http://www.usatoday.com/story/money/cars/2015/05/07/technology-car-break-ins/70939336/
Brief explanation: Sophisticated thieves are amplifying signals emitted from cars that are received by their corresponding car keys; the amplified signal travels potentially over hundreds of feet; the key's returned signals are (I would suspect; the article doesn't say) also amplified. Thus, a car lock can be opened even if the car's key is 'safely' in an owner's house or apartment. The article suggests keeping the keys in a refrigerator or microwave oven to shield it from those signals. One possible re-design solution, I think, would be for the key fob to retransmit a portion of the car-emitted signal instantly; the car would detect the timing of its emitted signal compared to the key-returned signal. Thus, the car would know how far away the key is from the car, foiling this system, by setting a maximum distance between the car and the key. This actually isn't a new idea: Supermarkets have microwave-detection door openers, and I suspect that their active range is determined by rapidly sweeping the frequency of the transmitter, and then mixing (non-linear mixing; not to be confused with linear mixing, done to audio) the returned signal. The further the target is, away from the transmitter, the higher the detected frequency. This makes the trigger point precisely dependent on the distance, not simply the amplitude of the returned signal. Jim Bell
