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Preface

This booklet provides the background fora better understanding of the Traffic Alertand Collision Avoidance System (TCAS II)by personnel involved in theimplementation and operation of TCAS II.This booklet is an update of a similarbooklet published in 1990 by the FederalAviation Administration (FAA). Thisupdate describes TCAS II Version 7.


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Table of Contents

PREFACE......................................................................................................................................................2

THE TCAS SOLUTION...............................................................................................................................5

BACKGROUND .............................................................................................................................................6

TCAS II DEVELOPMENT..............................................................................................................................7IN-SERVICE OPERATIONAL EVALUATIONS ..................................................................................................8

TOWARD A REQUIREMENT FORWORLDWIDE CARRIAGE ............................................................................9

STANDARDS AND GUIDANCE MATERIAL ...................................................................................................10

TCAS II TECHNICAL DESCRIPTION ..................................................................................................11

SYSTEM COMPONENTS...............................................................................................................................11

Mode S/TCAS Control Panel ........... .......... ........... ........... .......... ........... ........... .......... ........... ......... ......11

Antennas ............. .......... ........... .......... .......... ........... .......... ........... .......... .......... ........... .......... .......... .....12

Cockpit Presentation............................................................................................................................12Traffic Display................................................................................................................................................ 12Resolution Advisory Display .......................................................................................................................... 14

TARGET SURVEILLANCE .....................................................................................................................17

MODE S SURVEILLANCE............................................................................................................................17

MODE CSURVEILLANCE ...........................................................................................................................17

INTERFERENCE LIMITING...........................................................................................................................20

ELECTROMAGNETIC COMPATIBILITY ........................................................................................................20

COLLISION AVOIDANCE CONCEPTS ................................................................................................21

SENSITIVITY LEVEL ...................................................................................................................................21

TAU ...........................................................................................................................................................22

PROTECTED VOLUME ................................................................................................................................23

CAS LOGIC FUNCTIONS ........................................................................................................................26

TRACKING .................................................................................................................................................26

TRAFFIC ADVISORY...................................................................................................................................27THREAT DETECTION..................................................................................................................................27

RESOLUTION ADVISORY SELECTION .........................................................................................................28

TCAS/TCAS COORDINATION...................................................................................................................30

ADVISORY ANNUNCIATION .......................................................................................................................31

AIR/GROUND COMMUNICATIONS..............................................................................................................32

TRAFFIC ADVISORY DISPLAY....................................................................................................................32

RESOLUTION ADVISORY DISPLAYS ...........................................................................................................32

AURAL ANNUNCIATIONS...........................................................................................................................32

PERFORMANCE MONITORING ....................................................................................................................32

USE OF TCAS.............................................................................................................................................34

REGULATIONS AND OPERATIONAL GUIDANCE ..........................................................................................34

Controllers Responsibilities.................................................................................................................34

Pilot Responsibilities ...... ........... ........... ........... .......... ........... ........... ........... ........... .......... .......... ..........35

OPERATIONAL EXPERIENCE.......................................................................................................................37

TRAINING PROGRAMS ...............................................................................................................................38

Pilot Training Programs.......... ........... .......... ........... .......... ........... .......... ........... .......... ........... .......... ...39

Controller Training Programs.............................................................................................................39

SUMMARY..................................................................................................................................................40


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ABBREVIATIONS .....................................................................................................................................41

GLOSSARY.................................................................................................................................................42

BIBLIOGRAPHY .......................................................................................................................................45


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The TCAS Solution

After many years of extensive analysis,development, and flight evaluation by the

Federal Aviation Administration (FAA),other countries Civil Aviation Authorities(CAAs), and the aviation industry, a solutionhas been found to reduce the risk of midaircollisions between aircraft. This solution isknown as the Traffic Alert and CollisionAvoidance System or TCAS. In theinternational arena, the system is known asthe Airborne Collision Avoidance System orACAS.

TCAS is a family of airborne devices thatfunction independently of the ground-based

air traffic control (ATC) system and providecollision avoidance protection for a broadspectrum of aircraft types.

TCAS I provides traffic advisories (TA) andproximity warning of nearby traffic to assistthe pilot in the visual acquisition of intruderaircraft. TCAS I is mandated for use in theUnited States for turbine-powered, passenger-carrying aircraft having more than 10 and lessthan 31 seats. TCAS I is also used by anumber of general aviation fixed and rotary

wing aircraft.

TCAS II provides traffic advisories andresolution advisories (RA), i.e.,recommended escape maneuvers, in thevertical dimension to either increase ormaintain the existing vertical separationbetween aircraft. Airline aircraft, includingregional airline aircraft with more than 30seats, and general aviation turbine-poweredaircraft use TCAS II equipment.

The TCAS concept uses the same radar

beacon transponders installed on aircraft tooperate with ATC ground-based radars. Thelevel of protection provided by TCASequipment depends on the type of transponderthe target aircraft is carrying. The level ofprotection is outlined in Table 1. It should benoted that TCAS provides no protection

against aircraft that do not have an operatingtransponder.

Table 1. TCAS Levels of ProtectionTable 1. TCAS Levels of ProtectionTable 1. TCAS Levels of ProtectionTable 1. TCAS Levels of Protection

Own Aircraft EquiOwn Aircraft EquiOwn Aircraft EquiOwn Aircraft Equippppmentmentmentment

TCAS ITCAS ITCAS ITCAS I TCAS IITCAS IITCAS IITCAS IIMode AXPDRONLY

TA TA

Mode Cor MODES XPDR

TA TA andVertical RA

TCAS I TA TA andVertical RA

TargetAircraftEqu

TargetAircraftEqu

TargetAircraftEqu

TargetAircraftEquipment

ipment

ipment

ipment

TCAS II TA TA and

CoordinatedVertical RA

Based on a Congressional mandate (PublicLaw 100-223), the FAA has issued a rule thatrequires all passenger-carrying aircraft withmore than 30 seats be equipped withTCAS II.

Since the early 1990s, an operationalevaluation, known as the TCAS TransitionProgram (TTP), has collected and analyzed a

significant amount of data related to theperformance and use of TCAS II in both theU.S. National Airspace System (NAS) and inother airspace worldwide. As a result of theseanalyses, changes to TCAS II have beendeveloped, tested, and implemented. Thelatest changes, collectively known as TCASII Version 7, were certified in early 2000 andare now being implemented by the industry.

TCAS II Version 7 is the only version ofTCAS II that complies with the ICAOStandards and Recommended Practices

(SARPs) for ACAS II. As such, Version 7 iscurrently being mandated for carriage incertain countries or regions, e.g., Europe,Australia, and India, and has been mandatedfor carriage in 2003 by the International CivilAviation Organization (ICAO).


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BackgroundBackgroundBackgroundBackground

The development of an effective airbornecollision avoidance system has been a goal ofthe aviation industry for a number of years.As air traffic has continued to grow over the

years, development of and improvements toATC systems and procedures have made itpossible for controllers and pilots to copewith this increase in operations, whilemaintaining the necessary levels of flightsafety. However, the risk of airborne collisionremains. That is why, as early as the 1950s,the concept and initial development of anairborne collision avoidance system, acting asa last resort, was being considered.

A series of midair collisions that occurred in

the United States, has been the impetus forthe development and refinement of anairborne collision avoidance system. Thesetragic milestones included the followingcollisions:

In 1956, the collision between twoairliners over the Grand Canyonspurred both the airlines and theaviation authorities to initiate systemdevelopment studies for an effectivesystem.

In 1978, the collision between a light

aircraft and an airliner over San Diegoled the FAA to initiate thedevelopment ofTCAS.

Finally, in 1986, the collision betweena DC-9 and a private aircraft overCerritos, California, resulted in aCongressional mandate that requiredsome categories of American andforeign aircraft to be equipped withTCAS for flight operations in U.S.airspace.

In parallel to the development of TCASequipment in the United States, ICAO hasbeen working since the early 1980s todevelop standards for ACAS. ICAOofficially recognized ACAS on 11

November 1993. Its descriptive definitionappears in Annex 2 of the Convention on

International Civil Aviation and its use isregulated in Procedures for Air NavigationServices ----- Aircraft Operations (PANS-OPS)and Procedures for Air Navigation Services----- Rules of the Air and Air Traffic Services(PANS-RAC). In November 1995, the

SARPs and Guidance Material for ACAS IIwere approved, and they appear in Annex 10of the Convention on International CivilAviation.

During the late 1950s and early 1960s,collision avoidance development effortsincluded an emphasis on passive andnoncooperating systems. These conceptsproved to be impractical. One majoroperational problem that could not beovercome with these designs was the need for

nonconflicting, complementary avoidancemaneuvers that require a high-integritycommunications link between aircraftinvolved in the conflict.

