So the test for amplitude is aided by the fact that the signal strength received at the car increases by a factor of four if the distance is cut in half. Thus, you have a nice margin for setting your threshold.
With measuring the time, however, presuming that radio signal will travel on the order of one foot per nanosecond, you have much less of a threshold tolerance. If the unlock takes place within two feet of the car, that is two nanoseconds. If the key sits 20 feet away, that is a 20-nanosecond one-way travel. So this solution would need to be able to distinguish between a four nanosecond gap (round trip time) and a 40-nanosecond round-trip time.
Add to that the turnaround time in the car CPU which I would imagine to be some number of milliseconds, would 10 ms be reasonable?
Thus, the electronics in the car needs to distinguish between 10ms + 4 ns vs 10ms + 40ms. And given jitter in any modern CPU/memory/OS/electronics device, I would bet that the jitter totally swamps that.
With measuring the time, however, presuming that radio signal will travel on the order of one foot per nanosecond, you have much less of a threshold tolerance. If the unlock takes place within two feet of the car, that is two nanoseconds. If the key sits 20 feet away, that is a 20-nanosecond one-way travel. So this solution would need to be able to distinguish between a four nanosecond gap (round trip time) and a 40-nanosecond round-trip time.
Add to that the turnaround time in the car CPU which I would imagine to be some number of milliseconds, would 10 ms be reasonable?
Thus, the electronics in the car needs to distinguish between 10ms + 4 ns vs 10ms + 40ms. And given jitter in any modern CPU/memory/OS/electronics device, I would bet that the jitter totally swamps that.
(Keep in mind that this is a BOEC https://en.wikipedia.org/wiki/Back-of-the-envelope_calculati...)