Portable Collision Avoidance System

PCAS, which stands for Portable Collision Avoidance System, is an aircraft collision avoidance system similar in function to TCAS (Traffic Collision Avoidance System). TCAS is the industry standard for commercial collision avoidance systems but PCAS is gaining recognition as an effective means of collision avoidance for general aviation and is in use the world over by independent pilots in personally owned or rented light aircraft as well as by flight schools and flying clubs.

PCAS is a trademark of Zaon Flight Systems, Inc. The original PCAS technology was developed in 1999 and today the fourth generation of PCAS technology is in use. Through this technology, transponder-equipped aircraft are detected and ranged, and the altitude is decoded. PCAS G4 technology has advanced to the point that highly accurate range, relative altitude, and quadrant direction can be accurately detected in a portable cockpit device.

"The information and diagrams below have been used by permission"

How does PCAS basically work?

ATC ground stations and active TCAS systems transmit interrogation pulses on an uplink frequency of 1030 Megahertz. Aircraft transponders reply on a downlink frequency of 1090 Megahertz. PCAS devices detect these transponder responses, then analyze and display conflict information.

How does PCAS differ from TCAS or TIS?

PCAS is passive, portable, and much less expensive than traditional aircraft detection systems, such as TCAS, L3 Skywatch and Ryan TCAD. TCAS is an active system transmitting interrogation pulses and monitoring responses. TIS requires the use of ground based mode S sensor sites which will then transmit traffic information to mode S client systems. Several US sensor sites are scheduled for closure and the TIS service is generally unavailable in Canada. TCAS operates with more precision than PCAS but is also significantly more expensiveFact|date=May 2007.

How PCAS works in detail

;Step 1To start the cycle, an interrogation is sent out from ground-based RADAR stations and/or TCAS or other actively interrogating systems in your area. This signal is sent on 1030 MHz. For TCAS, this interrogation range can have a radius of 40 miles from the interrogation source. The Ground RADAR range can be 200 miles or more.

;Step 2The transponder on any aircraft within range of the interrogation replies on 1090 MHz with their squawk code (known as Mode A) and altitude code (or Mode C). The altitude information is sent in an encoded format.

Mode S transponders also reply on this frequency, and encoded within the Mode S transmission is the Mode A (squawk) and Mode C (altitude) information.

Military aircraft also respond on this frequency but use a different transmission protocol (see Step 3).

Your aircraft’s transponder should also reply. However, the XRX unit watches for this signal and will not report it as a threat aircraft. The unit may use this information to establish base altitude for use in step 4.

;Step 3Any aircraft reply within the XRX detection window (maximum 6 miles) will be received. The range is computed based on the amplitude of the received signal, the altitude code is decoded, and the signal angle-of-arrival is determined. XRX will recognize interrogations from TCAS, Skywatch, and any other “active” system, military protocols, and Mode S transmissions.

;Step 4The altitude of the aircraft (in the example, 2500 ft.) is compared to your local altitude (e.g. 1500 ft.) and the relative altitude is calculated (e.g., 1000 ft. above you). With relative direction, altitude and range determined, XRX displays this information and stores it in memory.

;Step 5 If additional aircraft are within detection range, the above process is repeated for each aircraft. The top threat is displayed on the left of the traffic screen and the second and third threats are displayed on the right.

The greatest threat is determined by looking at aircraft within the detection window you set up and comparing primarily the vertical separation (+/- relative altitude), and secondarily the range to the aircraft currently being displayed. XRX uses algorithms to determine which of two or more aircraft is a greater threat.

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