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In 1956, the Civil Aeronautics Administration Technical Development Center reported
that results of tests 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
collision between two airliners over the Grand Canyon on June 30, 1956. In 1978 a light
aircraft collided with an airliner over San Diego. By this time US pilots began to warm up
to the idea of a collision avoidance system. Ultimately, the final impetus that led to
congressional legislation mandating Traffic Alert/Collision Avoidance System (TCAS) was
the August 31, 1986 midair collision involving and Aeromexico DC-9 and a private airplane
in U.S. airspace over Cerritos, California near Los Angeles International 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 and differences between the systems will also be examined as well as pros
and cons. Additionally, the future of TCAS will be discussed.
EARLY AIRBORNE COLLISION AVOIDANCE SYSTEMS
As mentioned previously, TCAS or other similar devices have been in various stages of research and development since the early to mid 50s. Research findings during this time identified that the greatest danger of a collision lies in one aircraft overtaking another. The research also found that a warning to a pilot that potential collision danger exists is not sufficient information for prevention of a collision, and that relative bearing of an existing collision threat must be known to the pilot to give him enough time to see 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 problem was that of time-distance because of the potential rapid closure rates of jet aircraft converging nose to nose. Tests showed that when pilots initiate a sudden climb in a jet aircraft traveling at 400 knots, the aircraft 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 of equipment and installing it in aircraft to protect against midair collisions. The research became known as Airborne Collision Avoidance System research or ACAS. One of the earliest collision avoidance systems that was proposed, developed in the 1950s, was a three range device for high-speed jet aircraft. This was an adjustable device that would lessen the false alerts in congested areas. The shortest range was used in a congested terminal environment. The medium range was used for lower altitude flights with the long range being used while cruising in the flight levels. The adjustments in the system were made by changing the maximum range and altitude before a conflict alert signal was received. The earliest systems were based on equipment that attempted to calculate miss-distance, or the distance at which point the system would recommend an evasive maneuver. Obviously, if the miss-distance was at a minimum, an evasive maneuver was suggested. For the system to operate accurately, it required that relative bearing angle and closure rate between aircraft be calculated. This presented problems in turbulent conditions and the 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 different approach and used time to determine how long before participating aircraft obtained their separation minimum with there still being enough time to escape. This system was deemed more efficient because it did not try to predict miss-distance, therefore the problem of accurate bearing measurement which plagued the previous model was not a factor. Before reaching minimum separation and in enough time to evade the intruder, an alarm would sound and tell the pilot to climb or dive. The vertical component of the system operated with a small UHF transmitter which periodically transmits a series of pulses. The pulses were spaced at different intervals based on the aircrafts altitude. The receiver in the system interpreted the altitude of any similarly equipped aircraft in the vicinity. Further analysis was required if an aircraft was detected at or near the same altitude.5 During the development of the Bendix system, Dr. John Smiley Morrell discovered and first used the concept 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 and the rate at which the range changed. Engineers did this by devising a system called the ground-bounce ranging system. A transmitter sent a split signal one that traveled directly to the receiver and another that bounced 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, the aircraft were separated considerably, and if the delay was long, the aircraft were within close proximity of each other. If the ratio of range to range-rate reached twice the minimum escape time for the type aircraft on which the system was installed, the alarm sounded and issue an instruction to climb or descend depending 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 aircraft carried a very accurate and expensive cesium-rubidium clock that was synchronized to a master clock. A pulse train of information (including the host airplanes altitude) would be transmitted at a time precisely allocated for that airplane. Based on the time differential measured when another airplane received the signal, EROS could determine the range and closing speed of the approaching airplane. The problem with this system, like all of the others that preceded it, was that it only protected against aircraft with the same equipment 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, aircraft would be equipped with airborne interrogators that would be able to interpret data from the transponders of nearby aircraft. These systems became known as the Beacon Collision Avoidance System or BCAS. In the 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 was given the name passive BCAS. This technique is based on listening for transponder replies from other nearby aircraft to two or more ground interrogators. By timing the receipt of these ground interrogations and replies from other aircraft, and using the known positions of the ground interrogators, a passive system 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 not work over the ocean or where there was limited radar coverage. However, with the electronic and navigational capabilities that exist today, there is hope for a passive TCAS system. This is because in some instances, aircraft know exactly where they are if navigating with INS, Loran-C, or GPS. In this case 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 relatively simple system to understand. Basically, the system identifies the location and tracks the progress of aircraft equipped with beacon transponders. Currently, there are three versions of the TCAS system in use or in some stage of development; TCAS I, II, and III. TCAS I, the simplest of the systems, is less expensive but also less capable than the others. It was designed primarily for general aviation use. The TCAS I transmitter sends signals and interrogates Mode-C transponders. The TCAS I receiver and display indicates approximate bearing and