One of the most important developments inthe collision avoidance concept was thederivation of the range/range rate, or tau,concept by Dr. John S. Morrell of Bendix.This concept is based on time, rather thandistance, to the closest point of approach inan encounter.

During the late 1960s and early 1970s,several manufacturers developed aircraftcollision avoidance systems based oninterrogator/transponder and time/frequencytechniques. Although these systemsfunctioned properly during staged aircraftencounter testing, the FAA and the airlinesjointly concluded that in normal airlineoperations, they would generate a high rate ofunnecessary alarms in dense terminal areas.This problem would have undermined thecredibility of the system with the flight

crews. In addition, each target aircraft wouldhave to be equipped with the same equipmentto provide protection to an equipped aircraft.

In the mid 1970s, the Beacon CollisionAvoidance System (BCAS) was developed.BCAS used reply data from the Air Traffic


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Control Radar Beacon System (ATCRBS)transponders to determine an intruders rangeand altitude. At that time, ATCRBStransponders were installed in all airline andmilitary aircraft and a large number ofgeneral aviation aircraft. Thus, any BCAS-

equipped aircraft would be able to detect andbe protected against the majority of otheraircraft in the air without imposing additionalequipment requirements on those otheraircraft. In addition, the discrete addresscommunications techniques used in theMode S transponders then under developmentpermitted two conflicting BCAS aircraft toperform coordinated escape maneuvers with ahigh degree of reliability.

TCAS II developmentTCAS II developmentTCAS II developmentTCAS II development

In 1981, the FAA made a decision to developand implement TCAS utilizing the basicBCAS design for interrogation and tracking,but providing additional capabilities.

TCAS is designed to work autonomously of

the aircraft navigation equipment and

independently of the ground systems used

to provide ATC services. TCAS interrogatesICAO-compliant transponders of all aircraftin the vicinity and based on the replies

received, tracks the slant range, altitude(when it is included in the reply message),and bearing of surrounding traffic. Fromseveral successive replies, TCAS calculates atime to reach the CPA (Closest Point ofApproach) with the intruder, by dividing therange by the closure rate. This time value isthe main parameter for issuing alerts. If thetransponder replies from nearby aircraftincludes their altitude, TCAS also computesthe time to reach co-altitude. TCAS can issuetwo types of alerts:

TAs to assist the pilot in the visualsearch for the intruder aircraft and toprepare the pilot for a potential RA;and

RAs to recommend maneuvers that willeither increase or maintain the existingvertical separation from an intruder

aircraft. When the intruder aircraft isalso fitted with TCAS II, both TCAScoordinate their RAs through theMode S data link to ensure thatcomplementary resolution senses areselected.

TCAS II is designed to operate in trafficdensities of up to 0.3 aircraft per squarenautical mile (nmi), i.e., 24 aircraft within a 5nmi radius, which is the highest trafficdensity envisioned over the next 20 years.

Development of the TCAS II collisionavoidance algorithms included thecompletion of millions of computersimulations to optimize the protectionprovided by the system, while minimizing thefrequency of unacceptable or nuisanceadvisories. In addition to these computersimulations, early versions of the collisionavoidance algorithms were evaluated via pilotin the loop simulations and during theoperation of prototype equipment in FAAaircraft throughout the NAS.

Extensive safety studies were also performedto estimate the safety improvements thatcould be expected with the introduction ofTCAS into service. These safety studies havebeen continuously updated throughout the

refinement of the collision avoidancealgorithms. The safety studies have shownthat TCAS II will resolve nearly all of thecritical near midair collisions involvingairline aircraft. However, TCAS cannothandle all situations. In particular, it isdependent on the accuracy of the threataircrafts reported altitude and on theexpectation that the threat aircraft will notmake an abrupt maneuver that defeats theTCAS RA. The safety study also shows thatTCAS II will induce some critical near midair

collisions, but overall, the number of nearmidair collisions with TCAS is less than 10%of the number that would have occurredwithout the presence of TCAS.

Extensive studies were also carried out toevaluate the interaction between TCAS and


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ATC. The analysis of ATC radar datashowed that in 90% of the cases, the verticaldisplacement required to resolve an RA wasless than 300 feet. Based on these studies, itwas concluded that the possibility of theresponse to a TCAS RA causing an aircraft to

infringe on the protected airspace for anotheraircraft was remote. However, operationalexperience has shown that the actualdisplacement resulting from an RA responseis often much greater than 300 feet, andTCAS has had an adverse affect on thecontrollers and the ATC system. Because ofthis operational experience, Version 7contains numerous changes and enhancementsto the collision avoidance algorithms, theaural annunciations, the RA displays, andpilot training programs to minimize thedisplacement while responding to an RA.

InInInIn----ServiServiServiService Operationalce Operationalce Operationalce OperationalEvaluEvaluEvaluEvaluaaaationstionstionstions

To ensure that TCAS performed as expectedin its intended operational environment,several operational evaluations of the systemhave been conducted. These evaluationsprovided a means for the pilots using TCASand the controllers responsible for providingseparation services to TCAS-equipped

aircraft to have a direct influence on the finalsystem design and performance requirements.

The initial operational evaluation of TCASwas conducted by Piedmont Airlines in 1982.Using a TCAS II prototype unit manufacturedby Dalmo Victor, Piedmont flewapproximately 900 hours in scheduled,revenue service while recording data on theperformance of TCAS. These recorded datawere analyzed to assess the frequency andsuitability of the TAs and RAs. During thisevaluation, the TCAS displays were notvisible to the pilots, and observers from theaviation industry flew with the aircraft tomonitor the system performance and toprovide technical and operational commentson its design.

In 1987, Piedmont flew an upgraded versionof the Dalmo Victor equipment forapproximately 1200 hours. During thisevaluation, the TCAS displays were visible tothe pilots and the pilots were permitted to usethe information provided to maneuver the

aircraft in response to RAs. This installationincluded a dedicated TCAS data recorder sothat quantitative data could be obtained on theperformance of TCAS. In addition, pilot andobservers completed questionnaires followingeach TA and RA so that assessments could bemade regarding the value of the system to theflight crews.

This evaluation also provided the basis for thedevelopment of avionics certification criteriafor production equipment, validated pilottraining guidelines, provided the justificationfor improvements to the TCAS algorithmsand displays, and validated the pilotprocedures for using the equipment.

Following the successful completion of thesecond Piedmont evaluation, the FAAinitiated the Limited Installation Program(LIP). Under the LIP, Bendix-King andHoneywell built and tested commercialquality, pre-production TCAS II equipmentthat was in compliance with the TCAS IIMinimum Operational Performance

Standards (MOPS). Engineering flight testsof this equipment were conducted on themanufacturers' aircraft, as well as FAAaircraft. Using data collected during theseflight tests, together with data collectedduring factory and ground testing, bothmanufacturers equipment was certified via aSupplemental Type Certificate (STC) for usein commercial, revenue service.

The Bendix-King units were operated byUnited Airlines on a B737-200 and a DC8-73

aircraft. Northwest Airlines operated theHoneywell equipment on two MD-80 aircraft.Over 2000 hours of operating experiencewere obtained with the United aircraft andapproximately 2500 hours of operatingexperience were obtained with the Northwestinstallations.


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The experience provided by these operationalevaluations resulted in further enhancementsto the TCAS II logic, improved testprocedures, and finalized the procedures forcertification of production equipment. The

most important information obtained from theoperational evaluations was the nearlyunanimous conclusion that TCAS II was safe,operationally effective, and ready for morewidespread implementation.

With the successful completion of these earlyoperational evaluations, there was a highdegree of confidence that a system withsufficient maturity was available to meet theCongressionally mandated implementation ofTCAS II in U.S. airspace.

As part of this mandated implementation , thelargest operational evaluation of TCAS,known as the TTP, was initiated. The TTPbegan in late 1991 and has continued throughthe initial implementation, the mandatedupgrade to Version 6.04A Enhanced, and isstill active as Version 7 enters operation. Inconjunction with the TTP in the U.S.,EUROCONTROL has conducted extensiveevaluations of TCAS operations in Europe,and the Japan Civil Aviation Bureau (JCAB)has conducted similar assessments of

TCAS II performance in Japanese andsurrounding airspace. Other countries alsoconducted operational evaluations as the useof TCAS increased during the past 10 years.

The system improvements suggested as aresult of these TCAS II evaluations led to thedevelopment and release of Version 6.04AEnhanced in 1993. The principal aim of thismodification was the reduction of nuisancealerts, which were occurring at low altitudesand during level-off encounters, and the

correction of a problem in the altitudecrossing logic.

After the implementation of Version 6.04AEnhanced, operational evaluations continuedwith the same objective, and proposedperformance improvements led to the

development of Version 7. The MOPS forVersion 7 was approved in December 1997and Version 7 units became available forinstallation in late 1999. Version 7 isexpected to further improve TCAScompatibility with the air traffic control

system throughout the world.

