Picture Signal: The function of the Detector is to remove the modulation from the intermediate-frequency signal after it has been amplified. (As indicated previously, the desired modulation information originally superimposed on the carrier wave was transferred to a lower radio-frequency signal-the intermediate frequency signal.) This demodulation is accomplished by using a rectifier and a filter. A diode commonly is used to produce a rectified i-f signal, which is filtered to obtain an audio-frequency signal. This audio signal is the output of the detector.
Near the end of World War I, Fessenden's heterodyne principle was incorporated by Edwin H. Armstrong (q.v.) and French engineer Lucien Levy in their invention of the superheterodyne circuit, which is used in all modern radio and television receivers. In this circuit the incoming radio signal is converted, by heterodyning, to a lower but superaudible frequency called the intermediate frequency. At the intermediate frequency the signal is amplified and filtered (to reject interference) and is finally detected to obtain the sound or picture signal. (The name "superheterodyne" is a contraction from super-audibleheterodyne.)
EPIRBs can be started manually or activated automatically when immersed in water. The current models transmit a digital distress signal on 406 MHz, and are commonly referred to as "406 EPIRBs." (The older models that transmitted a tone on 121.5 and 243 MHz are being phased out.) The EPIRB signal is detected by one of several orbiting or geostationary satellites (these are satellites that maintain a position in space that is fixed relative to the earth, like those used for TV networking). The satellite transmits the signal to a ground station if one is within view; if one is not in view, the signal is stored and retransmitted at the earliest opportunity. The distress signal is then relayed to the appropriate search and rescue coordination center. The distress signal contains an identification number that will tell the rescue authorities the name and type of vessel, and the name and address of the owner.
A 406 EPIRB must be registered (at no cost) when it is purchased. The signal from some EPIRBs can also include the vessel's position if the unit has an internal GPS receiver or is connected to the vessel's GPS set. A 406 EPIRB transmits a signal as well on 121.5 MHz that can be used for direction finding by searching aircraft and vessels. It is also equipped with a flashing strobe light.
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