Take a typical distress frequency in the HF range, by broadcasting a simple audible message such as "ABC123 position N50.12345 W020.12345" every 30 seconds during which the "emergency" conditions are being sensed, we'd have a very good chance of being received up to a 1,000 miles away and more at night. Also, other planes in the area will also have the opportunity to listen in and pass any information onto the authorities. Recorded replay of ATC messages another desperately lacking feature within modern airliner avionics (GA has had it for years).
The cost of the retrofit (pre-bureaucracy and certification) would be quite low IMO. We wouldn't need any subscription or ongoing running costs except that to run/man a handful of worldwide stations. Security-wise, i'm not sure if it requires security considerations to be honest. It would only ever be broadcast in an emergency anyway and besides ADS-B already gives the public far more than that!
A lot of re-inventing the wheel is being proposed here. My idea is simple, probably because I'm stupid!
