A vital tactical advantage of passive sonar is that it does not alert its targets as to its presence. Active Sonar Active sonar transmits a signal which is reflected from the hull of the submarine, giving its position, and not depending on noise emitted by the target. The transmissions do, however, signal the presence of the sonar to its desired targets, probably long before it detects them. It is used only sparingly by submarines, as it compromises their most valuable asset of stealth, but more often by surface ships, which inevitably radiate enough noise to reveal their presence. Sonobuoys can be either passive or active, with the passive versions most useful for initial detection, and the active for direction of air- delivered AS weapons.
Homing torpedoes use active sonar.
However, the ranges are limited by the variations in the earth's field and the inevitable presence of magnetic materials in the vicinity of the sensor. The short range is acceptable for detonating magnetic mines or torpedo warheads, and offers a means of fixing the exact position of a submarine by an aircraft already directed to the general area by other sensors. But the ranges are too short to be useful for initial searches. The only satisfactory weapons use explosives detonated as close as possible to the submarine's hull. The best means of delivery is by a torpedo, launched from a fixed-wing aircraft, helicopter, submarine, or surface ship.