Toward a Requirement forToward a Requirement forToward a Requirement forToward a Requirement forWorldwide CarriageWorldwide CarriageWorldwide CarriageWorldwide Carriage

The United States was the first member ofICAO to mandate carriage of an airbornecollision avoidance system for passengercarrying aircraft operating in its airspace.

Because of this mandate, the number of long-range aircraft fitted with TCAS II and

operating in European and Asian airspacecontinued to increase, although the systemcarriage and operation were not mandatory inthis airspace. As studies, operationalexperience, and evaluations continued todemonstrate the safety benefits of TCAS II,some non-U.S. airlines also equipped theirshort-haul fleets with TCAS.

In 1995, the EUROCONTROL Committee ofManagement approved an implementationpolicy and schedule for the mandatory

carriage of TCAS II in Europe. The EuropeanAir Traffic Control Harmonization andIntegration Program (EATCHIP) ProjectBoard then ratified this policy. The approvedpolicy requires the following:

From 1 January 2000, all civil fixed-wing, turbine-powered aircraft having amaximum take-off mass exceeding15,000 kg, or a maximum approvedpassenger seating configuration ofmore than 30, will be required to beequipped with TCAS II, Version 7; and

From 1 January 2005, all civil fixed-wing, turbine-powered aircraft having amaximum take-off mass exceeding5,700 kg, or a maximum approvedpassenger seating configuration ofmore that 19, will be required to beequipped with TCAS II, Version 7.


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Because of delays in obtaining Version 7equipment, a number of exemptions to the1 January 2000 date were granted byEUROCONTROL. Each of the exemptionsgranted have a unique end date for the

exemption, but all exemptions will expire on31 March 2001.

Other countries, including Argentina,Australia, Chile, Egypt, India, and Japan,have also mandated carriage of TCAS IIavionics on aircraft operating in theirrespective airspace.

The demonstrated safety benefits of theequipment, and the 1996 midair collisionbetween a Saudia Boeing 747 and aKazakhstan Ilyushin 76, resulted in an ICAOproposal for worldwide mandatory carriage ofACAS II on all aircraft, including cargoaircraft, beginning in 2003. To guarantee theeffectiveness of this mandate, ICAO has alsomandated the carriage and use of pressurealtitude reporting transponders, which are aprerequisite for generating RAs.

After the mid-air collision between a GermanAir Force Tupolev 154 and a U.S. Air ForceC-141 transport aircraft, off Namibia inSeptember 1997, urgent consideration was

given to the need to equip military transportaircraft with TCAS. Although only a limitednumber of countries have included militaryand other government-owned aircraft in theirmandates for TCAS carriage, severalcountries, including the United States, haveinitiated programs to equip tanker, transport,and cargo aircraft within their military fleetswith TCAS II Version 7.

Standards and GuidanceStandards and GuidanceStandards and GuidanceStandards and GuidanceMaterialMaterialMaterialMaterial

The data obtained from the FAA and industrysponsored studies, simulations, flight tests,and operational evaluations have enabledRTCA to publish the MOPS for TCAS II.The current version of the MOPS, DO-185A,

describes the standards, requirements, andtest procedures for TCAS Version 7.

RTCA has also published MOPS for TCAS I,DO-197A, which defines the requirementsand test procedures for TCAS I equipment

intended for use on airline aircraft operated inrevenue service.

The FAA has issued Technical StandardOrder (TSO) C118a that defines therequirements for the approval of TCAS Iequipment. A draft Advisory Circularoutlining the certification requirements andthe requirements for obtaining operationalapproval of the system has been prepared andis being used by the FAAs AircraftCertification Offices (ACO) as the basis forapproving TCAS I installations andoperation.

For TCAS II, TSO C119b and AdvisoryCircular 20-131a have been published for useby FAA airworthiness authorities incertifying the installation of TCAS II onvarious classes of aircraft. Advisory Circular120-55a defines the procedures for obtainingoperational approval for the use of TCAS II.While the FAA developed these documents,they have been used throughout the world bycivil aviation authorities to approve the

installation and use of TCAS.

ICAO SARPs and Guidance Material forACAS I and ACAS II have been published inAnnex 10. The procedures for use of ACAShave been published in PANS-RAC andPANS-OPS. These documents provideinternational standardization for collisionavoidance systems.

For the avionics, the Airlines ElectronicEngineering Committee (AEEC) has

completed work on ARINC Characteristic735 to define the form, fit, and function ofTCAS II units. Similar work on the Mode Stransponder has been competed, and theresults of that work are contained in ARINCCharacteristic 718.


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TCAS II TechnicalDescription

System componentsSystem componentsSystem componentsSystem components

Figure 1 is a block diagram of TCAS II. ATCAS II installation consists of the followingmajor components.

TCAS Computer UnitThe TCAS Computer Unit, or TCAS

Processor, performs airspace surveillance,intruder tracking, its own aircraft altitude

tracking, threat detection, RA maneuverdetermination and selection, and generationof advisories. The TCAS Processor usespressure altitude, radar altitude, and discreteaircraft status inputs from its own aircraft tocontrol the collision avoidance logicparameters that determine the protectionvolume around the TCAS aircraft. If atracked aircraft is a collision threat, theprocessor selects an avoidance maneuver thatwill provide adequate vertical miss distancefrom the intruder while minimizing theperturbations to the existing flight path. If thethreat aircraft is also equipped with TCAS II,the avoidance maneuver will be coordinated

with the threat aircraft.

Figure 1. TCAS II Block DiagramFigure 1. TCAS II Block DiagramFigure 1. TCAS II Block DiagramFigure 1. TCAS II Block Diagram

Mode S TransponderMode S TransponderMode S TransponderMode S Transponder

A Mode S transponder is required to beinstalled and operational for TCAS II to beoperational. If the Mode S transponder fails,the TCAS Performance Monitor will detectthis failure and automatically place TCAS

into Standby. The Mode S transponder

performs the normal functions to support theground-based ATC system and can work witheither an ATCRBS or a Mode S groundsensor. The Mode S transponder is also used

to provide air-to-air data exchange betweenTCAS-equipped aircraft so that coordinated,complementary RAs can be issued whenrequired.

Mode S/TCAS Control Panel

A single control panel is provided to allow

the flight crew to select and control all TCASequipment, including the TCAS Processor,the Mode S transponder, and in some cases,the TCAS displays. A typical control panelprovides four basic control positions:

StandStandStandStand----bybybyby: Power is applied to theTCAS Processor and the Mode Stransponder, but TCAS does not issueany interrogations and the transponderwill reply to only discreteinterrogations.

TransponderTransponderTransponderTransponder: The Mode S transponder

is fully operational and will reply to allappropriate ground and TCASinterrogations. TCAS remains inStandby.

TA OnlyTA OnlyTA OnlyTA Only: The Mode S transponder isfully operational. TCAS will operatenormally and issue the appropriate

TADisplay

TCASCOMPUTER

UNIT MODE STRANSPONDER

MODE S/TCASCONTROL

PANEL

RADis-play

RADis-play

PRESSUREALTITUDE

RADAR ALTITUDE &DISCRETE INPUTS

AURALANNUNCIATION

DIRECTIONALANTENNA(TOP)

BOTTOMOMNIANTENNA(OptionalDirectionalAntenna)


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interrogations and perform all trackingfunctions. However, TCAS will onlyissue TAs, and the RAs will beinhibited.

AutomaticAutomaticAutomaticAutomatic or TA/RATA/RATA/RATA/RA: The Mode Stransponder is fully operational. TCASwill operate normally and issue theappropriate interrogations and performall tracking functions. TCAS will issueTAs and RAs, when appropriate.

As indicated in Figure 1, all TCAS controlsignals are routed through the Mode Stransponder.

Antennas

The antennas used by TCAS II include adirectional antenna that is mounted on the topof the aircraft and either an omnidirectionalor a directional antenna mounted on thebottom of the aircraft. Most installations usethe optional directional antenna on the bottomof the aircraft.

These antennas transmit interrogations on1030 MHz at varying power levels in each offour 90 azimuth segments. The bottom-mounted antenna transmits fewerinterrogations and at a lower power than thetop-mounted antenna. These antennas alsoreceive transponder replies, at 1090 MHz,and send these replies to the TCAS Processor.The directional antennas permit thepartitioning of replies to reduce synchronousgarbling.

In addition to the two TCAS antennas, twoantennas are also required for the Mode Stransponder. One antenna is mounted on thetop of the aircraft while the other is mountedon the bottom. These antennas enable the

Mode S transponder to receive interrogationsat 1030 MHz and reply to the receivedinterrogations at 1090 MHz. The use of thetop- or bottom-mounted antenna isautomatically selected to optimize signalstrength and reduce multipath interference.

TCAS operation is automatically suppressedwhenever the Mode S transponder istransmitting to ensure that TCAS does nottrack its own aircraft.

