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Locating
the Source of the SignalThere are three (3) important issues relating to the problem of locating an ELT / EPIRB signal source, these typically include;
The execution of these three (3) steps will vary radically from tasking to tasking. The vast majority of ELT searches are in reality non-distress or accidental activations. Most of these are undamaged and located in clear areas like airports or marine related facilities.
Finding them, particularly from flying aircraft or well trained ground crews, is like "shooting fish in a barrel" and has led to exaggerated claims for both equipment and techniques. Unfortunately, the cases of real distress are also the ones likely to involve damaged ELT / EPIRB's perhaps located in awkward positions, rough terrain, wet or snowy weather, night operations, and difficult access, any or all of which will tax the best of our skills, equipment, and techniques.
Under so-called "IDEAL" conditions, the signal from an ELT:
(1) Radiates equally in all directions, and
(2) Takes a path directly from the transmitter to the receiver
The first characteristic means that the signal strength gets stronger as the ELT is approached, regardless of the direction of approach. Signal strength or "build and fade" location patterns use this principle and work extremely well in many homing situations.
Unfortunately, the condition of equal radiation in all directions of the distress signal, is one of the most radically violated in distress situations, and this procedure is slow at best and should never be depended upon as a primary search technique. The second condition says that the direction to the transmitter is the same as the direction of arrival of the signal at the receiver. All direction finders use this principle.
Sensitivity refers to the ability of a receiver to pick up a weak signal. Both a good receiver and a high gain antenna are required for a sensitive DF operations. The ELT signal is weak due in part to the low transmitter power, and there is no such thing as too much sensitivity when purchasing receivers or DF equipment.
Paul D Turner
