Furuno FR-8111 - Manual (Page 45)

2 – 1
Actual topography and radar picture
The radar cannot display topography in its actual
shape.
Figure 2-1 Radar picture
and actual topography
2.2 Range Resolution
Range resolution is a measure of the capability of
a radar to display as separate pips the echoes re-
ceived 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 ech-
oes on the display unless they are separated by a
distance greater than one-half the pulselength.
Figure 2-2 Range resolution
Chapter 2
RADAR OBSERVATION
2.1 Radar Picture, Target
Properties and Radar Pulse
Strength of reflection
The strength of the reflected echo depends not only
on the height and size of the target but also its
shape, material and the angle at which the radio
pulse strikes it.
Target size and strength of reflected
echo
The size of a target has little to do with the strength
of the reflected echo. If the radio pulse strikes the
target at a right angle, even a small target will re-
turn a strong echo, provided its material is a good
radar target.
Radar picture and how the pulse
strikes a target
A return echo will be weak if the angle at which a
radio pulse strikes a target is small. For example,
flat surfaces, such as sandy beaches, sandbars and
mudbanks have almost no area that can reflect en-
ergy back to the radar. Conical surfaces, such as a
lighthouse, generate a weak return echo because
their shape diffuses most of the radiated energy.
Radar sees only the near side of targets. For ex-
ample, it cannot show you what is behind a sea
wall or an island. The echo of a mountain peak
may appear on the radar as a peninsula or small
island.
Target material and reflected echo
Generally, steel objects return a very strong echo,
and reefs and water return a weak echo. The weak-
est echoes come from wood and fiberglass ob-
jects. In summary, non-metallic objects and flat
or conical objects are not good radar targets.
Black areas are
strong reflection
targets.
Coastline
difficult to
display.
Target
Transmitted
radar
pulse
Transmitted
radar
pulse
Target
Radar is able to display two
distinct target echoes.
Radar cannot display targets as
separate echoes because they
are within the pulselength.