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meter. If the meter readings decrease turn 180DEG and proceed to determine
two further positions of equal signal strength then return to the mid-position.
(v) When apparent that the beacon is close, descend, if possible, to an altitude
giving approximately 2000FT clearance above terrain within approximately 10
NM of the search aircraft.
(vi) By continuing the boxing-in technique the observer should be able to estimate
the position of the beacon to within an area of 24 square miles.
(vii) The beacon site may be located with greater accuracy by detuning the VHF
receiver to 121.45 or 121.4 whilst close to the beacon and noting the position
on the ground over which there is maximum searchmeter deflection.The receiver
may also be detuned to 121.55 or 121.6. With the search aircraft 500FT above
ground level and the receiver tuned to 121.3 or 121.7, the signal will only be
heard within 200-300 metres of the beacon site (DIAGRAM 11).
121.5 MHz ELT
121.45 OR 121.55 MHz
121.4 OR 121.6 MHz
NOTE: The Vertical scale has been increased
by factor of 10 to increase clarity
DIAGRAM 11 - DETUNING PATTERN
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EMERGENCY PROCEDURES 24-Nov-2005 EMERG - 13
LOBES
CLEAR AREA
INDICATES CLEAR
SIGNAL
INTENSITY OF STIPPLING INDICATES AMOUNT OF BACKGROUND HASH
DIAGRAM 12 - DISTORTED RADIATION PATTERN
Note: Where the signal from the beacon is broken and distorted, the most probable
cause is that the beacon is amongst obstacles such as wreckage, trees, rocks, etc.
which cause the signal to be absorbed or reflected, forming lobes.
With the search aircraft flying along the track indicated in Diagram 12, the searchmeter
needle would rise and fall, corresponding with the passage of the aircraft through
successive lobes where the signal could be heard. In between the lobes there would
be receiver noise and no signal. Provided the aircraft is several thousand feet above
terrain, the observer may assume that the signal is being received direct from the
beacon rather than by reflection from surrounding terrain. Thus by following a lobe
the search aircraft will be guided direct to the beacon, (See DIAGRAM 13).
SIGNAL
FADE
SIGNAL STRENGTH INCREASES
TOWARDS BEACON
POINTS OF
MAXIMUM SIGNAL
SIGNAL HEARD
NO SIGNAL
DIAGRAM 13 - INTERCEPTING A LOBE
f. When the ratio between the beacon signal and the background hash begins to favour
the latter, alter heading until only a clear signal can be heard. The signal from the
beacon should be continuous with clarity improving as the search aircraft approaches
the beacon.
g. Once a heading has been established, note the searchmeter readings.The readings
will increase as the aircraft approaches the beacon site, reaching a maximum when
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EMERGENCY PROCEDURES 24-Nov-2005 EMERG - 14
about to overfly. The position of the beacon may be located with greater accuracy
by progressively detuning the VHF receiver to 121.3 or 121.7MHZ.
2.5 Overwater Searches
2.5.1 The principles of an overwater search using a searchmeter are similar to those used for
a regular pattern. However, the following differences should be noted:
a. The radiation pattern from a beacon transmitting overwater will generally be much
larger than if the beacon were on land. This could be up to 90NM to an aircraft flying
at 10000FT, and 180NM at 30000FT.
b. Some older marine beacons have an interrupted carrier.
c. Where a search aircraft is operating at an altitude of l0000FT or less and has a
searchmeter reading of more than four, the pilot should check on the adjacent
frequency of 121.45 or 121.55. If the signal can be heard on that frequency, albeit
faintly, the search aircraft will be within 15 NM of the beacon.
2.6 Mountainous Terrain
2.6.1 A beacon which is activated in mountainous terrain may be likened to an electric light
bulb illuminated in a model of the same area. The pilot of an aircraft flying in areas where
the light is reflected would be able to receive signals which are reflected. From positions
where the light could be seen direct, signals would be received directly from the beacon
or, conversely, where there were no reflections and the light could not be seen, there
would be no signals. Signals radiating from a beacon in rough country are absorbed,
reflected and reinforced to form isolated patches of strong signal which tend to hinder
rather than assist the location of the beacon.
2.6.2 To avoid possible confusion from reflected signals, it is essential that the search aircraft
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