Direction finding - Knowledge

917:(AM) broadcast beacon or station (listening for the null is easier than listening for a peak signal, and normally produces a more accurate result). This null was symmetrical, and thus identified both the correct degree heading marked on the radio's compass rose as well as its 180-degree opposite. While this information provided a baseline from the station to the ship or aircraft, the navigator still needed to know beforehand if he was to the east or west of the station in order to avoid plotting a course 180-degrees in the wrong direction. By taking bearings to two or more broadcast stations and plotting the intersecting bearings, the navigator could locate the relative position of his ship or aircraft. 1177: 1159: 881: 1602: 1168: 868: 1263: 478:", or "huff-duff" systems. To avoid RDF, the Germans had developed a method of broadcasting short messages under 30 seconds, less than the 60 seconds that a trained Bellini-Tosi operator would need to determine the direction. However, this was useless against huff-duff systems, which located the signal with reasonable accuracy in seconds. The Germans did not become aware of this problem until the middle of the war, and did not take any serious steps to address it until 1944. By that time huff-duff had helped in about one-quarter of all successful attacks on the U-boat fleet. 392: 1623: 1556: 1580: 716: 3155: 1640: 1887: 2079: 436: 1863: 1534: 1146: 2087: 1875: 1061: 300: 147: 2071: 561: 1899: 1131: 510:, with loops wound around two perpendicular sides. Signals from the loops are sent into a phase comparison circuit, whose output phase directly indicates the direction of the signal. By sending this to any manner of display, and locking the signal using PLL, the direction to the broadcaster can be continuously displayed. Operation consists solely of tuning in the station, and is so automatic that these systems are normally referred to as 1657: 1331:

SL system was in a metal underground tank below the antennas. Seven underground tanks were installed, but only two SL systems were installed at Wymondham, Norfolk and Weaverthorp in Yorkshire. Problems were encountered resulting in the remaining five underground tanks being fitted with Adcock systems. The rotating SL antenna was turned by hand which meant successive measurements were a lot slower than turning the dial of a goniometer.

50: 1382:" FM broadcast radio transmissions. A network of remotely operated VHF direction finders are used mainly located around the major cities. The transmissions from mobile telephone handsets are also located by a form of direction finding using the comparative signal strength at the surrounding local "cell" receivers. This technique is often offered as evidence in UK criminal prosecutions and, almost certainly, for SIGINT purposes. 4654: 2095:

the relative powers of the signals received. When the signal is on the boresight of one of the antennas, the signal at the other antenna will be about 12 dB lower. When the signal direction is halfway between the two antennas, signal levels will be equal and approximately 3 dB lower than the boresight value. At other bearing angles, φ, some intermediate ratio of the signal levels will give the direction.

3147: 4628: 155: 404:, where the signals were re-created in the area between the coils. A separate loop antenna located in this area could then be used to hunt for the direction, without moving the main antennas. This made RDF so much more practical that it was soon being used for navigation on a wide scale, often as the first form of aerial navigation available, with ground stations homing in on the aircraft's radio set. 1355:, Essex, Goonhavern, Cornwall, Anstruther and Bowermadden in the Scottish Highlands. Groups were also built in Iceland, Nova Scotia and Jamaica. The anticipated improvements were not realised but later statistical work improved the system and the Goonhavern and Ford End groups continued to be used during the Cold War. The Royal Navy also deployed direction finding equipment on ships tasked to 4352: 1211:(520 – 1720 kHz) frequency incorporating the station's identifier that is used to confirm the station and its operational status. Since these radio signals are broadcast in all directions (omnidirectional) during the day, the signal itself does not include direction information, and these beacons are therefore referred to as 3997: 4668:

A necessary part of the DF analysis is to identify the channel which contains the largest signal and this is achieved by means of a fast comparator circuit. In addition to the DF process, other properties of the signal may be investigated, such as pulse duration, frequency, pulse repetition frequency

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The signals received by the antennas are first amplified by a low-noise preamplifier before detection by detector-log-video-amplifiers (DLVAs). The signal levels from the DLVAs are compared to determine the angle of arrival. By considering the signal levels on a logarithmic scale, as provided by the

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Two-channel DF, using two adjacent antennas of a circular array, is achieved by comparing the signal power of the largest signal with that of the second largest signal. The direction of an incoming signal, within the arc described by two antennas with a squint angle of Φ, may be obtained by comparing

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Events hosted by groups and organizations that involve the use of radio direction finding skills to locate transmitters at unknown locations have been popular since the end of World War II. Many of these events were first promoted in order to practice the use of radio direction finding techniques for

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By 1941 only a couple of illicit transmitters had been identified in the UK; these were German agents that had been "turned" and were transmitting under MI5 control. Many illicit transmissions had been logged emanating from German agents in occupied and neutral countries in Europe. The traffic became

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As the commercial medium wave broadcast band lies within the frequency capability of most RDF units, these stations and their transmitters can also be used for navigational fixes. While these commercial radio stations can be useful due to their high power and location near major cities, there may be

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Amplitude comparison has been popular as a method for DF because systems are relatively simple to implement, have good sensitivity and, very importantly, a high probability of signal detection. Typically, an array of four, or more, squinted directional antennas is used to give 360 degree coverage.

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m beam. The angle of the beam was combined with results from a radiogoniometer to provide a bearing. The bearing obtained was considerably sharper than that obtained with the U Adcock system, but there were ambiguities which prevented the installation of 7 proposed S.L DF systems. The operator of an

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navigational systems. However the low cost of ADF and RDF systems, and the continued existence of AM broadcast stations (as well as navigational beacons in countries outside North America) has allowed these devices to continue to function, primarily for use in small boats, as an adjunct or backup to

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RDF was once the primary form of aircraft and marine navigation. Strings of beacons formed "airways" from airport to airport, while marine NDBs and commercial AM broadcast stations provided navigational assistance to small watercraft approaching a landfall. In the United States, commercial AM radio

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A "sense antenna" is used to resolve the two direction possibilities; the sense aerial is a non-directional antenna configured to have the same sensitivity as the loop aerial. By adding the steady signal from the sense aerial to the alternating signal from the loop signal as it rotates, there is now

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Several distinct generations of RDF systems have been used over time, following new developments in electronics. Early systems used mechanically rotated antennas that compared signal strengths from different directions, and several electronic versions of the same concept followed. Modern systems use

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as they can travel very long distances and "over the horizon", which is valuable for ships when the line-of-sight may be only a few tens of kilometres. For aircraft, where the horizon at altitude may extend to hundreds of kilometres, higher frequencies can be used, allowing much smaller antennas. An

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systems use a series of small dipole antennas arranged in a ring and use electronic switching to rapidly select dipoles to feed into the receiver. The resulting signal is processed and produces an audio tone. The phase of that audio tone, compared to the antenna rotation, depends on the direction of

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Often, the wideband amplifiers are protected from local high power sources (as on a ship) by input limiters and/or filters. Similarly the amplifiers might contain notch filters to remove known, but unwanted, signals which could impairs the system's ability to process weaker signals. Some of these

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of the DF system. This results in the signal strength at the radar receiver being very much smaller than that at the DF receiver. Consequently, in spite of its poor sensitivity, a simple crystal-video DF receiver is, usually, able to detect the signal transmission from a radar at a greater range

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The DF receiver enjoys a detection range advantage over that of the radar receiver. This is because the signal strength at the DF receiver, due to a radar transmission, is proportional to 1/R whereas that at the radar receiver from the reflected return is proportional to σ/R, where R is the range

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are linear devices and so, in principle, could be used as receiver preamplifiers. However, the klystron was quite unsuitable as it was a narrow-band device and extremely noisy and the TWT, although potentially more suitable, has poor matching characteristics and large bulk, which made it unsuitable

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Single-channel DF uses a multi-antenna array with a single channel radio receiver. This approach to DF offers some advantages and drawbacks. Since it only uses one receiver, mobility and lower power consumption are benefits. Without the ability to look at each antenna simultaneously (which would be

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The bearing value, obtained using this equation, is independent of the antenna beamwidth (= 2.Ψ0), so this value does not have to be known for accurate bearing results to be obtained. Also, there is a smoothing affect, for bearing values near to the boresight of the middle antenna, so there is no

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When the signal is on the boresight of Antenna 1 (φ = 0), the signal from the other two antennas will equal and about 12 dB lower. When the signal direction is halfway between two antennas (φ = 30°), their signal levels will be equal and approximately 3 dB lower than the boresight value,

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became small enough to attach to wildlife, and is now widely deployed for a variety of wildlife studies. Most tracking of wild animals that have been affixed with radio transmitter equipment is done by a field researcher using a handheld radio direction finding device. When the researcher wants to

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Another experimental spaced loop station was built near Aberdeen in 1942 for the Air Ministry with a semi-underground concrete bunker. This, too, was abandoned because of operating difficulties. By 1944, a mobile version of the spaced loop had been developed and was used by RSS in France following

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uses a pair of monopole or dipole antennas that takes the vector difference of the received signal at each antenna so that there is only one output from each pair of antennas. Two of these pairs are co-located but perpendicularly oriented to produce what can be referred to as the N–S (North-South)

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in the signal, and it is used instead of the strongest signal direction, because small angular deflections of the loop aerial away from its null positions produce much more abrupt changes in received current than similar directional changes around the loop's strongest signal orientation. Since the

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Early RDF systems were useful largely for long wave signals. These signals are able to travel very long distances, which made them useful for long-range navigation. However, when the same technique was being applied to higher frequencies, unexpected difficulties arose due to the reflection of high

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A key improvement in the RDF concept was introduced by Ettore Bellini and Alessandro Tosi in 1909 (U.S. Patent 943,960). Their system used two such antennas, typically triangular loops, arranged at right angles. The signals from the antennas were sent into coils wrapped around a wooden frame about

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systems. This permits better identification of multiple targets and, also, gives improved directional accuracy. Also, the antennas are small so they can be assembled into compact arrays and, in addition, they can achieve well defined beam patterns which can provide the narrow beams with high gain

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The basic principle of the correlative interferometer consists in comparing the measured phase differences with the phase differences obtained for a DF antenna system of known configuration at a known wave angle (reference data set). For this, at least three antenna elements (with omnidirectional

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technique uses two antenna pairs to perform an amplitude comparison on the incoming signal. The popular Watson-Watt method uses an array of two orthogonal coils (magnetic dipoles) in the horizontal plane, often completed with an omnidirectional vertically polarized electric dipole to resolve 180°

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that substantiated the correct bearing and allowed the navigator to avoid plotting a bearing 180 degrees opposite the actual heading. The U.S. Navy RDF model SE 995 which used a sense antenna was in use during World War I. After World War II, there were many small and large firms making direction

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used as an antenna. When the antenna was aligned so it pointed at the signal it produced maximum gain, and produced zero signal when face on. This meant there was always an ambiguity in the location of the signal: it would produce the same output if the signal was in front or back of the antenna.

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Usually, the signal amplitudes in two adjacent channels of the array are compared, to obtain the bearing of an incoming wavefront but, sometimes, three adjacent channels are used to give improved accuracy. Although the gains of the antennas and their amplifying chains have to be closely matched,

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can detect the general direction of such naturally-occurring radio sources, sometimes correlating their location with objects visible with optical telescopes. Accurate measurement of the arrival time of radio impulses by two radio telescopes at different places on Earth, or the same telescope at

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aerial. A Yagi antenna uses multiple dipole elements, which include "reflector" and "director" dipole elements. The "reflector" is the longest dipole element and blocks nearly all the signal coming from behind it, hence a Yagi has no front vs. back directional ambiguity: The maximum signal only

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has quite pronounced directionality, so the source of a transmission can be determined by pointing it in the direction where the maximum signal level is obtained. Since the directional characteristics can be very broad, large antennas may be used to improve precision, or null techniques used to

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several miles between the location of the station and its transmitter, which can reduce the accuracy of the 'fix' when approaching the broadcast city. A second factor is that some AM radio stations are omnidirectional during the day, and switch to a reduced power, directional signal at night.

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In the United Kingdom a radio direction finding service is available on 121.5 MHz and 243.0 MHz to aircraft pilots who are in distress or are experiencing difficulties. The service is based on a number of radio DF units located at civil and military airports and certain HM Coastguard

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aircraft, RDF antennas are easy to identify as the circular loops mounted above or below the fuselage. Later loop antenna designs were enclosed in an aerodynamic, teardrop-shaped fairing. In ships and small boats, RDF receivers first employed large metal loop antennas, similar to aircraft, but

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Early microwave receivers were usually simple "crystal-video" receivers, which use a crystal detector followed by a video amplifier with a compressive characteristic to extend the dynamic range. Such a receiver was wideband but not very sensitive. However, this lack of sensitivity could be

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system, in which the bearing to the navigational aid is measured from the signal itself; therefore no specialized antenna with moving parts is required. Due to relatively low purchase, maintenance and calibration cost, NDBs are still used to mark locations of smaller aerodromes and important

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The move to microwave frequencies meant a reappraisal of the requirements of a DF system. Now, the receiver could no longer rely on a continuous signal stream on which to carry out measurements. Radars with their narrow beams would only illuminate the antennas of the DF system infrequently.

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Typically, the correlative interferometer DF system consists of more than five antenna elements. These are scanned one after the other via a specific switching matrix. In a multi-channel DF system n antenna elements are combined with m receiver channels to improve the DF-system performance.

