tcasii faa

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Preface

This booklet provides the background for a better understanding of the Traffic Alert and Collision Avoidance System (TCAS II) by personnel involved in theimplementation and operation of TCAS II. This booklet is an update of the TCASII Version 7.0 manual published in 2000 by the Federal Aviation Administration(FAA). It describes changes to the CAS logic introduced by Version 7.1 andupdates the information on requirements for use of TCAS II and operationalexperience.

Version 7.1 logic changes will improve TCAS Resolution Advisory (RA) sensereversal logic in vertical chase situations. In addition all Adjust Vertical Speed,

Adjust RAs are converted to Level-Off, Level-Off RAs to make it more clear that a reduction in vertical rate is required.

The Minimum Operational Performance Standards (MOPS) for TCAS II Version7.1 were approved in June 2008 and Version 7.1 units are expected to beoperating by 2010-2011. Version 6.04a and 7.0 units are also expected tocontinue operating for the foreseeable future where authorized.


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Preface................................................................................................... .............................. 2

The TCAS Solution............................................................... .............................................. 5

Early Collision Avoidance Systems................................................................................ 5TCAS II Development .................................................. .................................................. 6Initial In-Service Evaluations ................................................. ........................................ 7Version 6.0 / 6.04a Implementation............................................................ .................... 8Version 7.0 Implementation.................................................................................... ........ 8Version 7.1 Implementation.................................................................................... ........ 9Requirements for World-Wide Carriage.................................................... ..................... 9RVSM Considerations .................................................. ................................................ 10Standards and Guidance Material .................................................... ............................. 10

TCAS II Components ........................................................ ............................................... 11

TCAS Computer Unit ....................................................... ............................................ 11Mode S Transponder......................... ........................................................ .................... 11Mode S/TCAS Control Panel............................................................................ ............ 11Antennas ...................................................... ........................................................ ......... 12Cockpit Presentation................................................... .................................................. 12Traffic Display....................................................... ....................................................... 12Traffic Display Symbology..................................................... ...................................... 13Resolution Advisory (RA) Display......................................... ...................................... 14

Target Surveillance................................................ .................................................... ....... 17

Mode S Surveillance......... .................................................... ........................................ 17Mode C Surveillance.................................................................................................... . 17Interference Limiting ............................................... .................................................... . 20Electromagnetic Compatibility ......................................... ............................................ 20Hybrid Surveillance ........................................................ .............................................. 20

Collision Avoidance Concepts............................................. ............................................. 22

Sensitivity Level ............................................. .................................................... .......... 22Tau .................................................... ........................................................ .................... 23Protected Volume............................................................. ............................................. 26

CAS Logic Functions......................................................... ............................................... 26

Tracking ................................................ ........................................................ ................ 26Traffic Advisory.............................................................. .............................................. 28Threat Detection................................................................................ ............................ 28Resolution Advisory Selection .............................................. ....................................... 29

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TCAS/TCAS Coordination...................................................... ..................................... 34Air/Ground Communications......................................................................... ............... 34Traffic Advisory (TA) Display .............................................. ....................................... 35Resolution Advisory (RA) Displays ............................................... .............................. 35Aural Annunciations ................................................... .................................................. 35

Performance Monitoring.............................................. ................................................. 37

Requirements for Use ............................................. ................................................... ....... 37

Regulations and Operational Guidance ................................................... ..................... 37Training Programs .................................................. ...................................................... 41

Operational Experience......................................................... ............................................ 42

Performance Monitoring Programs............................................................................. .. 42Observed Performance.................................................. ................................................ 43

Summary................................................... ....................................................... ................. 46

Abbreviations........... ....................................................... .................................................. 47

Glossary ................................................ ................................................... ......................... 48

Bibliography ..................................................... ............................................................. ... 50


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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, TrafficAlert and Collision Avoidance System or TCAS was developed to reduce the risk of mid-air collisions between aircraft. In theinternational arena, this system is knownas the Airborne Collision AvoidanceSystem or ACAS.

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

air traffic control (ATC) system, and provide collision avoidance protection for a broad spectrum of aircraft types. All TCASsystems provide some degree of collisionthreat alerting, and a traffic display. TCAS Iand II differ primarily by their alertingcapability.

TCAS I provides traffic advisories (TAs) toassist the pilot in the visual acquisition of intruder aircraft. TCAS I is mandated for use in the U.S. for turbine powered,

passenger-carrying aircraft having more than10 and less than 31 seats. TCAS I is alsoinstalled on a number of general aviationfixed wing aircraft and helicopters.

TCAS II provides TAs and resolutionadvisories (RAs), i.e., recommended escapemaneuvers, in the vertical dimension toeither increase or maintain the existingvertical separation between aircraft.TCAS II is mandated by the U.S. for commercial aircraft, including regional

airline aircraft with more than 30 seats or amaximum takeoff weight greater than33,000 lbs. Although not mandated for general aviation use, many turbine-powered general aviation aircraft and somehelicopters are also equipped with TCAS II.

The TCAS concept makes use of the sameradar beacon transponders installed on

aircraft to operate with ATCs ground-based radars. The level of protection provided byTCAS equipment depends on the type of transponder the target aircraft is carrying.The level of protection is outlined inTable 1. It should be noted that TCASprovides no protection against aircraftthat do not have an operatingtransponder.

Table 1. TCAS Levels of Protection

Own Aircraft EquipmentTCAS I TCAS II

Mode AXPDR ONLY

TA TA

Mode Cor Mode SXPDR

TA TA and Vertical RA

TCAS I TA TA and Vertical RA

T a r g e

t A i r c r a f

t E q u i p m e n

t

TCAS II TA TA and Coordinated Vertical RA

Early Collision Avoidance

Systems

The 1956 collision between two airlinersover the Grand Canyon spurred both theairlines and aviation authorities to initiatedevelopment of an effective collisionavoidance system that would act as a lastresort when there is a failure in the ATC-

provided separation services. During thelate 1950s and early 1960s, collisionavoidance development efforts included anemphasis on passive and non-cooperating

systems. These concepts proved to beimpractical. One major operational problemthat could not be overcome with thesedesigns was the need for non-conflicting,complementary avoidance maneuvers whichrequire a high-integrity communications link

between aircraft involved in the conflict.

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One of the most important collisionavoidance concepts, attributed to Dr. John S.Morrell of Bendix, was the use of Tau whichis the slant range between aircraft divided bythe rate of closure or range rate. Thisconcept 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, FAA and the airlines

jointly concluded that in normal airlineoperations, they would generate a high rateof unnecessary alarms in dense terminalareas. This problem would haveundermined the credibility of the systemwith the flight crews. In addition, eachtarget aircraft would have needed to beequipped with the same equipment to

provide protection to an equipped aircraft.

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

transponders to determine an intrudersrange and altitude. At that time, ATCRBStransponders were installed in all airline and military aircraft and a large number of general aviation aircraft. Thus, any BCAS-equipped aircraft would be able to detectand be protected against the majority of other aircraft in the air without imposingadditional equipment requirements on thoseother aircraft. In addition, the discreteaddress communications techniques used inthe Mode S transponders then under

development permitted two conflictingBCAS aircraft to perform coordinated escape maneuvers with a high degree of reliability. In 1978, the collision between alight aircraft and an airliner over San Diegoserved to increase FAA's efforts to completedevelopment of an effective collisionavoidance system.

TCAS II Development

In 1981, FAA made a decision to developand implement TCAS utilizing the basicBCAS design for interrogation and trackingwith some additional capabilities. LikeBCAS, TCAS is designed to work independently of the aircraft navigationequipment and the ground systems used to

provide Air Traffic Control (ATC) services.TCAS interrogates ICAO complianttransponders of all aircraft in the vicinityand based on the replies received, tracks theslant range, altitude (when it is included inthe reply message), and relative bearing of surrounding traffic. From several successivereplies, TCAS calculates a time to reach theCPA (Closest Point of Approach) with theintruder, by dividing the range by theclosure rate. This time value is the main

parameter for issuing alerts. If thetransponder replies from nearby aircraftinclude their altitude, TCAS also computesthe time to reach co-altitude. TCAS canissue two types of alerts:

Traffic Advisories (TAs) to assist the pilot in the visual search for theintruder aircraft and to prepare the

pilot for a potential RA; and

Resolution Advisories (RAs) torecommend maneuvers that will either increase or maintain the existingvertical separation from an intruder aircraft. When the intruder aircraft isalso fitted with TCAS II, both TCASco-ordinate their RAs through theMode S data link to ensure thatcomplementary RAs are selected.

