Raymarine GPM400 Reference Manual page 92

8.1 The fishfinder display
The fishfinder displays a scrolling image of the seabed, updating
from the right as your vessel makes progress.
The fishfinder status icon is located in the top-right section of the
monitor:
Icon animated - connection to a DSM has been
successful.
Icon static - the DSM is connected but not transmitting.
Icon greyed-out - no DSM is connected.
(animated icon)
8.2 Background information
The fishfinder requires a Digital Sounder Module (DSM) to process
acoustic signals and build up a detailed picture of the seabed.
G-Series Reference Manual
A transducer on the bottom of the boat sends pulses of sound
waves into the water and measures the time it takes for the sound
wave to travel to the bottom and back. The returning echoes are
affected by bottom structure and by any other objects in their path,
for example reefs, wrecks, shoals or fish.
The strength of the echoes is indicated on the display by different
colors. You can use this information to determine the shape and
make-up of the seabed, or the size of shoals, or of other objects.
8.3 Operating the fishfinder
The fishfinder provides you with four preset configurations, avail-
able on the default toolbar. These are tailored to provide optimal
operation in different circumstances - you should be able to find a
preset that gives you excellent results in any situation.
The presets are labelled single, dual, shallow and deep. When you
make a fishfinder window active, the title bar shows you which pre-
set is selected, along with its settings.
Each preset has been configured to provide the best operating
parameters for the fishfinder. However, it is possible to adjust the
presets if necessary.
To select a preset mode
1. Press the appropriate softkey.
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