International Civil Aviation Organization

World-wide CNS/ATM Systems

Implementation Conference

Rio de Janeiro, 11 – 15 May 1998

International Civil Aviation Organization

World-wide CNS/ATM Systems

Implementation Conference

Rio de Janeiro, 11 – 15 May 1998

WW/IMP–WP/41

11/5/98

Item 1

Agenda Item 1:	Overview of CNS/ATM systems implementation

AIRBORNE COLLISION AVOIDANCE SYSTEMS (ACAS)

(Presented by the Secretariat)

Information Paper

1.	INTRODUCTION

1.1	Airborne collision avoidance system (ACAS) is defined as "an aircraft system based on secondary surveillance radar (SSR) transponder signals which operates independently of ground-based equipment to provide advice to the pilot on potential conflicting aircraft that are equipped with SSR transponders".

2.	ACAS STANDARDIZATION

2.1	Standards and Recommended Practices (SARPs) for ACAS are contained in Annex 10, Volume IV, including the technical characteristics for surveillance requirements, threat detection and co-ordination with ACAS-equipped threats. Guidance material on ACAS is also contained in Annex 10, Volume IV.

2.2	ACAS I provides an information as an aid to 'see and avoid'. This system is not intended for international implementation and is not standardized by ICAO.

2.3	ACAS II provides vertical resolution advisories (RA) in addition to traffic advisories (TA). Latest provisions being included in Annex 10 with Amendment 73 (1998) provide significant enhancements to the collision avoidance logic (reduction of false alarms) as well as improved compatibility with ATM in terms of electromagnetic interference control on SSR frequencies and overall performance in a reduced vertical separation minima (RVSM) environment.

2.4	ACAS III will provide vertical and horizontal RA in addition to TA. ACAS III Standards are part of the future work of the ICAO SICAS Panel with a target date of 2000.

3.

3.1	ACAS information is intended to assist pilots in the safe operation of aircraft. The carriage of ACAS by aircraft shall not be a factor in determining the need for ATS in that area. The procedures for the provision of ATS to ACAS-equipped aircraft shall be identical to those applicable to non-ACAS-equipped aircraft.

3.2	International operational evaluations have been conducted for many years in the United States, in Japan and in Europe. Implementation guidelines and training objectives both for pilot and controller have been developed accordingly. ACAS II safety analysis results demonstrate that ACAS II significantly reduces the risk of collision, but cannot wholly eliminate it, and in common with any system which manoeuvres aircraft in close proximity, can itself induce some risk of collision.

3.3	The operation of ACAS is based on and requires wide spread use of pressure-altitude reporting SSR transponders (Mode A/C or Mode S). Any mandate to carry ACAS must be associated with/preceded by a mandate to carry and operate such a transponder.

4.	LEVEL OF IMPLEMENTATION

4.1	ACAS has been mandated in the United States and is being implemented in other States, e.g. India, Japan and Australia. In Europe the Member States of the European Civil Aviation Conference (ECAC) mandated carriage and operation of ACAS II from 1 January 2000.

4.2	The maturity is such that ACAS is ready for global fitment. While recommending today the carriage of ACAS II and in order to provide enough leading time for implementation, ICAO is requiring the carriage of ACAS II for international commercial aviation aircraft from 1 January 2003 (Amendment to Annex 6, Part I, March 1998).

4.3	Some 20 000 aircraft are equipped with Mode S transponders among which 10 000 are also ACAS equipped. The number of aircraft equipped with an SSR transponder is greater than 200 000 worldwide. The level of equipage should increase associated with the deployment of Mode S and ACAS in the regions.

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