TCAS II was designed to operate in trafficdensities of up to 0.3 aircraft per square

nautical mile (nmi), i.e., 24 aircraft within a5 nmi radius, which was the highest trafficdensity envisioned over the next 20 years.

Development of the TCAS II collisionavoidance algorithms included thecompletion of millions of computer simulations to optimize the protection

provided by the system, while minimizing

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the frequency of unacceptable or nuisanceadvisories. In addition to these computer simulations, early versions of the collisionavoidance algorithms were evaluated via

pilot in the loop simulations and during theoperation of prototype equipment in FAAaircraft throughout the NAS.

Extensive safety studies were also performed to estimate the safetyimprovements that could be expected withthe introduction of TCAS into service.These safety studies have been continuouslyupdated throughout the refinement of thecollision avoidance algorithms. The safetystudies have shown that TCAS II willresolve nearly all of the critical near mid-air collisions involving TCAS-equipped aircraft. However, TCAS cannot handle allsituations. In particular, it is dependent onthe accuracy of the threat aircrafts reported altitude and on the expectation that thethreat aircraft will not maneuver in a manner that defeats the TCAS RA. Achievingadequate separation is also contingent on the

pilot responding as the CAS logic expects.The safety study also showed that TCAS IIwill induce some critical near mid-air collisions, but overall, the number of near mid-air collisions with TCAS is less than ten

percent of the number that would haveoccurred without the presence of TCAS.

Extensive studies were also carried out toevaluate the interaction between TCAS and 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 aircraftto infringe on the protected airspace for

another aircraft was remote.

Initial In-Service Evaluations

To ensure that TCAS performed as expected in its intended operational environment,several operational evaluations of the systemhave been conducted. These evaluations

provided a means for the pilots using TCAS

and the controllers responsible for providingseparation services to TCAS-equipped aircraft to have a direct influence on thefinal system design and performancerequirements.The initial operational evaluation of TCASwas conducted by Piedmont Airlines in1982. Using a TCAS II prototype unitmanufactured by Dalmo Victor, Piedmontflew approximately 900 hours in scheduled,revenue service while recording data on the

performance of TCAS. These recorded datawere analyzed to assess the frequency and suitability 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 to

provide technical and operational commentson its design.

In 1987, Piedmont flew an upgraded versionof the Dalmo Victor equipment for approximately 1200 hours. During thisevaluation, the TCAS displays were visibleto the pilots and the pilots were permitted touse the information provided to maneuver the aircraft in response to RAs. Thisinstallation included a dedicated TCAS datarecorder so that quantitative data could be

obtained on the performance of TCAS. Inaddition, pilots and observers completed questionnaires following each TA and RAso that assessments could be made regardingthe utility of the system to the flight crews.

This evaluation also provided the basis for the development of avionics certificationcriteria for production equipment, validated

pilot training guidelines, provided justification for improvements to the TCASalgorithms and displays, and validated pilot

procedures for using the equipment.

Following the successful completion of thesecond Piedmont evaluation, FAA initiated the Limited Installation Program (LIP).Under the LIP, Bendix-King and Honeywell

built and tested commercial quality, pre- production TCAS II equipment that was incompliance with the TCAS II Minimum

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Operational Performance Standards(MOPS). Engineering flight tests of thisequipment were conducted on themanufacturers' aircraft as well as FAAaircraft. Using data collected during theseflight tests, along with data collected duringfactory and ground testing, bothmanufacturers equipment were certified viaa limited Supplemental Type Certificate(STC) for use in commercial revenueservice.

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-80aircraft. Over 2000 hours of operatingexperience were obtained with the United aircraft and approximately 2500 hours of operating experience were obtained with the

Northwest installations.

The experience provided by theseoperational evaluations resulted in further enhancements to the TCAS II logic,improved test procedures, and finalized the

procedures for certification of productionequipment. The most important informationobtained from the operational evaluationswas the nearly unanimous conclusion that

TCAS II was safe, operationally effective,and ready for more widespread implementation.

Version 6.0 / 6.04a Implementation

In 1986 the collision between a DC-9 and a private aircraft over Cerritos, Californiaresulted in a Congressional mandate (PublicLaw 100-223) that required some categoriesof U.S. and foreign aircraft to be equipped with TCAS II for flight operations in U.S.airspace. Based on Public Law 100-223,FAA issued a rule in 1989 that required all

passenger carrying aircraft with more than30 seats flying in U.S. airspace to beequipped with TCAS II by the end of 1991.This law was subsequently modified byPublic Law 101-236 to extend the deadlinefor full equipage until the end of 1993.Based on the successful results of the in-

service evaluations, RTCA published Version 6.0 of the TCAS II MOPS (DO-185) in September 1989 and Version 6.0units were put into full-time revenue servicein the U.S. starting in June 1990.As part of the mandated implementation, anextensive operational evaluation of TCAS,known as the TCAS Transition Program(TTP), was initiated in late 1991. Inconjunction with the TTP in the U.S.,EUROCONTROL conducted extensiveevaluations of TCAS operations in Europeand the Japan Civil Aviation Bureau (JCAB)conducted similar assessments of TCAS II

performance in Japanese and surroundingairspace. Other countries also conducted operational evaluations as the use of TCAS

began to increase.

The system improvements suggested as aresult of these TCAS II evaluations led tothe development and release of Version6.04a of the TCAS II MOPS (DO-185)

published by RTCA in May 1993. The principal aim of this modification was thereduction of nuisance alerts which wereoccurring at low altitudes and during level-off maneuvers and the correction of a

problem in the altitude crossing logic.

Version 7.0 Implementation

The results of the TTP evaluation of Version6.04a indicated that the actual verticaldisplacement resulting from an RA responsewas often much greater than 300 feet, and TCAS was having an adverse affect on thecontrollers and the ATC system. This led tothe development of Version 7.0 and numerous changes and enhancements to thecollision avoidance algorithms, auralannunciations, RA displays, and pilottraining programs to: (1) reduce the number of RAs issued, and (2) minimize altitudedisplacement while responding to an RA.Also included were: horizontal miss distancefiltering to reduce the number of unnecessary RAs, more sophisticated multi-threat logic, changes to reduce nuisancerepetitive TAs on RVSM routes in slowclosure situations, changes to increase the

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efficiency of the surveillance logic, and provision for sense reversals in TCAS-TCAS encounters.

The MOPS for Version 7.0 (DO-185A) wasapproved in December 1997 and Version 7.0units began to be installed in the U.S. on avoluntary basis in late 1999.

Version 7.1 Implementation

Based on an extensive analysis of TCAS IIVersion 7.0 performance since 2000

performed primarily in Europe, additionalchanges to improve the RA logic wereidentified. In response to a near mid-air thatoccurred in Japan in 2001 and a mid-air thatoccurred at Ueberlingen, Germany, near theSwiss border in July 2002, a change wasmade to permit additional sense reversalRAs in order to address certain verticalchase geometries. It should be noted that ineach of these cases, the pilots maneuvered opposite to the displayed RA. Separate fromthe Japan and Ueberlingen accidents, areview of other operational experience had shown that pilots occasionally maneuver inthe opposite direction from that indicated byan "Adjust Vertical Speed, Adjust" (AVSA)RA. To mitigate the risk of pilots increasingtheir vertical rate in response to an AVSARA, all AVSA RAs were replaced by LevelOff, Level Off" (LOLO) RAs.

Extensive validation of these changes was performed by the Europeans and the U.S.with the end result being publication of Version 7.1 of the MOPS (DO-185B) inJune 2008. Version 7.1 units are expected to

be operating by 2010-2011. It should benoted that Version 6.04a and 7.0 units areexpected to remain operating for theforeseeable future where authorized.

Requirements for World-WideCarriage

The U.S. was the first ICAO member Stateto mandate carriage of an airborne collision

avoidance system for passenger carryingaircraft operating in its airspace.

Because of this mandate, the number of longrange aircraft being fitted with TCAS II and operating in European and Asian airspacecontinued to increase even though systemcarriage and operation was not mandatory inthat airspace. As studies, operationalexperience, and evaluations continued todemonstrate the safety benefits of TCAS II,some non-U.S. airlines also equipped their short-haul fleets with TCAS.