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Similar beacons located in coastal areas are also used for maritime radio navigation, as almost every ship was equipped with a direction finder (Appleyard 1988). Very few maritime radio navigation beacons remain active today (2008) as ships have abandoned navigation via RDF in favor of GPS

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In spite of the system being presented publicly, and its measurements widely reported in the UK, its impact on the art of RDF seems to be strangely subdued. Development was limited until the mid-1930s, when the various British forces began widespread development and deployment of these

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company in 1905. This consisted of a number of horizontal wires or rods arranged to point outward from a common center point. A movable switch could connect opposite pairs of these wires to form a dipole, and by rotating the switch the operator could hunt for the strongest signal. The

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pointing in different directions. At first, this system was used by land and marine-based radio operators, using a simple rotatable loop antenna linked to a degree indicator. This system was later adopted for both ships and aircraft, and was widely used in the 1930s and 1940s. On

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Antennas for DF have to meet different requirements from those for a radar or communication link, where an antenna with a narrow beam and high gain is usually an advantage. However, when carrying out direction finding, the bearing of the source may be unknown, so antennas with wide

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techniques compare the arrival time of a radio wave at two or more different antennas and deduce the direction of arrival from this timing information. This method can use mechanically simple non-moving omnidirectional antenna elements fed into a multiple channel receiver system.

1299:. In addition to the fixed stations, RSS ran a fleet of mobile DF vehicles around the UK. If a transmitter was identified by the fixed DF stations or voluntary interceptors, the mobile units were sent to the area to home in on the source. The mobile units were HF Adcock systems. 1350:

introduced a variation on the shore based HF DF stations in 1944 to track U-boats in the North Atlantic. They built groups of five DF stations, so that bearings from individual stations in the group could be combined and a mean taken. Four such groups were built in Britain at

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operate on a standard 457 kHz, and are designed to help locate people and equipment buried by avalanches. Since the power of the beacon is so low the directionality of the radio signal is dominated by small scale field effects and can be quite complicated to locate.

4623:{\displaystyle \Delta \phi _{rms}={\frac {1}{-2.ln(0.5)}}.{\frac {\Psi _{0}^{2}}{\Phi ^{2}}}.{\sqrt {{\bigg (}\phi +{\frac {\Phi }{2}}{\bigg )}^{2}.{\frac {1}{SNR_{2}}}+{\frac {4.\phi ^{2}}{SNR_{1}}}+{\bigg (}\phi -{\frac {\Phi }{2}}{\bigg )}^{2}.{\frac {1}{SNR_{3}}}}}} 3165:

For three-channel DF, with three antennas squinted at angles Φ, the direction of the incoming signal is obtained by comparing the signal power of the channel containing the largest signal with the signal powers of the two adjacent channels, situated at each side of it.

2102:(dB) relative to the boresight value), then there is a linear relationship between the bearing angle φ and the power level difference, i.e. φ ∝ (P1(dB) - P2(dB)), where P1(dB) and P2(dB) are the outputs of two adjacent channels. The thumbnail shows a typical plot. 1294:

signals reflected back from the ionised layers in the upper atmosphere. Even with the expanded network, some areas were not adequately covered and for this reason up to 1700 voluntary interceptors (radio amateurs) were recruited to detect illicit transmissions by

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a valuable source of intelligence, so the control of RSS was subsequently passed to MI6 who were responsible for secret intelligence originating from outside the UK. The direction finding and interception operation increased in volume and importance until 1945.

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For adjacent processing using, say, Channel 1 and Channel 2, the bearing uncertainty (angle noise), Δø (rms), is given below. In these results, square-law detection is assumed and the SNR figures are for signals at video (baseband), for the bearing angle φ.

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occurs when the narrowest end of the Yagi is aimed in the direction from which the radio waves are arriving. With a sufficient number of shorter "director" elements, a Yagi's maximum direction can be made to approach the sharpness of a small loop's null.

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strikes as a method to indicate the direction of thunderstorms for sailors and airmen. He had long worked with conventional RDF systems, but these were difficult to use with the fleeting signals from the lightning. He had early on suggested the use of an

609:). Using two or more measurements from different locations, the location of an unknown transmitter can be determined; alternately, using two or more measurements of known transmitters, the location of a vehicle can be determined. RDF is widely used as a 998:

induced on the signal by sampling around the elements of a circular array. The original method used a single antenna that physically moved in a circle but the modern approach uses a multi-antenna circular array with each antenna sampled in succession.

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Many of the causes of bearing error, such as mechanical imperfections in the antenna structure, poor gain matching of receiver gains, or non-ideal antenna gain patterns may be compensated by calibration procedures and corrective look-up tables, but

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To give 360° coverage, antennas of a circular array are chosen, in pairs, according to the signal levels received at each antenna. If there are N antennas in the array, at angular spacing (squint angle) Φ, then Φ = 2π/N radians (= 360/N degrees).

4669:(PRF) and modulation characteristics. The comparator operation usually includes hysteresis, to avoid jitter in the selection process when the bearing of the incoming signal is such that two adjacent channels contain signals of similar amplitude. 4166:

To obtain more precise predictions at a given bearing, the actual S:N ratios of the signals of interest are used. (The results may be derived assuming that noise induced errors are approximated by relating differentials to uncorrelated noise).

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were fitted with a medium frequency direction finding antenna (MF/DF) (the antenna was fitted in front of the bridge) and high frequency direction finding (HF/DF, "Huffduff") Type FH 4 antenna (the antenna was fitted on top of the mainmast).

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reception characteristics) must form a non-collinear basis. The comparison is made for different azimuth and elevation values of the reference data set. The bearing result is obtained from a correlative and stochastic evaluation for which the

320:, which worked in the opposite sense, reaching maximum gain at right angles and zero when aligned. RDF systems using mechanically swung loop or dipole antennas were common by the turn of the 20th century. Prominent examples were patented by 751:

responsive to those arriving edge-on. This is caused by the phase of the received signal: The difference in electrical phase along the rim of the loop at any instant causes a difference in the voltages induced on either side of the loop.

538:, which has been in force since 1999. The striking cross frame antenna with attached auxiliary antenna can only be found on the signal masts of some older ships because they do not interfere there and dismantling would be too expensive. 1367:

A comprehensive reference on World War II wireless direction finding was written by Roland Keen, who was head of the engineering department of RSS at Hanslope Park. The DF systems mentioned here are described in detail in his 1947 book

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and ships. Historically emergency location transmitters only sent a tone signal and relied on direction finding by search aircraft to locate the beacon. Modern emergency beacons transmit a unique identification signal that can include

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is the overall gain of each channel, including antenna boresight gain, and is assumed to be the same in all three channels. As before, in these equations, angles are in radians, Φ = 360/N degrees = 2 π/N radians and A = -ln(0.5).

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The US military used a shore based version of the spaced loop DF in World War II called "DAB". The loops were placed at the ends of a beam, all of which was located inside a wooden hut with the electronics in a large cabinet with

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DF by phase comparison methods can give better bearing accuracy, but the processing is more complex. Systems using a single rotating dish antenna are more sensitive, small and relatively easy to implement, but have poor PoI.

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is a very common design. For longwave use, this resulted in loop antennas tens of feet on a side, often with more than one loop connected together to improve the signal. Another solution to this problem was developed by the

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Al-Sharabi K.I.A. and Muhammad D.F., "Design of Wideband Radio Direction Finder Based on Amplitude Comparison", Al-Rafidain Engineering, Vol. 19, Oct 2011, pp.77-86 (Find at: www.iasj.net/iasj?func=fulltext&aid=26752

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system, in which the direction to the beacon can be extracted from the signal itself, hence the distinction with non-directional beacons. Use of marine NDBs was largely supplanted in North America by the development of

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In general, in order to cater for modern circumstances, a broadband microwave DF system is required to have high sensitivity and have 360° coverage in order to have the ability to detect single pulses (often called

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In particular, the ability to compare the phase of signals led to phase-comparison RDF, which is perhaps the most widely used technique today. In this system the loop antenna is replaced with a single square-shaped

208:, 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. 2008:, by the Radar, to increase the processing gain of its receiver. On the other hand, the DF system can regain some advantage by using sensitive, low-noise, receivers and by using Stealth practices to reduce its 416:. The RDF station might now receive the same signal from two or more locations, especially during the day, which caused serious problems trying to determine the location. This led to the 1919 introduction of the 924:

antennas, which made the sets more portable and less bulky. Some were later partially automated by means of a motorized antenna (ADF). A key breakthrough was the introduction of a secondary vertical whip or

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systems used large RDF receivers to determine directions. Later radar systems generally used a single antenna for broadcast and reception, and determined direction from the direction the antenna was facing.

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frequencies were developed in the 1940s, in response to the growing numbers of transmitters operating at these higher frequencies. This required the design of new antennas and receivers for the DF systems.

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were used to mark "airways" intersections and to define departure and approach procedures. Since the signal transmitted contains no information about bearing or distance, these beacons are referred to as

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Turning the plane of the loop to "face" the signal so that the arriving phases are identical around the entire rim will not induce any current flow in the loop. So simply turning the antenna to produce a

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on an insulating frame, or a metal ring that forms the antenna's loop element itself; often the diameter of the loop is a tenth of a wavelength or smaller at the target frequency. Such an antenna will be

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careful design and construction and effective calibration procedures can compensate for shortfalls in the hardware. Overall bearing accuracies of 2° to 10° (rms) have been reported using the method.

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DLVAs, a large dynamic range is achieved and, in addition, the direction finding calculations are simplified when the main lobes of antenna patterns have a Gaussian characteristic, as shown earlier.

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Furthermore, some radars wishing to avoid detection (those of smugglers, hostile ships and missiles) would radiate their signals infrequently and often at low power. Such a system is referred to as a

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Mills R.F. and Prescott G.E., "Detectability Models for Multiple Access Low-Probability-of-Intercept Networks", IEEE Trans on Aerospace and Electronic System, Vol.36, No.3, July 2000, pp> 848-858.

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Early radio direction finders used mechanically rotated antennas that compared signal strengths, and several electronic versions of the same concept followed. Modern systems use the comparison of

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of the signal; very long wavelengths (low frequencies) require very large antennas, and are generally used only on ground-based systems. These wavelengths are nevertheless very useful for marine

2033:. In other applications, such as microwave links, the transmitter's antenna may never point at the DF receiver at all, so reception is only possible by means of the signal leakage from antenna 769:

only one position as the loop rotates 360° at which there is zero current. This acts as a phase reference point, allowing the correct null point to be identified, removing the 180° ambiguity. A

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allowed much higher frequencies to be used economically, which led to widespread use of VHF and UHF signals. All of these changes led to new methods of RDF, and its much more widespread use.

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In the case of 3-channel processing, an expression which is applicable when the S:N ratios in all three channels exceeds unity (when ln(1 + 1/SNR) ≈ 1/SNR is true in all three channels), is

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may be only a few tens of kilometres. For aerial use, where the horizon may extend to hundreds of kilometres, higher frequencies can be used, allowing the use of much smaller antennas. An

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for multi-channel systems using a preamplifier per antenna. However, a system has been demonstrated, in which a single TWT preamplifier selectively selects signals from an antenna array.

1814:. Preferably, when using amplitude comparison methods for direction finding, the main lobe should approximate to a Gaussian characteristic. Although the figure also shows the presence of 1290:

developed this into a larger network. One of the problems with providing coverage of an area the size of the UK was installing sufficient DF stations to cover the entire area to receive

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stations were required to broadcast their station identifier once per hour for use by pilots and mariners as an aid to navigation. In the 1950s, aviation NDBs were augmented by the

681: 1771:(SW) band) and great increase in signal spectrum, compared to the congested RF bands already in use. In addition to being able to accommodate many more signals, the ability to use 3992:{\displaystyle \phi ={\frac {\Delta _{1,2}-\Delta _{1,3}}{\Delta _{1,2}+\Delta _{1,3}}}.{\frac {\Phi }{2}}={\frac {\Delta _{2,3}}{\Delta _{1,2}+\Delta _{1,3}}}.{\frac {\Phi }{2}}} 1601: 971:

the case if one were to use multiple receivers, also known as N-channel DF) more complex operations need to occur at the antenna in order to present the signal to the receiver.

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null direction gives a clearer indication of the signal direction – the null is "sharper" than the max – with loop aerial the null direction is used to locate a signal source.

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followed. With these new devices, low-noise receiver preamplifiers became possible, which greatly increased the sensitivity, and hence the detection range, of DF systems.

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are capable of very wide bandwidths and have a nominal half-power beamwidth of about 70deg, making them very suitable for antenna arrays containing 4, 5 or 6 antennas.

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suite (ESM), where the directional information obtained augments other signal identification processes. In aircraft, a DF system provides additional information for the

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National Air Warfare Center, "Electronic Warfare and Radar Systems", NAWCWD TP 8347, 4th Ed., 2013. Find at: www.microwaves101.com/encyclopedias/ew-and-radar-handbook)

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In practice, the advantage is reduced by the ratio of antenna gains (typically they are 36 dB and 10 dB for the Radar and ESM, respectively) and the use of

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lifeboats in the UK, and Search and Rescue helicopters have direction finding receivers for marine VHF signals and the 121.5 MHz homing signals incorporated in

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signals. Longwave in particular had good long-distance transmission characteristics due to their limited interaction with the ground, and thereby provided excellent

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Over time, it became necessary to improve the performance of microwave DF systems in order to counter the evasive tactics being employed by some operators, such as

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In World War II considerable effort was expended on identifying secret transmitters in the United Kingdom (UK) by direction finding. The work was undertaken by the

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For the antennas in a circular array, three antennas are selected according to the signal levels received, with the largest signal present at the central channel.

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Intensive research work was carried out in the 1930s in order to develop transmitting tubes specifically for the microwave band which included, in particular, the

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discontinuity in bearing values there, as an incoming signals moves from left to right (or vice versa) through boresight, as can occur with 2-channel processing.

