PCAS (Portable Collision Avoidance Systems)
from Zaon Flight Systems

How it Works
To explain how the system works, consider the following:

Step One

To 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 Two

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). 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 and MRX units watch 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 Three

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

Step Four

The altitude of the aircraft (in the example, 2500 ft.) is compared to your local altitude (i.e., 1500 ft.) and the relative altitude is calculated (i.e., 1000 ft. above you). With relative direction, altitude and range determined, XRX and MRX display this information and stores it in memory (MRX cannot calculate or display relative direction).



Step Five

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 on Screen A, 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 and MRX units use patent-pending SmartLogic algorithms to determine which of two or more aircraft is a greater threat. See flight scenarios in manual for more information.

Coverage
XRX is a passive system, meaning it listens to replies from other aircraft. Other aircraft are responding to interrogations. There are two types of interrogation networks: Ground RADAR installations and TCAS- or other active system-equipped aircraft.

Ground RADAR installations provide ATC with traffic data by interrogating outward from the sweeping antenna, pictured here. The interrogation is sent on 1030 MHz. Your transponder, and the transponders of the other aircraft in the area, listen on 1030 MHz and give the response data on 1090 MHz. The range of each installation depends on the type of surrounding terrain and geography, but is usually 100-200 miles in diameter.


Click to enlarge
US Ground RADAR installations and approximate surrounding coverage areas.
(Source: FAA)

Overhead, airliners, corporate jets and other aircraft have systems that interrogate, called active systems, which make up an even better network of interrogation. Examples of active systems are TCAS, Skywatch, and Ryan 9900B systems. Because these systems are airborne, they are not limited by geographical features and provide an excellent platform for interrogation. They too interrogate in 1090 MHz, and transponders respond on 1090 MHz. PCAS listens on 1090 MHz and intercepts this data.


Click to enlarge
Typical TCAS or other active system network coverage.

Each dot represents an airliner with an interrogation range of 100 NM (200 NM diameter).
(Source: Flight Explorer)

A passive system which does not interrogate, such as XRX, only works if the interrogations are present. However, it is important to note that the combined effect of both overlapping networks provides for a nearly 100% coverage in the United States. The same type of systems exist in countries abroad, especially the same airborne TCAS-based systems. The most important thing to remember is that interrogations happen virtually everywhere you may fly.