Originally Posted by
peekay4
With 1090 MHz ADS-B you cannot block your tail number. So in practice any blocking would be done voluntarily by sites like FR24 if requested by the aircraft owner.
However, anyone with a $30 ADS-B receiver can always decode the raw ADS-B stream, get the ICAO addresses, then perform tail number mapping.
With 978 MHz ADS-B (UAT) there is an anonymity function for VFR aircraft but that won't prevent enthusiasts / journalists / bad guys from noticing flight patterns such as the ones described here.
We use rotating call signs in our transponders. Problem solved (for now).