How proximity fuzes changed anti-aircraft fire

Earlier anti-aircraft shells depended on crews or fire-control equipment setting a time at which the round should burst near a predicted aircraft position. The American wartime proximity fuze carried a small radar emitter and detonated when it sensed a nearby target. That removed much of the error involved in estimating the correct time of flight. A burst near the aircraft consequently became evidence of a useful engagement rather than merely a shell exploding at its preset time. The watch point explains why proximity-fuzed dual-purpose guns were such an important late-war improvement.