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Blake B. (ed.), "Manta", " Sceptre" and " Cutlass" ESM Systems, Jane's Radar and Electronic Warfare Systems, 1st Ed., Jane's Information Group, 1989, pp.344 -345

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Antennas are generally sensitive to signals only when they have a length that is a significant portion of the wavelength, or larger. Most antennas are at least

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By the early 1900s, many experimenters were looking for ways to use this concept for locating the position of a transmitter. Early radio systems generally used

1511:", "mobile T-hunting" or "fox hunting" takes place in a larger geographic area, such as the metropolitan area of a large city, and most participants travel in 420:(UK Patent 130,490), which consisted of four separate monopole antennas instead of two loops, eliminating the horizontal components and thus filtering out the 196:

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

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Modern positioning methods such as GPS, DGPS, radar and the now-outdated Loran C have radio direction finding methods that are imprecise for today's needs.

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At the end of World War II a number of RSS DF stations continued to operate into the Cold War under the control of GCHQ the British SIGINT organisation.

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W.G. Wade of the National Bureau of Standards uses a large multi-loop antenna to perform RDF in this 1919 photo. This is a fairly small unit for the era.

2864: 6621: 1919:, horns may be used. The bandwidths of horn antennas may be increased by using double-ridged waveguide feeds and by using horns with internal ridges. 215:(low frequencies) require very large antennas, and are generally used only on ground-based systems. These wavelengths are nevertheless used for marine 5379: 3173:

with the third signal now about 24 dB lower. At other bearing angles, ø, some intermediate ratios of the signal levels will give the direction.

1043:

is at a maximum. If the direction finding antenna elements have a directional antenna pattern, then the amplitude may be included in the comparison.

1955: 845:

directional circuits used to measure direction by comparing the differences in two or more matched reference antennas' received signals, used in old

383:

overcame this problem, to a point, by mounting antennas on ships and sailing in circles. Such systems were unwieldily and impractical for many uses.

5027: 4079: 1430:

locate a particular animal, the location of the animal can be triangulated by determining the direction to the transmitter from several locations.

4324:{\displaystyle \Delta \phi _{RMS}={\frac {\Phi }{2}}.{\frac {\Psi _{0}^{2}}{-ln(0.5).\Phi }}.{\sqrt {{\frac {1}{SNR_{1}}}+{\frac {1}{SNR_{2}}}}}} 943: 346:

that pointed directly to the transmitter. Methods of performing RDF on longwave signals was a major area of research during the 1900s and 1910s.

200:, the location of a radio source can be determined by measuring its direction from two or more locations. Radio direction finding is used in 901:

In use, the RDF operator would first tune the receiver to the correct frequency, then manually turn the loop, either listening or watching an

1026:

and E–W (East-West) signals that will then be passed to the receiver. In the receiver, the bearing angle can then be computed by taking the

760:

in the desired signal will establish two possible directions (front and back) from which the radio waves could be arriving. This is called a

242:. The ability to locate the position of an enemy transmitter has been invaluable since World War I, and played a key role in World War II's 6530: 6194: 5357:

Stove A.G. Hume A.L. and Baker C.j., "Low probability of intercept radar strategies", IEE Proc. Sonar Navig., Vol. 151, No. 5, October 2004

1318:

m lattice tower antennas. In 1941, RSS began experimenting with spaced loop direction finders, developed by the Marconi company and the UK

951: 5294:

East P.W., "ESM Range Advantage", IEE Proceedings F - Communications, Radar and Signal Processing, Vol.132, No.4, Jul 1985, pp. 223 - 225

6177: 6144: 6049: 1959: 1579: 1449:

systems and their resulting trajectories. These systems can be used for defensive purposes and also to gain intelligence on operation of

1390: 1319: 192:

or may be an inadvertant source, a naturally-occurring radio source, or an illicit or enemy system. Radio direction finding differs from

5435:

Ly P.Q.C, "Fast and Unabiguous Direction Finding for Digital Radar Intercept Receivers", Univ. of Adelaide, Dec. 2013, p. 16. Find at:

5515:

Blake B. (ed.), " Cutlass ESM Equipment", Jane's Radar and Electronic Warfare Systems, 3rd Ed., Jane's Information Group, 1991, p. 406

6593: 6234: 5597: 1282:(RSS also MI8). Initially three U Adcock HF DF stations were set up in 1939 by the General Post Office. With the declaration of war, 5524:

Streetly M., "SPS-N 5000 ESM System", Jane's Radar and Electronic Warfare Systems, 10th Ed., Jane's Information Group, 1998, p. 396

2848:{\displaystyle \ln {\Big (}{\frac {P_{1}}{P_{2}}}{\Big )}=\ln(P_{1})-\ln(P_{2})={\frac {A}{\Psi _{0}^{2}}}.(\Phi ^{2}-2\Phi \phi )} 1421:

is a widely applied research technique for studying the movement of animals. The technique was first used in the early 1960s, when

1343:

display at the centre of the beam and everything being supported on a central axis. The beam was rotated manually by the operator.

4343:

are the video (base-band) signal-to-noise values for the channels for Antenna 1 and Antenna 2, when square-law detection is used.

6626: 5548:

Pasternack, " Broadband Log Video Amplifiers". Find at: www.pasternack.com/pages/Featured_Products/broadband-log-video-amplifiers

5333:

Wise J.C., "A Perspective on EW Receiver Design", Tech. Report APA-TR-2009-1102, J.C. Wise and Associates, Nov. 2009, Find at:.

4066:

will always be a degrading factor. As all systems generate thermal noise then, when the level of the incoming signal is low, the

2098:

If the antenna main lobe patterns have a Gaussian characteristic, and the signal powers are described in logarithmic terms (e.g.

1782:

Once microwave techniques had become established, there was rapid expansion into the band by both military and commercial users.

707:

is used which is more sensitive in certain directions than in others. Many antenna designs exhibit this property. For example, a

17: 5557:

American Microwave Corporation, DLVA Model: LVD-218-50. Find at: www.americanmic.com/catalog/detector-log-video-amplifiers-dlva/

3162:

Improvements in bearing accuracy may be achieved if amplitude data from a third antenna are included in the bearing processing.

114: 6422: 6392: 6137: 6032: 5988: 4881: 86: 6575: 5064: 395:

This Royal Navy model is typical of B–T goniometers. The two sets of "field coils" and the rotating "sense coil" are visible.

5273:

Lipkin H.J., "Crystal-Video Receivers", MIT Radiation Series Vol 23, Microwave Receivers, Chapter 19 pp.504-506. Find at:

2997:

This shows the linear relationship between the output level difference, expressed logarithmically, and the bearing angle ø.

1555: 6224: 5983: 684:

systems consist of a number of small antennas fixed to a circular card, with all of the processing performed by software.

424:

being reflected down from the ionosphere. Adcock antennas were widely used with Bellini–Tosi detectors from the 1920s on.

5738: 1954:

Transistors suitable for microwave frequencies became available towards the end of the 1950s. The first of these was the

1134: 825:

directing received signals from a very narrow angle into a small receiving element mounted at the focus of the parabola.

93: 1837:

are the horizontal and vertical antenna beamwidths, respectively, in degrees. For a circular aperture, with beamwidth BW

1185:

R-5/ARN7 radio compass components, with the radio control box (left), indicator (center), and radio compass unit (right)

6168: 4798: 2030: 1701: 67: 2124: 1743:(TWT). Following the successful development of these tubes, large scale production occurred in the following decade. 5884: 5690: 5614: 5582: 5456:

Stott G.F., "DF Algorithms for ESM", Military Microwaves '88 Conference Proceedings, London, July 1988, pp. 463 – 468

5009: 4960: 1963: 695:

systems used separate omnidirectional broadcasters and large RDF receivers to determine the location of the targets.

405: 133: 5436: 4645:

are the video signal-to-noise values for Channel 1, Channel 2, and Channel 3 respectively, for the bearing angle φ.

1471:

Earth-based receivers can detect radio signals emanating from distant stars or regions of ionized gas. Receivers in

6636: 6525: 6517: 5919: 5844: 1253: 649: 475: 247: 100: 4932:

A paper on the technology and practice of the HF/DF systems used by the Royal Navy against U-boats in World War II

691:

sets were also referred to as RDF, which was a deception tactic. However, the terminology was not inaccurate; the

6535: 4941:

Gebhard, Louis A "Evolution of Naval Radio-Electronics and Contributions of the Naval Research Laboratory" (1979)

1821:

Typically, the boresight gain of an antenna is related to the beam width. For a rectangular horn, Gain ≈ 30000/BW

1504: 1485: 991: 636:

systems and methodologies. The ability to locate the position of an enemy transmitter has been invaluable since

616:

RDF systems can be used with any radio source, although the size of the receiver antennas are a function of the

6466: 6291: 6206: 5786: 5274: 3430: 3306: 3190: 2296: 1751:

Microwave signals have short wavelengths, which results in greatly improved target resolution when compared to

1622: 1533: 1176: 71: 5669: 1476:

different times in Earth's orbit around the Sun, may also allow estimation of the distance to a radio object.

1158: 82: 6661: 6160: 1639: 880: 522:

the signal. Doppler RDF systems have widely replaced the huff-duff system for location of fleeting signals.

427:

The US Army Air Corps in 1931 tested a primitive radio compass that used commercial stations as the beacon.

6646: 4767: 1975: 1590: 1500: 921: 5285:

Kiely D.G., "Advances in microwave direction finding", Proc. IEE, Vol. 113, No.11, Nov 1964, pp. 1967–1711

1767:

Other advantages of the newly available microwave band were the absence of fading (often a problem in the

6656: 6583: 5038: 1690: 5625: 5536:

MITEQ, "IF Signal Processing Components and Subsystems", Application Notes" pp. 33-51, (2010), Find at:

946:. By the 1960s, many of these radios were actually made by Japanese electronics manufacturers, such as 6671: 5000:

deRosa, L.A. (1979). "Direction Finding". In J.A. Biyd; D.B. Harris; D.D. King; H.W. Welch Jr. (eds.).

1656: 1040: 511: 224: 31: 1453:

belonging to other nations. These same techniques are used for detection and tracking of conventional

1262: 1167: 6651: 5478:

Schwartz M., "Information Transmission, Modulation and Noise", McGraw-Hill, N.Y.,4th Ed., 1990, p.525

5234:

Kingsley S. and Quegan S., "Understanding Radar Systems", McGraw -Hill 1992, SciTech Publishing, 1999

4763: 4753: 4718: 1228: 1109: 867: 568: 220: 2675:{\displaystyle {\frac {P_{1}}{P_{2}}}={\frac {\exp {\big }}{\exp {\Big }^{2}{\Big ]}}}=\exp {\Big }} 1886: 994:

is a phase based DF method that produces a bearing estimate on the received signal by measuring the

466:. When the office was moved, his new location at a radio research station provided him with both an 6498: 6229: 1806:

of a typical antenna gain characteristic, in the horizontal plane. The half-power beamwidth of the

1127:

stations. These stations can obtain a "fix" of the aircraft and transmit it by radio to the pilot.

1378:

Most direction finding effort within the UK now (2009) is directed towards locating unauthorised "

6666: 6641: 6550: 6540: 6488: 6219: 5869: 5839: 5731: 5506:

East P, "Microwave Intercept Receiver Sensitivity Estimation", Racal Defence Systems Report, 1998

5324:

Woolier D.F., "System considerations for naval ESM", IEE Proc. Vol. 132, Pt. F, No. 4, July 1985.

5213:

Morgan T.E., "Spiral Horns for ESM", IEE proc., Vol. 132, Pt. F., No. 4, July 1985, pp. 245 - 251

343: 60: 38: 1862: 837:

are generally used for highly accurate direction finding systems. The modern systems are called

219:

as they can travel very long distances "over the horizon", which is valuable for ships when the

6565: 6503: 6483: 6432: 6189: 4070:

in the receiver channels will be poor, and the accuracy of the bearing prediction will suffer.

3181:

For a signal incoming at a bearing ø, taken here to be to the right of boresight of Antenna 1:

1694: 1544: 1356: 1310:

m vertical antennas surrounding a small wooden operators hut containing a receiver and a radio-

1279: 1212: 1117: 1096: 1073: 838: 645: 552:

and PLB beacons, although modern GPS-EPIRBS and AIS beacons are slowly making these redundant.

243: 228: 107: 5680: 5394:

Tsui J.B., "Microwave Receivers with Electronic Warfare Applications", Kreiber, Florida, 1992"

6452: 6118: 4067: 1997:

than that at which the Radar's own receiver is able to detect the presence of the DF system.

1426: 1405: 1360: 1314:

which was adjusted to obtain the bearing. MF stations were also used which used four guyed 30

1271: 889: 888:

One form of radio direction finding works by comparing the signal strength of a directional

773:

exhibits similar properties, as a small loop, although its null direction is not as "sharp".

704: 5408:

East P.W., "Microwave System Design Tools with EW Applications", Artech House, 2nd Ed., 2008

1874: 6555: 6493: 6379: 6335: 6320: 6269: 6214: 6044: 6027: 6000: 5763: 5640: 4728: 1740: 846: 790: 633: 588: 391: 239: 235:

broadcasters, was in the 20th century a feature of most aircraft, but is being phased out.

5304: 5116:

Richardson D, "Techniques and Equipment of Electronic Warfare", Arco Publishing N.Y., 1985

871:

The RDF antenna on this B-17F is located in the prominent teardrop housing under the nose.

