Identifying False Echo Returns - Raymarine SL70RC PLUS Series User Manual

Chapter 3: Standard Radar Operations 3-5

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.
Main lobe
Side lobe Side lobe
Antenna
Arc True echo Side echoes
D1638-2
Figure 3-3: Side Lobes