Furuno FCR-21x9-BB Operator's Manual page 115

2. RADAR, CHART RADAR OPERATION
Bearing resolution
Bearing resolution is the ability of the radar to display as separate pips the echoes re-
ceived from two targets which are at the same range and close together. It is propor-
tional 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 pulse length 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
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range and bearing resolution are radar reflectors having an echoing area of 10 m .
Bearing accuracy
One of the most important features of the radar is how accurately the bearing of a tar-
get 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 se-
lecting 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 vari-
able range marker (VRM). The fixed range rings appear on the screen with a prede-
termined 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 in-
ner edge of the target, allowing the operator to obtain more accurate range measure-
ments.
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