Not sure the exact way transmitters and receivers work but I understand that on, for example, 72MHz we use crystals to divide the fractional range.

In these days of advanced miniturisation and technology, why dont the radio manufacturers add an authenticating code to the transmitted radio signals to identify a unique transmitter to its partnered receiver. This would eliminate any lockout problems caused by the very likely event of someone switching on whilst on the same channel as someone flying.

An onboard microprocessor (or a more advanced existing processor) in the receiver could verify the signal as coming from the correct transmitter (in addition to the existing channel as it does now).

I really dont see this as being a big deal, however, I dont profess to knowing all the facts on the subject.