8: 6588: 6462: 6279: 6252: 6106: 6010: 5758: 4738: 4703: 4686: 2046: 2009: 1993: 1942: 1508: 5644: 5626:"Influence of jitter in ADC on precision of direction-finding by digital antenna arrays" 5346: 3795:{\displaystyle \ln(P_{1})-\ln(P_{3})={\frac {A}{\Psi _{0}^{2}}}.(\Phi ^{2}+2\Phi \phi )} 3672:{\displaystyle \ln(P_{1})-\ln(P_{2})={\frac {A}{\Psi _{0}^{2}}}.(\Phi ^{2}-2\Phi \phi )} 1898: 544:

Radio direction finding networks also no longer exist. However rescue vessels, such as

6560: 6545: 6437: 6412: 6350: 6301: 6286: 5995: 5962: 5936: 5914: 5909: 5879: 5864: 5724: 5656: 1064:

A portable, battery operated GT-302 Accumatic automatic direction finder for marine use

1014: 959: 935: 665: 450: 340: 263: 4820: 6631: 6111: 5780: 5686: 5660: 5610: 5578: 5537: 5243: 5167: 5060: 5005: 4956: 4723: 4661:

A schematic of a possible DF system, employing six antennas, is shown in the figure.

3131:{\displaystyle \phi ={\frac {\Psi _{0}^{2}}{6.0202\Phi }}.{\big }+{\frac {\Phi }{2}}} 2252: 2005: 1796: 1776: 1516: 1492: 1422: 814: 491: 205: 189: 5079:

Tsui J.B., "Microwave Receivers with Electronic Warfare Applications", Kreiber, 1992

719:

The crossed-loops antenna atop the mast of a tug boat is a direction-finding design.

490:

led to greatly improved methods of comparing the phase of signals. In addition, the

6457: 6407: 6076: 5808: 5648: 2013: 1933:

tolerated because of the "range advantage" enjoyed by the DF receiver (see below).

1736: 1507:

or by its international abbreviation ARDF. Another form of the activity, known as "

1340: 854: 806: 610: 487: 370: 321: 216: 201: 5204:

Stutzman W.L. & Thiele G.A., "Antenna Theory and Design", 2nd Ed., Wiley 1998.

2987:{\displaystyle \phi ={\frac {\Psi _{0}^{2}}{2A.\Phi }}.{\big }+{\frac {\Phi }{2}}} 2114:

If the main lobes of the antennas have a Gausian characteristic, then the output P

1799:

gain. In addition, the antennas are required to cover a wide band of frequencies.

1239:

Today many NDBs have been decommissioned in favor of faster and far more accurate

715: 281:

sets were referred to as RDF, which is often stated was a deception. In fact, the

6345: 6274: 6096: 6071: 6005: 5967: 5904: 5572: 5437:

https://digital.library.adelaide.edu.au/dspace/bitstream/2440/90332/4/02whole.pdf

4733: 4708: 2001: 1772: 1768: 1472: 1466: 1442: 462:

to display these near instantly, but was unable to find one while working at the

5334: 5246:

and Olver A.D., "Corrugated horns for microwave antennas", Peter Perigrinus 1984

5192:

Tutorial, "Advantages of Microwaves", Microwave Engineering Introduction article

3004:(dBs) (where dBs are referred to boresight gain) by using ln(X) = X(dB)/(10.\log 6442: 6262: 5774: 5707: 5497:

Martino A. De, "Introduction to Modern EW Systems", 2nd Ed., Artech House 2012

4748: 1909: 1512: 1247: 1022: 1008: 962:

were two of the larger manufacturers of RDF radios and navigation instruments.

770: 677: 564: 467: 417: 317: 308: 274: 5652: 4148:{\displaystyle \Delta \phi _{RMS}=0.724{\frac {2.\Psi _{0}}{\sqrt {SNR_{0}}}}} 1274:

which radiated interfering signals due to feedback, a big problem at the time.

188:

to determine the direction to a radio source. The source may be a cooperating

6615: 6360: 6340: 6325: 6054: 6022: 5924: 5899: 5818: 4918: 4063: 2017: 1496: 1418: 1402:

location data that can aid in finding the exact location of the transmitter.

1267: 995: 673: 494:(PLL) allowed for easy tuning in of signals, which would not drift. Improved 325: 270: 197: 5275:

https://archive.org/details/MITRadiationLaboratorySeries23MicrowaveReceivers

530:

The various procedures for radio direction finding to determine position at

6447: 6365: 6355: 6296: 6101: 5889: 5874: 5813: 5798: 5791: 3154: 1853: 1717: 1438: 1379: 1108:

world. Starting in the 1950s, these beacons were generally replaced by the

926: 894: 842: 834: 818: 782: 735: 729: 708: 661: 657: 641: 507: 459: 401: 312: 255: 6129: 5699: 2070: 974:

The two main categories that a single channel DF algorithm falls into are

435: 6257: 6091: 5957: 5803: 5604: 4772: 4713: 4653: 2078: 1296: 1208: 914: 664:

for highly accurate results. These are generally integrated into a wider

637: 495: 440: 332: 259: 163: 27:

Measurement of the direction from which a received signal was transmitted

2086: 1145: 150:

Radiotriangulation scheme using two direction-finding antennas (A and B)

6330: 6086: 6066: 6059: 6039: 5946: 5380:"EW Acquisition Systems - probability of intercept and intercept times" 4893: 1971: 1803: 1347: 1311: 1200: 1196: 1027: 850: 810: 793: 692: 625:

automatic direction finder, often capable of being tuned to commercial

621: 617: 499: 463: 413: 282: 212: 185: 2286:

The second antenna, squinted at Phi and with the same boresight gain G

1818:, these are not a major concern when antennas are used in a DF array. 1060: 470:

and a suitable oscilloscope, and he presented his new system in 1926.

299: 30:"Direction finder" redirects here. For the air navigation device, see 6311: 6306: 6244: 6081: 5894: 5823: 5747: 5148:

Beck, A. H. W., "Thermionic Valves", Cambridge University Press, 1953

4743: 4697: 2263: 2038: 2034: 1967: 1916: 1807: 1792: 1761: 1705: 1682: 1204: 1130: 947: 938:, Gladding (and its marine division, Pearce-Simpson), Ray Jefferson, 910: 560: 454: 146: 5054: 800: 49: 6315: 5929: 5028:"457 kHz Electromagnetism and the Future of Avalanche Transceivers" 4691: 4674: 4046: 3001: 2099: 1815: 1732: 1454: 1394: 1352: 1105: 955: 939: 931: 822: 626: 517:

Other systems have been developed where more accuracy is required.

421: 336: 232: 211:

RDF systems can be used with any radio source, although very long

6015: 1568: 1450: 1291: 1090: 902: 652:" systems were directly or indirectly responsible for 24% of all 380: 375: 250:" systems were directly or indirectly responsible for 24% of all 159: 4882:"Broadcast Station Can Guide Flyer", April 1931, Popular Science 3146: 5671:

History of Communications-Electronics in the United States Navy

5623: 4821:"Radar (Radio Direction Finding) – The Eyes of Fighter Command" 4042: 1849: 1562: 1446: 1266:

British Post Office RDF lorry from 1927 for finding unlicensed

1257: 828: 653: 251: 154: 5107:

Lipsky S.E., "Microwave Passive Direction Finding", Wiley 1987

2041:

may only radiate a high data rate sequence very occasionally.

1724:, and planar construction, very few tubes could operate above 307:

The earliest experiments in RDF were carried out in 1888 when

4163:

is the signal-to-noise ratio that would apply at boresight.

4073:

In general, a guide to bearing uncertainty is given by >

1757: 1676: 1503:. The most popular form of the sport, worldwide, is known as 1233: 1086: 688: 594: 549: 535: 531: 278: 204:

for ships and aircraft, to locate emergency transmitters for

193: 5716: 5609:(2nd ed.). Routledge & Kegan Paul. pp. 68–69. 1248:

Location of illegal, secret or hostile transmitters – SIGINT

821:

can be used. Dish antennas are highly directional, with the

747:

sensitive to signals that are perpendicular to its face and

5222:

Milligan T.A., "Modern Antenna Design", 2nd Ed., Wiley 2005

4758: 1721: 1191:

Radio transmitters for air and sea navigation are known as

860: 739: 545: 449:

A major improvement in the RDF technique was introduced by

5262:

Electronic Intelligence: The Interception of Radar Signals

898:

usually mounted atop a portable battery-powered receiver.

884:

World War II US Navy high frequency radio direction finder

629:

transmitters, is a feature of almost all modern aircraft.

328:

in 1904 (U.S. Patent 771,819), among many other examples.

5382:, Watkins-Johnson Tech-notes Vol. 3, No. 3, May/June 1976 5134:

Gilmour jnr. A.S., "Microwave Tubes", Artech House, 1986

4919:"HF/DF An Allied Weapon against German U-boats 1939–1945" 4815: 4813: 4811: 1966:(HEMT). Initially, discrete transistors were embedded in 1725: 1711: 1630: 1609: 1399: 1287: 1283: 1240: 1085:

was the abbreviation used to describe the predecessor to

786: 486:

Several developments in electronics during and after the

4648: 3556:

As earlier, these can be expanded and combined to give:

1746: 909:(the direction at which a given signal is weakest) of a 6601:

Counterintelligence and counter-terrorism organizations

5305:

https://radar-engineer.com/files/Lecture_ES_Sensors.pdf

2049:) and achieve a high "Probability of Intercept" (PoI). 1752: 1689:

In Naval systems, the DF capability became part of the

1002: 597:

source. The act of measuring the direction is known as

277:

which are generally simpler to automate. Early British

5605:

Appleyard, S.F.; Linford, R.S.; Yarwood, P.J. (1988).

5345:

Davidson K., "Low Probability of Intercept", find at:

4808: 4694:, a cold war US Air Force HF direction finding system. 776: 262:

for highly accurate results, and are part of a larger

5347:

http://radar-engineer.com/files/Lecture_LPI_Radar.pdf

5303:

Davidson K., "Electronic Support Sensors". Find at:

5025: 4955:(8th ed.). Sutton Publishing. pp. 104–105. 4355: 4180: 4082: 3818: 3688: 3565: 3433: 3309: 3193: 3017: 2867: 2694: 2423: 2299: 2127: 1848:

Two antenna types, popular for DF, are cavity-backed

632:

For the military, RDF systems are a key component of

5157:

Baden Fuller A. J., "Microwaves"Pergamon Press, 1979

4851: 3176: 5183:

