Direction finding
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Direction finding (DF) refers to the establishment of the direction from which a received signal was transmitted. This can refer to radio or other forms of wireless communication. By combining the direction information from two or more suitably spaced receivers, the source of a transmission may be located in space via triangulation. This is called a cross-cut or fix.
Direction finding often requires an antenna that is directional - that is, more sensitive in certain directions than in others. Many antenna designs exhibit this property. For example, a Yagi antenna has quite pronounced directionality, so the source of a transmission can be determined simply by pointing it in the direction where the maximum signal level is obtained. However, to establish direction to great accuracy requires much more sophisticated techniques.
A simple form of directional antenna is the loop aerial. This consists of an open loop of wire on an insulating former, or a metal ring that forms the antenna elements itself. Such an antenna will be most sensitive to signals that are normal to its face, and least responsive to those meeting edge-on. Simply by turning the antenna to obtain maximum signal will establish two posible directions that the signal could be emanating from. By placing a reflector or shield on one side will eliminate one of the two possibilities. A dipole antenna exhibits similar properties, and is the basis for the Yagi antenna, which is familiar as the common VHF or UHF television aerial. For much higher frequencies still, parabolic antennas can be used, which are highly directional, focusing received signals from a very narrow angle to a receiving element at the centre.
More sophisticated techniques such as phased arrays are generally used for highly accurate direction finding such as that used in signals intelligence (SIGINT).
Some academic research has centered on the use of Software-defined_radios to perform the DF operations using receiveres with one or more separate channels in conjunction with multiple antenna arrays.
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Single Channel DF
Single-channel DF refers to the use of a multi-antenna array with a single channel radio receiver. This approach to DF obviously offers some advantages and drawbacks. Since it only uses one receiver, mobility and lower power consumption are obvious benefits but without the ability to look at each antenna simulataneously (which would be the case is one were to use multiple receivers) more complex operations need to occur at the antenna in order to present the signal to the receiver.
The two main categories that a single channel DF algorithm falls into are amplitude comparison and phase comparison. Some algorithms can be hybrids of the two.
Pseudo-Doppler DF Technique
The psuedo-doppler technique is a phase based DF method that produces a bearing estimate on the received signal by measuring the doppler shift 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.
Watson-Watt w/ Adcock Antenna Array
The Watson-Watt technique uses two Adcock antenna pairs to perform an amplitude comparison on the incoming signal. An Adcock antenna pair is 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 the 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) and E-W (East-West) signals that will then be passed to the reciver. In the receiver, the bearing angle can then be computed by taking the arctangent of the ratio of the N-S to E-W signal.
Related Links
- Radio Direction Finding Applications Literature (http://www.rdfproducts.com/ap_index.htm)(RDF Products)