Look for idea how to detect the falling object proximity to Earth before landing. The problem is speed, when for getting appropriate accuracy of required 2 meters plus minus 5 cm , at the 360-720 km/h corresponds to the 100-200 m/s thus 5 cm longs 0.5 ms to 0.25 ms. Thus, seems acoustic sensors are ultimate too slow, capacitative and inductive - too far field, infrared..... probably, microwave.... probably, however both are radiating strong fields by own and that is not good for the task where all spectra emissions ought be masked. What may be the simplest foolproof (and cheap) circuit for such task? Thought about steel ball in the thread, but experiments shown the wind is making false trigger.