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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RF Basics Technologies. Can you answer this question? Editorial Team - everything RF Nov 19, Radio triangulation using two antennas A and B The process of triangulating a signal can be done manually or automatically. 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. These signals form sum and difference radiation patterns.
The ratio of the sum and difference signals and knowledge of the sum and difference patterns are used to determine the direction of the transmitter. Phase information is used to determine on which side of the sum pattern the transmitter is. An advantage of this system is in its capability to determine the direction of a transmitter after receiving one pulse. Such pulse could be a mere few microseconds. Accuracies of 10meter over a Km distance has been reported.
The relative difference in the phase of the signal received by two omni directional antennas spaced a set distance apart can be used to determine direction or angle of arrival of a RF signal.
In the omni directional case, the interferometer does not have a way to determine if the signal arrives from the front or the back of the antennas. As the frequency increases or electrical separation increases ambiguities appear as the phase differences wrap. If the antennas are too close electrical separation very small then the resulting phase difference will be very small and the system will not be able to determine the AOA.
The frequency range of use is thus determined by the separation of the antennas and the noise figure of the receivers. The image below shows two omni directional antennas with the incident signal circulating around them.
The relative phase of the incident signal between the two antennas is shown in the image on the right. This phase difference is used to determine the direction of the incident signal. An Adcock antenna uses two crossed loop antennas. The bearing of the RF signal is determined using the level of the signals received at each antenna. The method to process the information from a Adcock array is referred to as Watson-Watt. This is the best-known method of radio direction finding. In a more general application four closely spaced omni directional antennas positioned in a square can be used to form an Adcock array.
The figure below shows the typical radiation pattern of an Adcock array. The received signal rotates around the array. The relative amplitude of the signal received by the two crossed loop antennas or combined omni directional antennas is shown on the right.
It is this relative phase that resolves the ambiguity. With the technological improvements in receivers and digital processors, all the information produced by multiple antenna elements can be used to improve the performance of RDF systems. Typically, the bearing is calculated using the phase differences of the signals received at the various antennas in the correlative array. The correlative algorithm compares the phase differences of the incoming RF signals at each antenna to a set of calibration phases stored in the processor to determine the most likely AOA.
The correction function correlates the relative phases and magnitudes for some correlators of the received signals with the correlation table over all possible angles; the maximum of the correlation function indicates the AOA. Implemented as a dual-band vertical stack, the low frequency band is enhanced with patented hybrid loop-Adcock technology to provide improved sensitivity vs traditional designs. The antenna presents patterns suitable for the Watson Watt estimation method, as well as 3-channel correlative using integrated band switch and control electronics.
External and internal RF chain calibration is provided along with a digital compass and active GPS antenna. A variety of mast mountable, fixed site and tactical wideband direction finding antenna arrays are available covering 20 - MHz. Designed for two-channel phase-sensitive receivers, correlative interferometer algorithms yield typical angle of arrival uncertainty of about one degree in azimuth. Collapsible to a 44 pound, 66x16x10 inch volume, this high sensitivity portable direction finding antenna system covers 20 to MHz.
Two-channel receivers with phase-sensitive correlative interferometry are ideal for use with this array. An optional whip can be attached on top for omnidirectional HF monitoring. Date of Publication: Jul DOI: Need Help?
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