During the Cold War, the Royal Canadian Air Force and the United States Air Force jointly built and operated radar picket stations to detect Soviet bombers, and the United States Navy expanded the naval radar picket concept. The wartime radar picket destroyers (DDR) were retained, and additional DDRs, destroyer escorts (DER), submarines (SSR, SSRN), and auxiliaries (AGR) were converted and built 1946–1959. The naval concepts were: 1) every carrier group would have radar pickets deployed around it for early warning of the increasing threat of Soviet air-to-surface missile attack, and 2) radar pickets would form barriers off the North American coasts, thus extending the land based lines. While on station, all of these assets - other than those assigned to fleet defense - were operationally controlled by the Aerospace Defense Command and after May 1958 NORAD.