A radar picket is a radar-equipped station, ship, submarine, aircraft, or vehicle used to increase the radar detection range around a force to protect it from surprise attack, typically air attack. Radar picket vessels may also be equipped to direct friendly fighters to intercept the enemy. In British terminology the radar picket function is called aircraft direction. Often several detached radar units encircle a force to provide increased cover in all directions. Airborne radar pickets are generally referred to as airborne early warning.
Radar picket ships first came into being in the US Navy during World War II to aid in the Allied advance to Japan. The number of radar pickets was increased significantly after the first major employment of kamikaze aircraft by the Japanese in the Battle of Leyte Gulf in October 1944. Fletcher- and Sumner-class destroyers were pressed into service with few modifications at first. Later, additional radars and fighter direction equipment were fitted, along with more light anti-aircraft (AA) guns for self-defense, usually sacrificing torpedo tubes to make room for the new equipment, particularly the large height-finding radars of the era. Deploying some distance from the force to be protected along likely directions of attack, radar pickets were the nearest ships to the Japanese airfields. Thus, they were usually the first vessels seen by incoming waves of kamikazes, and were often heavily attacked.
The radar picket system saw its ultimate development in World War II in the Battle of Okinawa. A ring of 15 radar picket stations was established around Okinawa to cover all possible approaches to the island and the attacking fleet. Initially, a typical picket station had one or two destroyers supported by two landing ships, usually landing craft support (large) (LCS(L)) or landing ship medium (rocket) (LSM(R)), for additional AA firepower. Eventually, the number of destroyers and supporting ships were doubled at the most threatened stations, and combat air patrols were provided as well. In early 1945, 26 new construction Gearing-class destroyers were ordered as radar pickets without torpedo tubes, to allow for extra radar and AA equipment, but only some of these were ready in time to serve off Okinawa. Seven destroyer escorts were also completed as radar pickets. The radar picket mission was vital, but it was also costly to the ships performing it. Out of 101 destroyers assigned to radar picket stations, 10 were sunk and 32 were damaged by kamikaze attacks. The 88 LCS(L)s assigned to picket stations had two sunk and 11 damaged by kamikazes, while the 11 LSM(R)s had three sunk and two damaged.
From 1943 Nazi Germany's Kriegsmarine operated several radar-equipped night fighter guide ships (Nachtjagdleitschiffe), including NJL Togo. which was equipped with a FuMG A1 Freya radar for early warning and a Würzburg-Riese gun laying radar, plus night fighter communications equipment. From October 1943, the NJL Togo cruised the Baltic Sea under the operational control of the Luftwaffe. In March 1944, after the three great Soviet bombing raids on Helsinki, she arrived in the Gulf of Finland to provide night fighter cover for Tallinn and Helsinki. Also, the Imperial Japanese Navy briefly modified two Ha-101 class submarines as dedicated radar pickets in the first half of 1945, but reconverted them to an even more important role as tanker submarines in June of that year.
During the Cold War, the United States Navy expanded the radar picket concept. The wartime radar picket destroyers (DDR) were retained, and additional DDRs, destroyer escorts (DER), and submarines (SSR) were converted and built 1946-1955. The concept was that every carrier group would have radar pickets deployed around it for early warning of the increasing threat of Soviet air-to-surface missile attack.
The 26 Gearing-class DDRs were supplemented by nine additional conversions during the early 1950s. The seven wartime DERs were not considered worth modernizing and were relegated to secondary roles. However, twelve additional DER conversions were performed 1954-58. Ten of these were conversions of diesel-powered DEs, which had a longer at-sea endurance than their steam-powered equivalents.
The slow DERs were used in combination with Guardian-class radar picket ships (converted Liberty ships) and Lockheed WV-2 Warning Star aircraft to extend the Distant Early Warning (DEW) line in the North Atlantic and North Pacific, to warn of Soviet bomber attacks. These assets formed two Barrier Forces known as BarLant and BarPac and operated 1955-1965. The aircraft patrolled lines extending from Argentia, Newfoundland to the Azores in the Atlantic, and from Midway to Adak, Alaska in the Pacific. The DERs maintained picket stations near these lines. The Guardian-class backed up the outer barriers with picket stations 400-500 miles from each coast. There were also three oil-rig-type offshore radar stations known as "Texas Towers" off the New England coast. While on station, all of these assets were operationally controlled by the Aerospace Defense Command.
The high casualties off Okinawa gave rise to the radar picket submarine, which had the option of diving when under attack. It was planned to employ converted radar picket submarines should the invasion of Japan become necessary. Two submarines received rudimentary radar conversions during the war, and in 1946 two more extensive conversions were performed. The radar equipment of these diesel submarines took the place of torpedoes and their tubes in the stern torpedo rooms. By 1953, a total of 10 SSR conversions had been performed, with radar suites called Migraine I, II, and III, the most extensive conversion adding a 24-foot (7.3 m) compartment as an expanded combat information center (CIC). In 1956 two large, purpose-built diesel SSRs, the Sailfish class, were commissioned. These were designed for a high surface speed with the intent of scouting in advance of carrier groups. However, the SSRs did not fare well in this mission. Their maximum surfaced speed of 21 knots was too slow to effectively operate with a carrier group, although it was sufficient for amphibious group operations. It was thought that nuclear power would solve this problem. The largest, most capable, and most expensive radar picket submarine was the nuclear-powered USS Triton , commissioned in 1959. The longest submarine built by the United States until the Ohio class Trident missile submarines of the 1980s, Triton's two reactors allowed her to exceed 30 knots on the surface.
