traffic alert and collision avoindance system

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TRAFFIC ALERT/COLLISION AVOIDANCE SYSTEM

INTRODUCTION

In 1956, the Civil Aeronautics Administration Technical Development Center reported that results oftests that had been conducted over the last four years indicate that only general use of proximity warning

devices would substantially reduce the steadily increasing threat of mid-air collisions.

1 The general interest in such devices was initially spawned by the ever-increasing growth in air traffic.

However, the catalyst for more in-depth research was an accident that occurred as a result of a collisionbetween two airliners over the Grand Canyon on June 30, 1956. In 1978 a light aircraft collided with anairliner over San Diego. By this time US pilots began to warm up to the idea of a collision avoidancesystem. Ultimately, the final impetus that led to congressional legislation mandating TrafficAlert/Collision Avoidance System (TCAS) was the August 31, 1986 midair collision involving andAeromexico DC-9 and a private airplane in U.S. airspace over Cerritos, California near Los AngelesInternational Airport.

2 Throughout this period, many versions of midair collision avoidance devices were proposed.

This discussion will explore the evolution of TCAS as we know it today. The specific characteristics anddifferences between the systems will also be examined as well as pros and cons. Additionally, the futureof TCAS will be discussed.

EARLY AIRBORNE COLLISION AVOIDANCE SYSTEMS

As mentioned previously, TCAS or other similar devices have been in various stages of research anddevelopment since the early to mid 50s. Research findings during this time identified that the greatestdanger of a collision lies in one aircraft overtaking another. The research also found that a warning to apilot that potential collision danger exists is not sufficient information for prevention of a collision, andthat relative bearing of an existing collision threat must be known to the pilot to give him enough time tosee the other aircraft and execute an avoidance maneuver. Finally, it was discovered that most collisions

occurred in terminal areas. The critical element in approaching a solution to the midair collision problemwas that of time-distance because of the potential rapid closure rates of jet aircraft converging nose tonose. Tests showed that when pilots initiate a sudden climb in a jet aircraft traveling at 400 knots, theaircraft would travel approximately one mile before the it would respond and start to climb.

3 Early warning was critical to reducing midair collisions.

With these findings in mind, scientist began to explore the possibilities of developing a new piece ofequipment and installing it in aircraft to protect against midair collisions. The research became known asAirborne Collision Avoidance System research or ACAS. One of the earliest collision avoidance systemsthat was proposed, developed in the 1950s, was a three range device for high-speed jet aircraft. This wasan adjustable device that would lessen the false alerts in congested areas. The shortest range was used in acongested terminal environment. The medium range was used for lower altitude flights with the longrange being used while cruising in the flight levels. The adjustments in the system were made bychanging the maximum range and altitude before a conflict alert signal was received. The earliest systemswere based on equipment that attempted to calculate miss-distance, or the distance at which point thesystem would recommend an evasive maneuver. Obviously, if the miss-distance was at a minimum, anevasive maneuver was suggested. For the system to operate accurately, it required that relative bearingangle and closure rate between aircraft be calculated. This presented problems in turbulent conditions andthe size of the equipment required was considered excessive. It never found a market.

4 Another type system, developed by Bendix Radio in the late 50s and early 60s, took a differentapproach and used time to determine how long before participating aircraft obtained their separationminimum with there still being enough time to escape. This system was deemed more efficient because itdid not try to predict miss-distance, therefore the problem of accurate bearing measurement whichplagued the previous model was not a factor. Before reaching minimum separation and in enough time toevade the intruder, an alarm would sound and tell the pilot to climb or dive. The vertical component of thesystem operated with a small UHF transmitter which periodically transmits a series of pulses. The pulses


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were spaced at different intervals based on the aircrafts altitude. The receiver in the system interpretedthe altitude of any similarly equipped aircraft in the vicinity. Further analysis was required if an aircraftwas detected at or near the same altitude.

5 During the development of the Bendix system, Dr. John Smiley Morrell discovered and first used theconcept of Tau. Tau is based on time, not distance. Mathematically, Tau is expressed as the range to the

intruder divided by its closure rate or range-rate.

