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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.

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 

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.

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. 

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).