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UNDERSTANDING PCAS24
What does XRX detect exactly?
To explain how the system works, consider the following illustrations:
1 > 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 or more from the interrogation source. The Ground RADAR range can
be 200 miles or more.
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). 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, PCAS 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.
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