In 1995, the EUROCONTROL Committeeof Management approved an implementation

policy and schedule for the mandatorycarriage of TCAS II in Europe. TheEuropean Air Traffic Control Harmonizationand Integration Program (EATCHIP) ProjectBoard then ratified this policy. Theapproved policy requires that:

From 1 January 2000, all civil fixed-wing turbine-powered aircraft havinga maximum take-off mass exceeding15,000 kg, or a maximum approved

passenger seating configuration of more than 30, will be required to beequipped with TCAS II, Version 7.0;

From 1 January 2005, all civil fixed-wing, turbine-powered aircraft havinga maximum take-off mass exceeding5,700 kg, or a maximum approved

passenger seating configuration of more that 19, will be required to beequipped with TCAS II, Version 7.0.

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

The demonstrated safety benefits of theequipment, and the 1996 mid-air collision

between a Saudia Boeing 747 and aKazakhstan Ilyushin 76, resulted in anICAO requirement for world-widemandatory carriage of ACAS II on allaircraft, including cargo aircraft, beginning

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in 2003. To guarantee the effectiveness of this mandate, ICAO has also mandated thecarriage and use of pressure altitudereporting transponders, which are a pre-requisite for the generation of RAs.After the mid-air collision between aGerman Air Force Tupolev 154 and a U.S.Air Force C-141 transport aircraft, off

Namibia in September 1997, urgentconsideration was given to the need to equipmilitary transport aircraft with TCAS.Several countries, including the U.S.,initiated programs to equip tanker, transportand cargo aircraft within their military fleetswith TCAS II Version 7.0.

In the U.S. effective Jan 1, 2005, for thoseaircraft required to carry TCAS II, Version7.0 must be installed in all new installations.For installations of TCAS II made prior toJan 1, 2005 under certain conditions,Version 6.04a can continue to be used.

RVSM Considerations

With the creation of Reduced VerticalSeparation Minimum (RVSM) airspace, aminimum requirement for TCAS equipagewas established. Specifically, in order tooperate an aircraft with TCAS II in RVSMairspace, it must meet TSO-C119b (Version7.0) or a later version. In the US, operationsoutside RVSM airspace with TCAS II can

be conducted using Version 6.04a.

Standards and Guidance Material

The data obtained from FAA and industrysponsored studies, simulations, flight tests,and operational evaluations have enabled RTCA to publish the MOPS for TCAS II.The current version of the MOPS, DO-185B, describes the standards, requirements,and test procedures for TCAS Version 7.1.EUROCAE ED-143 is the equivalentdocument for ACAS II.

RTCA has also published MOPS for TCAS I, DO-197A, which defines therequirements and test procedures for TCAS I

equipment intended for use on airlineaircraft operated in revenue service. TheFAA has issued Technical Standard Order (TSO) C118a that defines the requirementsfor the approval of TCAS I equipment. Adraft Advisory Circular outlining thecertification requirements and therequirements for obtaining operationalapproval of the system has been prepared and is being used by the FAAs AircraftCertification Offices (ACO) as the basis for approving TCAS I installations and operation.

For TCAS II, TSO C119c and AdvisoryCircular 20-151A have been published for use by FAA airworthiness authorities incertifying the installation of TCAS II onvarious classes of aircraft. AdvisoryCircular 120-55C defines the procedures for obtaining air carrier operational approval for the use of TCAS II. While FAA developed these documents, they have been used throughout the world by civil aviationauthorities to approve the installation and use of TCAS, or as the basis for development of State-specific requirementsand guidance.

ICAO Standards and Recommended

Practices (SARPs) and Guidance Materialfor ACAS I and ACAS II have been

published in Annex 10. The procedures for use of ACAS have been published in PANS-OPS Document 8168 and guidance to air traffic controllers, along with the

phraseology for reporting TCAS RAs have been published in PANS-ATM, Document4444. These documents provideinternational standardization for collisionavoidance systems.

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

published ARINC Characteristic 735A thatdefines the form, fit, and function of TCASII units. The AEEC has also published ARINC Characteristic 718B for the Mode Stransponder. Note: A Mode S transponder is required as part of a TCAS II installation.

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TCAS II Components

A block diagram of TCAS II is shown inFigure 1. A TCAS II installation consists of the following major components.

Figure 1. TCAS II Block Diagram

TCAS Computer Unit

The TCAS Computer Unit, or TCASProcessor performs airspace surveillance,intruder tracking, own aircraft altitudetracking, threat detection, RA maneuver

determination and selection, and generationof advisories. The TCAS Processor uses

pressure altitude, radar altitude, and discreteaircraft status inputs from own aircraft tocontrol the collision avoidance logic

parameters that determine the protectionvolume around the TCAS. If a tracked aircraft selects an avoidance maneuver aircraft that will provide adequate verticalmiss distance from the intruder whilegenerally minimizing the perturbations tothe existing flight path. If the threat aircraftis also equipped with TCAS II, theavoidance maneuver will be coordinated with the threat aircraft.

DIRECTIONALANTENNA (TOP)

RADAR ALTITUDE &DISCRETE INPUTS

PRESSUREALTITUDE

TCASCOMPUTER

UNITMode S Transponder

MODE 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 TCASinto Standby. The Mode S transponder

performs the normal functions to support theground-based ATC system and can work with either an ATCRBS or a Mode S ground sensor. The Mode S transponder is alsoused to provide air-to-air data exchange

between TCAS-equipped aircraft so thatcoordinated, complementary RAs can beissued when required.

Mode S/TCAS Control Panel

A single control panel is provided to allowthe flight crew to select and control allTCAS equipment including the TCASProcessor, the Mode S transponder, and insome cases, the TCAS displays. A typicalcontrol panel provides four (4) basic control

positions:

Stand-by: Power is applied to theTCAS Processor and the Mode Stransponder, but TCAS does not issueany interrogations and the transponder will reply to only discreteinterrogations. The transponder stilltransmits squitters. Note: If theaircraft is on the ground and

RADisplay

RADisplay

AURALANNUNCIATION

TADisplay

BOTTOMOMNI

ANTENNA(Optional

DirectionalAntenna)

MODE S/TCASCONTROL

PANEL

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If implemented on a part-time basis, thedisplay will automatically activate whenever a TA or an RA is issued. Currentimplementations include dedicated trafficdisplays; display of the traffic informationon shared weather radar displays, map

presentation displays, Engine Indication and Crew Alerting System (EICAS) displays,

Navigation Display (ND), and other displays such as a Cockpit Display of Traffic Information (CDTI) used inconjunction with Automatic DependentSurveillance - Broadcast (ADS-B)applications.

A majority of the traffic displays also provide the pilot with the capability to selectmultiple ranges and to select the altitude

band for the traffic to be displayed. Thesecapabilities allow the pilot to display trafficat longer ranges and with greater altitudeseparation while in cruise flight, whileretaining the capability to select lower display ranges in terminal areas to reducethe amount of display clutter.

Traffic Display Symbology

Figure 2 shows the various traffic symbolsused on the traffic display. Note thatalthough some minor TCAS symbologydifferences may exist on TCAS/CDTIshared displays, the basic TA and RA shapesand colors remain unchanged.

Both color and shape are used to assist the pilot in interpreting the displayed information. Own-aircraft is depicted as awhite or cyan airplane-like symbol. Thelocation of own aircraft symbol on thedisplay is dependent on the displayimplementation. Other aircraft are depicted

using geometric symbols, depending on their threat status, as follows:

n unfilled diamond, shown in either cyan or white, but not the same color as own-aircraft symbol, is used todepict "Other" non-threat traffic.

filled diamond, shown in either cyanor white, but not the same color as

own-aircraft symbol, is used to depictProximate Traffic. Proximate Trafficis non-threat traffic that is within 6nmi and 1200 ft from own aircraft.

Figure 2. Standardized Symbology forUse on the Traffic Display

filled amber or yellow circle is used to display intruders that have caused aTA to be issued.

filled red square is used to displayintruders that have caused an RA to beissued.

At a given time during operation, displayed traffic is likely to be Other. When a TA or RA occurs, TA, RA and Proximate traffic,within the selected display range, arerequired to be displayed. The display of Other traffic is recommended to assist the

pilot in visually acquiring the intruder causing the RA or TA. Although Proximate

+11

- 09

- 05

Own-aircraft. Airplane-like symbol, in white or cyan.

Other Traffic , altitudeunknown. Unfilled diamond in white or cyan

Proximate Traffic , 1100 feetabove and descending.

Filled diamond in white or cyan

Traffic Advisory (TA),900 feet below and level.Filled yellow/amber circle.

