A portable collision avoidance system (PCAS) is a collision avoidance system similar to the industry standard traffic collision avoidance system (TCAS), widely used in commercial aviation. PCAS, manufactured by Zaon, offers general aviation pilots, especially those flying under VFR, increased situational awareness by detecting nearby transponder-equipped aircraft, providing information on relative height, distance, and movement. Advanced versions integrate with EFIS to display aircraft on GPS maps, reducing workload in busy airspace. Developed in 1999 by pilot Zane Hovey, PCAS technology now offers precise range, altitude, and directional data. It gained broader recognition through its sponsorship of The Aviators (TV series). Its main competitor is FLARM.
Basic operation
ATC ground stations and active TCAS systems transmit interrogation pulses on an uplink frequency of 1,030 MHz. Aircraft transponders reply on a downlink frequency of 1,090 MHz. PCAS devices detect these transponder responses, then analyze and display conflict information.
Differences between PCAS and TCAS
PCAS is passive and less expensive than active aircraft detection systems, such as TCAS. TCAS operates with more precision than PCAS but is also more expensive and usually requires 'permanent' in-aircraft installation (requiring, in the United States, an FAA-approved mechanic to install). Class 2 TCAS gives mandatory instructions (called Resolution Advisories) whereas PCAS only alerts the pilot and may give a suggestion as to how to act.3 A very well known general aviation organization completed an evaluation of the PCAS XRX system to demonstrate the capabilities.4
Detailed operation
Step 1
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 1,030 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 2
The 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).
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).
A PCAS-containing aircraft's own 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 3
The PCAS unit computes range (maximum 6 miles) based on the amplitude of the received signal, the altitude code is decoded, and the signal angle-of-arrival is determined to a resolution of "quadrants" (ahead, behind, left, or right) using a directional antenna array.5 XRX will recognize interrogations from TCAS, Skywatch, and any other "active" system, military protocols, and Mode S transmissions.
Step 4
The altitude of the aircraft (in the example, 2,500 ft.) is compared to the altitude of the PCAS altitude (e.g., 1,500 ft.) and the relative altitude is calculated (e.g., 1,000 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 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.6
Models
- Zaon PCAS XRX (2016)7
References
"Portable Traffic System Maker Zaon Ceases Operation". 7 November 2013. https://www.avweb.com/ownership/portable-traffic-system-maker-zaon-ceases-operation/ ↩
US 8072374, Zane Hovey, "Automatic dependent surveillance systems and methods", published 2011-12-06, issued 2011-12-06, assigned to Zane Hovey https://patents.google.com/patent/US8072374B2/en?assignee=Zane+Hovey ↩
"Avidyne TAS600:Beats Skywatch in Value - Aviation Consumer Article". www.aviationconsumer.com. 16 January 2006. http://www.aviationconsumer.com/issues/36_2/accessories/5524-1.html ↩
"Zaon XRX Traffic: Affordable Azimuth - Aviation Consumer Article". www.aviationconsumer.com. 18 July 2006. Archived from the original on 2 April 2008.(subscription required) https://web.archive.org/web/20080402234035/http://www.aviationconsumer.com/issues/36_8/avionicsreport/5580-1.html ↩
"ZAON XRX owner's manual" (PDF). Zaon. Retrieved 11 March 2015. https://www.aircraftspruce.com/catalog/pdf/xrxmanual.pdf ↩
"Zaon PCAS XRX - Plane & Pilot Magazine". 6 February 2016. http://www.planeandpilotmag.com/products/tech-talk/zaon-pcas-xrx.html ↩
"Zaon PCAS XRX". February 2008. https://www.planeandpilotmag.com/article/zaon-pcas-xrx/ ↩