Raymarine RL70C, Owner'S Handbook Manual

Page: 69 / 218

Chapter 3 Standard Radar Operations 55
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 minimised 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.
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.
True echo Side echoes
Main lobe
Antenna
Arc
Side lobe Side lobe
D1638-2
Identifying False
Echo Returns