Raymarine a67, Installation And Operation Instructions Manual, Page: 187 / 330

17.6 Transducer bandwidth
The fishfinder application displays the fishfinder frequency,
center frequency or CHIRP mode depending on the connected
sonar module and transducer.
Example of CHIRP sonar module in CHIRP mode.
• When using a CHIRP sonar module set to CHIRP mode and
a wide band transducer the CHIRP mode is shown in the
fishfinder application title bar and the center frequency is
displayed on-screen.
• When using a CHIRP sonar module set to a non-CHIRP
mode and a wide band transducer the center frequency for
the transducer is shown in the fishfinder application title bar
and is displayed on-screen.
• When using a CHIRP sonar module connected to a
non-CHIRP (traditional) transducer the CHIRP sonar module
will behave the same a non-CHIRP (traditional) sonar module.
• When using a non-Chirp sonar module the transducer’s
operating frequency is displayed in the fishfinder application
title bar and is displayed on-screen.
Note: When using a CHIRP sonar module in non-CHIRP
mode only the transducer’s center frequency is displayed, this
maybe different to the actual frequency transmitted.
17.7 The sonar image
Interpreting the seabed using sonar
It is important to understand how to correctly interpret the
seabed structure represented in the fishfinder display.
The seabed usually produces a strong echo.
The following images show how different seabed conditions are
represented in the sonar display:
D
6
8
5
5
-3
1
2
3
Item Description
1
A hard bottom (sand) produces a
thin line.
2 A soft bottom (mud or seaweed
cover) produces a wide line.
3 A rocky or uneven bottom or a wreck
produces an irregular image with
peaks and troughs.
The dark layers indicate a good echo; the lighter areas indicate
weaker echoes. This could mean that the upper layer is soft
and therefore allowing sound waves to pass to the more solid
layer below.
It is also possible that the sound waves are making two complete
trips – hitting the seabed, bouncing off the vessel, then reflecting
off the seabed again. This can happen if the water is shallow,
the seabed is hard, or the gain is set to high.
Factors influencing the sonar display
The quality and accuracy of the display can be influenced by a
number of factors including vessel speed, depth, object size,
background noise and transducer frequency.
Vessel speed
The shape of the target changes along with your speed. Slower
speeds return flatter, more horizontal marks. Higher speeds
cause the target to thicken and arch slightly, until at fast speeds
the mark resembles a double vertical line.
Target depth
The closer the target to the surface, the larger the mark on
screen.
The depth of individual targets can be displayed by switching
on the
Target Depth ID in the fishfinder menu Menu >
Presentation . The number of target depths displayed is
influenced by the fish alarm sensitivity level.
Fishfinder application
187