6 The Bendix system only needed to determine the range of the aircraft at or near the same altitude andthe rate at which the range changed. Engineers did this by devising a system called the ground-bounceranging system. A transmitter sent a split signal one that traveled directly to the receiver and another thatbounced off of the ground, then to the aircraft. The time delay between the direct signal and the ground-reflected signal was calculated to determine how far apart the aircraft were. If the delay was short, theaircraft were separated considerably, and if the delay was long, the aircraft were within close proximity ofeach other. If the ratio of range to range-rate reached twice the minimum escape time for the type aircrafton which the system was installed, the alarm sounded and issue an instruction to climb or descenddepending on whether the intruder was higher or lower.

7 Numerous other systems were considered for development. Eliminate Range-zero System (EROS) was

developed for fast moving, fighter-type aircraft. EROS used time-frequency techniques. Each aircraftcarried a very accurate and expensive cesium-rubidium clock that was synchronized to a master clock. Apulse train of information (including the host airplanes altitude) would be transmitted at a time preciselyallocated for that airplane. Based on the time differential measured when another airplane received thesignal, EROS could determine the range and closing speed of the approaching airplane. The problem withthis system, like all of the others that preceded it, was that it only protected against aircraft with the sameequipment on board. Since the system was so costly, it was considered impractical and was never used.

8 Since the mid 70s, efforts have concentrated on the use of hardware already installed on most aircraft,namely the transponder of the Air Traffic Control Radar Beacon System (ATCRBS). Basically, aircraftwould be equipped with airborne interrogators that would be able to interpret data from the transpondersof nearby aircraft. These systems became known as the Beacon Collision Avoidance System or BCAS. Inthe late 70s, George Litchford, a New York electronics engineer, came up with a theory that a passive

anti-collision system could eavesdrop on ground interrogators and locate and track nearby aircraft. It wasgiven the name passive BCAS. This technique is based on listening for transponder replies from othernearby aircraft to two or more ground interrogators. By timing the receipt of these ground interrogationsand replies from other aircraft, and using the known positions of the ground interrogators, a passivesystem calculated the relative positions and altitudes of other aircraft.

9 Passive BCAS never went into full production because it was considered too complex and would notwork over the ocean or where there was limited radar coverage. However, with the electronic andnavigational capabilities that exist today, there is hope for a passive TCAS system. This is because insome instances, aircraft know exactly where they are if navigating with INS, Loran-C, or GPS. In thiscase a passive system would only need to receive a signal from one ground interrogator.

TRAFFIC ALERT/COLLISION AVOIDANCE SYSTEM

Building on this and other work, the FAA launched the TCAS program in 1981. TCAS is a relativelysimple system to understand. Basically, the system identifies the location and tracks the progress of


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aircraft equipped with beacon transponders. Currently, there are three versions of the TCAS system in useor in some stage of development; TCAS I, II, and III. TCAS I, the simplest of the systems, is lessexpensive but also less capable than the others. It was designed primarily for general aviation use. TheTCAS I transmitter sends signals and interrogates Mode-C transponders. The TCAS I receiver anddisplay indicates approximate bearing and relative altitude of all aircraft within the selected range, usuallyabout forty miles. Further, the system uses color coded dots to indicate which aircraft in the area pose a

potential threat. This is referred to as a Traffic Advisory (TA). When a pilot receives a TA, it is up tohim/her to visually identify the intruder and is allowed to deviate up to + 300 feet. Lateral deviation is notauthorized. In instrument conditions, the pilot is required to notify air traffic control for assistance inresolving the conflict.

10 TCAS II on the other hand is a more comprehensive system than TCAS I.

This system was required to be installed on all commercial air carriers operating in the United States byDecember 31, 1993. It offers all of the same benefits but it will also issue a Resolution Advisory (RA) tothe pilot. In other words, the intruder target is plotted and the system is able to tell whether the aircraft ifclimbing, diving, or in straight and level flight. Once this is determined, the system will advise the pilot toexecute an evasive maneuver that will deconflict the aircraft from the intruder. There are two types ofRAs, preventive and positive. Preventive RAs instruct the pilot not to change altitude or heading to avoid

a potential conflict. Positive RAs instruct the pilot to climb or descend at a predetermined rate of 2500feet per minute to avoid a conflict.

