Using Sonar; Understanding Sonar - Garmin GPSMAP 392, 398, 492, 498 Owner's Manual

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This section covers the additional sonar options for the
GPSMAP 398/498 when used with a transducer and the
GPSMAP 392/492 when used with a GSD 22, GSD 21, GSD 20
and tranducer. Read the Installation Instructions to properly install
the GSD 22, GSD 21, or GSD 20 and transducer.
NOTE: You MUST have a transducer installed and connected
to your GPSMAP 398/498 to use the sonar features.
NOTE: You MUST have a GSD 22, GSD 21, or GSD 20 and a
transducer installed and connected to your GPSMAP 392/492 to
use the sonar features. Ultrascroll is not available with the GSD 20.

Understanding Sonar

The transducer mounted on your vessel sends sound waves
down into the water in a cone shape, similar to a flashlight beam
(covering a smaller circular area at the top and angling out to a
larger circular area at the bottom). These sound waves reflect off
of any object that they hit, and then the waves travel back up to the
transducer. These objects could be fish, branches, the bottom, or
any other object that has a density that is different from the water.
The transducer receives the sound wave information and then sends
the information to the GSD 22, GSD 21, GSD 20 sounder module
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ONAR
and then to the GPSMAP 392/398/492/498. The
GPSMAP 392/398/492/498 displays the information on the screen
for you to see and interpret. The type of transducer and the settings
that you choose determine how the information is displayed on the
screen.
Using Dual Frequency
Dual frequency works best for deep water (off-shore) applications.
The 200 kHz frequency allows you access to great detail about
what is underneath your boat. The 50 kHz frequency provides
less detailed information, but penetrates to greater depths with
the same power. You can choose either or both frequencies to get
the information that you want. If you choose dual frequency, the
transducer alternates between the 50 kHz and 200 kHz signals. This
capability allows you to see the best picture available at any depth.
Using Dual Beam
Dual beam works best for shallow water (for example, inland)
applications. A dual beam transducer can transmit a narrow or a
wide beam. The water area covered by the transmitted sound waves
is determined by the beam width of the transducer and the water
depth. The narrow beam provides crisp detail of what is under your
boat, and is very helpful if you are fishing in deeper water where
the beam covers more area (for example, at a 30-foot depth, the
narrow beam covers the area of about a 7-foot circle).
GPSMAP 392/398/492/498 Owner's Manual