Cockpit Presentation

The TCAS interface with the pilots isprovided by two displays ----- the trafficdisplay and the RA display. These twodisplays can be implemented in a number ofways, including displays that incorporate bothdisplays into a single, physical unit.Regardless of the implementation, theinformation displayed is identical. Thestandards for both the traffic display and theRA display are defined in DO-185A.

Traffic DisplayTraffic DisplayTraffic DisplayTraffic Display

The traffic display, which can beimplemented on either a part-time or full-timebasis, depicts the position of nearby traffic,


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relative to its own aircraft. It is designed toprovide information that will assist the pilotin visual acquisition of other aircraft. Ifimplemented on a part-time basis, the displaywill automatically activate whenever a TA oran RA is issued. Current implementations

include dedicated traffic displays; display ofthe traffic information on shared weatherradar displays, MAP displays, EngineIndication and Crew Alerting System(EICAS) displays; and other multifunctiondisplays.

A majority of the traffic displays also providethe pilot with the capability to select multipleranges and to select the altitude band for thetraffic to be displayed. These capabilitiesallow the pilot to display traffic at longerranges and with greater altitude separationwhile in cruise flight, while retaining thecapability to select lower display ranges interminal areas to reduce the amount ofdisplay clutter.

Traffic Display Symbology

Both color and shape are used to assist thepilot in interpreting the displayedinformation.

The own aircraft is depicted as either a white

or cyan arrowhead or airplane-like symbol.The location of the own aircraft symbol onthe display is dependent on the displayimplementation. Other aircraft are depictedusing geometric symbols, depending on theirthreat status, as follows:

n unfilled diamond (), shown ineither cyan or white, but not the samecolor as the own aircraft symbol, isused to depict non-threat traffic.

filled diamond (), shown in either

cyan or white, but not the same color asthe own aircraft symbol, is used todepict Proximate Traffic. ProximateTraffic is non-threat traffic that is

within 6 nmi and 1200 ft from ownaircraft.

filled amber or yellow circle () isused to display intruders that havecaused a TA to be issued.

A filled red square (&) is used todisplay intruders that have caused an

RA to be issued.

Each symbol is displayed on the screenaccording to its relative position to ownaircraft. To aid the pilot in determining therange to a displayed aircraft, the trafficdisplay provides range markings at one-halfthe selected scale and at the full scale.Additional range markings may be providedat closer ranges, e.g., 2 nmi, on some displayimplementations. The selected display rangeis also shown on the display. The rangemarkings and range annunciation aredisplayed in the same color as the ownaircraft symbol unless the traffic display isintegrated with an existing display thatalready provides range markings, e.g., a MAPdisplay.

Vertical speed information and altitudeinformation are also provided for alldisplayed traffic that are reporting altitude.Relative altitude is displayed in hundreds offeet above the symbol if the intruder is aboveown aircraft and below the symbol if the

intruder is below own aircraft. When theintruder is above the own aircraft, the relative

altitude information is preceded by a ++++ sign.When the intruder is below the own aircraft, a------------ sign precedes the relative altitudeinformation. In some aircraft, the flight levelof the intruder can be displayed instead of itsrelative altitude. The flight level is shownabove the traffic symbol if the intruder isabove the own aircraft and below the trafficsymbol is the intruder is below the ownaircraft. If the intruder is not reporting its

altitude, no altitude information in shown forthe traffic symbol. The altitude information isdisplayed in the same color as the aircraftsymbol.

An arrow is displayed immediately to theright of a traffic symbol when the target


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aircraft is reporting its altitude and isclimbing or descending at more than 600fpm. An up arrow is used for a climbingaircraft; a down arrow is used for adescending aircraft. The arrow is displayed inthe same color as the aircraft symbol.

When an aircraft causing a TA or RA isbeyond the currently selected range of thetraffic display, half TA or RA symbols willbe displayed at the edge of the display at theproper relative bearing. In someimplementations, a written message such asTRAFFIC, TFC, or TCAS is displayed on thetraffic display if the intruder is beyond theselected display range. The half symbol or thewritten message will remain displayed untilthe traffic moves within the selected displayrange; the pilot increases the range on avariable range display to allow the intruder tobe displayed; or the pilot selects a displaymode that allows traffic to be displayed.

In some instances, TCAS may not have areliable bearing for an intruder causing a TAor RA. Because bearing information is usedfor display purposes only, the lack of bearinginformation does not affect the ability ofTCAS to issue TAs and RAs. When a No-Bearing TA or RA is issued, the threat level,as well as the range, relative altitude, and

vertical rate of the intruder, are written on thetraffic display. This text is shown in red foran RA and in amber or yellow for a TA. Forexample, if an RA was issued against anintruder at a range of 4.5 nmi and with arelative altitude of +1200 feet anddescending, the No Bearing indication onthe traffic display would be:

RA 4.5 +12RA 4.5 +12RA 4.5 +12RA 4.5 +12

Figure 2 shows the use of the various traffic

symbology used on the traffic display.

Resolution AdvisoResolution AdvisoResolution AdvisoResolution Advisory Displayry Displayry Displayry Display

The RA display provides the pilot withinformation on the vertical speed or pitchangle to fly or avoid to resolve an encounter.The RA display is typically implemented on

an instantaneous vertical speed indicator(IVSI); a vertical speed tape that is part of aPrimary Flight Display (PFD); or using pitchcues displayed on the PFD. RA guidance hasalso been implemented on a Heads-UpDisplay (HUD). The implementations usingthe IVSI or a vertical speed tape use red andgreen lights or markings to indicate thevertical speeds to be avoided (red) and thedesired vertical speed to be flown (green). Animplementation using pitch cues uses aunique shape on the PFD to show the pitch

angle to be flown or avoided to resolve anencounter. HUD implementations also use aunique shape to indicate the flight path to beflown or avoided to resolve an encounter.

In general, the round-dial IVSIimplementation is used on the older nonglassaircraft. However, some operators haveimplemented this display in their glassaircraft to provide a common display acrosstheir fleet types. Some IVSI implementationsuse mechanical instruments with a series ofred and green LEDs around the perimeter of

the display, while other implementations usean LCD display that draws the red and greenarcs at the appropriate locations. The LCDdisplay implementations also have thecapability to provide both the traffic and RAdisplay on a single instrument.

On glass aircraft equipped with a PFD, someairframe manufacturers have implemented theRA display on the vertical speed tape; somehave elected to provide pitch cues; and otherimplementations provide both pitch cues and

a vertical speed tape.

The standards for the implementation of RAdisplays are provided in DO-185A. Inaddition to the implementations outlinedabove, DO-185A defines requirements for


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15

implementation of the RA display via theflight director and a HUD.

Two RA displays are required ----- one in theprimary field of view of each pilot.

Figure 3 shows an RA display implementedon an LCD display that also provides trafficinformation. Figure 4 shows the two possibleimplementations on the PFD.

Figure 2. Standardized Symbology for Use

on the Traffic Display

Own Aircraft. AirplaneSymbol in White or

Cyan

- 02

Proximity Traffic, 200 Feet

Below and Descending.

Solid Diamond in White or

Cyan.

++ 07 Traffic Advisory (Intruder).700 Feet above and level.Solid Amber Circle.

- 01

Resolution Advisory

(Threat). 100 Feet Below

and Climbing. Solid Red

Square

Non Intruding Traffic

Altitude Unknown

Open Diamond in White

or Cyan


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16

Figure 3. TCAS RA Display Implemented on an IVSI

3104020

315

305

320

300

340

360

280

.818 STD

AP1

A/THR

FL 310

SPEED ALT L-NAV

7

2

Pitch Cue Implementation Vertical Speed Tape Implementation

Figure 4. TCAS RA Displays Implemented on a PFD


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17

Target Surveillance

TCAS, independent of any ground inputs,performs surveillance of nearby aircraft to

provide information on the position andaltitude of these aircraft so the collisionavoidance algorithms can perform theirfunction. The TCAS surveillance function

operates by issuing interrogations at 1030 MHzthat transponders on nearby aircraft respond toat 1090 MHz. These replies are received anddecoded by the surveillance portion of theTCAS software and the information is then

provided to the collision avoidance algorithms.

TCAS has a requirement to provide reliable

surveillance out to a range of 14 nmi and intraffic densities of up to 0.3 aircraft per square

nautical mile. The surveillance functionprovides the range, altitude, and bearing ofnearby aircraft to the collision avoidancefunction so threat determinations can be madeand so the information displayed on the traffic

display is accurate. The TCAS surveillance iscompatible with both the ATCRBS andMode S transponders.

TCAS can simultaneously track at least 30

transponder-equipped aircraft within itssurveillance range.

Because TCAS surveillance operates on thesame frequencies as that used by the ground-

based ATC radars, there is a requirementimposed on TCAS that it not interfere with the

functions of the ATC radars. Several designfeatures have been developed and implementedto allow TCAS to provide reliable surveillance

without degrading the performance of the ATCradars.