11 TCAS II is capable of interrogating Mode-C and Mode-S. In the case of both aircraft having Mode-Sinterrogation capability, the TCAS II systems communicate with one another and issue deconflicted RAs.

12 Since this system costs up to $200,000 per aircraft, manufacturers have built in an upgrade capabilityto the next generation TCAS III. This system will be virtually the same as TCAS II but will allow pilotswho receive RAs to execute lateral deviations to evade intruders. This will be possible because thedirectional antenna on TCAS III will be more accurate and will have a smaller bearing error. There arealso hopes that the new antenna will cut down on false alarms since it can more accurately determine anintruders location. Another upgrade that is proposed has to do with the Mode-S data link. Through thislink, a system will be capable of transmitting the aircrafts GPS position and velocity vector to other

TCAS-equipped aircraft thus providing much more accurate information.

13 A FEW PROBLEMS AND SOLUTIONS

Needless to say, there were a few problems that occurred in the development of TCAS. There was aproblem with the directional capabilities of the antenna used with the system. Signal clutter was also a bigproblem. Additionally, software upgrades had to be developed to lessen the number of false alarms. Thenlastly, but certainly not least, there were the problems of getting pilots and controllers used to the system.The antenna problem was a complex one. The typical spinning antennas that are located on airportsprovide directional information to controllers. This data is available because the antenna rotates 360degrees at such a rate that the locations of aircraft can be pinpointed every time the antenna makes arevolution. This philosophy is impractical for airborne interrogators though. So, engineers developed anantenna that contains a number of small antenna elements arranged in a circle around a center element.

Fed with the proper signal, they transmit an interrogating pulse simultaneously in all directions. But whenthe responses arrive, they strike at slightly different times. By comparing these patterns, of the returningsignals at each element, the computer can find the directions from whence the signals came.

14 Signal clutter was another problem that had to be overcome. During early work on TCAS, engineerswere worried that in crowded terminal areas with many transponders replying to multiple signals, thesystem would become overloaded with overlapping signals and clutter. This problem was overcome witha process called the whisper-shout and with a directional antenna. The whisper-shout method ofinterrogation allows the transmitter to send signals in two strengths. A low power signal (the whisper) istransmitted and only highly sensitive transponders, or transponders close by, can receive it and respond.Then the transmitter sends a stronger signal (the shout) which triggers responses from less sensitivetransponders or those that are further away. The operative element in this system is a mechanism thatprohibits the transponders that responded to the whisper from responding to the shout and vice-a-versa,

thus reducing the number of transponders responding at one time. A directional antenna was alsoincorporated into the system. This antenna, described in the previous paragraph has the ability to transmit


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in only one quadrant at a time thus reducing the number of signals being interrogated at any given time.These two components were key elements in the development of TCAS and prevent system overloadeven in the most crowded terminal areas.

15 There was not much for support for TCAS II when it was first introduced because of the large numberof false conflict alerts. These were particularly disturbing because many alerts occurred when aircraft

were on final approach, one of the busiest and most critical phases of flight. Version 6.00 was the originalsoftware for TCAS II. When using this software, some very interesting problems occurred. False conflictalerts were being triggered by transponders on ships and bridges. Additionally, parallel final approachcourses less than 5000 feet apart were causing false alerts. It has even been reported that a pilots ownaircraft can cause a false alarm. In this situation the pilot found himself trying to outmaneuver himself.All of these are software problems and have been addressed in the latest version, 6.04.

