What is Direction Finding?

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- everything RF

Nov 19, 2020

Direction Finding or radio direction finding (RDF) is the process of determining the direction and origin of an RF signal. This can be done for any mode of wireless communication such as radios, radar signals, drone remote controllers, monitoring (ELINT/SIGINT) etc.

Direction Finding uses one or more receivers that determine the location of the signal source using triangulation. Each receiver uses a directional antenna to determine the angle from at which the signal is received. When this angle is taken from three or more different locations, the origin of a signal can be determined. Triangulation uses the position of the receiver and the angle to the signal from a reference point with decent accuracy and does not require any clocks. It is used extensively in navigation of ship and aircraft, emergency services, wildlife tracking, etc.

Radio triangulation using two antennas (A and B)

The process of triangulating a signal can be done manually or automatically. When being done manually, the operator(s) selects 3 or more different locations. The amplitude of the received signal and the angle at which the amplitude is maximum are measured at each location. The information collected can then be used to pinpoint the origin of the signal.

Manual direction finding is an easy, low-cost process that requires almost no setup. However, the effectiveness and accuracy depend strongly on the skill of the operator.

Visual representation of the manual direction-finding method

Automatic direction, though more complex is usually more accurate as it uses more advanced techniques to identify the origin of the signal. The most common techniques are Doppler, Watson-Watt and Correlative Interferometry.

Doppler based Direction Finding Systems

Doppler based Direction Finding systems use the fundamentals of the Doppler effect, which is the change in the frequency perceived by a receiver when the receiver and the source are in motion relative to each other. These systems produce bearings by measuring doppler shift in the received signal by sampling around the elements of a circular array. In the past, Doppler RDF systems used an antenna that would physically rotate but modern Doppler RDF systems utilize multi-antenna circular array with each antenna sampled in sequence.

Watson-Watt Technique

Watson-Watt is an amplitude-based technique which was developed shortly after World War I. It uses two Adcock antenna pairs to compare amplitude on the incoming signal. An Adcock antenna pair is a pair of monopole or dipole antennas that measure the vector difference of the received signal at each antenna. Two of these pairs are co-located but perpendicularly oriented to produce N-S (North-South) and E-W (East-West) signals that will then be passed to the receiver.

Correlative Interferometry

Correlative Interferometry is a phase-based direction finding technique which calculates bearing based on the phase difference obtained for a DF antenna system of known configuration at a known wave angle. It has an odd number of antennas arranged in a circular pattern and the bearing is obtained from the data for which the correlation co-efficient in maximum.

The size of the antennas used for direction finding vary with the frequency of the signal. Lower frequency (higher wavelength) signals require larger antennas and are usually ground based. However, these frequencies are very important in marine navigation as lower frequency results in longer range and better immunity from noise. High frequency (low wavelength) signals require smaller antennas and are generally used in aircrafts with an automatic direction finder being a feature of almost all modern aircrafts.

Direction finding antennas placed at a suitable distance from each other

Direction Finding is extensively used for military purpose as the ability to locate enemy transmitters can help soldiers pinpoint their location. It has played a key role for the Military since World War I and is still used widely. Modern systems are quite evolved though and they often use phase array antennas that allow beamforming for providing accurate results along with the use of Doppler techniques.

Multipath can be termed as the biggest challenge in radio direction finding. It is a relatively bigger challenge in urban and mountainous regions where the received signal can be a reflection of the original signal. The ability of a direction finding system to deal with multipath determines the quality of that system.

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