Mode S SurveillanceMode S SurveillanceMode S SurveillanceMode S Surveillance

Because of the selective address feature of theMode S system, TCAS surveillance of Mode S

equipped aircraft is relatively straightforward.TCAS listens for the spontaneous

transmissions, or squitters, that are generatedonce per second by the Mode S transponder.

Among other information, the squittercontains the unique Mode S address of the

sending aircraft.

Following the receipt and decoding of asquitter message, TCAS sends a Mode Sinterrogation to the Mode S address contained

in the squitter. The Mode S transponderreplies to this interrogation and the replyinformation is used by TCAS to determine

the range, bearing, and altitude of the Mode Saircraft.

To minimize interference with other aircraft

and ATC on the 1030/1090 MHz channels,the rate at which a Mode S aircraft is

interrogated by TCAS is dependent on therange and closure rate between the twoaircraft. As the target aircraft approaches the

area where a TA may be required, theinterrogation rate increases to once persecond. At extended ranges, a target isinterrogated at least once every five seconds.

TCAS tracks the range and altitude of eachMode S target. These target reports are

provided to the collision avoidance logic foruse in the detection and advisory logic and

for presentation to the pilot on the trafficdisplay. The relative bearing of the target isalso provided to the collision avoidance logicso that the targets position can be properlyshown on the traffic display. The bearinginformation is not used by the collision

avoidance logic for threat detection andadvisory selection.

Mode C SurveillanceMode C SurveillanceMode C SurveillanceMode C Surveillance

TCAS uses a modified Mode C interrogation

known as the Mode C Only All Call tointerrogate nearby Mode A/C transponders.The nominal interrogation rate for thesetransponders is once per second. Because

TCAS does not use Mode A interrogations,the Mode A transponder codes of nearby

aircraft are not known to TCAS.


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18

Aircraft that are not equipped with an operatingaltitude encoder reply to these interrogations

with no data contained in the altitude field ofthe reply. TCAS uses the framing pulses of the

reply to initiate and maintain a range andbearing track on these targets. As with the

Mode S tracks, these replies are passed to thecollision avoidance logic for traffic advisorydetection and for presentation on the traffic

display.

The replies from aircraft that are capable of

providing their Mode C altitude are tracked inrange, altitude, and bearing. These target

reports are passed to the collision avoidancelogic for TA and RA detection and for

presentation on the traffic display.

TCAS surveillance of Mode C targets iscomplicated by problems of synchronous andnonsynchronous garbling, as well as reflections

of signals from the ground (multipath). When aMode C Only All Call interrogation is issued

by TCAS, all Mode C transponders that detectthe interrogation will reply. Because of thelength of the reply message (21 microseconds),

all Mode C equipped aircraft within a rangedifference of 1.7 nmi from the TCAS aircraftwill generate replies that garble, or overlapeach other, when received by TCAS. This is

shown in Figure 5 and is called synchronousgarble. Various techniques have beenincorporated into TCAS to cope with thiscondition.

Figure 5. Synchronous Garble Area

Hardware degarblers can reliably decode upto three overlapping replies, and the

combined use of variable interrogation powerlevels and suppression pulses reduces the

number of transponders that reply to a singleinterrogation. This technique, known as

whisper-shout (WS) takes advantage ofdifferences between the receiver sensitivity oftransponders and the transponder antenna

gains of target aircraft.

A low power level is used for the first

interrogation step in a WS sequence. Duringthe next WS step, a suppression pulse is firsttransmitted at a slightly lower level than thefirst interrogation. The suppression pulse is

followed two microseconds later by aninterrogation at a slightly higher power level.

This action suppresses most of thetransponders that had replied to the previousinterrogation, but elicits replies from an

additional group of transponder that did notreply to the previous interrogation. As shownin Figure 6, the WS procedure is followed

progressively in 24 steps, to separate theMode C replies into several groups, and thus

reduces the possibility of garbling. The WSsequence is transmitted once during eachsurveillance update period, which isnominally one second.

Another technique used to reducesynchronous garble is the use directionaltransmissions to further reduce the number of

potential overlapping replies. This techniqueis shown in Figure 7. Slightly overlapping

coverage must be provided in all directions toensure 360 degree coverage. Synchronousgarble is also reduced by the use of the ModeC Only All Call interrogation. Thisinterrogation inhibits Mode S transponders

from replying to a Mode C interrogation.

Nonsynchronous garble is caused by thereceipt of undesired transponder replies thatwere generated in response to interrogations

from ground sensors or other TCASinterrogations. These so-calledfruitreplies are

transitory so they are typically identified and

TCAS

Other Mode A/C

Aircraft that can

cause garble

Target of

Interest

1.71.7

nmi nmi


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Figure 6. Whisper-Shout Interrogation Sequence

Figure 7. Directional Transmission

discarded by correlation algorithms in the

surveillance logic. Operational experience

with TCAS has shown that the probability ofinitiating and maintaining a track based onfruit replies is extremely remote.

Avoiding the initiation of surveillance tracksbased on multipath replies is anotherimportant consideration in the design of theTCAS surveillance. Multipath results in thedetection of more than one reply to the same

interrogation, generally of lower power, fromthe same aircraft. It is caused by a reflectedinterrogation and usually occurs over flatterrain. To control multipath, the direct-path

power level is used to raise the minimum

triggering level (MTL) of the TCAS receiverenough to discriminate against the delayed and

lower power reflections. This technique,referred to as Dynamic MTL (DMTL), isshown in Figure 8. As shown in Figure 8, the

four-pulse direct reply is above the DMTLlevel, while the delayed, lower-powermultipath reply is below the DMTL threshold,

and is thus rejected by TCAS.

REPLY REGION


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20

F1 F2

Figure 8. Dynamic Thresholding of

ATCRBS Replies

Interference LimitingInterference LimitingInterference LimitingInterference Limiting

Interference limiting is a necessary part of thesurveillance function. To ensure that notransponder is suppressed by TCAS activityfor more than 2% of the time, and that TCASdoes not create an unacceptably high fruit rate

for the ground-based ATC radars, multipleTCAS units within detection range of one

another, i.e., approximately 30 nmi, aredesigned to limit their own transmissions

under certain conditions. As the number ofsuch TCAS units within this region increases,the interrogation rate and power allocation for

each of them must decrease to preventundesired interference with the ATC radars.

To achieve this, every TCAS unit counts thenumber of other TCAS units within detection

range. This is done by periodicallytransmitting TCAS broadcast messages thatinclude the Mode S address of the transmitting

aircraft every eight seconds. Mode S

transponders are designed to accept thebroadcast messages without replying. Thesemessages are monitored by the TCASinterference limiting algorithms to develop an

estimate of the number of TCAS units withindetection range. The number of total TCAS

units is used by each TCAS to limit theinterrogation rate and power as required.

While interference limiting has been anintegral part of TCAS since its inception,

initial operational experience with TCASindicated that refinements were necessary in

the surveillance design to meet the above-stated requirements. In Version 7, the

interference limiting algorithms have beenmodified to address problems seen duringoperation. These modifications account for

different distributions in TCAS aircraft in theterminal area because of the increased trafficdensity near airports.

The modifications also inhibit the interference

algorithms at altitudes above Flight Level (FL)180 and provide longer surveillance ranges in

high-density traffic environments. A keyfeature of the modifications is the guarantee

that reliable surveillance will always beavailable out to a range of six nautical miles.In high density traffic areas at altitudes below

FL180, the interrogation rate will be reducedfrom one per second to once every fiveseconds for non-threat aircraft that are at leastthree nautical miles away and are at more than60 seconds from closest point of approach

(CPA).

Electromagnetic CompatibilityElectromagnetic CompatibilityElectromagnetic CompatibilityElectromagnetic Compatibility

TCAS incorporates a number of designfeatures to ensure that TCAS does notinterfere with other radio services that operate

in the 1030/1090 MHz frequency band. Thedesign of the Mode S waveforms used byTCAS provide compatibility with the Mode A

and Mode C interrogations of the ground-based secondary surveillance radar system and

the frequency spectrum of Mode Stransmissions is controlled to protect adjacentdistance measuring equipment (DME)

channels.

The interference limiting features of TCASalso help to ensure electromagneticcompatibility with the ATC radar system. An

extensive series of analyses, equipment test,and computer simulations have shown that the

surveillance design contained in the Version 7

20.3 s

Reply Pulses

D namicMTL

Multipath

Minimum

Triggering Level

(MTL)


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21

software have demonstrated that operationallysignificant interference will not occur between

TCAS, secondary surveillance radar, andDME systems.

