False Echoes - Furuno FR-1500 Operator's Manual

Bearing resolution
Bearing resolution is the ability of the radar to display as separate pips the echoes received from
two targets which are at the same range and close together. It is proportional to the scanner length
and reciprocally proportional to the wavelength. The length of the scanner radiator should be
chosen for a bearing resolution better than 2.5° (IMO Resolution). This condition is normally
satisfied with a radiator of 1.2 m (4 ft) or longer in the X-band. The S-band radar requires a radiator
of nominal 12 feet (3.6 m) in our product range.
Range resolution
Range resolution is the ability to display as separate pips the echoes received from two targets
which are on the same bearing and close to each other. This is determined by pulselength only.
Practically, a 0.08 microsecond pulse offers the discrimination better than 35 m as do so with all
FURUNO radars.
Test targets for determining the range and bearing resolution are radar reflectors having an
echoing area of 10 m .
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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 shown below.
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 A/C
SEA control.
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