Resolution Advisory (RA),500 feet below and climbing.Filled red square.

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status of traffic can be derived by the pilotfrom the relative range and altitude, thesymbol indication allows this state to bedetermined perceptually, from a quick glance.

Each symbol is displayed on the screen,according to its relative position to ownaircraft. To aid the pilot in determining therange to a displayed aircraft, the trafficdisplay provides range markings at one-half the selected scale and at the full scale.Additional range markings may be provided at closer ranges, e.g., 2 nmi, on some displayimplementations. The selected displayrange is also shown on the display. Therange markings and range annunciation aredisplayed in the same color as own aircraftsymbol unless the traffic display isintegrated with an existing display thatalready provides range markings, e.g., aMAP display.

Relative altitude is displayed in hundreds of feet above the symbol if the intruder isabove own aircraft and below the symbol if the intruder is below own aircraft. When theintruder is above own aircraft, the relativealtitude information is preceded by a + sign.When the intruder is below own aircraft, a sign precedes the relative altitudeinformation. In some aircraft, the flightlevel of the intruder can be displayed instead of its relative altitude. The flight level isshown above the traffic symbol if theintruder is above own aircraft and below thetraffic symbol is the intruder is below ownaircraft. If the intruder is not reporting itsaltitude, no altitude information in shownfor the traffic symbol. The altitudeinformation is displayed in the same color asthe aircraft symbol.

An arrow is displayed immediately to theright of a traffic symbol when the targetaircraft is reporting its altitude and isclimbing or descending at more than 500fpm. An up arrow is used for a climbingaircraft; a down arrow is used for adescending aircraft. The arrow is displayed in the same color as the aircraft symbol.

When an aircraft causing a TA or RA is beyond the currently selected range of thetraffic display, half TA or RA symbols will

be displayed at the edge of the display at the proper relative bearing. In someimplementations, a written message such asTRAFFIC, TFC, or TCAS isdisplayed on the traffic display if theintruder is beyond the selected displayrange, or if the selected display mode doesnot support the display of traffic. The half symbol or the written message will remaindisplayed until the traffic moves within theselected display range, the pilot increasesthe range on a variable range display toallow the intruder to be displayed, or the

pilot selects a display mode that allowstraffic to be displayed.

In some instances, TCAS may not have areliable bearing for an intruder causing a TAor RA. Since bearing information is used for display purposes only, the lack of

bearing information does not affect theability of TCAS to issue TAs and RAs.When a No-Bearing TA or RA is issued,the threat level, as well as the range, relativealtitude, and vertical rate of the intruder arewritten on the traffic display. This text is

shown in red for an RA and in amber or yellow for a TA. For example, if an RA wasissued against an intruder at a range of 4.5nmi and with a relative altitude of +1200feet and descending, the No Bearingindication on the traffic display would be:

RA 4.5 +12

Resolut ion Advisory (RA) 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 onan 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 guidancehas also been implemented on a Head-UpDisplay (HUD). The implementations using

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the IVSI or a vertical speed tape utilize red and green lights or markings to indicate thevertical speeds to be avoided (red) and thedesired vertical speed to be flown (green).An implementation using pitch cues uses aunique shape on the PFD to show the pitchangle 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 "non-glass cockpit" aircraft. However, someoperators have implemented this display intheir "glass-cockpit" aircraft to provide acommon display across their fleet types.Some IVSI implementations use mechanicalinstruments with a series of red and greenLEDs around the perimeter of the display,while other implementations use a LCDdisplay that draw the red and green arcs atthe appropriate locations. The LCD displayimplementations also have the capability to

provide both the traffic and RA display on asingle instrument.

On "glass-cockpit" aircraft equipped with aPFD, some airframe manufacturers haveimplemented the RA display on the verticalspeed tape; some have elected to provide

pitch cues; and other implementations provide both pitch cues and a vertical speed tape.

The standards for the implementation of RAdisplays are provided in DO-185B. Inaddition to the implementations outlined above, DO-185B defines requirements for implementation of the RA display via theflight director. Two RA displays arerequired; one in the primary view of each

pilot. Figure 3 shows an RA displayimplemented on a LCD that also providestraffic information. Figure 4 shows two

possible implementations on the PFD.

Figure 3. TCAS RA Display Implemented on an IVSI


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310 4020

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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Target Surveillance

TCAS, independent of any ground inputs, performs surveillance of nearby aircraft to provide information on the position and altitude of these aircraft so that the collisionavoidance algorithms can perform their function. The TCAS surveillance functionoperates by issuing interrogations at 1030MHz that transponders on nearby aircraftrespond to at 1090 MHz. These replies arereceived and decoded by the surveillance

portion of the TCAS software and theinformation is then provided to the collisionavoidance algorithms.

TCAS has a requirement to provide reliablesurveillance out to a range of 14 nmi and intraffic densities of up to 0.3 aircraft per square nautical mile. The surveillancefunction provides the range, altitude, and

bearing of nearby aircraft to the collisionavoidance function so that threatdeterminations can be made and so that theinformation displayed on the traffic display isaccurate. The TCAS surveillance iscompatible with both the ATCRBS and Mode S transponders. TCAS can

simultaneously track up to 30 transponder-equipped aircraft within a nominal range of 30 nmi.

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 withthe functions of the ATC radars. Severaldesign features have been developed and implemented to allow TCAS to providereliable surveillance without degrading the

performance of the ATC radars.

Mode S Surveillance

Because of the selective address feature of the Mode S system, TCAS surveillance of Mode S equipped aircraft is relativelystraightforward. TCAS listens for thespontaneous transmissions, or squitters, that

are generated once per second by the Mode Stransponder. Among other information, thesquitter contains the unique Mode S addressof the sending aircraft. The Mode S addressis known internationally as the ICAO 24-bitaircraft address.

Following the receipt and decoding of asquitter message, TCAS sends Mode Sinterrogations to the Mode S addresscontained in the squitter. These interrogationsgenerally occur once per second. DO-185Brequires interrogations to be transmitted atleast once every five seconds The Mode Stransponder replies to these interrogationsand the reply information is used by TCAS todetermine range, bearing, and altitude of theMode S aircraft.

To minimize interference with other aircraftand ATC on the 1030/1090 MHz channels,the rate at which a Mode S aircraft isinterrogated by TCAS is dependent on therange and closure rate between the twoaircraft. At extended ranges, a target isinterrogated once every five seconds. As thetarget aircraft approaches the area where aTA may be required, the interrogation rateincreases to once per second.

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

provided to the collision avoidance logic for use 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 logicfor use in the Horizontal Miss Distance Filter and so that the targets position can be

properly shown on the traffic display.

Mode C Surveillance

TCAS uses a modified Mode C interrogationknown as the Mode C Only All Call tointerrogate nearby Mode A/C transponders.The nominal interrogation rate for thesetransponders is once per second. SinceTCAS does not use Mode A interrogations,the Mode A transponder codes of nearbyaircraft are not known to TCAS.

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18

Hardware degarblers can reliably decode upto three overlapping replies. In addition, thecombined use of variable interrogation power levels and suppression pulses reduces thenumber of transponders that reply to a singleinterrogation. This technique, known aswhisper-shout (WS), takes advantage of differences between the receiver sensitivity of transponders and the transponder antennagains of target aircraft.

Aircraft that are not equipped with anoperating altitude encoder reply to theseinterrogations with no data contained in thealtitude field of the reply. TCAS uses theframing pulses of the reply to initiate and maintain a range and bearing track on thesetargets. As with the Mode S tracks, thesereplies are passed to the collision avoidancelogic for traffic advisory detection and for

presentation on the traffic display.A low power level is used for the firstinterrogation 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 isfollowed 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 anadditional group of transponders that did notreply to the previous interrogation. As shownin Figure 6, the WS procedure is followed

progressively in 24 steps in the forward direction to separate the Mode C replies intoseveral groups and thus reducing the

possibility of garbling. WS sequences withfewer steps are used for the two sides and theaft direction. The WS sequence is transmitted

once during each surveillance update period,which is nominally one second.

The replies from aircraft that are capable of providing their Mode C altitude are tracked in range, altitude, and bearing. These targetreports are passed to the collision avoidancelogic for possible TA and RA selection and for presentation on the traffic display.

