Identifying False Echo Returns - Raymarine SL72 Owner's Handbook Manual

Chapter 3: Standard Radar Operations
.t
Table 3-3:
Control
GAIN
MULTI
TARGETS
Soft Key

Identifying False Echo Returns

Not all echoes are direct returns to the radar antenna. Occasionally signals
appear at positions where there is no actual target. These are called false
echoes, and may be caused by side lobes, ghost images, indirect echoes or
multiple echoes.
This section briefly describes the echo patterns that can be produced by false
echoes and the likely cause. It should be noted that the radar operator, through
observation, practice, and experience, can generally detect these conditions
very quickly. These effects can usually be minimized using the radar controls.
Side Lobes
A small part of the RF energy from each transmitted pulse is radiated outside
the single narrow beam. This produces side lobe patterns, shown in
Figure 3-3.
Side lobes have no effect on distant or small surface objects. However, the
echo from a large object at short range may produce an arc on the radar screen
similar to a range ring or a series of echoes forming a broken arc. Side-lobe
echoes normally occur at ranges below 3nm, and can be reduced by adjusting
the SEA control.
Arc
Figure 3-3:
Controls to Adjust the Radar Picture
Option Function
GAIN Controls the strength of the target returns
SEA Reduces sea returns
RAIN Reduces close rain or snow returns
FTC Reduces distant rain or snow returns
TUNE Fine tunes the receive
INT REJ Turns interference rejection on/off
EXPANSION Turns target expansion on/off
WAKES Turns wakes on/off and selects wakes length
Side lobe
True echo Side echoes
Side Lobes
Main lobe
Side lobe
Antenna
D1638-2
3-5
Default Setting
AUTO
AUTO-HARBOUR
OFF
OFF
AUTO
ON
OFF
OFF