Gupta K.C., "Microwaves", New Age Intnl. Pub., 2012

4863: 4839: 4827: 4056: 2118:(φ), as a function of bearing angle φ, is given by 1140: 1081:, was once the primary aviation navigational aid. ( 74:. Unsourced material may be challenged and removed. 5592:Elliott, Peter (1972). "The Lend-Lease Captains". 5538:https://nardamiteq.com/docs/MITEQ_IFsignal_c17.pdf 5315:Connor F.R., "Antennas", Edward Arnold, 1972, p.8. 5168:http://maximus-randd.com/tv-tuner-history-pt1.html 4622: 4323: 4147: 3991: 3794: 3671: 3537: 3413: 3289: 3130: 2986: 2847: 2674: 2403: 2232: 2109: 1359:in order to try to locate German submarines, e.g. 853:-mounted direction finding system was designed by 723: 5055:Titterington, B.; Williams, D.; Dean, D. (2007). 4700:, a cold war US Navy HF direction finding system. 4579: 4555: 4471: 4447: 3530: 3517: 3484: 3468: 3406: 3393: 3360: 3344: 3282: 3269: 3244: 3228: 2734: 2703: 2667: 2607: 2588: 2524: 2396: 2383: 2350: 2334: 2225: 2212: 2187: 2171: 1956:metal oxide semiconductor field effect transistor 1936: 801:Parabolic antennas for extremely high frequencies 534:are no longer part of the maritime safety system 6613: 5166:Hooijmans P., "Philip's tuner history". Find at 2233:{\displaystyle P_{1}(\phi )=G_{0}.\exp {\Bigr }} 2023: 1795:are usually chosen, even though they have lower 1499:purposes, or to practice locating the source of 1149:Historic advertisement for Kolster radio compass 703:In one type of direction finding, a directional 680:which are generally simpler to automate. Modern 518: 5701:Radio Direction Finding Applications Literature 5624:M. Bondarenko and V.I. Slyusar. (August 2011). 5035:International Snow Science Workshop (ISSW 2000) 408:were widespread from the 1920s into the 1950s. 254:sunk during the war. Modern systems often used 227:, which could be tuned to radio beacons called 5591: 1322:. These consisted of two parallel loops 1 to 2 985: 656:sunk during the war. Modern systems often use 6145: 5732: 5335:https://ausairpower.net/APA-Maritime-ESM.html 3110: 3056: 2966: 2912: 2575: 2537: 2509: 2462: 2052: 1524:Selection of radio direction-finding stations 1445:techniques are utilized to track launches of 1033: 982:. Some algorithms can be hybrids of the two. 920:Later, RDF sets were equipped with rotatable 439:FH4 "Huff-duff" equipment on the museum ship 5469:, Edward Arnold, London, 2nd ed. 1982, p. 44 4999: 1958:(MOSFET). Others followed, for example, the 1391:Emergency position-indicating rescue beacons 829:Electronic analysis of two antennas' signals 734:A simple form of directional antenna is the 613:system, especially with boats and aircraft. 587:) is a device for finding the direction, or 6159: 5633:Radioelectronics and Communications Systems 5230: 5228: 5200: 5198: 4950: 1960:metal-semiconductor field-effect transistor 1949: 1306:The HF Adcock stations consisted of four 10 805:For much higher frequencies still, such as 294: 158:Direction finding antenna near the city of 6152: 6138: 5739: 5725: 5574:A History of the Marconi Company 1874-1965 5374: 5372: 5130: 5128: 5126: 5124: 5122: 1677:Direction finding at microwave frequencies 1326:m square on the ends of a rotatable 3 to 8 930:finding equipment for mariners, including 857:for the U.S. Government as early as 1972. 648:. It is estimated that the UK's advanced " 246:. It is estimated that the UK's advanced " 6622:Automatic identification and data capture 5598:International Naval Research Organization 5103: 5101: 5099: 5097: 5095: 5093: 5091: 5089: 5087: 5085: 4969: 3538:{\displaystyle P_{3}=G_{T}.\exp {\Bigr }} 3414:{\displaystyle P_{2}=G_{T}.\exp {\Bigr }} 3290:{\displaystyle P_{1}=G_{T}.\exp {\Bigr }} 2404:{\displaystyle P_{2}=G_{0}.\exp {\Bigr }} 1519:with radio direction-finding techniques. 134:Learn how and when to remove this message 5532: 5530: 5256: 5254: 5252: 5225: 5195: 5179: 5177: 5175: 4652: 4159:for a signal at crossover, but where SNR 3153: 3145: 2085: 2077: 2069: 1270:transmitters. It was also used to find 1261: 1144: 1129: 1059: 879: 866: 714: 559: 434: 390: 298: 153: 145: 6566:Scientific & Technical intelligence 5422: 5420: 5418: 5416: 5414: 5404: 5402: 5400: 5390: 5388: 5369: 5144: 5142: 5140: 5119: 5004:. Los Altos, CA: Peninsula Publishing. 3000:Natural logarithms can be converted to 1515:while attempting to locate one or more 1030:of the ratio of the N–S to E–W signal. 238:For the military, RDF is a key tool of 14: 6614: 5989:Chinese espionage in the United States 5667: 5082: 5057:Radio Orienteering – The ARDF Handbook 4857: 3150:Three-port DF, polar plot (normalized) 2685:The natural logarithm of the ratio is 1922: 1712:Brief history of microwave development 833:More sophisticated techniques such as 6133: 5720: 5678: 5570: 5527: 5249: 5172: 5026:J. Hereford & B. Edgerly (2000). 4917:Bauer, Arthur O. (27 December 2004). 4916: 4869: 4845: 4833: 4649:A typical DF system with six antennas 3158:Three-port DF, log scale (normalized) 3008:(e)), so the equation can be written 1747:The advantages of microwave operation 519:Pseudo-doppler radio direction finder 453:as part of his experiments to locate 6388:By alliances, nations and industries 5984:Chinese intelligence activity abroad 5411: 5397: 5385: 5137: 4984: 2074:Two-port DF, polar plot (normalized) 1915:For larger arrays, needing narrower 1614:(Emergency location beacon aircraft) 1417:Location of radio-tagged animals by 1412: 1003:Watson–Watt, or Adcock-antenna array 965: 311:discovered the directionality of an 72:adding citations to reliable sources 43: 6050:Interpersonal (HUMINT) intelligence 4989:(4th ed.). London, UK: Iliffe. 4792:"Next Gen Implementation Plan 2013" 3141: 2082:Two-port DF, log scale (normalized) 1702:low-probability-of-intercept radars 1055: 777:Yagi antenna for higher frequencies 738:. This consists of an open loop of 481: 24: 5059:. Radio Society of Great Britain. 4568: 4460: 4429: 4414: 4356: 4255: 4219: 4205: 4181: 4115: 4083: 4041:The difference values here are in 3981: 3958: 3939: 3922: 3908: 3885: 3866: 3848: 3829: 3783: 3768: 3745: 3660: 3645: 3622: 3503: 3492: 3379: 3368: 3255: 3120: 3045: 3027: 2976: 2901: 2877: 2836: 2821: 2798: 2656: 2641: 2618: 2563: 2545: 2488: 2369: 2358: 2198: 2031:low-probability-of-intercept radar 1981: 1927: 1785: 1195:and are the radio equivalent to a 958:In aircraft equipment, Bendix and 905:to determine the direction of the 324:in 1902 (U.S. Patent 716,134) and 25: 6683: 5885:Computer and network surveillance 5709:Doppler Systems Application Notes 3177:Basic equations for three-port DF 2065: 1964:high electron mobility transistor 1433: 359:of the wavelength, more commonly 5920:Short-range agent communications 5682:Probing the Sky with Radio Waves 4894:"Die Geschichte des Funkpeilens" 4057:Bearing uncertainty due to noise 1897: 1885: 1873: 1861: 1802:The figure shows the normalized 1779:techniques now became possible. 1655: 1638: 1621: 1600: 1578: 1554: 1532: 1385: 1335:the D-Day invasion of Normandy. 1254:High frequency direction finding 1175: 1166: 1157: 1141:Maritime and aircraft navigation 575:aerial visible above the cockpit 476:high-frequency direction finding 48: 6536:Words of estimative probability 6398:Operational platforms by nation 5685:. University of Chicago Press. 5563: 5551: 5542: 5518: 5509: 5500: 5491: 5481: 5472: 5459: 5450: 5441: 5429: 5360: 5351: 5339: 5327: 5318: 5309: 5297: 5288: 5279: 5267: 5237: 5216: 5207: 5186: 5160: 5151: 5110: 5073: 5048: 5018: 4993: 4978: 4944: 4935: 2110:Basic equations for two-port DF 1505:Amateur Radio Direction Finding 1486:Amateur radio direction finding 1050: 724:Null finding with loop antennas 682:pseudo-Doppler direction finder 525: 386: 59:needs additional citations for 6627:Radio-frequency identification 4910: 4886: 4875: 4784: 4402: 4396: 4249: 4243: 3789: 3764: 3733: 3720: 3708: 3695: 3666: 3641: 3610: 3597: 3585: 3572: 3105: 3096: 3080: 3071: 2961: 2948: 2936: 2923: 2842: 2817: 2786: 2773: 2761: 2748: 2662: 2637: 2554: 2542: 2498: 2476: 2144: 2138: 1937:Klystron and TWT preamplifiers 1320:National Physical Laboratories 640:, and it played a key role in 406:Bellini–Tosi direction finders 316:Later experimenters also used 13: 1: 5746: 4778: 2024:The new demands on DF systems 1976:microwave integrated circuits 1393:are widely deployed on civil 5674:. US Navy. pp. 261–265. 5607:Marine Electronic Navigation 1591:Aircraft emergency frequency 1501:radio frequency interference 1460: 875: 712:improve angular resolution. 555: 498:and the introduction of the 430: 7: 6584:Intelligence cycle security 4680: 2262:is one half the half-power 2090:Power Diff. (dB) v. Bearing 1691:Electronic Support Measures 1083:Range and Direction Finding 986:Pseudo-doppler DF technique 956:Koden Electronics Co., Ltd. 698: 412:frequency signals from the 10: 6688: 5002:Electronic Countermeasures 4987:Wireless Direction Finding 2282:and angles are in radians. 2053:DF by amplitude comparison 1483: 1464: 1370:Wireless Direction Finding 1251: 1199:. The transmitter sends a 1115: 1113:helicopter landing sites. 1034:Correlative interferometer 1006: 861:Time difference of arrival 785:is familiar as the common 727: 512:automatic direction finder 289: 225:automatic direction finder 36: 32:Automatic direction finder 29: 6574: 6516: 6476: 6421: 6378: 6243: 6205: 6185: 6176: 6167: 5976: 5945: 5855: 5832: 5754: 5679:Yeang, Chen-Pang (2013). 5653:10.3103/S0735272711080061 4754:Real-time locating system 4719:Indoor positioning system 2414:Comparing signal levels, 2255:gain (i.e. when ø = 0), 569:Lockheed Model 10 Electra 5668:Howeth, Linwood (1963). 