Collision Avoidance

Concepts

Airborne collision avoidance is a complexproblem. It has taken many years to developan operationally acceptable solution and the

refinement of the system continues tomaximize the compatibility between TCAS,

ATC systems throughout the world, andexisting cockpit procedures. The heart ofcollision avoidance is the collision avoidancesystem logic or the CAS logic. To explain theoperation of the CAS logic, the basic CAS

concepts of sensitivity level, tau, and protectedvolume need to be understood.

Sensitivity LevelSensitivity LevelSensitivity LevelSensitivity Level

Effective CAS logic operation requires atrade-off between necessary protection andunnecessary advisories. This trade-off isaccomplished by controlling the sensitivity

level (SL), which controls the time or tauthresholds for TA and RA issuance, and

therefore the dimensions of the protectedairspace around each TCAS-equipped aircraft.The higher the SL, the larger the amount of

protected airspace. However, as the amount ofprotected airspace increases, the incidence ofunnecessary alerts has the potential to

increase.

TCAS uses two means of determining theoperating SL.

1. Pilot Selection. The TCAS Control Panel

provides a means for the pilot to selectthree operating modes:

When the Control Panel switch isplaced in the Standby Position, TCASis operating in SL1. In SL1, TCASdoes not transmit any interrogations.

SL1 is normally selected only whenthe aircraft is on the ground or ifTCAS has failed. The pilot selection

of Standby on the Control Panel isnormally the only way that SL1 will

be selected.

When the pilot selects TA-ONLY onthe control panel, TCAS is placed intoSL2. While in SL2, TCAS performs

all surveillance functions and willissue TAs, as required. RAs areinhibited in SL2.

When the pilot selects TA-RA or theequivalent mode on the control panel,

the TCAS logic automatically selectsthe appropriate SL based on thealtitude of the own aircraft. Table 2

provides the altitude threshold atwhich TCAS automatically changes

SL and the associated SL for thataltitude band. In these SLs, TCAS

performs all surveillance functionsand will issue TAs and RAs, asrequired

2. Ground-Based Selection. Although the useof ground-based control of SL has not

been agreed to between pilots, controllers,and the FAA and is not envisioned for use

in U.S. airspace, the capability for ground-based selection of SL is included in the

TCAS design. This design feature allowsthe operating SL to be selected from theground by using a Mode S uplink

message. The TCAS design allows theselection of any SL shown in Table 2 withthe exception of SL1.

When the pilot has selected the TA-RA modeon the Control Panel, the operating SL is

automatically selected via inputs from theaircrafts radar or pressure altimeter. SL2 will

be selected when the TCAS aircraft is below

1,000 feet above ground level (AGL) (100feet) as determined by the radar altimeter

input. As previously stated, when in SL2, RAsare inhibited and only TAs will be issued.

In SL3 through SL7, RAs are enabled and

issued at the times shown in Table 2. SL3 isset based on inputs from the radar altimeter,while the remaining SLs are set based on


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Table 2. Sensitivity Level Definition and Alarm Thresholds

Tau (Seconds) DMOD (nmi) Altitude Threshold

(feet)

Own Altitude

(feet)

SL

TA RA TA RA TA RA (ALIM)

< 1000 2 20 N/A 0.30 N/A 850 N/A1000 - 2350 3 25 15 0.33 0.20 850 300

2350 5000 4 30 20 0.48 0.35 850 300

5000 10000 5 40 25 0.75 0.55 850 350

10000 20000 6 45 30 1.00 0.80 850 400

20000 42000 7 48 35 1.30 1.10 850 600

> 42000 7 48 35 1.30 1.10 1200 700

pressure altitude using inputs from the ownaircraft barometric altimeter.

TauTauTauTau

TCAS uses time-to-go to CPA, rather thandistance, to determine when a TA or an RAshould be issued. TCAS uses the time to CPA

to determine the range tau and the time tocoaltitude to determine the vertical tau. Tau is

an approximation of the time, in seconds, toCPA or to the aircraft being at the samealtitude. 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 bythe combined vertical speed of the two aircraft(feet/minute) times 60.

TCAS II operation is based on the tau concept

for all alerting functions. Table 2 provides theTA and RA tau thresholds used in each

sensitivity level. The boundary lines shown inFigure 9 indicate the combinations of range andclosure rate that would trigger a TA with a 40-

second range tau and an RA with a 25-secondrange tau. This represents the range taus used in

SL5. Similar graphs can be generated for othersensitivity levels. Figure 10 shows thecombinations of altitude separation andcombined vertical speeds that would trigger a

TA with a 40-second vertical tau and an RAwith a 25-second vertical tau.

In events where the rate of closure is very low,as shown in Figure 11, an intruder aircraft can

come very close in range without crossing therange tau boundaries and thus, without causing

a TA or an RA to be issued. To provideprotection in these types of advisories, therange tau boundaries are modified as shown in

Figure 12. This modification is referred to asDMOD and allows TCAS to use a fixed-range

threshold to issue TAs and RAs in these slowclosure encounters. The value of DMOD varies

with the different sensitivity levels and thevalues used to issue TAs and RAs are shown inTable 2.

When the combined vertical speed of the TCASand the intruder aircraft is low, TCAS will use

a fixed-altitude threshold to determine whethera TA or an RA should be issued. As withDMOD, the fixed altitude thresholds vary withsensitivity level, and the TA and RA thresholdsare shown in Table 2.

For either a TA or an RA to be issued, both therange and vertical criteria, in terms of tau or thefixed thresholds, must be satisfied only one of

the criteria is satisfied, TCAS will not issue anadvisory.


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Protected VolumeProtected VolumeProtected VolumeProtected Volume

A protected volume of airspace surrounds eachTCAS-equipped aircraft. The tau and DMOD

criteria described above shape the horizontalboundaries of this volume. The vertical tau and

the fixed altitude thresholds determine thevertical dimensions of the protected volume.

The horizontal dimensions of the protectedairspace are not based on distance, but on tau.

Thus, the size of the protected volume dependson the speed and heading of the aircraft

involved in the encounter.

TCAS II is designed to provide collisionavoidance protection in the case of any twoaircraft that are closing horizontally at any rate

up to 1200 knots and vertically up to 10,000feet per minute (fpm).


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Figure 9. TA/RA Range Tau

Values for SL5

Figure 10. TA/RA Vertical Tau

Values for SL5

40Second Tau

25 Second Tau

0.00

1.00

2.00

3.00

4.00

5.00

6.00

0 100 200 300 400 500

Rate of Closure, Knots

Range,

NauticalMiles

40 Second Tau

25 Second Tau

0

1000

2000

3000

4000

5000

6000

7000

0 1000 2000 3000 4000 5000 6000 7000 8000 9000 10000

Combined Vertical Speed, fpm

AltitudeSepar

ation,Feet


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25

Figure 11. Need for Modified Tau

Figure 12. Modified TA/RA Range Tau Values for SL5

40 Second Tau (TA)

25 Second Tau (RA)

0

1

2

3

4

5

6

0 100 200 300 400 500

Rate of Closure, Knots

Range,NauticalMiles

Slow Vertical Closure Rate

Slow HorizontalClosure Rate

Slow Overtake


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CAS Logic Functions

Surveillance

Tracking

Traffic advisory

Threat detection

Advisory annunciation

TargetOwn aircraft

Altitude testRange test

Sense selection Strength selection

Horizontal filtering

Resolution advisory

TCAS/TCAS

coordination

Performance Monitor

Figure 13. CAS Logic Functions

The logic functions employed by TCAS toperform its collision avoidance function areshown in Figure 13. The following

descriptions of these functions are intendedto provide a general level of understandingof these functions. The nature of providingan effective collision avoidance systemresults in the need to have numerous special

conditions spread throughout the functionsand these are dependent on encounter

geometry, range and altitude thresholds, andaircraft performance. These specialconditions are beyond the scope of this

document. A complete description of theCAS logic and additional details of itsdesign and performance are contained inRTCA DO-185A.

TrackingTrackingTrackingTracking

Using range, altitude (when available), and

bearing from nearby aircraft that areprovided to CAS by the Surveillance

function, the CAS logic initiates andmaintains a three-dimensional track of eachaircraft and uses this information todetermine the time to CPA and the altitudeof each aircraft at CPA. The CAS logic uses

the altitude information to estimate thevertical speed of each nearby aircraft andmaintains a vertical track for each aircraft.The altitude tracking can use altitude that isquantized in either 100- or 25-foot

increments. The CAS tracking function isdesigned to track aircraft with vertical ratesof up to 10,000 fpm.

The CAS logic also uses the data from its

own aircraft pressure altitude to determinethe own aircraft altitude, vertical speed, andrelative altitude of each aircraft. The CASlogic uses the altitude source on the ownaircraft that provides the finest resolution.

The own aircraft data can be provided ineither one, 25-, or 100-foot increments. The

outputs from the CAS tracking algorithm,i.e., range, range rate, relative altitude, and

vertical rate, are provided to the TA andThreat Detection logic so that the need for aTA or an RA can be determined.

