Furuno 851 MARK-2 - Manual (Page 32)

3-2
3.2 Target Properties and
Radar Wave Reflection
Generally, larger targets can be seen on
the radar display at greater ranges,
provided line-of-sight exists between the
scanner and target. However, a large target
with poor reflecting properties may not be
detected as easily as a smaller target with
better reflecting properties. For example,
you might expect a lighthouse to be a good
radar target because of its size. In actuality
the return echo is weak since the conical
shape diffuses most of the radiated energy.
A ship whose hull is made of conducting
materials, such as steel, will return a
relatively strong echo.
On the other hand, hulls made from wood
or fiberglass return much weaker echoes.
Vertical surfaces, such as a cliff, are good
targets provided they face the radar.
Conversely, horizontal and smooth
surfaces such as mudbanks, sandy
beaches, and gently sloping hills make
poor targets because they disperse rather
than reflect most of the energy that strikes
them.
The strongest radar echoes known come
from built-up areas, docks, etc., because
these targets are less subject to changes in
aspect. These types of targets have three
flat, smooth surfaces mutually at right
angles. Some radar buoys are arranged
this way so as to deliberately increase their
detection range.
3.3 Range Resolution
Range resolution is a measure of the
capability of a radar to display as separate
pips the echoes received from two targets
which are on the same bearing and are
close together.
The main factor which affects range
resolution is pulselength. Two targets on
the same bearing, close together, cannot
be seen as two distinct echoes on the
display unless they are separated by a
distance greater than one-half the
pulselength.
Transmitted
radar
pulse
Radar is able to display two
distinct target echoes.
Target
Transmitted
radar
pulse
Radar cannot display targets as
separate echoes because they
are within the pulselength.
Target
Figure 3-2 Range resolution
3.4 Bearing Resolution
Bearing resolution is a measure of the
capability of a radar to display as separate
targets the echoes received from two
targets which are at the same range and
are close together.
The principal factor which affects bearing
resolution is horizontal beamwidth. Two
targets at the same range must be
separated by more than one beamwidth to
appear as separate pips.