2268:A = -\ln(0.5), so that P 1950:Transistor preamplifiers 1716:Earlier in the century, 1479: 1207:(150 – 400 kHz) or 992:pseudo-doppler technique 660:antennas to allow rapid 295:Early mechanical systems 258:antennas to allow rapid 6637:Radio direction finding 6551:Intelligence assessment 6541:All-source intelligence 6161:Intelligence management 5870:Covert listening device 5840:Intelligence assessment 4953:Air Band Radio Handbook 3300:Channel 2 output is 2037:. In addition, covert 1841:, it is Gain ≈ 30000/BW 1213:non-directional beacons 1097:non-directional beacons 1069:Radio direction finding 1041:correlation coefficient 599:radio direction finding 344:ground wave propagation 229:non-directional beacons 178:radio direction finding 39:Direction determination 18:Radio Direction Finding 6235:Operational techniques 6190:Special reconnaissance 4975:Elliott (1972), p. 264 4673:issues are covered in 4658: 4624: 4325: 4149: 4068:signal-to-noise ratios 3993: 3796: 3673: 3539: 3415: 3291: 3159: 3151: 3132: 2988: 2849: 2676: 2405: 2234: 2091: 2083: 2075: 1695:Radar Warning Receiver 1586:RDF on 121.5 MHz 1406:Avalanche transceivers 1361:Captain class frigates 1357:anti-submarine warfare 1280:Radio Security Service 1275: 1272:regenerative receivers 1150: 1137: 1118:Non-directional beacon 1074:radio direction finder 1065: 885: 872: 720: 646:Battle of the Atlantic 581:radio direction finder 576: 446: 396: 304: 244:Battle of the Atlantic 166: 151: 6119:Targeted surveillance 5594:Warship International 5571:Baker, W. J. (2013). 4656: 4625: 4326: 4150: 3994: 3797: 3674: 3540: 3424:Channel 3 output is 3416: 3292: 3184:Channel 1 output is 3157: 3149: 3133: 2989: 2850: 2677: 2406: 2235: 2089: 2081: 2073: 1663:Maritime RDF station 1646:Maritime RDF station 1265: 1148: 1133: 1116:Further information: 1063: 883: 870: 718: 563: 438: 394: 302: 157: 149: 6662:Aircraft instruments 6556:Medical intelligence 6526:Competing hypotheses 6336:Industrial espionage 6321:Denial and deception 6045:Industrial espionage 5845:competing hypotheses 5264:, Artech House, 1985 4951:Smith, D.J. (2005). 4729:Phase interferometry 4353: 4178: 4080: 3816: 3686: 3563: 3431: 3307: 3191: 3015: 2865: 2692: 2421: 2297: 2125: 2004:techniques, such as 1892:Cavity backed spiral 1741:travelling wave tube 1023:Adcock antenna array 976:amplitude comparison 847:signals intelligence 634:signals intelligence 601:or sometimes simply 240:signals intelligence 68:improve this article 37:For other uses, see 6647:American inventions 6589:Counterintelligence 6504:Technical (TECHINT) 6499:Open-source (OSINT) 6489:Geospatial (GEOINT) 6463:Casualty estimation 5645:2011RCSys..54..436B 4739:Radio determination 4704:Battle of the Beams 4687:Amplitude monopulse 4427: 4232: 3758: 3635: 3040: 2890: 2811: 2631: 2047:amplitude monopulse 2010:radar cross-section 1994:radar cross-section 1923:Microwave receivers 1608:Aerial 121.5/156.8 1509:transmitter hunting 1441:and other advanced 849:(SIGINT). A modern 83:"Direction finding" 6657:Italian inventions 6561:Military geography 6546:Basic intelligence 6484:Financial (FININT) 6425:signature (MASINT) 6351:One-way voice link 6287:Concealment device 5996:Cold War espionage 5963:Front organization 5937:Surveillance tools 5915:Phone surveillance 5910:One-way voice link 5865:Concealment device 4898:www.seefunknetz.de 4659: 4657:Six-port DF system 4620: 4413: 4321: 4218: 4145: 3989: 3792: 3744: 3669: 3621: 3535: 3411: 3287: 3160: 3152: 3128: 3026: 2984: 2876: 2845: 2797: 2672: 2617: 2401: 2230: 2092: 2084: 2076: 1868:Antenna polar plot 1681:DF techniques for 1517:radio transmitters 1423:radio transmitters 1276: 1203:transmission on a 1151: 1138: 1066: 886: 873: 815:parabolic antennas 721: 678:doppler techniques 672:the comparison of 666:electronic warfare 577: 571:with the circular 451:Robert Watson-Watt 447: 397: 341:great circle route 305: 275:doppler techniques 264:electronic warfare 167: 152: 6672:Navigational aids 6609: 6608: 6512: 6511: 6403:Direction finding 6393:In modern history 6374: 6373: 6127: 6126: 5781:Agent provocateur 5712:(Doppler Systems) 5244:Clarricoats P.J.B 5066:978-1-905086-27-6 5044:on July 22, 2011. 4724:MUSIC (algorithm) 4618: 4616: 4574: 4548: 4508: 4466: 4438: 4406: 4319: 4317: 4289: 4259: 4211: 4143: 4142: 3987: 3974: 3914: 3901: 3759: 3636: 3512: 3388: 3264: 3126: 3049: 2982: 2905: 2812: 2730: 2632: 2594: 2446: 2378: 2253:antenna boresight 2207: 2006:Chirp compression 1941:The klystron and 1797:antenna boresight 1777:frequency hopping 1667: 1649: 1615: 1594: 1572: 1548: 1493:disaster response 1413:Wildlife tracking 966:Single-channel DF 922:ferrite loopstick 603:direction finding 492:phase-locked loop 313:open loop of wire 206:search and rescue 190:radio transmitter 184:), is the use of 170:Direction finding 144: 143: 136: 118: 16:(Redirected from 6679: 6652:Radio navigation 6408:Traffic analysis 6380:Signals (SIGINT) 6215:Asset recruiting 6183: 6182: 6174: 6173: 6154: 6147: 6140: 6131: 6130: 5741: 5734: 5727: 5718: 5717: 5696: 5675: 5664: 5630: 5620: 5601: 5588: 5558: 5555: 5549: 5546: 5540: 5534: 5525: 5522: 5516: 5513: 5507: 5504: 5498: 5495: 5489: 5485: 5479: 5476: 5470: 5463: 5457: 5454: 5448: 5445: 5439: 5433: 5427: 5424: 5409: 5406: 5395: 5392: 5383: 5376: 5367: 5364: 5358: 5355: 5349: 5343: 5337: 5331: 5325: 5322: 5316: 5313: 5307: 5301: 5295: 5292: 5286: 5283: 5277: 5271: 5265: 5258: 5247: 5241: 5235: 5232: 5223: 5220: 5214: 5211: 5205: 5202: 5193: 5190: 5184: 5181: 5170: 5164: 5158: 5155: 5149: 5146: 5135: 5132: 5117: 5114: 5108: 5105: 5080: 5077: 5071: 5070: 5052: 5046: 5045: 5043: 5037:. Archived from 5032: 5022: 5016: 5015: 4997: 4991: 4990: 4985:Keen, R (1947). 4982: 4976: 4973: 4967: 4966: 4948: 4942: 4939: 4933: 4931: 4929: 4928: 4923: 4914: 4908: 4907: 4905: 4904: 4890: 4884: 4879: 4873: 4867: 4861: 4855: 4849: 4843: 4837: 4831: 4825: 4824: 4817: 4806: 4805: 4803: 4797:. Archived from 4796: 4788: 4629: 4627: 4626: 4621: 4619: 4617: 4615: 4614: 4613: 4594: 4589: 4588: 4583: 4582: 4575: 4567: 4559: 4558: 4549: 4547: 4546: 4545: 4529: 4528: 4527: 4514: 4509: 4507: 4506: 4505: 4486: 4481: 4480: 4475: 4474: 4467: 4459: 4451: 4450: 4444: 4439: 4437: 4436: 4426: 4421: 4412: 4407: 4405: 4379: 4374: 4373: 4330: 4328: 4327: 4322: 4320: 4318: 4316: 4315: 4314: 4295: 4290: 4288: 4287: 4286: 4267: 4265: 4260: 4258: 4231: 4226: 4217: 4212: 4204: 4199: 4198: 4154: 4152: 4151: 4146: 4144: 4141: 4140: 4125: 4124: 4123: 4122: 4109: 4101: 4100: 4045:but could be in 3998: 3996: 3995: 3990: 3988: 3980: 3975: 3973: 3972: 3971: 3953: 3952: 3936: 3935: 3920: 3915: 3907: 3902: 3900: 3899: 3898: 3880: 3879: 3863: 3862: 3861: 3843: 3842: 3826: 3809:and rearranging 3801: 3799: 3798: 3793: 3776: 3775: 3760: 3757: 3752: 3740: 3732: 3731: 3707: 3706: 3678: 3676: 3675: 3670: 3653: 3652: 3637: 3634: 3629: 3617: 3609: 3608: 3584: 3583: 3544: 3542: 3541: 3536: 3534: 3533: 3527: 3526: 3521: 3520: 3513: 3511: 3510: 3501: 3490: 3488: 3487: 3472: 3471: 3456: 3455: 3443: 3442: 3420: 3418: 3417: 3412: 3410: 3409: 3403: 3402: 3397: 3396: 3389: 3387: 3386: 3377: 3366: 3364: 3363: 3348: 3347: 3332: 3331: 3319: 3318: 3296: 3294: 3293: 3288: 3286: 3285: 3279: 3278: 3273: 3272: 3265: 3263: 3262: 3250: 3248: 3247: 3232: 3231: 3216: 3215: 3203: 3202: 3142:Three-channel DF 3137: 3135: 3134: 3129: 3127: 3119: 3114: 3113: 3095: 3094: 3070: 3069: 3060: 3059: 3050: 3048: 3039: 3034: 3025: 2993: 2991: 2990: 2985: 2983: 2975: 2970: 2969: 2960: 2959: 2935: 2934: 2916: 2915: 2906: 2904: 2889: 2884: 2875: 2854: 2852: 2851: 2846: 2829: 2828: 2813: 2810: 2805: 2793: 2785: 2784: 2760: 2759: 2738: 2737: 2731: 2729: 2728: 2719: 2718: 2709: 2707: 2706: 2681: 2679: 2678: 2673: 2671: 2670: 2649: 2648: 2633: 2630: 2625: 2613: 2611: 2610: 2595: 2593: 2592: 2591: 2585: 2584: 2579: 2578: 2571: 2570: 2561: 2541: 2540: 2528: 2527: 2514: 2513: 2512: 2506: 2505: 2496: 2495: 2486: 2466: 2465: 2452: 2447: 2445: 2444: 2435: 2434: 2425: 2410: 2408: 2407: 2402: 2400: 2399: 2393: 2392: 2387: 2386: 2379: 2377: 2376: 2367: 2356: 2354: 2353: 2338: 2337: 2322: 2321: 2309: 2308: 2290:gives an output 2276:= 0.5 when ø = Ψ 2239: 2237: 2236: 2231: 2229: 2228: 2222: 2221: 2216: 2215: 2208: 2206: 2205: 2193: 2191: 2190: 2175: 2174: 2159: 2158: 2137: 2136: 2014:Stealth aircraft 1901: 1889: 1880:Antenna log plot 1877: 1865: 1737:cavity magnetron 1665: 1659: 1647: 1642: 1629:RDF station 410 1625: 1613: 1604: 1588: 1582: 1566: 1558: 1542: 1536: 1473:radio telescopes 1341:cathode ray tube 1329: 1325: 1317: 1309: 1179: 1170: 1161: 1056:Radio navigation 980:phase comparison 855:ESL Incorporated 807:millimeter waves 783:Yagi-Uda antenna 611:radio navigation 488:Second World War 482:Post-war systems 371:half-wave dipole 368: 367: 363: 358: 357: 353: 322:John Stone Stone 217:radio navigation 202:radio navigation 139: 132: 128: 125: 119: 117: 76: 52: 44: 21: 6687: 6686: 6682: 6681: 6680: 6678: 6677: 6676: 6612: 6611: 6610: 6605: 6570: 6508: 6494:Imagery (IMINT) 6472: 6433:Electro-optical 6424: 6423:Measurement and 6417: 6370: 6346:Numbers station 6275:Black operation 6239: 6201: 6163: 6158: 6128: 6123: 6097:Sting operation 6006:Black operation 5972: 5968:Limited hangout 5950: 5941: 5905:Numbers station 5857: 5851: 5828: 5768: 5750: 5745: 5715: 5693: 5628: 5617: 5585: 5566: 5561: 5556: 5552: 5547: 5543: 5535: 5528: 5523: 5519: 5514: 5510: 5505: 5501: 5496: 5492: 5486: 5482: 5477: 5473: 5464: 5460: 5455: 5451: 5446: 5442: 5434: 5430: 5425: 5412: 5407: 5398: 5393: 5386: 5377: 5370: 5365: 5361: 5356: 5352: 5344: 5340: 5332: 5328: 5323: 5319: 5314: 5310: 5302: 5298: 5293: 5289: 5284: 5280: 5272: 5268: 5259: 5250: 5242: 5238: 5233: 5226: 5221: 5217: 5212: 5208: 5203: 5196: 5191: 5187: 5182: 5173: 5165: 5161: 5156: 5152: 5147: 5138: 5133: 5120: 5115: 5111: 5106: 5083: 5078: 5074: 5067: 5053: 5049: 5041: 5030: 5023: 5019: 5012: 4998: 4994: 4983: 4979: 4974: 4970: 4963: 4949: 4945: 4940: 4936: 4926: 4924: 4921: 4915: 4911: 4902: 4900: 4892: 4891: 4887: 4880: 4876: 4868: 4864: 4856: 4852: 4844: 4840: 4832: 4828: 4819: 4818: 4809: 4801: 4794: 4790: 4789: 4785: 4781: 4734:Position fixing 4709:Electric beacon 4683: 4651: 4644: 4640: 4636: 4609: 4605: 4598: 4593: 4584: 4578: 4577: 4576: 4566: 4554: 4553: 4541: 4537: 4530: 4523: 4519: 4515: 4513: 4501: 4497: 4490: 4485: 4476: 4470: 4469: 4468: 4458: 4446: 4445: 4443: 4432: 4428: 4422: 4417: 4411: 4383: 4378: 4363: 4359: 4354: 4351: 4350: 4342: 4338: 4310: 4306: 4299: 4294: 4282: 4278: 4271: 4266: 4264: 4233: 4227: 4222: 4216: 4203: 4188: 4184: 4179: 4176: 4175: 4162: 4136: 4132: 4118: 4114: 4110: 4108: 4090: 4086: 4081: 4078: 4077: 4059: 4037: 4033: 4029: 4025: 4021: 4017: 4013: 4009: 4005: 3979: 3961: 3957: 3942: 3938: 3937: 3925: 3921: 3919: 3906: 3888: 3884: 3869: 3865: 3864: 3851: 3847: 3832: 3828: 3827: 3825: 3817: 3814: 3813: 3808: 