Running without ADS-B around major airports would be unacceptable. I think drones are going to be required to have them before 2020 which makes me think that isn't quite right.
With ATC notified, the helicopter could revert to Mode-C [0] transponder mode, using an assigned transponder code and ATC would still track them safely. TCAS [1] would also still function safely when interrogated by secondary radar.
ADS-B reduces controller workload as they don't have to manually tag individual aircraft, especially in a high traffic area. ADS-B also works without secondary radar interrogation.
When there is a high speed chase near an airport, sometimes the tower controller will request the helicopter remain under a certain altitude or clear of a specific area.
Additionally, websites like FlightAware.com have limited direct ADS-B coverage and also display the FAA 5-minute delayed feed. I've never heard of a "criminal" in the US using an ADS-B receiver to evade police helicopters in real time.
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.
I track several of these aircraft a week, and they usually don't have ADS-B. I track them via multilateration, combining my receiver's data with data from several other receivers (as part of the Flightaware network: http://flightaware.com/adsb/)
Many commercial operations don't have ADS-B installed (I wish! That would make my life easier.). They use other systems like MLAT and primary radar to track them.
If they make drones carry ADS-B transponders, it will only be heavy and high-flying drones. Signal becomes noise after so long, and they don't want to overcrowd 1090MHz.
