Furuno FR-2105 Operator's Manual page 110

X-band and S-band
In fair weather, the above equation 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
2
echoing area of 10 m .
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 marker 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.
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