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

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.