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M
AbnormalPropagation
The maximum radar
detection
range
depends upon the height
of the
scanner and the height of a
target
as described
in
the
section of
"The
Horizon for Radar
Beam Radiation". lf
a
so-called
"duct"
occurs on the sea surface
due to a certain weather
condition,
however,
the
radar
beam may
propagate
to an abnormally
long
distance, at which a target may
be detected by the radar.
For instance, assuming that
the radar range is 6 NM
(on
the
repetition frequency
of
1100
Hz), the
first
pulse
is reflected
from
a iarget at about 76 NM or more and
received
during the next
pulse
repetition
time.
In
this case,
a
false
echo appears at a
position
that is
about
76
NM shorter than
the actual distance. lfthe
false echo appears at 5 NM
on the
radar
display, the true distance
of
the target
is
5
+
76
=
81 NM.
On the
radar
range scale of 1.5
NM
(on
the repetition frequency of
1900 Hz), a false echo may
appear at a
position
that is about
43 NM
shorter than the actual
dislance.
This type
of
false echo
can
be discriminated by changing
over the range
scale
(the
repetition
frequency), because the
distance
of the
target changes accordingly.
tvll
Radar Interface
When another radar equipment
using the
same
frequency
band
as
that
on own ship is near
own
ship, a radar
interference
pattern may
appear
on the radar display.
This interference
pattern
consists of a
number
of spots,
which appear, in various forms.
These
spots do not always
appear at the same
places,
so that they can be discriminated
from the target
echoes.
(See
Fig.
6.6)
Fig.6.6
l.-
:
-' -'l-
tt
o-c
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