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RADIOLOCATION J. Mateu - J. Berenguer

RADIOLOCATION

Airborne Collision Avoidance System(ACAS)

Jordi Mateu – Jordi Berenguer


Course Contents

I. Introduction to Air Navigation and Surveillance Functions.

II. Pulsed Radar.III. Radar Clutter.IV. Doppler based Radars.V. Tracking Radars.VI. Secondary Surveillance Radar (SSR).VII. Automatic Dependent Surveillance (ADS).VIII.Airborne Collision Avoidance System (ACAS).IX. Enhanced Ground Proximity Warning Systems.X. Weather Radar.


Airborne Collision Avoidance System (ACAS)

• The Airborne Collision Avoidance System (ACAS) IIconcept (realized as Traffic alert and Collision Avoidance System (TCAS) II equipment) is an airborne avionics system which acts independently of ATC as a last resort safety net to mitigate the risk of midair collision.

• ACAS tracks aircraft in the surrounding airspace throughthe replies from their ATC transponders.

• If the system diagnoses a risk of impending collision it issues a Resolution Advisory (RA) to the flight crew, which directs the pilots how best to regulate or adjust theirvertical rate so as to avoid a collision.


Airborne Collision Avoidance System (ACAS)

• ACAS II version 7.0 has been mandated in Europe since 1 January 2005 by all civil fixed-wing turbine-engined aircraft having a maximum take-off mass exceeding 5700 kg or a maximum approved passenger seating configuration of more than 19.

• Amendment 85 to ICAO Annex 10 (volume IV) published in October 2010 introduced a provision stating that all new ACAS installations after1 January 2014 shall be compliant with version 7.1 and after 1 January 2017 all ACAS units shall be compliant with version 7.1.

• In December 2011, the European Commission published an Implementing Rule mandating the carriage of ACAS II version 7.1 within European Union airspace earlier than the dates stipulated in ICAOAnnex 10: from 1 December 2015 by all civil aircraft with a maximum certified take-off mass over 5700 kg or authorized to carry more than 19 passengers, with the exception of unmanned aircraft systems.


ACAS AND TCAS

TCAS II is the only implementation that meets the ACAS ICAO Standards and Recommended Practices (SARPs). The term ACAS II is typically used when referring to the standard or concept and TCAS II when referring to the implementation.However, often both terms are used interchangeably.

ACAS PRINCIPLES

ACAS is designed to work both autonomously and independently of the aircraft navigationequipment and any ground systems used for the provision of air traffic services.

Through antennas, ACAS interrogates the ICAO standard compliant transponders of aircraft in thevicinity.

Based upon the replies received, the system tracks the slant range, altitude (when it is included in the reply message) and bearing of surrounding traffic.


ACAS ALERTS ISSUES

ACAS can issue two types of alerts:

• Traffic Advisories (TAs)– Helps the pilots in the visual acquisition of the intruder aircraft– Alerts them to be ready for a potential resolution advisory.

• Resolution Advisories (RAs)– Are avoidance maneuvers recommended to the pilot.– An RA will tell the pilot the range of vertical rates within which the

aircraft should be flown to avoid the threat aircraft.– An RA can be generated against all aircraft equipped with an altitude

reporting transponder (Mode S or Mode A/C).– The intruder does not need to be fitted with ACAS II. – When the intruder aircraft is also fitted with an ACAS II system, both

systems coordinate their RAs through the Mode S data link, in order to select complementary resolution senses.






ACAS STANDARDS – ICAO Annex 10

• ACAS I– Measures range and closure rates of other aircraft.– Alerts pilot when intruder aircraft enters protected volume of airspace and becomes a potential threat (Traffic

Advisory - TA).– Provides information as an aid to "see and avoid" action, but does not include the capability for generating

Resolution Advisory (RAs). – ACAS I is not, nor has it ever been, mandated in Europe and there are no operational rules regarding the use of

ACAS I.

• ACAS II provides vertical RAs in addition to TAs.– Measures range and closure rates of other aircraft.– Alerts pilot when intruder aircraft enters protected volume of airspace and becomes a potential threat (Traffic

Advisory - TA).– Provides avoidance maneuvers for pilots in vertical sense if intruder becomes a threat (Resolution Advisory - RA)– Coordinates and provides complementary RAs (if both aircraft have Mode S transponders

• ACAS III -Sometimes referred to as TCAS IV- provides vertical and horizontal RAs in addition to TAs.– Provides vertical and horizontal RAs in addition to TAs.– Although ACAS III is mentioned as a future system in ICAO Annex 10, ACAS III is unlikely to materialize due to

difficulties the current surveillance systems have with horizontal tracking.

ACAS XResearch is being conducted to develop a future collision avoidance system under the working name of ACAS X.If developed and certified, ACAS X will not be commercially available before the 2020s.It is unclear at this stage whether ACAS X would provide any horizontal resolutions.


