You can also have three antennas in the car, and have high speed electronics measure the RF delay as received on each antenna. From there the key can tell how

You can also have three antennas in the car, and have high speed electronics measure the RF delay as received on each antenna. From there the key can tell how far away it is from the car (similar to how GPS works), and it would only respond to the challenge when it is in the proper position.