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Bearing accuracy
One of the most important features of the radar
is how accurately the bearing of a target can be
measured. The accuracy of bearing
measurement basically depends on the
narrowness of the radar beam. However, the
bearing is usually taken relative to the ship’s
heading, and thus, proper adjustment of the
heading line at installation is an important factor
in ensuring bearing accuracy. To minimize error
when measuring the bearing of a target, put the
target echo at the extreme position on the
screen by selecting a suitable range.
Range measurement
Measurement of the range to a target is also a
very important function of the radar. Generally,
there are two means of measuring range: the
fixed range rings and the variable range marker
(VRM). The fixed range rings appear on the
screen with a predetermined interval and
provide a rough estimate of the range to a target.
The variable range marker’s diameter is
increased or decreased so that the marker
touches the inner edge of the target, allowing
the operator to obtain more accurate range
measurements.
3.2 False Echoes
Occasionally echo signals appear on the screen
at positions where there is no target or
disappear even if there are targets. They are,
however, recognized if you understand the
reason why they are displayed. Typical false
echoes are described in this paragraph.
Shadow sectors
Funnels, stacks, masts, or derricks in the path of
the antenna block the radar beam. If the angle
subtended at the antenna is more than a few
degrees, a non-detecting sector may be
produced. Within this sector targets can not be
detected.
Radar
antenna
Radar
mast
Shadow sector
Figure 3-2 Shadow sectors
Multiple echoes
Multiple echoes occur when a transmitted pulse
returns from a solid object like a large ship,
bridge, or breakwater. A second, a third or more
echoes may be observed on the display at
double, triple or other multiples of the actual
range of the target as shown below. Multiple
reflection echoes can be reduced and often
removed by decreasing the gain (sensitivity) or
properly adjusting the [STC] key.
Own ship
Target
Tr ue
echo
Multiple
echo
Figure 3-3 Multiple echoes