From 1955 to 1965 the United States Navy employed Guardian-class radar picket ships (converted from the former boxed aircraft transport version of the Liberty ship) to extend the DEW Line seaward. Sixteen of these were stationed on the East and West Coasts: eight at Treasure Island, California and eight at Davisville, Rhode Island. The ships' names matched the mission: USS Guardian (AGR -1), USS Lookout (AGR-2), USS Skywatcher (AGR-3), USS Searcher (AGR-4), USS Investigator (AGR-9), USS Outpost (AGR-10), USS Protector (AGR-11), and USS Vigil (AGR-12) on the East Coast and USS Scanner (AGR-5), USS Locator (AGR-6), USS Picket (AGR-7), USS Interceptor (AGR-8), USS Interdictor (AGR-13), USS Interpreter (AGR-14), USS Tracer (AGR-15) and USS Watchman (AGR-16) on the West Coast. The hull classification symbol of the ships was initially YAGR, changed to AGR in 1958. The standard crew consisted of 13 officers, eight chief petty officers, and 125 enlisted. Typical station duty was about 30-45 days out and 15 days in port.
Picket stations were about 400-500 miles off each coast and provided an overlapping radar or electronic barrier against approaching aircraft. While on station, the ships' operational control shifted from the Navy to the Air Force and NORAD. While on station, each ship stayed within a specific radius of its assigned picket station, reporting and tracking all aircraft contacts. Each ship carried qualified air controllers to direct intercept aircraft sent out to engage contacts. While on station other duties such as search and rescue, weather reporting, and miscellaneous duties were assigned. The National Marine Fisheries Service even provided fishing gear so that the crew could fish for tuna during the season, and the ships sent daily reports of fish caught for research purposes.
The introduction of the Grumman WF-2 Tracer (later the E-1 Tracer) carrier-based airborne early warning aircraft in 1958 doomed the radar picket as a carrier escort. Airborne radar had evolved to the point where it could warn of an incoming attack more efficiently than a surface ship. In 1961 the DDRs and SSRs were withdrawn. All but six DDRs received anti-submarine warfare conversions under the FRAM I and FRAM II programs and were redesignated as DDs; the remaining six were somewhat modernized under FRAM II and retained in the DDR role. The SSRs were converted to other roles or scrapped. Triton was left without a mission. Some alternatives were considered, including serving as an underwater national command post, but she eventually became the first US nuclear submarine to be decommissioned, in 1969. The DERs and AGRs stayed on the DEW Line patrols until 1965.
By 1965, the development of over-the-horizon radar made the barrier forces and Guardian-class radar ships obsolete. Ground-based systems then had the capability to see beyond their once state-of-the-art radar systems. The final use of the radar picket concept by the US Navy was in the Vietnam War. The Gulf of Tonkin Positive Identification Radar Advisory Zone (PIRAZ) guided missile destroyer leaders (aka frigates) (redesignated as cruisers in 1975) and cruisers provided significant air control and air defense in that war.
The British Royal Navy constructed or converted two types of dedicated aircraft direction ships in the late 1950s and early 1960s. Four World War II Battle-class destroyers and four Weapon-class destroyers were converted 1959-1962 as Fast Air Detection Escorts to accompany fast carrier groups. Also, four Type 61 Salisbury-class frigates were commissioned 1957-1960 to accompany slow carrier or amphibious groups. However, the aircraft direction function was short-lived. With the mid-1960s decision to phase out the fast carriers, the Battle-class ships were placed in reserve 1966-1968 and were scrapped or converted to non-combat roles by 1974. The Salisbury-class were relegated to non-combat roles or sold by the end of 1978.
Twenty T43-class minesweepers were converted to KVN-50 class radar picket ships between 1955 and 1959. Modifications involved replacing the aft gun turret with a Knife Rest-A or Big Net radar. Most were retired during the 1970s or relegated to training duties, with the last withdrawn in 1987.
Four Whiskey-class submarines were converted to Project 640 radar picket boats between 1959 and 1963 by fitting a Boat Sail radar in an enlarged conning tower. These were known to NATO as "Whiskey Canvas Bag" from the canvas coverings often put over the radar when NATO aircraft approached. While the US radar picket submarines were intended for fleet defense, the Project 640 boats were intended to provide warning of air attacks on Soviet coastal territory.:119
14 further T43-class minesweepers were converted to Project 258 KVN-6 class radar picket ships between 1973 and 1977 with Kaktus radars. Some were later modified to Project 258M ships with Rubka (NATO: Strut Curve) radars.
Three other projects were cancelled before conversions were made.