The CAS tracker also uses the differencebetween its own aircraft pressure altitude

and radar altitude to estimate theapproximate elevation of the ground above

mean sea level. This ground estimation logicfunctions whenever the own aircraft is

below 1750 ft AGL. The ground level

estimate is then subtracted from the pressure

altitude received from each nearby Mode C-equipped aircraft to determine theapproximate altitude of each aircraft abovethe ground. If this difference is less than 360

feet, TCAS considers the reporting aircraftto be on the ground. If TCAS determines the

intruder to be on the ground, it inhibits thegeneration of advisories against this aircraft.


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This methodology is shown graphically inFigure 14.

A Mode S-equipped aircraft is considered to

be on the ground if the on-the-ground statusbit indicates the aircraft is on the ground.

Radaraltimeter

1750 feet above ground(Threshold below which TCAS checks for targets on the ground)

360-foot allowance

Barometricaltimeter

Ground level

Standard altimeter setting Estimated elevation of ground

Declared

airborne

TCAS

Declared

on ground

Declared

on ground

Figure 14. Mode C Target on Ground

Determination

Traffic AdvisoryTraffic AdvisoryTraffic AdvisoryTraffic Advisory

Using the tracks for nearby aircraft, rangeand altitude tests are performed for eachaltitude-reporting target. Nonaltitude

reporting aircraft are assumed to be

coaltitude and only range tests areperformed on these targets. The range test isbased on tau, and the TA tau must be lessthan the threshold shown in Table 2. In

addition, the current or projected verticalseparation at CPA must be within the TAaltitude threshold shown in Table 2 for atarget to be declared an intruder. If the TAlogic declares an aircraft to be an intruder, a

TA will be issued against that aircraft.

A nonaltitude reporting aircraft will be

declared an intruder if the range test aloneshows that the calculated tau is within the

RA tau threshold associated with the currentSL being used as shown in Table 2.

Version 7 includes changes to ensure that atargets TA status is maintained in slow

closure rate encounters by invoking morestringent requirements for removing a TA.

These changes address problems reported inwhich multiple TAs were issued against the

same target in parallel approach encountersand in RVSM airspace.

Threat DetectionThreat DetectionThreat DetectionThreat Detection

Range and altitude tests are performed oneach altitude-reporting intruder. If the RA

tau and either the time to co-altitude orrelative altitude criteria associated with the

current SL are met, the intruder is declared athreat. Depending on the geometry of theencounter and the quality and age of thevertical track data, an RA may be delayed ornot selected at all. RAs cannot be generated

for nonaltitude reporting intruders.

Version 7 includes changes in the ThreatDetection logic to improve the performanceof this portion of the logic. These changes

include the following:

Declaring the own aircraft to be on theground when the input from the radar

altimeter is valid and below 50 feetAGL. This precludes completereliance on the own aircrafts weight-on-wheels switch that has been shown

to be unreliable in some aircraft. Preventing the SL from decreasing

during a coordinated encounter tomaintain the continuity of a displayed

RA, and thus prevent multiple RAsfrom being issued against the sameintruder.

Inhibiting threat declaration againstintruder aircraft with vertical rates inexcess of 10,000 fpm.

Reducing alert thresholds to account

for the reduction in vertical separationto 1000 feet above FL290 in RVSMairspace.

Modifying the criteria used to reducethe frequency of bump-up or highvertical rate encounters. Thismodification allows a level aircraft to

delay the issuance of an RA for up tofive seconds to allow additional time


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for detecting a level-off maneuver by aclimbing or descending aircraft.

Introducing a horizontal miss distance(HMD) filter to reduce the number ofRAs against intruder aircraft having a

large horizontal separation at CPA.The HMD filter can also weaken anRA prior to ALIM being obtained to

minimize altitude displacement whenthe filter is confident that the

horizontal separation at CPA will belarge.

Resolution Advisory SelectionResolution Advisory SelectionResolution Advisory SelectionResolution Advisory Selection

When an intruder is declared a threat, a twostep process is used to select the appropriate

RA for the encounter geometry. The firststep in the process is to select the RA sense,i.e., upward or downward. Based on the

range and altitude tracks of the intruder, theCAS logic models the intruders flight pathfrom its present position to CPA. The CASlogic then models upward and downwardsense RAs for own aircraft, as shown in

Figure 15, to determine which senseprovides the most vertical separation atCPA. In the encounter shown in Figure 15,the downward sense logic will be selected

because it provides greater verticalseparation.

B

A

TCAS

Threat

CPA

downward

upward

Figure 15. RA Sense Selection

In encounters where either of the sensesresults in the TCAS aircraft crossing throughthe intruders altitude, TCAS is designed to

select the nonaltitude crossing sense if thenoncrossing sense provides the desiredvertical separation, known as ALIM, atCPA. The value of ALIM varies with SLand the value for each SL is shown in

Table 2. If the nonaltitude crossing senseprovides at least ALIM feet of separation at

CPA, this sense will be selected even if thealtitude-crossing sense provides greater

separation. If ALIM cannot be obtained inthe nonaltitude crossing sense, an altitude

crossing RA will be issued. Figure 16 showsan example of encounters in which thealtitude crossing and nonaltitude crossing

RA senses are modeled and the noncrossingRA sense is selected.

ALIMThreat

TCASCPA

ALIM

RA Climb

issued

Figure 16. Selection of Noncrossing RA

Sense

The second step in selecting an RA is tochoose the strength of the advisory. TCAS is

designed to select the RA strength that is theleast disruptive to the existing flight path,

while still providing ALIM feet of

separation. Table 3 provides a list ofpossible advisories that can be issued as the

initial RA when only a single intruder isinvolved in the encounter. After the initial

RA is selected, the CAS logic continuouslymonitors the vertical separation that will be

provided at CPA and if necessary, the initial

RA will be modified.

A new feature was implemented inVersion 7 to reduce the frequency of RAsthat reverse the existing vertical rate of the

own aircraft. When two TCAS-equippedaircraft are converging vertically with

opposite rates and are currently wellseparated in altitude, TCAS will first issue avertical speed limit (Negative) RA toreinforce the pilots likely intention to leveloff at adjacent flight levels. If no response to

this initial RA is detected, or if eitheraircraft accelerates toward the other aircraft,


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29

the initial RA will strengthen as required.This change was implemented to reduce the

frequency of initial RAs that reversed thevertical rate of the own aircraft (e.g., posted

a climb RA for a descending aircraft)because pilots did not follow a majority of

these RAs, and those that were followed,were considered to be disruptive bycontrollers.

In some events, the intruder aircraft willmaneuver vertically in a manner that thwarts

the effectiveness of the issued RA. In thesecases, the initial RA will be modified to

either increase the strength or reverse thesense of the initial RA. The RA issued when

an increased strength RA is required isdependent on the initial RA that was issued.

Figure 17 depicts an encounter where it isnecessary to increase the climb rate from the1500 fpm required by the initial RA to 2500

fpm. This is an example of an IncreaseClimb RA. Figure 18 depicts an encounterwhere an initial Descend RA requiresreversal to a Climb RA after the intrudermaneuvers.

In a coordinated encounter in which anaircraft appears to ignore an initialnonaltitude crossing RA, Version 7 will

inhibit Increase Rate RAs for this aircraftand only consider RA reversals if the otheraircraft maneuvers.

Version 7 permits sense reversals incoordinated encounters. This sense reversal

logic is very similar to that previouslyavailable in encounters with non-TCASthreats. In TCAS-TCAS encounters, RAreversals are not permitted for the first nineseconds after the initial RA to allow time for

both aircraft to initiate their RA response.

RA reversals are not permitted if the aircraftare within 300 feet of each other and thereversal would result in an altitude crossingRA. In coordinated encounters, the logic that

considers issuing an Increase Rate RA latein an altitude crossing RA is disabled.

Because of aircraft climb performancelimitations at high altitude or in some flap

and landing gear configurations, an aircraftinstallation may be configured to inhibit

Climb or Increase Climb RA under someconditions. These inhibit conditions can be

provided via program pins in the TCASconnector or in real-time via an input from a

Flight Management System (FMS). If theseRAs are inhibited, the RA Selection Criteriawill not consider them in the RA selection

and will choose an alternative upward senseRA if the downward sense RA does not

provide adequate vertical separation.

