Direction finding (DF), or radio direction finding (RDF), is the use of radio waves to determine the direction to a radio source. The source may be a cooperating radio transmitter or may be an inadvertant source, a naturally-occurring radio source, or an illicit or enemy system. Radio direction finding differs from radar in that only the direction is determined by any one receiver; a radar system usually also gives a distance to the object of interest, as well as direction. By triangulation, the location of a radio source can be determined by measuring its direction from two or more locations. Radio direction finding is used in radio navigation for ships and aircraft, to locate emergency transmitters for search and rescue, for tracking wildlife, and to locate illegal or interfering transmitters. During the Second World War, radio direction finding was used by both sides to locate and direct aircraft, surface ships, and submarines.
High-frequency direction finding, usually known by its abbreviation HF/DF or nickname huff-duff, is a type of radio direction finder (RDF) introduced in World War II. High frequency (HF) refers to a radio band that can effectively communicate over long distances; for example, between U-boats and their land-based headquarters. HF/DF was primarily used to catch enemy radios while they transmitted, although it was also used to locate friendly aircraft as a navigation aid. The basic technique remains in use as one of the fundamental disciplines of signals intelligence, although typically incorporated into a larger suite of radio systems and radars instead of being a stand-alone system.
United States Army Security Agency (USASA) Field Station Augsburg was the site of a Wullenweber AN/FLR-9 (V8) radio direction finder, established during the Cold War. Field Station Augsburg was located on Gablingen Kaserne, near the village of Gablingen just north of Augsburg in Bavaria, West Germany. It was one of nearly 20 field stations positioned strategically around the world by the U.S. Armed Forces during the Cold War. Field Station Augsburg opened in 1970 and closed in 1998, at which time it was turned over to the German government.
Canadian Forces Station Leitrim, also referred to as CFS Leitrim, is a Canadian Forces Station located in the neighbourhood of Leitrim in Ottawa, Ontario. It is concerned with the interception, decrypting and processing of communication for the Communications Security Establishment and the Canadian Forces, and forms part of the ECHELON system.
Sugar Grove Station is a National Security Agency (NSA) communications site located near Sugar Grove in Pendleton County, West Virginia. According to a 2005 article in The New York Times, the site intercepts all international communications entering the Eastern United States. Activities at the site previously involved the Navy Information Operations Command (NAVIOCOM). In April 2013, the Chief of Naval Operations ordered that the NAVIOCOM support base be closed by September 30, 2015, as "a result of the determination by the resource sponsor National Security Agency to relocate the command's mission." The naval base is being repurposed as a privately owned healthcare facility for veterans, while the NSA listening station, to the south, continues to operate.
Galeta Island is an island located on the Atlantic side of the Republic of Panama, just east of the city of Colón, Panama.
San Vito dei Normanni Air Station was a United States Air Force facility located 7 miles (11 km) west of Brindisi, Apulia, between the port of Brindisi and the town of San Vito dei Normanni, Italy.
Silver Strand Training Complex (SSTC) is a training facility for U.S. Special Operations Forces. Located between Imperial Beach and Silver Strand State Beach near San Diego. The area is part of Naval Base Coronado and commanded by that base's Commanding Officer. While just north of Imperial Beach, it is within the city limits of Coronado. The facility was known by locals as the "elephant cage" which is a nickname for the large Wullenweber direction finder antenna. The antenna was used to provide direction finding, primary communication links for U.S. Navy submarines. The antenna was finally dismantled in 2015, even though it was scheduled to be removed in fiscal year 2007.
Nauen Transmitter Station in Nauen, Havelland district, Brandenburg, Germany, is the oldest continuously operating radio transmitting installation in the world. Germany's first high power radio transmitter, it was founded on 1 April 1906 by Telefunken corporation and operated as a longwave radiotelegraphy station through World War II, and during World War I became Germany's main link with the outside world when its submarine communications cables were cut. Upgraded with shortwave transmitters in the 1920s it was Germany's most advanced long range radio station, continually upgraded with the latest equipment and serving as an experimental station for Telefunken to test new technology. At the end of World War II, invading Russian troops dismantled and removed the transmitting equipment. During the Cold War it served as the GDR's international shortwave station Radio Berlin International (RBI), and was the East Bloc's second most powerful radio station, disseminating Communist propaganda to other countries. Since German Reunification in 1991 it has been operated by Deutsche Telekom, Germany's state telecommunication service. The original 1920 transmitter building designed by architect Herman Muthesius is still used; it is one of the many remaining buildings designed by that architect that is a protected cultural heritage site.