16 Through Mitre Corporations new logic-software version, Delta airlines, the first voluntary user,reported an 80 percent reduction in TCAS conflict alerts. Additionally, the number of Bump-up alertshave been reduced. Bump-up alerts occur when the TCAS of a descending aircraft calls for it to climbto avoid a fast-climbing aircraft below, not knowing that the aircraft will level off at a lower altitude. Thiswas a common occurrence at Dallas-Fort Worth airport because arriving and departing aircraft use thesame fixes.

17 Additionally, the buffer requirements or thresholds between participating aircraft were lowered, thusreducing the number of false conflict alerts.We are all resistant to change. It is just a fact of life. This was especially the case with TCAS. WhenTCAS was first introduced, it was viewed as a nuisance more than anything else. This was because theusers considered the system unreliable. Pilots viewed it as just another instrument they had to watch in analready busy cockpit. They, in some cases, became complacent and began to totally disregard TCASconflict alerts which defeats the whole purpose of the system. By reducing the number of unnecessaryTCAS alerts, the new software is expected to increase the confidence of flight crews in respondingregularly to TCAS alerts. Already, with the new software upgrade, pilots opinions are beginning to sway.They have begun to consider TCAS as a way for them to increase their situational awareness. It givesthem the big picture on a screen in the cockpit; something they had to develop mentally before.

18 Additionally, it has been reported that TCAS has been used to avoid wake turbulence by getting tooclose to heavy aircraft.

THE FUTURE OF TCAS

TCAS was developed to help reduce the potential for midair collisions. However, the time could somedaycome when the system actually helps to relieve congestion and expedite traffic as well. An example ofthis was tested on several occasions in 1993. The In-Trail Climb (ITC) is intended to reduce fuelconsumption and reduce separation criteria for transoceanic flights. This maneuver permits a trailingaircraft at a lower altitude to climb through the altitude of a preceding aircraft using TCAS II as aseparation maintenance aid. This is substantial because it allows aircraft to climb to more fuel efficient orless turbulent cruising altitudes earlier in their flights. During the first test last year, a United DC-10 wasable to save 2000 lbs of fuel.

19 Other prospects for TCAS include reduced separation on transoceanic routes, reduced spacing fordepartures in instrument conditions, and could permit aircraft to establish and maintain separationintervals on final during approaches.

20 Another intriguing prospect for the use of TCAS is that of being able transmit GPS coordinates andaltitude via Mode-S datalink. This information could be used to enhance the effectiveness and accuracy ofTCAS and could also be transmitted to air traffic control by means other than conventional radar. Thesystem would also be able to be incorporated rapidly and at a minimum cost because only a softwareupgrade would be required for those already using TCAS II. This again, would be especially useful fortransoceanic flights by relaying position information from aircraft to air traffic control centers.

21 REFERENCES

Ashley, Steven, TCAS: Can it Stop Midair Collisions?, Popular Science, August 1988, pp. 36-40, 80.


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Doty, L. L., CAA Details Results of Collision Tests, Aviation Week, November 5, 1956, p. 38.

Klass, Philip J., Airlines Initial Use of TCAS Suggests Need for Minor Changes, Aviation Week &Space Technology, April 8, 1991, pp. 36-37.

Klass, Philip J., Anti-Collision System Appears Promising, Aviation Week, February 15, 1960, pp. 67-75.

Klass, Philip J., Bendix, BFGoodrich, Trimble Vie for TCAS I Business, Aviation Week & SpaceTechnology, January 11, 1993, pp. 45-47.

Klass, Philip J., Extensive Airline Use of TCAS Pinpoints Desirable Software Changes, Aviation Week& Space Technology, January 27, 1992, pp. 48-51.

Klass, Philip J., NAVSATS Promise New ATC Horizons, Aviation Week & Space Technology,January 18, 1993, pp. 29-30.

Klass, Philip J., Novel ATC Technique to Undergo Tests, Aviation Week & Space Technology, August

16, 1993, pp. 38-39.

Klass, Philip J., New TCAS Software Cuts Conflict Alerts, Aviation Week & Space Technology,September 20, 1993, p. 44.

McClellan, J. Mac, Collision Vision, Flying, May 1989, p. 54-56.

