One of the earliest military applications for radio was in direction-finding (DF), which makes it possible to locate the positions of enemy aircraft and ships using four major components: an antenna, a receiver, a processor or processors, and a control and output system. Examples of radio DF equipment in use at the beginning of the twenty-first century include the OUTBOARD (Organizational Unit Tactical Baseline Operational Area Radio Detection) system of the U.S. Navy. Direction finding often uses triangulation, which is based on laws of plane trigonometry.

Direction finding and triangulation. A direction finder can be any electronic device used to locate a source of electronic emissions such as a ship or aircraft. In everyday usage by military, security, and intelligence services, direction finding is virtually synonymous with radio DF. Direction finding usually involves a radio receiver linked to a revolving antenna, which scans for the strongest possible signal in the area.

Assuming two stationary transmitters can be located, direction finding can be used to locate one's position by means of triangulation. The latter is based on the trigonometric principle that, for any triangle, when one side and two angles are known, the other angle and two sides can be calculated. To establish the measure for two angles of a triangle on Earth's surface, it is necessary to use a surveying device known as a theodolite, or some electronic equivalent. The measured and known side of the triangle is known as the baseline.

Components of a DF system. The simplest DF system must contain an antenna, receiver, at least one processor, and control/output systems. The antenna must be versatile, so as to address a variety of requirements, some of which seem almost at cross-purposes to one another. It must be omnidirectional, or capable of receiving input from 360 degrees, yet capable of pinpointing the locations of specific signals from the range of radio noise it receives. Additionally, it must make possible the reception of signals over the widest possible area, yet receive these on an ultra-accurate pencil beam. Given these various requirements, modern DF systems often use not one antenna but an array, or they may make use of a phased-array antenna, which can quickly change its pattern of radiation using electronic means.

Receivers may be either single-channel, dual-channel, or N-channel. In a single-channel receiver, a switch sequentially selects one antenna from an array, while in the dual-channel model, switching may be used to select pairs from three or more antennas. N-channel receivers are capable of operating across multiple antennas without the requirement of switching.

Once the signal is received, it is necessary to calculate the location of the emitter by comparing signal properties such as amplitude. For this operation, a processor is used. With multiple or phased-array antennas, the operator may need not a single processor, but an array of distributed digital signal processors. With twenty-first century technology, it is possible for machines to perform a variety of complex calculations in real time or near-real time. Lastly, there is the control/output system, which includes a variety of subcomponents such as functions for the input and preparation of data, as well as various other operations requiring a workable interface between operator and equipment.

Radio DF in history. The use of radio direction finding dates back to World War I, when both the Allies and the forces of the Central Powers used it to locate enemy positions on the ground. The essential principles of direction-finding were established at that time, well before radio entered commercial use in the early 1920s.

During the interwar period, the British Royal Navy used radio DF extensively with the aid of listening stations. The latter had been established in the wake of escalating international conflicts, including the Italian invasion of Ethiopia (which potentially threatened British-controlled lands in east Africa) and the Spanish Civil War, during which Italian submarines threatened British vessels transporting supplies to Republican forces.

By the late 1930s, the British had begun using high-frequency direction finding (HF/DF or "Huff Duff") equipment on their warships. This technology had benefited from improvements by Canadian engineers, who created a means of automatically recording the directional bearings of transmissions by radio. During the Second World War, the Royal Navy successfully used HF/DF to locate German submarines in the north Atlantic.

Across the ocean, the U.S. Navy received help from French scientists who had escaped the Nazi and Vichy regimes, and who assisted Navy technicians in developing a means of visual imaging to record the bearings of a vessel emitting transmissions. This equipment, tested in 1940 and operational by the latter part of 1942, also made it possible to maintain a track on an enemy U-boat even after the latter had stopped transmitting.

DF in the Cold War and modern era. The Germans themselves made advances in antenna technology, but because of their failure to accurately assess Allied DF capabilities, the principal beneficiaries of these developments would later be their wartime adversaries. During the 1950s and 1960s, the U.S. military adapted German Wullenweber antenna systems for use in Vietnam and other theatres of the Cold War. The United States also made used of the Wullenwebers (sometimes referred to as circularly disposed dipole antenna arrays or CDDAs) for land-based electronic eavesdropping, taking advantage of their wide operational range of 3,200 miles (5,150 km). Today, abandoned Wullenwebers—nicknamed "rings of poles," "dinosaur cages," or "elephant cages"—dot the globe, an almost poignant visual symbol of the long-vacated superpower conflict.

At the turn of the twenty-first century, radio DF equipment was a standard feature of U.S. Navy vessels. By 2000, the OUTBOARD system had been in use on naval vessels for many years, and was slated for an upgrade through the Cooperative OUTBOARD Logistics Update (COBLU) program. OUTBOARD made use of high-frequency deck-edge antennas and VHF (very high frequency) mast antennas, as well as a receiver that automatically searched for, received, collected, and analyzed signals. Thus, it combined the receiving and processing functions in a single piece of equipment. Its control/output system is capable of collecting processed cryptologic transmissions and transmitting intelligence to other members of the battle group via data links.

PERIODICALS:

Cochran, William W. "Direction Finding at Ultra High Frequency (UHF): Improved Accuracy." Wildlife Society Bulletin 29, no. 2 (summer 2001): 594.

ELECTRONIC:

Herskovitz, Don. "A Sampling of Direction-Finding Systems." Journal of Electronic Defense 23, no. 8 (August 2000): 57–65.

Rivers, Brendan. "U.S. Navy Orders OUTBOARD Update." Journal of Electronic Defense 23, no. 8 (August 2000): 31.

Robinson, Clarence O., Jr. "Position-Fixing Methods Use Broadband Direction Finders." Signal 53, no. 2 (October 1998): 71–74.

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