relative altitude of all aircraft within the selected range, usually about 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 to him/her to visually identify the intruder and is allowed to deviate up to + 300 feet. Lateral deviation is not authorized. In instrument conditions, the pilot is required to notify air traffic control for assistance in resolving 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 by December 31, 1993. It offers all of the same benefits but it will also issue a Resolution Advisory (RA) to the pilot. In other words, the intruder target is plotted and the system is able to tell whether the aircraft if climbing, diving, or in straight and level flight. Once this is determined, the system will advise the pilot to execute an evasive maneuver that will deconflict the aircraft from the intruder. There are two types of RAs, 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 2500 feet 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-S interrogation 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 capability to the next generation TCAS III. This system will be virtually the same as TCAS II but will allow pilots who receive RAs to execute lateral deviations to evade intruders. This will be possible because the directional antenna on TCAS III will be more accurate and will have a smaller bearing error. There are also hopes that the new antenna will cut down on false alarms since it can more accurately determine an intruders location. Another upgrade that is proposed has to do with the Mode-S data link. Through this link, 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 a problem with the directional capabilities of the antenna used with the system. Signal clutter was also a big problem. Additionally, software upgrades had to be developed to lessen the number of false alarms. Then lastly, 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 airports provide directional information to controllers. This data is available because the antenna rotates 360 degrees at such a rate that the locations of aircraft can be pinpointed every time the antenna makes a revolution. This philosophy is impractical for airborne interrogators though. So, engineers developed an antenna 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 when the responses arrive, they strike at slightly different times. By comparing these patterns, of the returning signals 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, engineers were worried that in crowded terminal areas with many transponders replying to multiple signals, the system would become overloaded with overlapping signals and clutter. This problem was overcome with a process called the whisper-shout and with a directional antenna. The whisper-shout method of interrogation allows the transmitter to send signals in two strengths. A low power signal (the whisper) is transmitted 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 sensitive transponders or those that are further away. The operative element in this system is a mechanism that prohibits 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 also incorporated into the system. This antenna, described in the previous paragraph has the ability to transmit 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 overload even 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 number of 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 original software for TCAS II. When using this software, some very interesting problems occurred. False conflict alerts were being triggered by transponders on ships and bridges. Additionally, parallel final approach courses less than 5000 feet apart were causing false alerts. It has even been reported that a pilots own aircraft 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 alerts have been reduced. Bump-up alerts occur when the TCAS of a descending aircraft calls for it to climb to avoid a fast-climbing aircraft below, not knowing that the aircraft will level off at a lower altitude. This was a common occurrence at Dallas-Fort Worth airport because arriving and departing aircraft use the same fixes.17 Additionally, the buffer requirements or thresholds between participating aircraft were lowered, thus reducing 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. When TCAS was first introduced, it was viewed as a nuisance more than anything else. This was because the users considered the system unreliable. Pilots viewed it as just another instrument they had to watch in an already busy cockpit. They, in some cases, became complacent and began to totally disregard TCAS conflict alerts which defeats the whole purpose of the system. By reducing the number of unnecessary TCAS alerts, the new software is expected to increase the confidence of flight crews in responding regularly 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 gives them 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 too close to heavy aircraft.
THE FUTURE OF TCAS
TCAS was developed to help reduce the potential for midair collisions. However, the time could someday come when the system actually helps to relieve congestion and expedite traffic as well. An example of this was tested on several occasions in 1993. The In-Trail Climb (ITC) is intended to reduce fuel consumption and reduce separation criteria for transoceanic flights. This maneuver permits a trailing aircraft at a lower altitude to climb through the altitude of a preceding aircraft using TCAS II as a separation maintenance aid. This is substantial because it allows aircraft to climb to more fuel efficient or less turbulent cruising altitudes earlier in their flights. During the first test last year, a United DC-10 was able to save 2000 lbs of fuel.19 Other prospects for TCAS include reduced separation on transoceanic routes, reduced spacing for departures in instrument conditions, and could permit aircraft to establish and maintain separation intervals on final during approaches.20
Another intriguing prospect for the use of TCAS is that of being able transmit GPS coordinates and altitude via Mode-S datalink. This information could be used to enhance the effectiveness and accuracy of TCAS and could also be transmitted to air traffic control by means other than conventional radar. The system would also be able to be incorporated rapidly and at a minimum cost because only a software upgrade would be required for those already using TCAS II. This again, would be especially useful for transoceanic flights by relaying position information from aircraft to air traffic control centers.21
Ashley, Steven, TCAS: Can it Stop Midair Collisions?, Popular Science, August 1988, pp. 36-40, 80.
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 & Space Technology, 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.
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Updated: September 09, 2006