TCAS Versions

• TCAS II version 6.04a – old version of TCAS II– Never mandated in Europe – Still used by some military aircraft or foreign aircraft (which do not fall within

the current European mandate)

• TCAS II version 7.0 currently mandated in Europe– To be gradually phased out and replaced by version 7.1

• TCAS II version 7.1 mandated in Europe– For all new aircraft after 1 March 2012 – For all aircraft after 1 December 2015

• TCAS II version 7.1 mandated by ICAO (Annex 10):– For all new installations after 1 January 2014 – For all units after 1 January 2017

Note: TCAS versions are backwards compatible


History of implementation 1956-1999


History of implementation 2000-2011


ACAS I

• Provides only advisories to aid visual acquisition. • RAs are not issued. • It provides three levels of advisories:

– Other Traffic. – Proximate Advisories (PA). – Traffic Advisories (TA).

• TAs are issued based on either tau (time to go) or proximity to an intruder aircraft, using two sensitivity levels.

• Nominally, all intruding transponder equipped aircraft within 5 nmi are detected and shown on a traffic display.

• The display of a TA is accompanied by an aural alert “Traffic, traffic” to inform the crew a TA has been displayed.

• The aural annunciations are inhibited if own aircraft is below 400 feetAGL (on an aircraft equipped with a radio altimeter) or when the landing gear is extended (if no radio altimeter is installed).

• When TCAS I is installed on a fixed-gear aircraft without a radio altimeter, the aural annunciations will never be inhibited.


TCAS – Protected volume of airspace

Time to go

The range tau is equal to the slant range (nmi) divided by the closing speed (knots) multiplied by 3600.

The vertical tau is equal to the altitude separation (feet) divided by the vertical closing speed of the two aircraft (feet/minute) times 60.


TCAS – Protected volume of airspace


TCAS II VERSION 7.0

• It was approved in December 1997 and became available at the beginning of 1999.

• Version 7.0 further improved TCAS II compatibility with the air traffic control system.

• The most significant enhancements brought by version 7.0 were: – The introduction of a horizontal miss distance filter; – 25-foot vertical tracking; – Sophisticated multi-threat logic; – Compatibility with Reduced Vertical Separation Minima (RVSM)

operations; – The reduction of electromagnetic interference; – Allowing RA reversals in coordinated encounters; – Simplified aural annunciations.


TCAS

• Sistema independent del Control de Trànsit Aeri(ATC).

• Es basa en la utilització dels transponders de Radar Secundari que incorporen les alters aeronaus.

• Consisteix en interrogar les altres aeronaus, i calcular la seva distància i altitude, per:– Emetre alertes de peril: TRAFFIC ADVISORY (TA),

(TCAS I i TCAS II)

– Emetre resolucions de maniobra: RESOLUTION ADVISORY (RA), (TCAS II)


TCAS II – Block diagram• Directive antenna located at the top of the aircraft.

– Transmit interrogations on 1030 MHz at varying power levels in each of four 90° azimuth segments.

• Omnidirectional antenna located at the bottom of the aircraft

– Transmits fewer interrogations and at a lower power than the top-mounted antenna.

– These antennas also receive transponder replies, at 1090 MHz, and send these replies to the TCAS Processor.

• Mode S/TCAS Control panel– Stand-by– Transponder– TA Only– Automatic or TA/RA

• Mode S Transponder– Performs the normal functions as ATCRBS or a Mode S.– Provides air-to-air data exchange between TCAS-equipped

aircraft so that coordinated, complementary RAs can be issued when required.

– One antenna is mounted on the top of the aircraft while the other is mounted on the bottom.

– These antennas enable the Mode S transponder to receive interrogations at 1030 MHz and reply to the received interrogations at 1090 MHz.

– New transponder-TCAS integrated systems only require two antennas that are shared by the transponder and TCAS.

• TCAS computer– performs airspace surveillance, intruder tracking, own

aircraft altitude tracking, threat detection, RA maneuver determination and selection, and generation of advisories.

– uses pressure altitude, radar altitude, and discrete aircraft status inputs

• Traffic (TA) and Resolution (RA) announcements• Aural output messages• Traffic Display

– Displayed traffic information also includes traffic vertical speed indications, and Proximate, TA, and RA status.


TCAS II – Block diagram


Antennas


Interrogation process


Garbling

Due to the fact that the reply duration of a transponder is of 21 µs, a Synchronous Garbling can be created by all transponders within a radius of:

R =𝑐𝜏

2=

21 · 10 · 3 · 10

2= 𝟏, 𝟕 𝐧𝐦𝐢

The synchronous garbling can be reduced:• by slightly overlapping coverage.• by the use of the Mode C Only All Call

interrogation, which inhibits Mode S transponders from replying to a Mode C interrogation.


