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Coaxial switch technology for collision avoidance systems.

Coaxial Switch Technology for Collision Avoidance Systems


The traffic alert and collision avoidance system (TCAS II) is an independent airborne system that joins the ground-based air traffic control (ATC) system and the pilot's see-and-avoid concept as cockpit aids to warn of possible collisions. The FAA has been directed by Congress to require all commercial aircraft with more than 30 seats that fly over the US to have TCAS II units installed by 1992. There are three versions of TCAS available, including TCAS II, which warns the pilots when they are on possible collision courses with other aircraft and tells them to climb or dive or warns them not to climb or dive; TCAS III, which tells pilots to turn right or left and up or down; and TCAS I, which is a simple, low cost version that provides a proximity warning indicator for general aviation aircraft. TCAS II will be retrofitted on an estimated 3500 to 4000 commercial aircraft that have more than 30 passenger seats. At an estimated cost of $200,000 to $300,000 per aircraft, TCAS will cost the airlines close to $1 B. Anything that can reduce these costs and improve the systems' reliability will speed the implementation of TCAS and make flying safer. By using microwave coaxial switches, airlines can reduce the number of antennas and cables required in implementing TCAS, thereby reducing costs and improving the systems' reliability.

TCAS Systems

Two air traffic control radar beacon system (ATCRBS or mode C) transponders are standard equipment for every airliner. Many other airplanes carry these transponders as well since they are required for flying within the controlled air space near most major airports. The transponder sends out a signal giving the airplane's identification number and altitude every time it is swept by the ATC ground radars. The return signal tells the radar the airplane's range and bearing. Over 20 years ago, work was begun on a passive anti-collision system that could monitor these ground-radar interrogations, and locate and track nearby aircraft. This system was not successful because it did not function in areas without radar coverage. In 1981, the FAA launched the current TCAS program. Within the last six years companies such as Bendix/King Air Transport Avionics Division, Sperry Dalmo Victor and Collins Air Transport Division of Rockwell International Corp. have all developed TCAS II systems.

In operation the TCAS system performs very much like the ATC radars. It scans the surrounding air space once every second with a broadcasted signal. This scan triggers a response from any ATC transponders within 15 nautical miles to the front and eight nautical miles to the rear of the aircraft. The latest generation of transponders (mode S) can be addressed individually and exchange data from airplane-to-airplane. Using an electronically steerable directional antenna, TCAS determines the direction of nearby aircraft. The return signal provides altitude. The time for interrogation, plus response, yields range information. Since TCAS is interrogating once every second, the on-board computer can calculate flight paths of approaching airplanes and compare it to its own flight path to see if there is any danger of collision. These data are displayed in graphic format for the pilot. In case of possible collision, TCAS alerts and directs the pilot with special warning lights and a synthesized voice message. TCAS does this for any and all transponder equipped airplanes in the area.

The TCAS directional antenna consists of a number of small antenna elements arranged in a circle around a center element. The antenna transmits an interrogating pulse in either one direction or omnidirectionally. The computer finds the direction of the return signals by comparing the phase patterns at each element of the antenna.

One of the early concerns of the TCAS engineers was signal clutter; in crowded areas with many transponder replies, TCAS would be overloaded and the response garbled. The FAA, through contracts with the MITRE Corp. and the Lincoln Laboratory, has come up with a solution. The answer is a concept called whisper-shout. Whisper-shout is a thresholding technique in which nearby aircraft are queried in groups by stronger and stronger levels of interrogation pulses. The transmitter sends out a low level signal that only nearby aircraft respond to. A higher level signal then is transmitted that aircraft farther away answer. Included with this signal is a coded pulse that suppresses answers from those transponders that have previously replied. Another solution to clutter is the use of the directional antenna to query only one quadrant at a time. The use of these two methods has overcome signal clutter. The use of whisper-shout required the development of the mode S transponder. This advanced transponder includes a built-in address unique to each airplane. This address allows a ground ATC radar or a TCAS to interrogate an area and receive replies from only one aircraft at a time. This feature is important when two airplanes are on a collision course and instructions are being transmitted back and forth as to which will climb and which will dive.

To determine which approaching aircraft to warn the pilot about, the computer software calculates the number of seconds to closest approach. This concept, called [Tau], can be expressed as the range to intruder divided by his closure rate. Based on [Tau], plus the range, bearing and altitude data TCAS get from the intruder's transponder, the logic predicts whether the intruder will come into close proximity within 25 s or so. If it finds that the relative position at that later time will be too close, it can give two advisories, traffic alerts and resolution advisories. Traffic alerts provide bearing and distance information to the pilot when an aircraft is 35 to 45 s away and a resolution advisory tells the pilot how to maneuver to avoid collision.

TCAS Configurations

TCAS hardware consists of a computer, a transceiver, a directional antenna, an omnidirectional antenna, assorted cockpit displays (traffic alert and resolution advisories) and a mode S transponder with top and bottom antennas, as shown in Figure 1. Each aircraft is required to carry a back-up transponder that can be either mode S or mode C. If an aircraft has a mode C transponder as back-up, and its primary mode S transponder fails, then TCAS will provide warnings of impending collisions but will not be able to execute joint collision avoidance maneuvers. Microwave coaxial switches are used to allow the two transponders to share antennas. Airlines have the choice of using switches or extra antennas cabled directly to a transponder. Figure 2 shows two configurations commercial aircraft use for mode C transponders before TCAS is installed. Only a bottom antenna is required since transponder interrogations come only from ground-based radar. With TCAS installed, there are four configurations for transponders, as shown in Figure 3, that vary in the number of antennas, cables and switches used. The use of an SPDT switch reduces the number of antennas and cables required. The cost of cables and antennas, plus their installation, is substantially greater than the cost of a switch due to the extensive labor involved in routing cables and modifying air frames for an antenna. Also, the use of switches makes the system more reliable since the minimum operating life of a switch is over one million cycles, whereas an antenna is exposed to many inimical events, such as weather and physical forces. Another important consideration for airline companies is that future systems will be installed on aircraft and require antennas. With over a dozen antennas already installed on an aircraft (for radios, DME and Tacan), there is little room left for the additional antennas that will be required for systems such as global positioning systems (GPS) and Airlink (a Satcom link for voice and data).

Microwave Coaxial Switches

SPDT electro-mechanical coaxial switches are being installed on new Boeing aircraft, including the 757 and 747. Although it is not a requirement for TCAS, this switch model has internal terminations that provide a continuous 50 [Omega] load to the standby transponder. The use of three different RF connectors prevent mis-wiring during the installation of the switch.


The implementation of TCAS II will reduce the probability of a mid-air collision and improve the safety of commercial flight. The use of coaxial switches can reduce the cost of installation and improve both the reliability of TCAS and the flexibility of the airlines towards installing new kinds of systems in the future.

PHOTO : Fig. 1 A basic TCAS II system.

PHOTO : Fig. 2 The initial mode C transponder configuration.

PHOTO : Fig. 3 The final configurations of the switch and transponder.


[1]Steven Ashley, "TCAS: Can it Stop Mid-air Collision?", Popular Science, Vol. 233, Aug. 1988, p. 36(6).
COPYRIGHT 1991 Horizon House Publications, Inc.
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Author:Long, Michael
Publication:Microwave Journal
Date:Dec 1, 1991
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