TCAS surveillance of Mode C targets iscomplicated by problems of synchronous and non-synchronous garbling, as well asreflections of signals from the ground (multipath). When a Mode C Only All Callinterrogation is issued by TCAS, all Mode Ctransponders that detect the interrogation willreply. Because of the length of the replymessage (20.3 microseconds), all Mode Cequipped aircraft within a range difference of 1.7 nmi from the TCAS aircraft will generate

replies that garble, or overlap each other,when received by TCAS. This is shown inFigure 5 and is called synchronous garble.Various techniques have been incorporated into TCAS to cope with this condition.

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 overlappingcoverage must be provided in all directions toensure 360-degree coverage. Synchronousgarble is also reduced by the use of theMode C Only All Call interrogation. This

interrogation inhibits Mode S transpondersfrom replying to a Mode C interrogation.

Target of

Figure 5. Synchronous Garble Area

Non-synchronous garble is caused by thereceipt of undesired transponder replies thatwere generated in response to interrogationsfrom ground sensors or other TCASinterrogations. These so-called fruit repliesare transitory so they are typically identified

Interest

TCAS

1.7 nmi

Other Mode A/CAircraft that cancause garble


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Figure 6 . Whisper-Shout Interrogation Sequence

Figure 7. Directional Transmission

and discarded by correlation algorithms in thesurveillance logic. Operational experiencewith TCAS has shown that the probability of

initiating and maintaining a track based onfruit replies is extremely remote.

Avoiding the initiation of surveillance tracks based on multipath replies is another important consideration in the design of theTCAS surveillance. Multipath results in thedetection of more than one reply to the sameinterrogation, generally of lower power, from

the same aircraft. It is caused by a reflected interrogation and usually occurs over flatterrain. To control multipath, the direct-path

power level is used to raise the minimumtriggering level (MTL) of the TCAS receiver enough 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, the4-pulse direct reply is above the DMTL level,while the delayed, lower-power multipathreply is below the DMTL threshold, and is

thus rejected by TCAS.

REPLY REGION

Figure 8. Dynamic Thresholding of ATCRBS Replies

Reply Pulses

Dynamic MTL

Multipath MinimumTriggering LevelMTL

20.3 s

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Interference Limiting

Interference limiting is a necessary part of thesurveillance function. To ensure that notransponder is suppressed by TCAS activityfor more than two (2) percent of the time, and TCAS does not create an unacceptably highfruit rate for the ground-based ATC radars,multiple TCAS units within detection rangeof one another, i.e., approximately 30 nmi,are designed to limit their own transmissionsunder certain conditions. As the number of such TCAS units within this region increases,the interrogation rate and power allocationfor each TCAS unit must decrease to preventundesired interference with the ATC radars.

To achieve this, every TCAS unit counts thenumber of other TCAS units within detectionrange. This is accomplished by having eachTCAS unit periodically (every 8-10 seconds)transmit a TCAS broadcast message thatincludes the Mode S address of thetransmitting aircraft. Mode S transpondersare designed to accept the broadcastmessages without replying and pass the

broadcast messages to their associated TCASunits. The messages are then used by thereceiving TCAS interference limitingalgorithms to develop an estimate of thenumber of TCAS aircraft (NTA) withindetection range. NTA is used by each TCASto limit the interrogation rate and power asrequired.

While interference limiting has been anintegral part of TCAS since its inception,initial operational experience with TCASindicated that refinements were necessary inthe surveillance design to meet the above-stated requirements. In Version 7.0, threekey modifications were made to the

interference limiting algorithms:

(1) In addition to computing the number of nearby TCAS aircraft, each TCAS nowalso was required to estimate thedistribution of those nearby TCASaircraft. This allowed the algorithms toaccount for different distributions in

TCAS aircraft in the terminal (high-density) and en-route areas.

(2) For TCAS aircraft flying above FlightLevel (FL) 180, the interferencelimiting algorithms were simplified,allowing longer surveillance ranges for aircraft overflying high density trafficareas.

(3) A maximum allowable interferencelimiting power reduction wasintroduced to ensure that the TCASsurveillance range Is always adequatefor collision avoidance.

Electromagnetic Compatibility

TCAS incorporates a number of design

features to ensure that TCAS does notinterfere with other radio services thatoperate in the 1030/1090 MHz frequency

band. The design of the Mode S waveformsused by TCAS provide compatibility with theMode A and Mode C interrogations of the`ground based secondary surveillance radar system and the frequency spectrum of Mode S transmissions is controlled to protectadjacent distance measuring equipment(DME) channels.

The interference limiting features of TCASalso help to ensure electromagneticcompatibility with the ATC radar system. Anextensive series of analyses, equipment tests,and computer simulations of the Version 7.0and later surveillance software demonstrated that operationally significant interference willnot occur between TCAS, secondarysurveillance radar, and DME systems.

Hybrid Surveillance

Hybrid surveillance is a new feature whichmay be included as optional functionality inTCAS II units. Hybrid surveillance is amethod to decrease the Mode S surveillanceinterrogations by an aircraft's TCAS unit. .Specifically, TCAS units equipped withhybrid surveillance use passive surveillanceinstead of active surveillance to track intruders that meet validation criteria and are

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21

not projected once to be near-term collisionthreats. With active surveillance, TCAStransmits interrogations to the intruder'stransponder and the transponder replies

provide range, bearing, and altitude for theintruder. With passive surveillance, positiondata provided by an onboard navigationsource is broadcast from the intruder's ModeS transponder. The position data is typically

based on GPS and received on own ship bythe use of Mode S extended squitter, i.e. 1090MHz ADS-B, also known as 1090ES.Standards for Hybrid Surveillance have been

published in RTCA DO-300.

Once an intruder comes close to being acollision threat, it is tracked with activesurveillance. Figure 9 illustrates how thesystem transitions from passive surveillancewith validation to active surveillance as afunction of the collision potential. When theintruder is far from being a threat, it istracked with passive surveillance, and the

passive surveillance position is validated once per minute with a TCAS activeinterrogation. When the intruder is a near threat in altitude or range, but not both, it istracked with passive surveillance, and the

passive surveillance position is validated once every 10 seconds with an active TCAS interrogation. When the intruder is a near threat in altitude and range, it is tracked withactive surveillance at a 1 Hz interrogationrate. The criteria for transitioning from

passive to active surveillance were designed to ensure that all TCAS advisories will be

based on active surveillance.

The intent of hybrid surveillance is to reducethe TCAS interrogation rate through the

judicious use of the ADS-B data provided viathe Mode S extended squitter without anydegradation of the safety and effectiveness of the TCAS.

Passive surveillance.Intruder is n ot a near threat.

Validate intruder with TCAS activeinterrogation once per minute.

Act ive su rv eil lan ce.Intruder is a near threat in altitude and range.

TCAS active interrogatio n at 1 Hz.

Traffic Adv isory

Resolution Advi sory

Passive surveillance.Intruder is a near threat in altitude or range.

Validate intruder with TCASactive interro gation at 0.1 Hz.

Own ship

Intruder I n c r e a s i n g C o l l i s i o n P

o t e n t i a l

Passive surveillance.Intruder is n ot a near threat.

Validate intruder with TCAS activeinterrogation once per minute.

Act ive su rv eil lan ce.Intruder is a near threat in altitude and range.

TCAS active interrogatio n at 1 Hz.

Traffic Adv isory

Resolution Advi sory

Passive surveillance.Intruder is a near threat in altitude or range.

Validate intruder with TCASactive interro gation at 0.1 Hz.

Own ship

Intruder I n c r e a s i n g C o l l i s i o n P

o t e n t i a l

Figure 9. Transition from Passive to Active Surveillance


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Collision AvoidanceConcepts

Airborne collision avoidance is a complex

problem. It has taken many years to developan operationally acceptable solution and refinement of the system continues tomaximize the compatibility between TCAS,ATC systems throughout the world, and existing cockpit procedures. The heart of collision avoidance is the collision avoidancesystem logic, or the CAS logic. To explainthe operation of the CAS logic, the basicCAS concepts of sensitivity level, tau, and

protected volume need to be understood.

Sensitivity LevelEffective CAS logic operation requires atrade-off between necessary protection and unnecessary advisories. This trade-off isaccomplished by controlling the sensitivitylevel (SL), which controls the time or tauthresholds for TA and RA issuance, and therefore the dimensions of the protected airspace around each TCAS-equipped aircraft. The higher the SL, the larger theamount of protected airspace and the longer

the alerting thresholds. However, as theamount of protected airspace increases, theincidence of unnecessary 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 is

placed in the Standby Position,TCAS is operating in SL1. In SL1,TCAS does not transmit anyinterrogations. SL1 is normallyselected only when the aircraft is onthe ground or if TCAS has failed.The pilot selection of Standby on the

Control Panel is normally the onlyway that SL1 will be selected.