Reingold, Lester, TCAS: Not-Quite-Perfect Solution, Air Transport World, January 1992, pp. 78-80.

Westlake, Michael, How to Avoid Air Collisions, Far Eastern Economic Review, December 20, 1990,p. 66.

FAA Redirects TCAS-III Effort, Aviation Week & Space Technology, September 27, 1993, p. 37.

United to Test TCAS Use for Altitude Changes, Aviation Week & Space Technology, November 22,1993, p. 63.

Questions about this article? E-Mail Captain Rob "Woody" Ricker

TRAFFIC ALERT AND COLLISION AVOIDANCE SYSTEM

The Traffic Alert and Collision Avoidance System, or TCAS, is an instrument integrated into othersystems in an aircraft cockpit. It consists of hardware and software that together provide a set ofelectronic eyes so the pilot can "see" the traffic situation in the vicinity of the aircraft. Part of the TCAScapability is a display showing the pilot the relative positions and velocities of aircraft up to 40 milesaway. The instrument sounds an alarm when it determines that another aircraft will pass too closely to thesubject aircraft. TCAS provides a backup to the air traffic control systems regular separation processes.

The MITRE Corporation conducted early research into collision avoidance technologies under thesponsorship of the Federal Aviation Administration (FAA). TCAS is a direct descendant of thoseinvented at MITRE and elsewhere. To learn more about TCAS, and the people who invented it, readfurther, or click on the following sections:

Background Historical Perspective A Collision Avoidance System is Born Taking to the Skies: The Congressional Mandate Evolving to Meet Safety Needs The Final Generation


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Background

Since the early 1960s, MITRE's Center for Advanced Aviation System Development (CAASD) hasprovided the FAA with Air Traffic Control (ATC) system engineering support. As part of thislongstanding partnership, CAASD helped the FAA implement a collision avoidance system for aircraft.The resulting Traffic Alert and Collision Avoidance System, or TCAS, has become a standard for safety

in the United States and abroad. Its value is clear: no airline mid-air collisions have occurred in the UnitedStates since 1990, when the airlines began equipping their planes with TCAS.

From its inception, TCAS has dramatically improved pilots' chances of successfully averting the threat ofa mid-air collision. Pilots have come to rely on TCAS to give them the crucial data to avoid collisions. Astheir last line of defense, TCAS gives pilots the edge needed to ensure that their crew and passengers havethe safest flight possible.

The project benefited from the cooperative efforts of the FAA, airlines, and several other companies.CAASD designed and developed the collision avoidance logic at the heart of the system. TheMassachusetts Institute of Technology's Lincoln Laboratory developed air-to-air surveillance. The FAATechnical Center and a team of contractors, including The Analytical Sciences Corporation, ColemanResearch Corporation, and Rannoch Corporation, were responsible for software verification and

validation. The FAA Technical Center and ARINC Research handled operational evaluations.

Historical Perspective

On June 30, 1956, two planes collided over the Grand Canyon. In the wake of this and other such airbornedisasters, the industry realized they needed a system that could help prevent similar incidents. Companiessoon began designing collision avoidance systems, but two problems hampered their efforts.First, adoption of the proposed systems would require the airlines to equip their fleets with expensive newhardware. Second, there was still a lot of development left to do before an adequate system would beready.

In 1974, MITRE proposed an alternative. Using the transponders already installed in many aircraft forcommunication with the FAA's ground-based Air Traffic Control Radar Beacon System (ATCRBS),

developers took advantage of existing technologies to significantly hasten the design and implementationprocess. The Beacon-Based Collision Avoidance System (BCAS) was the predecessor of today's TCAS.This system sent interrogation signals to nearby aircraft similar to the FAA's radar system. Thetransponders then sent back response signals. The system interpreted these signals to determine thelocation, speed, and course of each plane and used the data to avoid a potential collision.

BCAS test results were promising. On the ground, MITRE equipped a trailer to receive transpondersignals as if it were an aircraft. BCAS lived up to expectations, prompting the FAA Technical Center totest the system on one of its aircraft. On the basis of these two tests, the FAA moved forward with furtherdevelopment of BCAS.

