ADS-B has always had an "anonymous" option for aircraft using the UAT uplink, as long as they're squawking 1200/VFR. (Similarly, there's long been a way to requ

ADS-B has always had an "anonymous" option for aircraft using the UAT uplink, as long as they're squawking 1200/VFR. (Similarly, there's long been a way to request that specific aircraft are hidden from the FAA's public radar feeds.) Aircraft using the 1090ES uplink can't take advantage of the privacy mode, because it's based on top of Mode S, which is an older protocol and never had privacy built in - it didn't need it before because Mode S just didn't include position data. (We'll ignore Mode S multilateration, which is a recent development.) The private ICAO addresses are a hack to provide similar functionality as UAT. > Why did they bother to invent a publicly broadcast address if it wasn't intended to be used that way!? Because those addresses were invented for Mode S, which didn't broadcast position. It was part of the secondary radar (transponder) system, and needed to be paired with primary radar to get something useful.