A.2.3 false echoes – Seiwa SWRx series User Manual

Page 49

User Manual

A.2.3 False echoes

Sometimes Radar will display targets on screen that do not exist in the real world.

You should be aware of howand why this happens.

A.2.3.1

Ghost echoes

Sometimes one large object very near your boat will appear as two different targets

on screen. One is the actual Radar echo. The other is a ghost echo generated by a re-

reflection of the original signal. It comes back to your own boat, bounces back to the

target, and then is picked up by the antenna on the second bounce. The actual echo

appears at the correct distance and bearing on the screen. The ghost echo appears

somewhere behind your boat. This type of false echo is also generated by re-reflec-

tion of waves from bridges, break walls or building along shore.

Target

Direct
reflection path

Secondary
reflection path

Mast etc.

Real echo

Ghost echo

Direction of ghost echo

Fig. A.2.3.1 - Ghost Echoes

A.2.3.2

Multiple echoes

If there is a large vertical reflecting surface near your boat, as in the case when

you pass alongside a large ship, Radar signals are repeatedly bounced back and

forth between your boat and the other object. Two to four images appear on the

screen at equal intervals in the same bearing. This is called a multiple echo. The

image appearing closest to you is the real echo. Multiple echoes will disappear as

you move away from the reflecting object or its bearing changes.

Real echo

Multiple
echoes

{

Fig. A.2.3.2 - Multiple Echoes

A.2.3.3

False echoes caused by side lobe

An antenna’s side lobe emissions are low power, and will not register distant

targets. However, if there is a strong reflecting target near your boat, it some-

times may appear as a circular-arc false echo on the screen.

WARNING

When near large targets or land, your boat's mast may sometimes appear as

circular-arc shaped false echo.