Garbling

The synchronous garbling can be reduced:

– By slightly overlapping coverage, making directional interrogations in four 90° sectors, with the directive antenna.

– By the use of the Mode C Only All Call interrogation, which inhibits Mode S transponders from replying to a Mode C interrogation.

– By avoiding the initiation of surveillance tracks based on multipath replies, with Dynamic Minimum Triggering Level (DMTL).

• The direct-path power level is used to raise the minimum triggering level (MTL) of the TCAS receiver enough to discriminate against the delayed and lower power reflections.

• The 4-pulse direct reply is above the DMTL level, while the delayed, lower-power multipath reply is below the DMTL threshold, and is thus rejected by TCAS.


Modes of operationStand-by:• Does not issue any interrogations and the transponder

will reply to only discrete interrogations. • The transponder still transmits squitters.• If the aircraft is on the ground and transmitting

extended squitters, it is not required to transmit short (acquisition) squitters.

Transponder: • Fully operational and will reply to all appropriate ground

and TCAS interrogations. • TCAS remains in Stand-by.

TA Only:• Mode S fully operational.• TCAS will operate normally and issue the appropriate

interrogations and perform all tracking functions. • However, TCAS will only issue TAs;• RAs will be inhibited.

Automatic or TA/RA:• Mode S transponder fully operational. • TCAS will operate normally and issue the appropriate

interrogations and perform all tracking functions. • TCAS will issue TAs and RAs when appropriate.• The sensitivity is automatically selected in terms of the

AGL.• The sensitivity is also determinated from the Tau

parameter (Range/Closure rate).


Sensitivity level

DMOD: Distance Modifier – Lateral Threshold used in the Range Test and Miss Distance Filter

ZTHR: Z Threshold – Vertical Threshold for Resolution Advisory


Protected airspace


Collisions Avoidance Logic

When both aircraft are equipped with TCAS II system, they make use of the mode S communication system in order to transmit coordination interrogations, guaranteeing the compliance between the RA generated.


Standardized Symbology for Use on the Traffic Display


Cockpit presentation


TCAS traffic display example: EFIS


TCAS RA display example: PFD & vertical speed tape


RA display example: IVSI combined with traffic display


RA aural alerts TCAS II version 7.0


TCAS II – VERSION 7.1


TCAS II – Change 7.1


Version 7.1 EUROCONTROL mandate• In 2011, the European Commission published Regulation No 1332/2011, subsequently amended

by Regulation No 2016/583, mandating, from 1 December 2015, the carriage of ACAS II (TCAS II) version 7.1 within European Union airspace by all civil airplanes with a MTOM exceeding 5700 kg or authorized to carry more than 19 passengers.

• Aircraft not referred above, but which are equipped on a voluntary basis with ACAS II, must be equipped with version 7.1. The mandate does not apply to unmanned aircraft systems.

AUR.ACAS.1005 Performance requirement(1) The following turbine-powered aeroplanes shall be equipped with collision avoidance logic version 7.1 of ACAS II:

(a) Airplanes with a maximum certificated take-off mass exceeding 5.700 kg;(b) Airplanes authorized to carry more than 19 passengers.

(2) Aircraft not referred to in point 1 which are equipped on a voluntary basis with ACAS II shall have collision avoidance logic version 7.1.(3) Point 1 shall not apply to unmanned aircraft systems.

AUR.ACAS.1010 ACAS II trainingOperators shall establish ACAS II operational procedures and training programs so that the flight crew is appropriately trained in the avoidance of collisions and becomes competent in the use of ACAS II equipment.’



• In this instance aircraft A should climb at a rate that will take it out of the red zone and maintain a rate that is within the green zone. However, as the diagram shows, pilots can sometimes overreact and with the result that the vertical speed achieved exceeds that required to safely avoid the threat. Rates as high as 6,000 ft/min have been observed.

• The maneuver shown may bring aircraft A into conflict with aircraft C.• The new ACAS II 7.1 version replaces the previous "adjust vertical speed, adjust" with the new

"Level off, level off" RA and reduce instances of level busts and not induce conflict with the third party.


RA aural alerts TCAS II version 7.1
















TCAS II – Change 7.1 – Hybrid surveillance

• Hybrid Surveillance further improves situational awareness and safety in the high density airspace and supports air traffic modernization efforts by reducing 1090 MHz frequency congestion.

• Hybrid Surveillance allows the TCAS Traffic Computer to further reduce active interrogations by using intruder position data received via ADS-B equipped aircraft.

• Hybrid Surveillance capability, which meets the qualification and requirements of the Hybrid Surveillance Minimum Operational Performance Standards (MOPS) document RTCA/DO-300, is identified as optional functionally within FAA TSO-C119c.


Hybrid surveillance

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