The international standardization of Airborne Collision Avoidance System (ACAS) is discussed in International Civil Aviation Organization. The work is now in a final stage after the recommendation by SICAS (SSR Improvements and Collision Avoidance Systems) Panel. To survey the effect of the system to the present ATC and aircraft operation system in Japan, The Collision Avoidance System Operational Survey Committee is organized. Civil Aviation Bureau, Universities, Airlines, Avionics Company, Defence Agency, and Electronic Navigation Research Institute participate to the committee.

The survey has three major objects. They are (1) Situation analysis of advisory generation, (2) Analysis of effects of Collision Avoidance System in such situations to ATC circumstances, (3) Operational issues to be solved. In this paper, (1) is mainly discussed.

In the survey, pilot and ATC comments are collected, and ground radar data are analyzed using those data. In this paper, the results of analysis of ground radar data are presented.

The error of angle of arrival (AOA) measurement that is observed with electromagnetic wave propagation around the cylindrical fuselage is studied. The AOA is measured with airborne antenna which is used for Airborne Collision Avoidance System. The antenna element is perpendicular to the fuselage surface.

Considerably large error is resulted by the calculation for the diffracted ray path geometry. This error is due to the bend of propagation path on the curved surface of fuselage, and can degrade the accuracy of any AOA measurement which is measuring the normal direction of wavefront. The antenna mounting method for error reduction is also discussed in this paper.

The Mode S transponder with illegal addresses are found in some airspaces. This paper discusses the effect of those address on the ACAS (Airborne Collision Avoidance Sysem) operation. The result of monitoring for those addresses in the Tokyo airspace is also reported.

ACAS functions with the Mode S addressed surveillance and communication can fail without unique addressing for each transponder. Then, the operation of Mode S transponders with illegal addresses are viewed as a potential safety problem.

The Tokyo airspace has been monitored for illegal Mode S addresses with modified ACAS experimental unit. Addresses are collected by passive reception of Mode S squitters. About five percents of the collected addresses are illegal.