Depth charges were the earliest weapons designed for use by ships against submerged submarines. These explosives were initially dropped as the ship moved over the presumed location of a submarine; but shipboard SONAR was unable to maintain contact with the submarine at close range. Various mortar-type projectors (including hedgehog and squid) were devised during World War II to allow a ship to maintain SONAR contact while lobbing explosive charges toward the submarine. During the cold war, rockets were developed to provide greater range with reduced recoil. Some rockets carry homing torpedoes to provide terminal guidance for the warhead.