards (MOPS) based TCAS IlI pseudocode and all 19 RA evaluation tests. Using the simple separation model, which was more suited for the encounters generated (i.e., co-altitude and no vertical rates) it was shown that for a typical installation (air carrier airframe and Mode S blade antenna located in close proximity), horizontal RAs would be issued only 9-15% of the time against intruders penetrating the TCAS II defined threat …