Smiths Kelvin Hughes Nucleus 3 5000 Series, System Handbook

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Spu ri ous Echoes
12 In built up areas and in narrow congested waters,
transmitted energy may be reflected along a number of
paths producing confusing spurious echo patterns on the
display. Spurious echoes may not always appear in the
same location and may not correlate.
NOTE:
Adjustment of the GAIN control helps to minimise these
spurious echoes.
Ghost Echoes
13 In a similar manner to false echoes, the proportion of
transmitted energy reflected off obstructions may be
directed towards a real target causing a ghost echo of the
real target to appear on the bearing at which the antenna is
pointing. The ghost echo will appear to be a real target and
behave in the same way. However, because the antenna is
not directed at the real target the returns from the ghost
target will be weaker than those of the real target. The range
of the ghost echo is the true distance of the real target.
NOTE:
The suspected Ghost target echo will appear on the display
at the same radius as the real target (see below). The VRM
facility can be used to confirm this. However, there is no
real way of determining whether the indicated target is a
ghost or a real target.
14
Other types of ghosting include echoes of groups of
targets (which appear to be real). When in the vicinity
of land masses, these may be from large inland objects and
may be caused by a combination of atmospheric conditions,
unusual propagation conditions and reflection.
15
Where obstructions occur in close proximity to the
antenna, the radar beam can be dispersed causing
target smearing to occur. This is indicated by a number of
weaker echoes appearing around a stronger target echo on
the display. When the antenna points directly at the target
the returns are at their strongest and these form the thickest
part of the arc shaped pattern on the display (see below).
Ra dar In ter fer ence
16 Other radar equipment using the same frequency, on
the same vessel or other vessels, may interfere with
the PPI presentation. Such interference produces a spiral
pattern emanating from the centre of the radar picture.
Careful positioning of the Antenna arrangement helps to
minimise interference from these radars. Cross pulse
blanking and the use of interference rejector circuits, when
selected, remove the spiral patterns from the display.
At mo spheric Con di tions
17 The propagation of radar signals in acute atmospheric
conditions may have an adverse effect on the PPI
display. A transmitted beam of energy normally travels in a
straight ‘line of sight’ path but certain conditions may
contribute to the beam bending upwards or downwards.
18 The effect of this condition (known as anomalous
propagation) is that when the beam is ‘bent’ upwards,
distant targets appear below the beam and consequently the
maximum detection range of the equipment is impaired.
19 When the transmitted beam is ‘bent’ downwards, the
beam tends to follow the earth’s surface and improves
maximum range performance, with the detection of targets
over the horizon.
20
Under specific atmospheric conditions the beam may
be ‘bent’ to reflect from the earth’s surface to the upper
atmosphere where, due to the presence of a layer of dense
air, the beam is reflected back to earth. This condition,
known as ‘ducting’, may happen several times and echoes
may be obtained over great distances. However, these
echoes may return several transmissions later and are
displayed as false ranges on the display.
21 S e v e r e a t m o s p h e r i c c o n d i t i o n s , s u c h a s
thunderstorms or acute static build up during
pre-storm situations, also have the affect of distorting the
PPI presentation due to the contorting effect of the electrical
disturbances in the atmosphere.
May 01 3.2 Original
KH 2020
Chap 3
Obstruction
Rotating
Antenna
Ghost
Target
Common
Target Radius
Real
Target
CD-2527
CD-2529
Ownship
Target
Smearing