Radar Range And Image Quality - Lighthouse a65 Installation And Operation Instructions Manual

20.4 Radar range and image quality

Maximum radar range
The usable range of the radar is limited by factors
such as the height of the scanner, and height of the
target.
Maximum radar range is essentially line-of-sight, so
is limited by the height of the scanner and the height
of the target as illustrated below:
a
1
1
R ma x = 2.23 (
Item
1
2
3
a
1
a
2
R
max
h
H
The table below shows typical maximum radar
ranges for various radar antenna heights and target
heights. Remember that although the radar horizon
is greater than the optical horizon, the radar can only
detect targets if a large enough target is above the
radar horizon.
Antenna height Target height
(meters) (meters)
3 3
3 10
5 3
5 10
Radar image quality
A number of factors can affect the quality of a radar
image, including echoes, sea clutter, and other
interference.
Not all radar echoes are produced by valid targets.
Spurious or missing echoes may be caused by:
• Side lobes.
• Indirect echoes.
• Multiple echoes.
• Blind sectors.
• Sea, rain, or snow clutter.
• Interference.
296
R ma x
a
2
2
h + H )
Description
Radar equipped vessel.
Curvature of the earth.
Target (Cliff).
Radar horizon of antenna.
Radar horizon of target.
Maximum radar range in
nautical miles.
R = a + a
max 1 2
Radar antenna height in
metres.
Target height in metres.
Maximum range
(Nautical miles)
7.7
10.9
8.8
12
Through observation, practice, and experience, you
can generally detect these conditions very quickly
and use the radar controls to minimize them.
Side Lobes
Side lobe patterns are produced by small amounts of
energy from the transmitted pulses that are radiated
outside the narrow main beam. The effects of side
lobes are most noticeable with targets at short
ranges (normally below 3 nm), and in particular with
larger objects. Side lobe echoes form either arcs on
the radar screen similar to range rings, or a series of
echoes forming a broken arc.
3
1
D1643-4
2
3
Item Description
1 Main lobe
2
Side lobes
3 Antenna
4 Arc
5
True echo
6 Side echoes
Indirect Echoes
There are several types of indirect echoes or ghost
images. These sometimes have the appearance of
true echoes, but in general they are intermittent and
poorly defined.
1
3
Item Description
1 False echo
2 True echo
3 Passing ship
4 Mast or funnel
5 True echo
6 False echo
Multiple Echoes
Multiple echoes are not very common but can occur
if there is a large target with a wide vertical surface at
a comparatively short range. The transmitted signal
4
2
6
2
4
a Series / c Series / e Series
6
5
D1638-5
5
6
D1641-5