A Collision Avoidance System Is Born

In 1981, the FAA chose to pursue the onboard design approach used in BCAS rather than a ground-basedcollision avoidance system which was also under consideration. At that point, BCAS was renamed TCAS.

There are two different versions of TCAS, for use on different classes of aircraft. The first, TCAS I,indicates the bearing and relative altitude of all aircraft within a selected range (generally 10 to 20 miles).With color-coded symbols, the display indicates which aircraft pose potential threats. This constitutes theTraffic Advisory (TA) portion of the system. When pilots receive a TA, they must visually identify theintruding aircraft and may alter their plane's altitude by up to 300 feet. TCAS I does not offer solutions,but does supply pilots with important data so that they can determine the best course of action. Anillustration of TCAS range and altitude criteria shows the horizontal and vertical distances to monitortraffic and issue advisories to maintain safe separation of aircraft.

In addition to a traffic display, the more comprehensive TCAS II also provides pilots with resolution

advisories (RAs) when needed. The system determines the course of each aircraft; climbing, descending,or flying straight and level. TCAS II then issues an RA advising the pilots to execute an evasive


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maneuver necessary to avoid the other aircraft, such as "Climb" or "Descend." If both planes are equippedwith TCAS II, then the two computers offer deconflicting RAs. In other words, the pilots do not receiveadvisories to make maneuvers that would effectively cancel each other out, resulting in a continued threat.

TCAS Diagram------------------------------------------------------------------------TCAS queries other aircraft, receives information, displays traffic,and reacts by warning pilots when there is a potential threat.------------------------------------------------------------------------

MITRE's key contribution to the development of TCAS was its work on the collision avoidance logic forTCAS II. The software uses the collected data on the flight patterns of other aircraft and determines ifthere is a potential collision threat. The system doesn't just show the other planes on a display like a radarscreen, but offers warnings and solutions in the form of traffic advisories (TAs) and resolution advisories

(RAs).

As CAASD's Dr. Andrew Zeitlin points out, "Because of the pilots' normal workload, we don't expectthem to spend all of their time looking at the screen. It's there when needed, but more important, it speaksup and advises them as they need to make a maneuver to avoid a collision."

Aside from the logic design, much of MITRE's work on TCAS involved creating and running computersimulations to test the system. "Because it's expensive to fly test encounters," says Dr. Zeitlin, "we havedeveloped some very powerful tools where we can generate millions of encounters on the computer andevaluate the logic exhaustively. We can also play back radar data from ordinary traffic and get a feel forhow the system works and how much disruption you get day to day or at different locations with ordinarytraffic." On occasion, MITRE has also assisted the FAA and other organizations in evaluating specialencounters. "For example, if somebody has a near-miss and they want to know what TCAS's role was or

what would TCAS have done in the encounter, we can simulate the encounter and give advice," saysZeitlin.

Taking to the Skies: The Congressional Mandate

On August 31, 1986, while TCAS was still in development, a collision occurred over Cerritos, California,involving an Aeromexico DC-9 and a small Piper aircraft carrying a family of three. The DC-9 wasdescending toward Los Angeles International Airport in clear skies, flying at 6,500 feet. The Piper hit theDC-9's tail, causing both aircraft to plummet from the sky.

The accident resulted in the deaths of all 67 people aboard the two planes, as well as 15 people on theground. In the aftermath of this accident, Congress passed a law requiring the FAA to mandate the use ofTCAS. By 1993, all carrier aircraft operating within U.S. airspace with more than 30 passenger seats were

equipped with TCAS II. Aircraft with 10 to 30 seats were required to employ TCAS I.


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Evolving to Meet Safety Needs

Although the airlines were using the more advanced version 6.01 of the TCAS logic, some improvementsstill needed to be made. The system was issuing RAs in some situations, such as final approach, whentraffic may be closer but is safely under control. Many pilots saw these RAs as a nuisance. The systemwas basically too sensitive, with unnecessary TAs and RAs even being triggered by transponders on

bridges and ships.

