Raymarine e7D, Installation And Operation Handbook, Page: 128 / 270

12.4 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
a
2
h
H
D1643-4
Rmax
Rmax = 2.23 ( h + H )
1
3
2
Item Description
1 Radar equipped vessel.
2 Curvature of the earth.
3 Target (Cliff).
a
1
Radar horizon of antenna.
a
2
Radar horizon of target.
R
max
Maximum radar range in nautical
miles.
R
max
= a
1
+ a
2
h Radar antenna height in metres.
H Target height in metres.
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
(meters) Target height
(meters) Maximum range
(Nautical miles)
3 3 7.7
3 10 10.9
5
3 8.8
5 10 12
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.
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.
D1638-5
1
2 2
3
4
5
6
6
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.
D1641-5
1
3
2
4
5
6
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 will be reflected back and forth between the
target and your own ship, resulting in multiple echoes, displayed
beyond the range of the true target echo, but on the same bearing.
D1642-4
1
2
Item Description
1 True echo
2 Multiple echoes
128 e7 / e7D / e95 / e97 / e125 / e127 / c95 / c97 / c125 / c127