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Chapter 3 TRUE AND FALSE ECHOES ON DISPLAY
3.4 FALSE ECHOES
Fig. 3.4-2
Radar
scanner
Funnel
3.4.4
FALSE ECHO BY MULTIPLE REFLECTION
When there is a large structure or ship with a high vertical surface near own ship as
shown in Fig. 3.4-3, multiple refection returns may appear on the radar display. These
echoes appear in the same intervals, of which the nearest echo is the true echo of the
target.
Fig. 3.4-3
3.4.5
SECOND TIME ECHOES
The maximum radar detection range depends upon the height of the scanner and the
height of a target as described in the Section "3.1 RADAR WAVE WITH THE
HORIZON". If a so-called "duct" occurs on the sea surface due to a certain weather
condition, however, the radar beam may propagate to a abnormally long distance, at
which a target may be detected by the radar.
For instance, assuming that the pulse length is MP3 (on the repetition frequency of 1400
Hz), the first pulse is reflected from a target at about 58 NM or more and received during
the next pulse repetition time. In this case, a false echo (second time echo) appears at a
position that is about 58 NM shorter than the actual distance. If the false echo appears at
5 NM on the radar display, the true distance of the target is 5+58=63 NM. On the pulse
3-6
Direct microwave
Secondary reflection
of microwave
HL
Actual target
False echo from funnel
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