Great guys, this is nearer the right direction.
Broadcasting GPS units sounds good, & the technology doesn't sound impossible, I wish I knew more about the electronics stuff, there are other markets for that as well, I'm sure the Mountain rescue services would appreciate a distress call with a pinpoint location included, especially when the poor sod that fell down the mountain doesn't know where he is. So going a step further & add a button that changes the signal to a distress call shouldn't be difficult. I'm sure a lot more people than just the aviation industry would be interested, mass market = cheaper?
The problem of azimuth resolution may not worry many people, just the knowledge that something is on a converging track would do me.
As for upsetting NATS etc, why should that be a problem? If it were self contained ie:- not hard wired to the aircraft, would that make it unregulatable?
As for a radar based device, how much wattage / amps is required to return a signal from say 10 miles? If that requires a battery the size of a porta-cabin then it's no use.