When gates are cascaded, the total propagation delay is approximately the sum of the individual delays, an effect which can become a problem in high-speed circuits.

The timing of both measurements is synchronised using a GPS signal, and the total propagation time (tp1 + tp2) of the outward and return signals (S1, S2) is stored in a memory.

When gates are cascaded, the total propagation delay is approximately the sum of the individual delays, an effect which can become a problem in high-speed circuits.

Intensified CCD (Charge Coupled Device) cameras (18, 20) (preferably two) are electronically shuttered after a time delay corresponding to the round trip propagation time to and from the object.