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

performs all surveillance functionsand will issue TAs as required. RAsare inhibited in SL2.

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

panel, the TCAS logic automaticallyselects the appropriate SL based onthe altitude of own aircraft. Table 2

provides the altitude threshold atwhich TCAS automatically changesSL 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 theuse of ground based control of SL has not

been agreed to between pilots,controllers, and FAA and is not currentlyused in U.S. airspace, the capability for ground based control of SL is included inthe TCAS design. This design featureallows the operating SL to be reduced from the ground by using a Mode S

uplink message. The TCAS designallows the selection of any SL shown inTable 2 with the exception of SL1.

When the pilot has selected the TA-RA modeon the Control Panel, the operating SL isautomatically selected via inputs from theaircrafts radar or pressure altimeter. SL2will be selected when the TCAS aircraft is

below 1,000 feet above ground level (AGL)(100 feet) as determined by the radar altimeter input. As previously stated, when

in SL2, RAs are 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

pressure altitude using inputs from ownaircraft barometric altimeter.

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Table 2. Sensitivity Level Definition and Alarm Thresholds

Tau

TCAS primarily uses time-to-go to CPArather than distance to determine when a TAor an RA should be issued. The time to CPAis called the range tau and the time to co-altitude is called the vertical tau. Tau is anapproximation of the time, in seconds, toCPA or to the aircraft being at the samealtitude. The range tau is equal to the slantrange (nmi) divided by the closing speed (knots) multiplied by 3600. The vertical tauis equal to the altitude separation (feet)divided by the vertical closing speed of thetwo aircraft (feet/minute) times 60.

TCAS II operation is based on the tauconcept for all alerting functions. A TA or anRA is displayed only when both the range tauand vertical tau are less than certain threshold values that depend on sensitivity level.Table 2 provides the TA and RA tauthresholds used in each sensitivity level.

The boundary lines shown in Figure 10indicate the combinations of range and rangeclosure rate that would trigger a TA with a40-second range tau and an RA with a 25-

second range tau. These are the range tausused in SL5. Similar graphs can be generated for other sensitivity levels.

A problem with this simple definition of tauis that in encounters where the rate of closureis very low, such as those shown inFigure 11, an intruder aircraft can come veryclose in range without crossing the range tau

boundaries and thus, without causing a TA or an RA to be issued. To provide protection inthese types of encounters, a modified definition of range tau is used that gives the

boundaries shown in Figure 12. At larger ranges and higher closure rates these boundaries are essentially equal to thosedefined by the basic tau concept. However, atclose ranges and at slower closure rates themodified tau boundaries converge to a non-zero range called DMOD. This modificationallows TCAS to issue TAs and RAs at or

before the fixed DMOD range threshold inthese slow-closure-rate encounters. Thevalue of DMOD varies with the differentsensitivity levels and the values used to issue

TAs and RAs are shown in Table 2.

There is a similar problem when the verticalclosure rate of the TCAS and the intruder aircraft is low, or when they are close butdiverging in altitude. To address that

problem, TCAS uses a fixed altitudethreshold, referred to as ZTHR, inconjunction with the vertical tau, todetermine whether a TA or an RA should beissued. As with DMOD, ZTHR varies withsensitivity level and the TA and RA

thresholds are shown in Table 2. Figure 13shows the combinations of altitude separationand vertical closure rate that would trigger aTA with a 40-second vertical tau and an RAwith a 25-second vertical tau, as appropriatefor SL5. The ZTHR values are reflected inthe level portion of the curve at low verticalclosure rates.

Tau (Seconds) DMOD (nmi) ZTHR (feet)Altitude Threshold

ALIM(feet)

Own Altitude (feet) SL

TA RA TA RA TA RA RA< 1000 (AGL) 2 20 N/A 0.30 N/A 850 N/A N/A

1000 - 2350 (AGL) 3 25 15 0.33 0.20 850 600 3002350 5000 4 30 20 0.48 0.35 850 600 300

5000 10000 5 40 25 0.75 0.55 850 600 35010000 20000 6 45 30 1.00 0.80 850 600 40020000 42000 7 48 35 1.30 1.10 850 700 600

> 42000 7 48 35 1.30 1.10 1200 800 700

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0

2

4

6

8

10

12

14

0 200 400 600 800 1000 1200Range Closure Rate, Knots

R a n g e ,

N a u

t i c a

l M i l e s

TA Range Boundaryfor SL=5

RA Range Boundaryfor SL=5

Figure 10. TA/RA Range Boundaries for SL5 using Unmodified Tau

Slow Overtake

Slow HorizontalClosure Rate

Slow Vertical Closure Rate

Figure 11. Need for Modified Tau

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0

2

4

6

8

10

12

14

0 200 400 600 800 1000 1200Range Closure Rate, Knots

R a n g e ,

N a u

t i c a

l M i l e

s

TA Range Boundaryfor SL=5

RA Range Boundaryfor SL=5

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

0

2000

4000

6000

8000

10000

0 2000 4000 6000 8000 10000 12000 14000 16000 18 000 20000

Ver t ical Closure Rate, fpm

A l t i t u

d e

S e p a r a

t i o n ,

f e e t

TA Altitude Boundaryfor SL=5

RA Altitude Bounda ryfor SL=5

Figure 13. TA/RA Vertical Tau Boundaries for SL5

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Protected Volume

A protected volume of airspace surroundseach TCAS-equipped aircraft. As shown inFigure 14, the tau and DMOD criteriadescribed above shape the horizontal

boundaries of this volume. The vertical tauand the fixed altitude thresholds determinethe vertical dimensions of the protected volume.

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

plus an estimate of the protected horizontalmiss distance. Thus, the size of the protected volume depends on the speed and heading of the aircraft involved in the encounter. Thehorizontal miss distance filter seeks toconstrain the volume in order to exclude RAsfor aircraft with sufficient lateral separation,and uses both range and bearing informationto accomplish this.

TCAS II is designed to provide collisionavoidance protection in the case of any twoaircraft that are closing horizontally at anyrate up to 1200 knots and vertically up to10,000 feet per minute (fpm).

CAS Logic Functions

The logic functions employed by TCAS to perform its collision avoidance function areshown in Figure 15. The followingdescriptions of these functions are intended to

provide a general level of understanding of these functions. The nature of providing aneffective collision avoidance system resultsin the need to have numerous specialconditions spread throughout the functions

and these are dependent on encounter geometry, range and altitude thresholds, and

aircraft performance. These specialconditions are beyond the scope of thisdocument. A complete description of theCAS logic and additional details of its designand performance are contained in RTCA DO-185B, Section 2.2.5.

Tracking

Using the range, altitude (when available),and bearing of nearby aircraft that are

provided to CAS by the Surveillancefunction, the CAS logic initiates and maintains a track on each aircraft.Successive range reports are used to computerange rate. Altitude information is used toestimate the vertical speed of each nearbyaltitude-reporting aircraft. The altitudetracking can use altitude that is quantized ineither 25 or 100 foot increments. The CAStracking function is designed to track aircraftwith vertical rates of up to 10,000 fpm. TheCAS logic uses the track information todetermine the time to CPA and the altitude of each aircraft at CPA.

The CAS logic also uses the data from itsown aircraft pressure altitude to determinethe own aircraft altitude, vertical speed, and the relative altitude of each aircraft. TheCAS logic uses the altitude source on ownaircraft that provides the finest resolution.Own aircraft data can be provided as either fine altitude reports with quantization lessthan 10-foot increments, or as coarse altitudereports with quantization up to 100-footincrements. The outputs from the CAStracking algorithm, i.e., range, range rate,relative altitude, and vertical rate are

provided to the Traffic Advisory and ThreatDetection logic to determine if a TA or anRA is needed.

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Not to Scale

Figure 14. TCAS Protection Volume

Figure 15. CAS Logic Functions

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The CAS tracker also uses the difference between its own aircraft pressure altitudeand radar altitude to estimate theapproximate elevation of the ground abovemean sea level. This ground estimationlogic functions whenever own aircraft is

below 1750 ft AGL. The ground levelestimate is then subtracted from the pressurealtitude received from each Mode Cequipped nearby aircraft to determine theapproximate altitude of each aircraft abovethe ground. If this difference is less than360 feet, TCAS considers the reportingaircraft to be on the ground. If TCASdetermines the intruder to be on the ground,it inhibits the generation of advisoriesagainst this aircraft. This methodology isshown graphically in Figure 16.