Increase Descent

TCAS

Threat

CPA

Descend

The threat increases itsdescent rate towards

TCAS aircraft after theinitial Descend RA is

issued

Figure 17. Increase Rate RA

Initial projection

TCAS

Threat

CPA

ReversalRA

Initial RA

Figure 18. RA Reversal

TCAS is designed to handle multiaircraftencounters, i.e., those encounters in which

more that one intruder is detected at thesame time. (It should be noted that in morethan 10 years of TCAS operation, less than a

half dozen true multiaircraft encounters havebeen recorded worldwide.) TCAS will

attempt to resolve these types of encountersby selecting a single RA that will provideadequate separation from each of theintruders. This RA can be any of the initialRAs shown in Table 3, or a combination of

upward and downward sense RAs, e.g., DoNot Climb and Do Not Descend. It is


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Table 3. Possible Initial RAs

UPWARD SENSE DOWNWARD SENSERA TYPE RA Required

Vertical RateRA Required Vertical

Rate

Positive Climb 1500 to 2000 fpm Descend -1500 to -2000 fpm

Positive Crossing Climb 1500 to 2000 fpm Crossing Descend -1500 to -2000 fpmPositive Maintain Climb 1500 to 4400 fpm Maintain Descend -1500 to -4400 fpm

Negative Do NotDescend

> 0 fpm Do Not Climb < 0 fpm

Negative Do NotDescend > 500fpm

> -500 fpm Do Not Climb > 500fpm

< + 500 fpm

Negative Do NotDescend >1000 fpm

> -1000 fpm Do Not Climb >1000 fpm

< + 1000 fpm

Negative Do NotDescend >2000 fpm

> -2000 fpm Do Not Climb >2000 fpm

< + 2000 fpm

possible that the RA selected in suchencounters may not provide ALIM separationfrom all intruders. Version 7 provides newcapabilities to the multiaircraft logic to allow

this logic to utilize Increase Rate RAs andRA Reversals to better resolve encounters.

During an RA, if the CAS logic determines

that the response to a Positive RA (seeTable 3) has provided ALIM feet of vertical

separation before CPA, the initial RA will beweakened to either a Do Not Descend RA(after an initial Climb RA) or a Do Not

Climb RA (after an initial Descend RA). Thisis done to minimize the displacement from

the TCAS aircrafts original altitude.Negative RAs will not be weakened and theinitial RA will be retained until CPA unless it

is necessary to strengthen the RA or reversethe RA sense.

TCAS is designed to inhibit Increase Descent

RAs below 1450 feet AGL; Descend RAsbelow 1100 feet AGL; and all RAs below1000100 feet AGL. If a Descend RA is

being displayed as the own aircraft descendsthrough 1100 feet AGL,the RA will bemodified to a Do Not Climb RA.

After CPA is passed and the range betweenthe TCAS aircraft and threat aircraft beginsto increase, all RAs are cancelled.

TCAS/TCAS CoordinationTCAS/TCAS CoordinationTCAS/TCAS CoordinationTCAS/TCAS Coordination

In a TCAS/TCAS encounter, each aircrafttransmits interrogations to the other via theMode S link to ensure the selection ofcomplementary RAs by the two aircraft. The

coordination interrogations use the same1030/1090 MHz channels used forsurveillance interrogations and replies andare transmitted once per second by eachaircraft for the duration of the RA.Coordination interrogations containinformation about an aircrafts intended RAsense to resolve the encounter with the otherTCAS-equipped intruder. The informationin the coordination interrogation isexpressed in the form of a complement. Forexample, when an aircraft selects an upward

sense RA, it will transmit a coordinationinterrogation to the other aircraft that restrictsthat aircrafts RA selection to those in thedownward sense. The strength of thedownward sense RA would be determined bythe threat aircraft based on the encountergeometry and the RA Selection logic.


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The basic rule for sense selection in aTCAS/TCAS encounter is that each TCAS

must check to see if it has received an intentmessage from the other aircraft before

selecting an RA sense. If an intent messagehas been received, TCAS selects the opposite

sense from that selected by the other aircraftand communicated via the coordinationinterrogation. If TCAS has not received an

intent message, the sense is selected based onthe encounter geometry in the same manneras would be done if the intruder were not

TCAS equipped.

In a majority of the TCAS/TCAS encounters,the two aircraft will declare the other aircraft

to be a threat at slightly different times. Inthese events, coordination proceeds in a

straightforward manner with the first aircraftdeclaring the other to be a threat, selecting itsRA sense based on the encounter geometry,

and transmitting its intent to the otheraircraft. At a later time, the second aircraftwill declare the other aircraft to be a threat,and having already received an intent fromthe first aircraft, will select a complementary

RA sense. The complementary sense that isselected will then be transmitted to the otheraircraft in a coordination interrogation.

Occasionally, the two aircraft declare eachother as threats simultaneously, andtherefore, both aircraft will select their RAsense based on the encounter geometry. Inthese encounters, there is a chance that bothaircraft will select the same sense. When this

happens, the aircraft with the higher Mode Saddress will detect the selection of the samesense and will reverse its sense.

Version 7 includes the capability for TCAS

to issue RA reversals in coordinated

encounters if the encounter geometry changesafter the initial RA is issued. The RAreversals in coordinated encounters are

annunciated to the pilot in the same way asRA reversals against non-TCAS intruders. In

a coordinated encounter, if the aircraft withthe low Mode S address has Version 7

installed, the low Mode S address can reversethe sense of its initial RA and communicate

this to the high Mode S address aircraft. Thehigh Mode S address aircraft will thenreverse its displayed RA. The aircraft with

the high Mode S address can be equippedwith either Version 6.04 or Version 7.

In a coordinated encounter, only one RAreversal based on changes in the encountergeometry can be issued. The initial RA sensewill not be reversed until it has been

displayed for at least nine seconds, unless thelow Mode S address aircraft has a vertical

rate higher than 2500 feet per minute and actscontrary to the RA. This delay is included inthe design to allow sufficient time for the two

aircraft to initiate a response to the initial RA.

Advisory AnnunciationAdvisory AnnunciationAdvisory AnnunciationAdvisory Annunciation

The CAS logic also performs the function ofsetting flags that control the displays and

aural annunciations. The traffic display, theRA display, and the aural devices use theseflags to alert the pilot to the presence of TAs

and RAs. Aural annunciations are inhibitedbelow 500100 feet AGL.

The TCAS aural annunciations are integratedwith other environmental aural alertsavailable on the aircraft. The priority schemeestablished for these aural alerts gives

windshear detection systems and groundproximity warning systems (GPWS) a higherannunciation priority than a TCAS alert.TCAS aural annunciations will be inhibitedduring the time that a windshear or GPWS

alert is active.


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Air/Ground CommunicationsAir/Ground CommunicationsAir/Ground CommunicationsAir/Ground Communications

Using the Mode S data link, TCAS candownlink RA reports to Mode S ground

sites. These reports can be provided in theMode S transponders 1090 MHz response

to an interrogation from the Mode S groundsensor requesting information and it can also

be provided automatically using the TCAS1030 MHz transmitter.

During the time an RA is displayed, TCASwill automatically generate a downlinkmessage containing information on the RA

being displayed to the crew. Thisinformation, known as the RA Broadcast, is

provided when an RA is initially issued andwhen the RA is updated. It is rebroadcastevery eight seconds using the TCAS 1030MHz transmitter. At the end of an RA, anindication will be provided to the ground

that the RA is no longer being displayed.

Traffic Advisory DisplayTraffic Advisory DisplayTraffic Advisory DisplayTraffic Advisory Display

The functions of the traffic advisory display

are to aid the flight crew in visuallyacquiring intruder aircraft; discriminating

between intruder aircraft and other nearby

aircraft; determining the horizontal positionof nearby aircraft; and providing confidence

in the performance of TCAS.

Traffic advisory displays have beenimplemented in a number of different waysand with varying levels of flexibility. The

requirements for the various means ofimplementing the traffic displays are

documented in RTCA DO-185A. Anoverview of the traffic display features andcapabilities was provided earlier in this

booklet.

Version 7 requirements inhibit the display ofintruders with relative altitudes of more than9900 feet if the pilot has selected thedisplay of relative altitude. This displayrange is the maximum possible because only

two digits are available to display therelative altitude.

Resolution Advisory DisplaysResolution Advisory DisplaysResolution Advisory DisplaysResolution Advisory Displays

The RA display is used by TCAS to advisethe pilot how to maneuver, or not maneuver

in some cases, to resolve the encounter asdetermined by the CAS logic. Examples for

various RA display implementations areshown in Figure 3 and Figure 4. Therequirements for RA displays are containedin RTCA DO-185A.

To accommodate physical limitations onsome IVSI displays, Version 7 will notallow the display of any Maintain Rate RAsthat call for vertical rates in excess of 4400fpm. Because of this, the logic will model

the minimum of the own aircrafts verticalrate and 4400 fpm if a Maintain Rate RA isrequired; and will select the sense that

provides the best separation, even if theselected sense is opposite the existing

vertical speed.

Aural AnnunciationsAural AnnunciationsAural AnnunciationsAural Annunciations

Whenever the collision avoidance logicissues a TA or an RA, a voice alert is issuedto ensure that the pilots are aware of theinformation being displayed on the traffic

and RA displays. These aural annunciationscan be provided via a dedicated speakerinstalled in th

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