The Radio Direction Finding with Advantage of the Software Defined Radio
The radio-frequency engineering recently has gone through extensive development. Software-Defined Radio (SDR) plays an important role in this development, bringing new possibilities to radio-frequency engineering and enables us to look at existing radio t
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Introduction
Radio Direction Finding (RDF) is the measurement of the direction from which a received signal was transmitted. This has many applications in different sectors. The RDF is used for example in the navigation of ships and aircrafts, for tracking wildlife, and to locate not only emergency transmitters but also illegal or interfering ones. Nowadays, most of the radio frequency bands are occupied. Fast classification and location of the source of an illegal or interfering transmitters could be very helpful, especially when transmitters or networks are being built and maintained. The classification and locating can be simplified thanks to software defined radio (SDR).
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Radio Direction Finding
The RDF is nearly as old as radio itself. Earliest experiments in the radio direction finding were carried out by Heinrich Hertz in 1888. Since then many types c IFIP International Federation for Information Processing 2016 Published by Springer International Publishing Switzerland 2016. All Rights Reserved K. Saeed and W. Homenda (Eds.): CISIM 2016, LNCS 9842, pp. 720–728, 2016. DOI: 10.1007/978-3-319-45378-1 62
The Radio Direction Finding with Advantage of the SDR
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of radio direction finding techniques were discovered. The mostl frequently used ones are the Watson-Watt and the Doppler technique. 2.1
Watson-Watt Radio Direction Finding System
The first one from the mentioned techniques is older and was introduced by Robert Watson-Watt before the World War II. It utilises an antenna array consisting of four equidistant vertical elements (known as Adcock antenna [8]) and two separated precisely calibrated receivers. The angle of arrival (AoA) is then determined from the signals level difference. The presence of two receivers is the main disadvantage of this technique [1]. 2.2
Doppler Radio Direction Finding System
The second technique uses the Doppler effect. This effect was described by Austrian physicist Christian Doppler in 1842 in Prague. Most people associate the Doppler effect with acoustic waves, but the theory is also applicable to radio waves as well. This well known effect describes a difference between the observed and emitted frequency of a wave for an observer which is moving relatively to the source of the waves. The earliest Doppler direction finding systems used a single antenna placed on a fast rotating turntable. As the receiving antenna approaches the transmitter, the frequency increases and as the antenna recedes from the transmitter, the frequency decreases. The receiver then observes a frequency modulated signal at a rate equal to the frequency of the rotation (rotation tone). The received signal is then de-modulated by FM receiver, which then produces the demodulated rotation tone. The angle of arrival is then determined from the phase offset between the original and the demodulated rotation tone [2].
Fig. 1. Pseudo-Doppler block diagram
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Spinning the turntable fast enough to produce high frequency signal efficiently is very difficult. Because of this, t
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