Find-A-Car

I'd love a radio direction finder car tracking system. OK, I'm absent-minded enough to lose my car in car parks on a semiregular basis. This idea would help me find my car again!

What you have is three class 1 Bluetooth modules, so a theoretical 100m range as I understand. The exact tech isn't important, Bluetooth merely seems possible. Modules are sold together and share serial numbers so the system can still operate if more than one person has it per carpark.

Modules 1 and 2 sit at opposite ends of the car and share a clock - either they're wired together or one has a clock and the other reads the time from it. For a retrofit, that seems more practical. This clock will need at least millisecond accuracy.

Module 3 is on your keyring and has a small buzzer and a button. Pressing the button sets the module to seek. If it can't get a response from either module, you're out of range. If it can, it checks the time with both modules. It then beeps every second for as long as you hold the button in with the pitch of the beep indicating the difference in time received from the two modules.

I know 2 alone isn't normally enough to get a position, so I'd envisage the user sweeping their keys to gain an initial direction. From then on, it should be enough. 3 transmitters begs the question of where to put the third transmitter...