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INFO: Full form for SMO is System Management Office in Space Science category
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A radio direction finder (RDF) is a device for finding the direction, or bearing, to a radio source. The act of measuring the direction is known as radio direction finding or sometimes simply direction finding (DF). 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 radio navigation system, especially with boats and aircraft.

RDF systems can be used with any radio source, although the size of the receiver antennas are a function of the wavelength 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 navigation 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 automatic direction finder, often capable of being tuned to commercial AM radio transmitters, is a feature of almost all modern aircraft.

For the military, RDF systems are a key component of signals intelligence systems and methodologies. The ability to locate the position of an enemy transmitter has been invaluable since World War I, and it played a key role in World War II's Battle of the Atlantic. It is estimated that the UK's advanced "huff-duff" systems were directly or indirectly responsible for 24% of all U-boats sunk during the war. Modern systems often use phased array antennas to allow rapid beam forming for highly accurate results. These are generally integrated into a wider electronic warfare suite.

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 the comparison of phase or doppler techniques which are generally simpler to automate. Modern pseudo-Doppler direction finder systems consist of a number of small antennas fixed to a circular card, with all of the processing performed by software.

Early British radar sets were also referred to as RDF, which was a deception tactic. However, the terminology was not inaccurate; the Chain Home systems used separate omnidirectional broadcasters and large RDF receivers to determine the location of the targets.

INFO: Full form for PSM is Propellant Storage Module in Space Science category
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INFO: Full form for PSM is Phase Sensitive Modulator in Space Science category
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INFO: Full form for PSM is Pyro Substitute Monitor in Space Science category
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INFO: Full form for PSM is Priority Select Module in Space Science category
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INFO: Full form for PSM is Project Security Manager in Space Science category
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INFO: Full form for PMM is Property Management Manual in Space Science category
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INFO: Full form for PIM is Pulse Interval Modulation in Space Science category
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INFO: Full form for PCB is Power Circuit Breaker in Space Science category
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INFO: Full form for PCM is Pressurized Cargo Module in Space Science category
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INFO: Full form for PCM is Power Control Mission in Space Science category
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INFO: Full form for PCB is Product Configuration Baseline in Space Science category
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INFO: Full form for PCB is Power Control Box in Space Science category
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INFO: Full form for SCM is Software Cost Model in Space Science category
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INFO: Full form for SCM is Site Configuration Message in Space Science category
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