Since 1945 the destroyer's dual antiaircraft-antisubmarine role has continued through the transition to guided missiles. Because of the high cost of cruisers, smaller escort ships became the backbone of lesser navies in the guided-missile age. The destroyer completed its transition, begun during World War II, from surface-ship killer to anti-aircraft escort. A modern destroyer's armament consists of surface-to-air missiles, anti-submarine torpedoes, anti-ship missiles, and one or two main guns of about four or five inches in caliber. Many destroyers carry submarine-hunting helicopters, and some carry cruise missiles. Like cruisers, they bristled with an array of sonar and radar sensors and were packed with electronic gear for the swift detection and identification of hostile targets and the computation of firing data. 