According to Dr. Zeitlin, "There was a growing tendency among pilots to ignore the advisory, even whenthey didn't necessarily have full knowledge of the situation. Everyone was concerned that one day theywould ignore one that was necessary."

In 1992, CAASD developed logic version 6.04 to alleviate these problems. Delta Airlines, the first carrierto voluntarily use the new logic, reported an 80 percent reduction in RAs. The following year, CAASDdeveloped an additional improvement to the logic, version 6.04A. Airlines began equipping their fleetswith this version in 1994.

The Final Generation

In 1997, CAASD finished work on one final major change to the TCAS logic, version 7. It was approvedby the RTCA standards committee and the FAA, and is the version that will be installed on all newaircraft. It has also been adopted by the International Civil Aviation Organization (ICAO) as theinternational standard. Initially, version 7 will be installed on aircraft serving European and some othercountries. American carriers who fly to these countries will have to upgrade from 6.04A to 7 on theirinternational planes, and can voluntarily upgrade the equipment already on their U.S. fleets.

According to program manager David Lubkowski, who led CAASD's TCAS software developmentgroup, the version 7 logic should yield at least a 20 percent reduction in RAs over the previous version."We also ran simulations using radar data from Europe, where they encounter more high-altitude en routeconflicts," he said. "The new 7.0 software resulted in a 40 percent reduction in unnecessary RAs." Thenew logic also significantly improves TCAS performance in several other important areas.

CAASD personnel have conducted safety studies to evaluate the performance of each successive versionof the TCAS logic. In a 1997 report on version 7, CAASD's Dr. Michael McLaughlin examined thereduced risk of collision in aircraft equipped with TCAS II versus the risk in aircraft without TCAS.Based on the likelihood of incursions into a protected zone around aircraft with a radius of 500 feet and aheight of 200 feet -- defined as Critical Near Mid-Air Collisions (NMACs) -- McLaughlin concluded that"TCAS should reduce NMAC probability by at least 90 to 98 percent," depending on whether one or bothaircraft in an encounter are equipped with TCAS.

Though NMACs, especially those involving commercial, passenger aircraft are already extremely rare,McLaughlin notes that "TCAS is intended to reduce their probability even further."

Although the FAA has said that version 7 will be the final logic for TCAS, CAASD continues to work onmany different air traffic control projects, and will undoubtedly play a role in the development of anyfuture collision avoidance systems.


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II-Morrow Develops an Alternative to TCAS Anti-Collision System

FAA may be hemming and hawing over how to implement the long overdue Automatic DependentSurveillance (Broadcast) ADS-B system that will be a cornerstone of its Free Flight program, butPortland-based II-Morrow, along with the Cargo Airlines Association, is making this technology a reality.

ADS-B is a surveillance system that is dependent upon each aircraft's automatically broadcasting itsidentification, position and altitude, hence the name ADS-B. Each aircraft also transmits its track andspeed, thereby allowing ADS-B equipment at an ATC station to display the flight path trend vector ofeach aircraft fitted with similar equipment.

II-Morrow and parent company UPS have demonstrated a proof-of-concept ADS-B system that will beinstalled on certain UPS air freighters starting this fall. II-Morrow's ADS-B development program isbeing sponsored by the Cargo Airlines Association as an alternative to TCAS, that is required to beinstalled on passenger carrying airliners, but which has not yet been mandated for cargo aircraft.

At first, ADS-B seems as though it's a technology that competes with TCAS. Actually, ADS-Bcomplements TCAS. While TCAS displays the location and relative altitude of intruders as symbols and,in the case of TCAS II, provides traffic conflict resolution advisories, ADS-B provides far more

information and it functions at a much longer range.

II-Morrow's ADS-B uses an Avionics Display Systems flat-panel MFD, a modified Honeywell AirTransport Mode S data link transponder and II-Morrow's specially designed VHF nav band and UHFTACAN datalink.

traffic alert and collision avoindance system

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