I wonder if a $12 TV Tuner RTL-SDRs can listen in to secondary radar triggered TCAS responses? (I wonder how much trouble you'd get in for setting up a USDR or HackRFOne to transmit your own "secondary radar" signals to make the TCAS respond?)
TCAS "responses" are only sent during a Resolution Advisory.
Mode-C and Mode-S responses are sent when interrogated by secondary radar. ADS-B transponders with "Extended Squitter" transmit regularly while active, so no need to transmit your own interrogation pulses.
Its possible to monitor Secondary Radar pulses, and Mode C returns. It would require multiple geographically separated receivers to triangulate. I don't think FlightAware supports Mode C, only Mode S(MLAT) and ADS-B.
If you transmitted your own "secondary radar" pulses, the FAA and/or the FCC would probably launch a criminal investigation, with the FBI involved. Making your own "secondary radar" would be pointless and high risk.
Interfering with radar equipment by transmitting in a controlled, safety and security critical band? On a bad day, that could land you a truckload of terrorism charges.