Radaraltimeter

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

360-foot allowance

Barometricaltimeter

Ground level

Standard altimeter setting Estimated elevation of ground

Declaredairborne

TCAS

Declaredon ground

Declaredon ground

Figure 16. Mode C Target on Ground

Determination

A Mode S equipped aircraft is considered to be on the ground if the on-the-ground status bit contained in either the squitter or transponder reply indicates the aircraft is onthe ground.

Traffic AdvisoryUsing the tracks for nearby aircraft, rangeand altitude tests are performed for eachaltitude-reporting target. The range test is

based on tau, and the TA tau must be lessthan the threshold shown in Table 2. Inaddition, the current or projected verticalseparation at CPA must be within the TA

altitude threshold shown in Table 2 for atarget to be declared an intruder. If theTraffic Advisory logic declares an aircraft to

be an intruder, a TA will be issued againstthat aircraft.

A non-altitude reporting aircraft will bedeclared an intruder if the range test aloneshows that the calculated tau is within theRA tau threshold associated with the currentSL being used as shown in Table 2.

Version 7.0 included changes to ensure thata targets TA status is maintained in slowclosure rate encounters by invoking morestringent requirements for removing a TA.These changes address problems reported inwhich multiple TAs were issued against thesame target in parallel approach encountersand in RVSM airspace.

Threat Detection

Range and altitude tests are performed oneach altitude-reporting intruder. If the RAtau and either the time to co-altitude or relative altitude criteria associated with thecurrent 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 or not selected at all. RAs cannot be generated for non-altitude reporting intruders.

Version 7.0 included changes in the ThreatDetection logic to improve the performanceof this portion of the logic. These changesincluded:

Declaring own aircraft to be on theground when the input from the radar altimeter is valid and below 50 feetAGL. This precludes completereliance on own aircrafts weight-on-wheels switch that has been shown to

be unreliable in some aircraft.

Preventing the SL from decreasingduring a coordinated encounter tomaintain the continuity of a displayed RA and thus, prevent multiple RAs

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from being issued against the sameintruder.

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

Reducing alert thresholds to accountfor 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 todelay the issuance of an RA to allowadditional time for detecting a level off maneuver by a climbing or descendingaircraft.

Introducing a horizontal miss distance(HMD) filter to reduce the number of RAs against intruder aircraft having alarge horizontal separation at CPA. As

part of the range test, the HMD filter can also terminate an RA prior toALIM being obtained to minimizealtitude displacement when the filter isconfident that the horizontal separationat CPA will be large.

Resolution Advisory Selection

Initial Resolution Advisory Selection

When an intruder is declared a threat, a twostep process is used to select the appropriateRA for the encounter geometry. The firststep in the process is to select the RA sense,i.e., upward or downward. Based on therange and altitude tracks of the intruder, theCAS logic models the intruders flight pathfrom its present position to CPA. The CASlogic then models upward and downward

sense RAs for own aircraft, as shown inFigure 17, to determine which sense

provides the most vertical separation atCPA.

Note: In modeling aircraft response to RAs,the expectation is the pilot will begin theinitial 0.25 g acceleration maneuver within

five seconds to an achieved rate of 1500

fpm. Pilot response with 0.35 g accelerationto an achieved rate of 2500 fpm is expected within 2.5 seconds for subsequent RAs.

B

A

TCAS

Thr eat

CPA

downwa rd

upward

Figure 17. RA Sense Selection

In the encounter shown in Figure 17, thedownward sense logic will be selected sinceit provides greater vertical separation. Inencounters where either of the senses resultsin the TCAS aircraft crossing through theintruders altitude, TCAS is designed toselect the non-altitude crossing sense if thenon-crossing sense provides the desired vertical separation (ALIM) at CPA. If thenon-altitude crossing sense provides at leastALIM feet of separation at CPA, this sensewill be selected even if the altitude crossingsense provides greater separation. If ALIMcannot be obtained in the non-altitudecrossing sense, an altitude crossing RA will

be issued. Figure 18 shows an example of encounters in which the altitude crossingand non-altitude crossing RA senses aremodeled and the non-crossing RA sense isselected.

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AL I MTh r eat

TCAS CPA

AL I M

RACl i mbi s s ued

Figure 18. Selection of Non-Crossing RA

Sense

Due to aircraft climb performancelimitations at high altitude or in some flapand landing gear configurations, an aircraftinstallation may be configured to inhibitClimb or Increase Climb RAs under someconditions. These inhibit conditions can be

provided via program pins in the TCASconnector or in real-time via an input from aFlight Management System (FMS). If theseRAs are inhibited, the RA Selection Criteriawill not consider them in the RA selectionand will choose an alternative upward senseRA if the downward sense RA does not

provide adequate vertical separation.TCAS is designed to inhibit IncreaseDescent RAs below 1450 feet AGL;Descend RAs below 1100 feet AGL; and allRAs below 1000100 feet AGL. If aDescend RA is being displayed as ownaircraft descends through 1100 feet AGL,the RA will be modified to a Do Not ClimbRA.

The second step in selecting an RA is tochoose the strength of the advisory. Thestrength of an RA is the degree of restriction

placed on the flight path by a vertical speed limit (negative) RA or the magnitude of thealtitude rate wanted for positive RAs.TCAS is designed to select the RA strengththat is the least disruptive to the existingflight path, while still providing ALIM feetof separation. An exception introduced inVersion 7.1 is noted below.

RAs can be classified as positive (e.g.,climb, descend) or negative (e.g., limit climbto 0 fpm, limit descend to 500 fpm). Theterm "Vertical Speed Limit" (VSL) isequivalent to "negative." RAs can also beclassified as preventive or corrective,depending on whether own aircraft is, or isnot, in conformance with the RA targetaltitude rate. Corrective RAs require achange in vertical speed; preventive RAs donot require a change in vertical speed.

A new feature was implemented in Version7.0 to reduce the frequency of initial RAsthat reverse the existing vertical rate of ownaircraft. When two TCAS-equipped aircraftare converging vertically with opposite ratesand are currently well separated in altitude,

TCAS will first issue a vertical speed limit(VSL or Negative) RA to reinforce the

pilots likely intention to level off atadjacent flight levels. If no response to thisinitial RA is detected, or if either aircraftaccelerates (vertically) toward the other aircraft, the initial RA will strengthen asrequired. This change was implemented toreduce the frequency of initial RAs thatreversed the vertical rate of own aircraft(e.g., displayed a climb RA for a descendingaircraft) because pilots did not follow a

majority of these RAs, and those that werefollowed, were considered to be disruptive

by controllers.

An exception to the least disruptive RArule was introduced in Version 7.1. After TCAS Version 7.0 was introduced into theairspace, opposite initial responses tocorrective VSL RAs (annunciated asAdjust Vertical Speed, Adjust) wereidentified. The correct response to an AVSARA is always a reduction in vertical speed.Several encounters were observed where the

pilot increased vertical speed, causingfurther reduction in separation between ownaircraft and the intruder. In Version 7.1, allcorrective VSL RAs requiring non-zerovertical rates (500, 1000, or 2000 fpm) arechanged to VSL 0 fpm RAs before beingdisplayed to the pilot and are annunciated asLevel Off, Level Off. The VSL 0 fpm RA

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is stronger than required, however thischange was made to make the intention of the corrective VSL, i.e., a move toward levelflight, unambiguously clear.

Table 3 provides a list of possible advisoriesthat can be issued as the initial RA whenonly a single intruder is involved in theencounter. Table 3 is derived from DO-185B, Table 2-16. After the initial RA isselected, the CAS logic continuouslymonitors the vertical separation that will be

provided at CPA and if necessary, the initialRA will be modified.

