Radar picket ships first came into being during the second world war to aid in the advance to Japan. As they were the nearest ships to the Japanese airfields, and hence the first seen, they were often heavily attacked by kamikaze aircraft.
Formerly, the United States Navy employed radar picket ships such as the USS Hissem (DER-400) to extend the DEW Line seaward. The designation DER indicated "Destroyer Escort Radar picket ship." DERs carried no armaments, but the guns were left in place to retain their appearances as warships. The British Royal Navy built a specialised class of radar picket frigate; the Type 61 or Salisbury class.
During the Cold War, the United States Navy advanced to radar picket submarines which had the option of diving when under attack themselves. The radar equipment of these diesel submarines took the place of torpedoes and their tubes in the rear torpedo rooms. The largest, most capable, and most expensive of these submarines was the nuclear-powered USS Triton. With the advance to lighter radar sets that required less power to operate and so could be carried in even carrier aircraft, the radar picket submarine became obsolete; in fact, the Triton was considered a boondoggle.