3805:Eliminating A/Ψ 3771: 3767: 3753: 3748: 3739: 3727: 3723: 3702: 3698: 3687: 3684: 3683: 3648: 3644: 3630: 3625: 3616: 3604: 3600: 3579: 3575: 3564: 3561: 3560: 3551: 3529: 3528: 3522: 3516: 3515: 3514: 3506: 3502: 3491: 3489: 3483: 3482: 3467: 3466: 3451: 3447: 3438: 3434: 3432: 3429: 3428: 3405: 3404: 3398: 3392: 3391: 3390: 3382: 3378: 3367: 3365: 3359: 3358: 3343: 3342: 3327: 3323: 3314: 3310: 3308: 3305: 3304: 3281: 3280: 3274: 3268: 3267: 3266: 3258: 3254: 3249: 3243: 3242: 3227: 3226: 3211: 3207: 3198: 3194: 3192: 3189: 3188: 3179: 3144: 3118: 3109: 3108: 3090: 3086: 3065: 3061: 3055: 3054: 3041: 3035: 3030: 3024: 3016: 3013: 3012: 3007: 2974: 2965: 2964: 2955: 2951: 2930: 2926: 2911: 2910: 2891: 2885: 2880: 2874: 2866: 2863: 2862: 2824: 2820: 2806: 2801: 2792: 2780: 2776: 2755: 2751: 2733: 2732: 2724: 2720: 2714: 2710: 2708: 2702: 2701: 2693: 2690: 2689: 2666: 2665: 2644: 2640: 2626: 2621: 2612: 2606: 2605: 2587: 2586: 2580: 2574: 2573: 2572: 2566: 2562: 2557: 2536: 2535: 2523: 2522: 2515: 2508: 2507: 2501: 2497: 2491: 2487: 2482: 2461: 2460: 2453: 2451: 2440: 2436: 2430: 2426: 2424: 2422: 2419: 2418: 2395: 2394: 2388: 2382: 2381: 2380: 2372: 2368: 2357: 2355: 2349: 2348: 2333: 2332: 2317: 2313: 2304: 2300: 2298: 2295: 2294: 2289: 2279: 2275: 2271: 2261: 2250: 2224: 2223: 2217: 2211: 2210: 2209: 2201: 2197: 2192: 2186: 2185: 2170: 2169: 2154: 2150: 2132: 2128: 2126: 2123: 2122: 2117: 2112: 2068: 2055: 2026: 2002:Spread spectrum 1984: 1982:Range advantage 1952: 1939: 1930: 1928:Early receivers 1925: 1910:Spiral antennas 1905: 1902: 1893: 1890: 1881: 1878: 1869: 1866: 1844: 1840: 1836: 1832: 1828: 1824: 1813: 1788: 1786:Antennas for DF 1773:Spread spectrum 1769:Shortwave radio 1749: 1714: 1679: 1674: 1673: 1672: 1669: 1668: 1664: 1660: 1651: 1650: 1643: 1634: 1633: 1626: 1617: 1616: 1612: 1605: 1596: 1595: 1587: 1583: 1574: 1573: 1565: 1559: 1550: 1549: 1541: 1537: 1526: 1525: 1488: 1482: 1469: 1467:Radio astronomy 1463: 1436: 1415: 1388: 1327: 1323: 1315: 1307: 1260: 1250: 1189: 1188: 1187: 1186: 1182: 1181: 1180: 1172: 1171: 1163: 1162: 1143: 1120: 1058: 1053: 1036: 1011: 1005: 988: 968: 927:'sense' antenna 878: 831: 823:parabolic shape 819:"dish" antennas 803: 779: 732: 726: 701: 558: 528: 484: 433: 389: 365: 361: 360: 355: 351: 350: 318:dipole antennas 297: 292: 140: 129: 123: 120: 77: 75: 65: 53: 42: 35: 28: 23: 22: 15: 12: 11: 5: 6685: 6675: 6674: 6669: 6667:Air navigation 6664: 6659: 6654: 6649: 6644: 6642:Geopositioning 6639: 6634: 6629: 6624: 6607: 6606: 6604: 6603: 6598: 6597: 6596: 6586: 6580: 6578: 6572: 6571: 6569: 6568: 6563: 6558: 6553: 6548: 6543: 6538: 6533: 6531:Target-centric 6528: 6522: 6520: 6514: 6513: 6510: 6509: 6507: 6506: 6501: 6496: 6491: 6486: 6480: 6478: 6474: 6473: 6471: 6470: 6460: 6455: 6453:Radiofrequency 6450: 6445: 6440: 6435: 6429: 6427: 6419: 6418: 6416: 6415: 6410: 6405: 6400: 6395: 6390: 6384: 6382: 6376: 6375: 6372: 6371: 6369: 6368: 6363: 6358: 6353: 6348: 6343: 6338: 6333: 6328: 6323: 6318: 6309: 6304: 6299: 6294: 6289: 6284: 6283: 6282: 6272: 6267: 6266: 6265: 6260: 6249: 6247: 6241: 6240: 6238: 6237: 6232: 6227: 6222: 6217: 6211: 6209: 6203: 6202: 6200: 6199: 6198: 6197: 6186: 6180: 6178:Human (HUMINT) 6171: 6165: 6164: 6157: 6156: 6149: 6142: 6134: 6125: 6124: 6122: 6121: 6116: 6115: 6114: 6109: 6099: 6094: 6089: 6084: 6079: 6074: 6069: 6064: 6063: 6062: 6057: 6047: 6042: 6037: 6036: 6035: 6030: 6020: 6019: 6018: 6013: 6003: 5998: 5993: 5992: 5991: 5980: 5978: 5974: 5973: 5971: 5970: 5965: 5960: 5954: 5952: 5943: 5942: 5940: 5939: 5934: 5933: 5932: 5922: 5917: 5912: 5907: 5902: 5897: 5892: 5887: 5882: 5877: 5872: 5867: 5861: 5859: 5858:communications 5853: 5852: 5850: 5849: 5848: 5847: 5836: 5834: 5830: 5829: 5827: 5826: 5821: 5816: 5811: 5806: 5801: 5796: 5795: 5794: 5784: 5777: 5775:Agent handling 5771: 5769: 5767: 5766: 5761: 5755: 5752: 5751: 5744: 5743: 5736: 5729: 5721: 5714: 5713: 5705: 5704:(RDF Products) 5697: 5691: 5676: 5665: 5639:(8): 436–445. 5621: 5615: 5602: 5589: 5583: 5567: 5565: 5562: 5560: 5559: 5550: 5541: 5526: 5517: 5508: 5499: 5490: 5480: 5471: 5465:Connor F. R., 5458: 5449: 5440: 5428: 5410: 5396: 5384: 5378:Hatcher B.R., 5368: 5359: 5350: 5338: 5326: 5317: 5308: 5296: 5287: 5278: 5266: 5248: 5236: 5224: 5215: 5206: 5194: 5185: 5171: 5159: 5150: 5136: 5118: 5109: 5081: 5072: 5065: 5047: 5017: 5010: 4992: 4977: 4968: 4961: 4943: 4934: 4909: 4885: 4874: 4872:, p. 188. 4862: 4860:, p. 261. 4850: 4848:, p. 150. 4838: 4836:, p. 187. 4826: 4807: 4804:on 2013-10-23. 4782: 4780: 4777: 4776: 4775: 4770: 4761: 4756: 4751: 4749:Radio location 4746: 4741: 4736: 4731: 4726: 4721: 4716: 4711: 4706: 4701: 4695: 4689: 4682: 4679: 4650: 4647: 4642: 4638: 4634: 4631: 4630: 4612: 4608: 4604: 4601: 4597: 4592: 4587: 4581: 4573: 4570: 4565: 4562: 4557: 4552: 4544: 4540: 4536: 4533: 4526: 4522: 4518: 4512: 4504: 4500: 4496: 4493: 4489: 4484: 4479: 4473: 4465: 4462: 4457: 4454: 4449: 4442: 4435: 4431: 4425: 4420: 4416: 4410: 4404: 4401: 4398: 4395: 4392: 4389: 4386: 4382: 4377: 4372: 4369: 4366: 4362: 4358: 4340: 4336: 4333: 4332: 4313: 4309: 4305: 4302: 4298: 4293: 4285: 4281: 4277: 4274: 4270: 4263: 4257: 4254: 4251: 4248: 4245: 4242: 4239: 4236: 4230: 4225: 4221: 4215: 4210: 4207: 4202: 4197: 4194: 4191: 4187: 4183: 4160: 4157: 4156: 4139: 4135: 4131: 4128: 4121: 4117: 4113: 4107: 4104: 4099: 4096: 4093: 4089: 4085: 4058: 4055: 4035: 4031: 4027: 4023: 4019: 4015: 4011: 4007: 4003: 4000: 3999: 3986: 3983: 3978: 3970: 3967: 3964: 3960: 3956: 3951: 3948: 3945: 3941: 3934: 3931: 3928: 3924: 3918: 3913: 3910: 3905: 3897: 3894: 3891: 3887: 3883: 3878: 3875: 3872: 3868: 3860: 3857: 3854: 3850: 3846: 3841: 3838: 3835: 3831: 3824: 3821: 3806: 3803: 3802: 3791: 3788: 3785: 3782: 3779: 3774: 3770: 3766: 3763: 3756: 3751: 3747: 3743: 3738: 3735: 3730: 3726: 3722: 3719: 3716: 3713: 3710: 3705: 3701: 3697: 3694: 3691: 3680: 3679: 3668: 3665: 3662: 3659: 3656: 3651: 3647: 3643: 3640: 3633: 3628: 3624: 3620: 3615: 3612: 3607: 3603: 3599: 3596: 3593: 3590: 3587: 3582: 3578: 3574: 3571: 3568: 3549: 3546: 3545: 3532: 3525: 3519: 3509: 3505: 3500: 3497: 3494: 3486: 3481: 3478: 3475: 3470: 3465: 3462: 3459: 3454: 3450: 3446: 3441: 3437: 3422: 3421: 3408: 3401: 3395: 3385: 3381: 3376: 3373: 3370: 3362: 3357: 3354: 3351: 3346: 3341: 3338: 3335: 3330: 3326: 3322: 3317: 3313: 3298: 3297: 3284: 3277: 3271: 3261: 3257: 3253: 3246: 3241: 3238: 3235: 3230: 3225: 3222: 3219: 3214: 3210: 3206: 3201: 3197: 3178: 3175: 3143: 3140: 3139: 3138: 3125: 3122: 3117: 3112: 3107: 3104: 3101: 3098: 3093: 3089: 3085: 3082: 3079: 3076: 3073: 3068: 3064: 3058: 3053: 3047: 3044: 3038: 3033: 3029: 3023: 3020: 3005: 2995: 2994: 2981: 2978: 2973: 2968: 2963: 2958: 2954: 2950: 2947: 2944: 2941: 2938: 2933: 2929: 2925: 2922: 2919: 2914: 2909: 2903: 2900: 2897: 2894: 2888: 2883: 2879: 2873: 2870: 2856: 2855: 2844: 2841: 2838: 2835: 2832: 2827: 2823: 2819: 2816: 2809: 2804: 2800: 2796: 2791: 2788: 2783: 2779: 2775: 2772: 2769: 2766: 2763: 2758: 2754: 2750: 2747: 2744: 2741: 2736: 2727: 2723: 2717: 2713: 2705: 2700: 2697: 2683: 2682: 2669: 2664: 2661: 2658: 2655: 2652: 2647: 2643: 2639: 2636: 2629: 2624: 2620: 2616: 2609: 2604: 2601: 2598: 2590: 2583: 2577: 2569: 2565: 2560: 2556: 2553: 2550: 2547: 2544: 2539: 2534: 2531: 2526: 2521: 2518: 2511: 2504: 2500: 2494: 2490: 2485: 2481: 2478: 2475: 2472: 2469: 2464: 2459: 2456: 2450: 2443: 2439: 2433: 2429: 2412: 2411: 2398: 2391: 2385: 2375: 2371: 2366: 2363: 2360: 2352: 2347: 2344: 2341: 2336: 2331: 2328: 2325: 2320: 2316: 2312: 2307: 2303: 2287: 2284: 2283: 2280: 2277: 2273: 2269: 2266: 2259: 2256: 2248: 2241: 2240: 2227: 2220: 2214: 2204: 2200: 2196: 2189: 2184: 2181: 2178: 2173: 2168: 2165: 2162: 2157: 2153: 2149: 2146: 2143: 2140: 2135: 2131: 2115: 2111: 2108: 2067: 2066:Two-channel DF 2064: 2054: 2051: 2025: 2022: 1983: 1980: 1974:circuits, but 1951: 1948: 1938: 1935: 1929: 1926: 1924: 1921: 1907: 1906: 1904:Pyramidal horn 1903: 1896: 1894: 1891: 1884: 1882: 1879: 1872: 1870: 1867: 1860: 1842: 1838: 1834: 1830: 1826: 1822: 1811: 1787: 1784: 1748: 1745: 1713: 1710: 1678: 1675: 1671: 1670: 1662: 1661: 1654: 1652: 1645: 1644: 1637: 1635: 1628: 1627: 1620: 1618: 1607: 1606: 1599: 1597: 1585: 1584: 1577: 1575: 1561: 1560: 1553: 1551: 1539: 1538: 1531: 1528: 1527: 1523: 1522: 1521: 1513:motor vehicles 1484:Main article: 1481: 1478: 1465:Main article: 1462: 1459: 1435: 1434:Reconnaissance 1432: 1414: 1411: 1387: 1384: 1249: 1246: 1236:in the 1970s. 1184: 1183: 1174: 1173: 1165: 1164: 1156: 1155: 1154: 1153: 1152: 1142: 1139: 1057: 1054: 1052: 1049: 1035: 1032: 1009:Adcock antenna 1007:Main article: 1004: 1001: 987: 984: 967: 964: 934:, Aqua Guide, 877: 874: 841:by analogy to 830: 827: 802: 799: 778: 775: 771:dipole antenna 728:Main article: 725: 722: 700: 697: 687:Early British 565:Amelia Earhart 557: 554: 527: 524: 483: 480: 468:Adcock antenna 432: 429: 418:Adcock antenna 400:the size of a 388: 385: 309:Heinrich Hertz 296: 293: 291: 288: 231:or commercial 142: 141: 56: 54: 47: 26: 9: 6: 4: 3: 2: 6684: 6673: 6670: 6668: 6665: 6663: 6660: 6658: 6655: 6653: 6650: 6648: 6645: 6643: 6640: 6638: 6635: 6633: 6630: 6628: 6625: 6623: 6620: 6619: 6617: 6602: 6599: 6595: 6594:organizations 6592: 6591: 6590: 6587: 6585: 6582: 6581: 6579: 6577: 6576:Dissemination 6573: 6567: 6564: 6562: 6559: 6557: 6554: 6552: 6549: 6547: 6544: 6542: 6539: 6537: 6534: 6532: 6529: 6527: 6524: 6523: 6521: 6519: 6515: 6505: 6502: 6500: 6497: 6495: 6492: 6490: 6487: 6485: 6482: 6481: 6479: 6475: 6468: 6464: 6461: 6459: 6456: 6454: 6451: 6449: 6446: 6444: 6441: 6439: 6436: 6434: 6431: 6430: 6428: 6426: 6420: 6414: 6411: 6409: 6406: 6404: 6401: 6399: 6396: 6394: 6391: 6389: 6386: 6385: 6383: 6381: 6377: 6367: 6364: 6362: 6361:Steganography 6359: 6357: 6354: 6352: 6349: 6347: 6344: 6342: 6341:Interrogation 6339: 6337: 6334: 6332: 6329: 6327: 6326:Eavesdropping 6324: 6322: 6319: 6317: 6313: 6310: 6308: 6305: 6303: 6300: 6298: 6295: 6293: 6290: 6288: 6285: 6281: 6278: 6277: 6276: 6273: 6271: 6268: 6264: 6261: 6259: 6256: 6255: 6254: 6251: 6250: 6248: 6246: 6242: 6236: 6233: 6231: 6230:Direct action 6228: 6226: 6225:Covert action 6223: 6221: 6218: 6216: 6213: 6212: 6210: 6208: 6204: 6196: 6195:organizations 6193: 6192: 6191: 6188: 6187: 6184: 6181: 6179: 6175: 6172: 6170: 6166: 6162: 6155: 6150: 6148: 6143: 6141: 6136: 6135: 6132: 6120: 6117: 6113: 6110: 6108: 6105: 6104: 6103: 6100: 6098: 6095: 6093: 6090: 6088: 6085: 6083: 6080: 6078: 6075: 6073: 6070: 6068: 6065: 6061: 6058: 6056: 6055:interrogation 6053: 6052: 6051: 6048: 6046: 6043: 6041: 6038: 6034: 6031: 6029: 6026: 6025: 6024: 6023:Eavesdropping 6021: 6017: 6014: 6012: 6009: 6008: 6007: 6004: 6002: 5999: 5997: 5994: 5990: 5987: 5986: 5985: 5982: 5981: 5979: 5975: 5969: 5966: 5964: 5961: 5959: 5956: 5955: 5953: 5948: 5944: 5938: 5935: 5931: 5928: 5927: 5926: 5925:Steganography 5923: 5921: 5918: 5916: 5913: 5911: 5908: 5906: 5903: 5901: 5900:Invisible ink 5898: 5896: 5893: 5891: 5888: 5886: 5883: 5881: 5878: 5876: 5873: 5871: 5868: 5866: 5863: 5862: 5860: 5854: 5846: 5843: 5842: 5841: 5838: 5837: 5835: 5831: 5825: 5822: 5820: 5819:Sleeper agent 5817: 5815: 5812: 5810: 5807: 5805: 5802: 5800: 5797: 5793: 5790: 5789: 5788: 5785: 5783: 5782: 5778: 5776: 5773: 5772: 5770: 5765: 5762: 5760: 5757: 5756: 5753: 5749: 5742: 5737: 5735: 5730: 5728: 5723: 5722: 5719: 5711: 5710: 5706: 5703: 5702: 5698: 5694: 5692:9780226015194 5688: 5684: 5683: 5677: 5673: 5672: 5666: 5662: 5658: 5654: 5650: 5646: 5642: 5638: 5634: 5627: 5622: 5618: 5616:0-7102-1271-2 5612: 5608: 5603: 5599: 5595: 5590: 5586: 5584:9781134526079 5580: 5577:. Routledge. 