Strengthening Advisories

In some events, the intruder aircraft willmaneuver vertically in a manner that thwartsthe effectiveness of the issued RA. In thesecases, the initial RA will be modified toeither increase the strength or reverse thesense of the initial RA. Reversed sense RAswill be discussed separately. A VSL isstrengthened by changing to a morerestrictive VSL or to a positive Climb or Descend RA. A Climb or Descend RA isstrengthened to an Increase Climb/DescentRA. An Increase Climb/Descent RA can


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Table 3. Possible Initial RAs for Single Threat

Upward Sense Downward Sense

RA TypeRA

Required

Vertical Rate(fpm) RA

Required

Vertical Rate(fpm)Positive

(Corrective) Climb 1500 to 2000 Descend -1500 to -2000

Positive(Corrective) Crossing Climb 1500 to 2000 Crossing Descend -1500 to -2000

Positive(Corrective)

Crossing MaintainClimb 1500 to 4400

Crossing MaintainDescend -1500 to -4400

Positive(Corrective) Maintain Climb 1500 to 4400 Maintain Descend -1500 to -4400

Negative(Corrective) Reduce Descent 0 Reduce Climb 0

*Negative(Corrective) Reduce Descent > -500 Reduce Climb < 500



Negative(Preventive) Do Not Descend > 0 Do Not Climb < 0

Negative(Preventive)

Do Not Descend >500 fpm > -500

Do Not Climb >500 fpm < 500







* These Initial RAs cannot occur in Version 7.1

only be issued after a Climb/Descend RAhas been displayed either as an initial RA, astrengthening of a negative RA, or a sensereversal RA.

Figure 19 depicts an encounter where it isnecessary to increase the descent rate fromthe 1500 fpm required by the initial RA to2500 fpm. This is an example of anIncrease Descent RA.

I nc r eas e Des cent

T CAS

Threat

CPA

De s c end

T he t hr ea t I ncr eas esi t s des c ent r at e

t owar ds T C AS aI r c r af taf t er t he i ni t i alDes cend R A i s I s s ued

Figure 19. Increase Rate RA

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Sense Reversals

Version 7.0 and later permit sense reversalsin coordinated encounters. This sensereversal logic is very similar to that

previously available in encounters with non-TCAS threats. Figure 20 depicts anencounter where an initial Climb RArequires reversal to a Descend RA after theintruder maneuvers.

I ni t i al pr oj ec t i on

TCAS

Th r eat

CPA

Rever s alRA

I ni t i al RA

Figure 20. RA Reversal

After the introduction of Version 7.0 into theairspace, a weakness in the sense reversallogic was discovered. This issue was firstobserved in encounters with two TCASequipped aircraft that were vertically closewith both aircraft climbing or descending inthe same vertical direction with one pilotfollowing the TCAS RA and the other pilotnot following the RA. Eventually thisencounter geometry was determined to be

problematic for encounters with a TCASequipped aircraft and a non-TCAS threat. InVersion 7.1, new reversal logic was added toaddress this situation. The new logicrecognizes the vertical chase with lowvertical miss distance geometry that canarise when either own aircraft or the threatmaneuvers contrary to their RA in acoordinated encounter, or when anunequipped threat moves so as to thwartown aircraft's RA.

Multiple-Threat Resolution Advisories

TCAS is designed to handle m ulti-threatencounters, i.e., those encounters in whichmore than one threat is active at the same

time. TCAS will attempt to resolve thesetypes of encounters by selecting a single or composite RA that will provide adequateseparation from each of the intruders. It is

possible that the RA selected in suchencounters may not provide ALIMseparation from all intruders. An initialmulti-threat RA can be any of the initialRAs shown in Table 3, or a combination of upward and downward sense negative RAs,e.g., Do Not Climb and Do Not Descend.Version 7.0 provided new capabilities to themulti-threat logic to allow this logic toutilize Increase Rate RAs and RA Reversalsto better resolve encounters.

Weakening Advisories

During an RA, if the CAS logic determinesthat the response to a Positive RA has

provided ALIM feet of vertical separation prior to CPA (i.e. the aircraft have becomesafely separated in altitude while not yetsafely separated in range) before CPA, theinitial RA will be weakened to either a Do

Not Descend RA (after an initial Climb RA)or a Do Not Climb RA (after an initialDescend RA). This is done to minimize thedisplacement from the TCAS aircraftsoriginal altitude.

In Version 7.0 and later, after ALIM feet of separation has been achieved, the resultingDo Not Descend or Do Not Climb RA isdesignated as corrective. In Version 7.0,the RA is annunciated as Adjust VerticalSpeed, Adjust. In Version 7.1, the RA isannunciated as Level Off, Level Off.(Version 6.04a keeps the original preventivedesignation, meaning that the RA isannunciated as Monitor Vertical Speed.)

In Version 7.0 and later, negative RAs willnot be weakened and the initial RA will beretained until CPA unless it is necessary tostrengthen the RA or reverse the RA sense.

After CPA is passed and the range betweenthe TCAS aircraft and threat aircraft beginsto increase, or if the horizontal miss distancefilter is able to determine prior to CPA that

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there will be sufficient horizontal missdistance, all RAs are cancelled.

TCAS/TCAS Coordination

In a TCAS/TCAS encounter, each aircrafttransmits interrogations to the other via theMode S link to ensure the selection of complementary RAs by the two aircraft.The coordination interrogations use thesame 1030/1090 MHz channels used for surveillance interrogations and replies and are 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 other TCAS-equipped intruder. The informationin the coordination interrogation isexpressed in the form of a complement. For example, when an aircraft selects an upward sense RA, it will transmit a coordinationinterrogation to the other aircraft thatrestricts that aircrafts RA selection to thosein the downward sense. The strength of thedownward sense RA would be determined

by the threat aircraft based on the encounter geometry and the RA Selection logic.

The basic rule for sense selection in aTCAS/TCAS encounter is that each TCASmust check to see if it has received an intentmessage from the other aircraft beforeselecting an RA sense. If an intent messagehas been received, TCAS selects theopposite sense from that selected by theother aircraft and communicated via thecoordination interrogation. (An exception tothis occurs in Version 7.0 and later if theintruder has selected an altitude crossingsense and own TCAS satisfies a set of conditions that allows it to reverse that sense

selection.) If TCAS has not received anintent message, the sense is selected based on the encounter geometry in the samemanner as would be done if the intruder were not TCAS equipped.

In a majority of the TCAS/TCASencounters, the two aircraft will declare theother aircraft to be a threat at slightly

different times. In these events,coordination proceeds in a straight-forward manner with the first aircraft declaring theother to be a threat, selecting its RA sense

based on the encounter geometry, and transmitting its intent to the other aircraft.At a later time, the second aircraft willdeclare the other aircraft to be a threat, and having already received an intent from thefirst aircraft, will select a complementaryRA sense. The complementary sense that isselected will then be transmitted to the other aircraft in a coordination interrogation.

Occasionally, the two aircraft declare eachother as threats simultaneously, and therefore both aircraft will select their RAsense based on the encounter geometry. Inthese encounters, there is a chance that bothaircraft will select the same sense. Whenthis happens, the aircraft with the higher Mode S address will detect the selection of the same sense and will reverse its sense.

Version 7.0 added the capability for TCASto issue RA reversals in coordinated encounters if the encounter geometry changeafter the initial RA is issued. The RAreversals in coordinated encounters areannunciated to the pilot in the same way as

RA reversals against non-TCAS intruders.In a coordinated encounter, if the aircraftwith the low Mode S address has Version7.0 or later installed, it can reverse the senseof its initial RA and communicate this to thehigh Mode S address aircraft. The highMode S address aircraft will then reverse itsdisplayed RA. The aircraft with the highMode S address can be equipped with anyversion of TCAS.

In a coordinated encounter, only one RA

reversal based on changes in the encounter geometry can be issued.

Air/Ground Communications

Using the Mode S data link, TCAS candownlink RA reports to Mode S ground sensors. This information is provided in theMode S transponders 1090 MHz response

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to an interrogation from a Mode S ground sensor requesting TCAS information.

For Version 7.0 and later, RA informationis also provided automatically using theTCAS 1030 MHz transmitter. This RABroadcast Message is an uplink format"message, but it is intended for 1030 MHzreceivers on the ground. This broadcast is

provided when an RA is initially displayed to the flight crew and when the RA isupdated, and is rebroadcast every eight (8)seconds in all directions.

For Version 7.0 and later, for 18 secondsafter the end of the RA, both the RA Reportand the RA Broadcast Message include anRA Terminator bit (RAT), indicating thatthe RA is no longer being displayed to the

pilot.

Traffic Advisory (TA) Display

The functions of the traffic advisory displayare to aid the flight crew in visuallyacquiring intruder aircraft; discriminating

between intruder aircraft and other nearbyaircraft; determining the horizontal positionof nearby aircraft; and providing confidencein the performance of TCAS.

Traffic advisory displays have beenimplemented in a number of different waysand with varying levels of flexibility. Therequirements for the various means of implementing the traffic displays aredocumented in RTCA DO-185B. Anovervie

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