Furuno FAR-21 7-BB Series, Operator'S Manual

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2. RADAR OBSERVATION
2-2
X-band and S-band
In fair weather, the equation on the previous page does not give a significant
difference between X- and S-band radars. However, in heavy precipitation
condition, an S-band radar would have better detection than an X-band radar.
Radar resolution
There are two important factors in radar resolution (discrimination): bearing
resolution and range resolution.
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 antenna length and reciprocally proportional to
the wavelength. The length of the antenna 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 about 12 feet (3.6 m) or longer.
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
2
.
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.