Basic Radar Theory; Side-Lobe Echoes; Indirect Echoes - Icom MR-570R Operating Manual

Radar uses a form of electromagnetic radiation, which
like light, can be reflected. Because of this property,
some objects may cause false echoes on the screen
where in fact no targets actually exist.

Side-lobe echoes

Radiation can escape on each side of the beam inside
the lobes. If a target reflects this radiation, it will be dis-
played on the screen as an echo.
Main beam
Side lobes

Indirect echoes

Indirect echoes may be returned from either a passing
ship, or returned from a reflecting surface, such as a
mast on your own ship.
Scanner
Mast or
similar obstruction
Target
Bridge
An echo is reflected at this point.
Target
Own ship

BASIC RADAR THEORY

These echoes may appear if a large vessel, bridge, or
tank is in proximity. Operators should be familiar with
the effects of these phenomena. In some cases,
echoes can be reduced.
Side-lobe echoes usually occur at short ranges and as
a result of large (strongly reflective) targets. They can
be reduced with proper adjustment of the [SEA] con-
trol. See p.10 for the [SEA] control.
False
echoes
An indirect echo from a reflecting surface will appear
on a different bearing from the direct (true) echo, but
the distance will be approximately the same for both.
True
echo
Indirect
echo
8
True
echo
True echo
False echo
23