5576: 5575: 5569: 5568: 5554: 5545: 5539: 5533: 5531: 5521: 5512: 5503: 5494: 5484: 5475: 5468: 5462: 5453: 5444: 5438: 5432: 5423: 5421: 5419: 5417: 5415: 5405: 5403: 5401: 5391: 5389: 5381: 5375: 5373: 5363: 5354: 5348: 5342: 5336: 5330: 5321: 5312: 5306: 5300: 5291: 5282: 5276: 5270: 5263: 5260:Wiley R. G., 5257: 5255: 5253: 5245: 5240: 5231: 5229: 5219: 5210: 5201: 5199: 5189: 5180: 5178: 5176: 5169: 5163: 5154: 5145: 5143: 5141: 5131: 5129: 5127: 5125: 5123: 5113: 5104: 5102: 5100: 5098: 5096: 5094: 5092: 5090: 5088: 5086: 5076: 5068: 5062: 5058: 5051: 5040: 5036: 5029: 5021: 5013: 5011:0-932146-00-7 5007: 5003: 4996: 4988: 4981: 4972: 4964: 4962:0-7509-3783-1 4958: 4954: 4947: 4938: 4920: 4913: 4899: 4895: 4889: 4883: 4878: 4871: 4866: 4859: 4854: 4847: 4842: 4835: 4830: 4822: 4816: 4814: 4812: 4800: 4793: 4787: 4783: 4774: 4771: 4769: 4765: 4762: 4760: 4757: 4755: 4752: 4750: 4747: 4745: 4742: 4740: 4737: 4735: 4732: 4730: 4727: 4725: 4722: 4720: 4717: 4715: 4712: 4710: 4707: 4705: 4702: 4699: 4696: 4693: 4690: 4688: 4685: 4684: 4678: 4676: 4670: 4666: 4662: 4655: 4646: 4610: 4606: 4602: 4599: 4595: 4590: 4585: 4571: 4563: 4560: 4550: 4542: 4538: 4534: 4531: 4524: 4520: 4516: 4510: 4502: 4498: 4494: 4491: 4487: 4482: 4477: 4463: 4455: 4452: 4440: 4433: 4423: 4418: 4408: 4399: 4393: 4390: 4387: 4384: 4380: 4375: 4370: 4367: 4364: 4360: 4349: 4348: 4347: 4344: 4311: 4307: 4303: 4300: 4296: 4291: 4283: 4279: 4275: 4272: 4268: 4261: 4252: 4246: 4240: 4237: 4234: 4228: 4223: 4213: 4208: 4200: 4195: 4192: 4189: 4185: 4174: 4173: 4172: 4168: 4164: 4137: 4133: 4129: 4126: 4119: 4111: 4105: 4102: 4097: 4094: 4091: 4087: 4076: 4075: 4074: 4071: 4069: 4065: 4064:thermal noise 4054: 4050: 4048: 4044: 4039: 3984: 3976: 3968: 3965: 3962: 3954: 3949: 3946: 3943: 3932: 3929: 3926: 3916: 3911: 3903: 3895: 3892: 3889: 3881: 3876: 3873: 3870: 3858: 3855: 3852: 3844: 3839: 3836: 3833: 3822: 3819: 3812: 3811: 3810: 3786: 3780: 3777: 3772: 3761: 3754: 3749: 3741: 3736: 3728: 3724: 3717: 3714: 3711: 3703: 3699: 3692: 3689: 3682: 3681: 3663: 3657: 3654: 3649: 3638: 3631: 3626: 3618: 3613: 3605: 3601: 3594: 3591: 3588: 3580: 3576: 3569: 3566: 3559: 3558: 3557: 3554: 3523: 3507: 3498: 3495: 3479: 3476: 3473: 3463: 3460: 3457: 3452: 3448: 3444: 3439: 3435: 3427: 3426: 3425: 3399: 3383: 3374: 3371: 3355: 3352: 3349: 3339: 3336: 3333: 3328: 3324: 3320: 3315: 3311: 3303: 3302: 3301: 3275: 3259: 3251: 3239: 3236: 3233: 3223: 3220: 3217: 3212: 3208: 3204: 3199: 3195: 3187: 3186: 3185: 3182: 3174: 3170: 3167: 3163: 3156: 3148: 3123: 3115: 3102: 3099: 3091: 3087: 3083: 3077: 3074: 3066: 3062: 3051: 3042: 3036: 3031: 3021: 3018: 3011: 3010: 3009: 3003: 2998: 2979: 2971: 2956: 2952: 2945: 2942: 2939: 2931: 2927: 2920: 2917: 2907: 2898: 2895: 2892: 2886: 2881: 2871: 2868: 2861: 2860: 2859: 2839: 2833: 2830: 2825: 2814: 2807: 2802: 2794: 2789: 2781: 2777: 2770: 2767: 2764: 2756: 2752: 2745: 2742: 2739: 2725: 2721: 2715: 2711: 2698: 2695: 2688: 2687: 2686: 2659: 2653: 2650: 2645: 2634: 2627: 2622: 2614: 2602: 2599: 2596: 2581: 2567: 2558: 2551: 2548: 2532: 2529: 2519: 2516: 2502: 2492: 2483: 2479: 2473: 2470: 2467: 2457: 2454: 2448: 2441: 2437: 2431: 2427: 2417: 2416: 2415: 2389: 2373: 2364: 2361: 2345: 2342: 2339: 2329: 2326: 2323: 2318: 2314: 2310: 2305: 2301: 2293: 2292: 2291: 2281: 2267: 2265: 2257: 2254: 2246: 2245: 2244: 2218: 2202: 2194: 2182: 2179: 2176: 2166: 2163: 2160: 2155: 2151: 2147: 2141: 2133: 2129: 2121: 2120: 2119: 2107: 2103: 2101: 2096: 2088: 2080: 2072: 2063: 2059: 2050: 2048: 2042: 2040: 2036: 2032: 2021: 2019: 2018:Stealth ships 2015: 2011: 2007: 2003: 1998: 1995: 1992:and σ is the 1989: 1988: 1979: 1977: 1973: 1969: 1965: 1961: 1957: 1947: 1944: 1934: 1920: 1918: 1913: 1911: 1900: 1895: 1888: 1883: 1876: 1871: 1864: 1859: 1858: 1857: 1855: 1854:horn antennas 1851: 1846: 1819: 1817: 1809: 1805: 1800: 1798: 1794: 1783: 1780: 1778: 1774: 1770: 1765: 1763: 1759: 1754: 1744: 1742: 1738: 1734: 1729: 1727: 1723: 1719: 1709: 1707: 1703: 1698: 1696: 1692: 1687: 1684: 1658: 1653: 1641: 1636: 1632: 1624: 1619: 1611: 1603: 1598: 1592: 1581: 1576: 1570: 1564: 1557: 1552: 1546: 1545:Galeta Island 1535: 1530: 1529: 1520: 1518: 1514: 1510: 1506: 1502: 1498: 1497:civil defense 1494: 1487: 1477: 1474: 1468: 1458: 1456: 1452: 1448: 1444: 1440: 1439:Phased arrays 1431: 1428: 1424: 1420: 1419:triangulation 1410: 1407: 1403: 1401: 1396: 1392: 1386:Emergency aid 1383: 1381: 1376: 1373: 1371: 1365: 1362: 1358: 1354: 1349: 1344: 1342: 1336: 1332: 1321: 1313: 1304: 1300: 1298: 1293: 1289: 1285: 1281: 1273: 1269: 1268:amateur radio 1264: 1259: 1255: 1245: 1242: 1237: 1235: 1230: 1224: 1220: 1218: 1214: 1210: 1206: 1202: 1198: 1194: 1178: 1169: 1160: 1147: 1136: 1135:ILS Localizer 1132: 1128: 1124: 1119: 1114: 1111: 1107: 1103: 1099: 1098: 1092: 1088: 1084: 1080: 1076: 1075: 1070: 1062: 1048: 1044: 1042: 1031: 1029: 1024: 1019: 1018:ambiguities. 1016: 1010: 1000: 997: 996:doppler shift 993: 983: 981: 977: 972: 963: 961: 957: 953: 949: 945: 941: 937: 933: 928: 923: 918: 916: 912: 908: 904: 899: 896: 891: 882: 869: 865: 862: 858: 856: 852: 848: 844: 840: 836: 835:phased arrays 826: 824: 820: 816: 812: 808: 798: 795: 792: 788: 784: 774: 772: 766: 763: 759: 753: 750: 746: 741: 737: 731: 717: 713: 710: 706: 696: 694: 690: 685: 683: 679: 675: 669: 667: 663: 659: 655: 651: 647: 643: 639: 635: 630: 628: 623: 619: 614: 612: 608: 604: 600: 596: 592: 591: 586: 582: 574: 570: 566: 562: 553: 551: 547: 542: 539: 537: 533: 523: 520: 515: 513: 509: 503: 501: 497: 493: 489: 479: 477: 471: 469: 465: 461: 456: 452: 445: 444: 437: 428: 425: 423: 419: 415: 409: 407: 403: 393: 384: 382: 377: 372: 347: 345: 342: 338: 334: 329: 327: 326:Lee de Forest 323: 319: 314: 310: 301: 287: 284: 280: 276: 272: 267: 265: 261: 257: 253: 249: 245: 241: 236: 234: 230: 226: 222: 221:line-of-sight 218: 214: 209: 207: 203: 199: 198:triangulation 195: 191: 187: 183: 179: 175: 171: 165: 161: 156: 148: 138: 135: 127: 116: 113: 109: 106: 102: 99: 95: 92: 88: 85: – 84: 80: 79:Find sources: 73: 69: 63: 62: 57:This article 55: 51: 46: 45: 40: 33: 19: 6402: 6366:Surveillance 6356:Resident spy 6297:Cryptography 6102:Surveillance 5890:Cyber spying 5875:Cryptography 5814:Resident spy 5799:Double agent 5779: 5708: 5700: 5681: 5670: 5636: 5632: 5606: 5593: 5573: 5564:Bibliography 5553: 5544: 5520: 5511: 5502: 5493: 5483: 5474: 5466: 5461: 5452: 5443: 5431: 5362: 5353: 5341: 5329: 5320: 5311: 5299: 5290: 5281: 5269: 5261: 5239: 5218: 5209: 5188: 5162: 5153: 5112: 5075: 5056: 5050: 5039:the original 5034: 5020: 5001: 4995: 4986: 4980: 4971: 4952: 4946: 4937: 4925:. Retrieved 4912: 4901:. 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631: 615: 606: 602: 598: 589: 584: 580: 578: 572: 543: 540: 529: 526:21st century 516: 508:ferrite core 504: 496:vacuum tubes 485: 472: 460:oscilloscope 448: 442: 426: 410: 398: 387:Bellini–Tosi 348: 330: 306: 268: 256:phased array 237: 210: 181: 177: 173: 169: 168: 130: 124:October 2023 121: 111: 104: 97: 90: 78: 66:Please help 61:verification 58: 6438:Geophysical 6220:Cell system 6207:Clandestine 6092:Stay-behind 6001:Recruitment 5958:Canary trap 5856:Devices and 5804:Field agent 4858:Howeth 1963 4773:Wullenweber 4714:Geolocation 1704:and covert 1567:(providing 1297:ground wave 1209:Medium wave 1015:Watson-Watt 960:Sperry-Rand 915:medium wave 839:goniometers 736:loop aerial 638:World War I 333:medium wave 260:beamforming 213:wavelengths 186:radio waves 164:Switzerland 6616:Categories 6467:earthquake 6331:False flag 6169:Collection 6087:Sexpionage 6067:COINTELPRO 6060:safe house 6040:False flag 5977:Operations 5951:techniques 5947:Tradecraft 4927:2008-01-26 4903:2023-08-11 4870:Yeang 2013 4846:Baker 2013 4834:Yeang 2013 4779:References 2039:Data links 2035:side lobes 2012:, as with 1972:microstrip 1917:beamwidths 1829:, where BW 1804:polar plot 1793:beamwidths 1762:Data links 1706:Data links 1348:Royal Navy 1312:goniometer 1252:See also: 1201:Morse Code 1197:lighthouse 1028:arctangent 952:Fuji Onkyo 851:helicopter 843:WW II 811:microwaves 794:television 693:Chain Home 622:navigation 618:wavelength 500:transistor 464:Met Office 414:ionosphere 283:Chain Home 94:newspapers 6458:Materials 6312:Defection 6307:Dead drop 6280:black bag 6245:Espionage 6011:black bag 5895:Dead drop 5824:Spymaster 5748:Espionage 5661:110506568 4744:Radio fix 4698:AN/FRD-10 4633:where SNR 4569:Φ 4564:− 4561:ϕ 4521:ϕ 4461:Φ 4453:ϕ 4430:Φ 4415:Ψ 4385:− 4361:ϕ 4357:Δ 4335:where SNR 4256:Φ 4235:− 4220:Ψ 4206:Φ 4186:ϕ 4182:Δ 4116:Ψ 4088:ϕ 4084:Δ 4034:) - \ln(P 4022:) - \ln(P 3982:Φ 3959:Δ 3940:Δ 3923:Δ 3909:Φ 3886:Δ 3867:Δ 3849:Δ 3845:− 3830:Δ 3820:ϕ 3787:ϕ 3784:Φ 3769:Φ 3746:Ψ 3718:⁡ 3712:− 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