Ministry of Defence Chicksands, or more simply MOD Chicksands, is a tri-service British Armed Forces facility in Bedfordshire, approximately 35 miles (56 km) north of London. It is named after Chicksands Priory, a 12th-century Gilbertine monastery located within the perimeter of the camp.
Wacław Struszyński was a Polish electronics engineer who made a vital contribution to the defeat of U-boats in the Battle of the Atlantic. He designed an exceptional radio antenna which enabled effective high frequency (HF) radio direction finding systems to be installed on Royal Navy convoy escort ships. Such direction finding systems were referred to as HF/DF or Huff-Duff, and enabled the bearings of U-boats to be determined when the U-boats made high frequency radio transmissions.
The United States Air Force's 381st Intelligence Squadron is an intelligence unit located at Joint Base Elmendorf-Richardson, Alaska. The unit operated the last operational AN/FLR-9 Circularly Disposed Antenna Array (CDAA), also known as an "Elephant Cage", until 25 May 2016.
Gablingen Kaserne is a former military facility in Gablingen near Augsburg, Germany, which was closed in 1998. Its primary use was signals intelligence collection during the Cold War.
The AN/FLR-9 is a type of very large circularly disposed antenna array, built at eight locations during the Cold War for HF/DF direction finding of high priority targets. The worldwide network, known collectively as "Iron Horse", could locate HF communications almost anywhere on Earth. Because of the exceptionally large size of its outer reflecting screen, the FLR-9 was commonly referred to by the nickname "Elephant Cage." Constructed in the early to mid 1960s, in May 2016 the last operational FLR-9 at Joint Base Elmendorf-Richardson in Alaska was decommissioned. It can be confused with the US Navy's AN/FRD-10, which also used a circularly disposed antenna array.
The FuG 200 Hohentwiel was a low-UHF band frequency maritime patrol radar system of the Luftwaffe in World War II. It was developed by C. Lorenz AG of Berlin starting in 1938 under the code name "Hohentwiel", an extinct volcano in the region of Baden-Württemberg in southern Germany.
The FuG 240 "Berlin" was an airborne interception radar system operating at the "lowest end" of the SHF radio band, which the German Luftwaffe introduced at the very end of World War II. It was the first German radar to be based on the cavity magnetron, which eliminated the need for the large multiple dipole-based antenna arrays seen on earlier radars, thereby greatly increasing the performance of the night fighters. Introduced by Telefunken in April 1945, only about 25 units saw service.
Curtain arrays are a class of large multielement directional radio transmitting wire antennas, used in the short-wave radio bands. They constitute a type of reflective array antenna, consisting of multiple wire dipole antennas, suspended in a vertical plane, often positioned in front of a "curtain" reflector made of a flat vertical screen of many long parallel wires. These are suspended by support wires strung between pairs of tall steel towers, reaching heights of up to 90 m high. Primarily employed for long-distance skywave transmission, they emit a beam of radio waves at a shallow angle into the sky just above the horizon, which is then reflected by the ionosphere back to Earth beyond the horizon. Curtain arrays are extensively used by international short-wave radio stations for broadcasting to large areas at transcontinental distances.
The AN/FRD-10 is a United States Navy circularly disposed antenna array (CDAA), built at a number of locations during the Cold War for high frequency radio direction finding (HF/DF) and signals intelligence. In the Joint Electronics Type Designation System, FRD stands for fixed ground, radio, direction finding. 14 sites were originally constructed as a part of the "Classic Bullseye" program. Two AN/FRD-10 systems were later installed in Canada. AN/FRD-10 systems were originally constructed in the early 1960s, but after the dissolution of the Soviet Union, the systems began to be shut down and demolished. The Naval Security Group operated and maintained the U.S. Navy AN/FRD-10 systems. The system had several nicknames including Fred-10 and Elephant or Dinosaur cages. As of 2015, none of the US Navy AN/FRD-10 sites are extant, but the two Canadian sites remain in service. The AN/FLR-9 was a system with a similar design and function, but operated by the US Air Force and Army.
Naval Computer and Telecommunications Station Guam is a United States Navy communications facility on the U.S. territory of Guam. It is located on the Naval Base Guam North Finegayan Telecommunications Site along Guam Highway 3 in Dededo, on the northwest coast of the island.
Project Boresight was a United States program that built radio-location systems to find the position of Russian submarines that were using burst communications. It was created as a top-secret program.
