NorthStar Navigation 438, 435 operation manual Fish arches, Gain, Changing between Auto and Manual

Fish arches

In good conditions and with Fish symbols Off, a fish passing through the cone-shaped ultrasonic pulse is displayed as a fish arch.

A fish arch occurs when a fish enters the weak edge of the sonar cone, generating a weak echo that is displayed as the first pixel of the fish arch. As the fish moves closer to the middle of the cone, the distance between the transducer and the fish reduces and the echo is displayed at progressively shallower depths, producing the start of an arch. When the fish passes directly beneath the middle of the cone, the echo becomes strongest and thickest. As the fish passes out of the middle of the cone the reverse happens with a progressively weaker and deeper echo.

There are many reasons why fish arches may not be seen. For example:

•Poor transducer installation (see Transom

Transducers Installation Guide).

•If the boat is anchored then fish will tend to show on the display as horizontal lines as they swim into and out of the transducer sonar beam. Slow speeds in deeper water give the best fish arch returns.

•Range is important. It will be much easier to see fish arches when using zoom mode to concentrate on a particular section of water, rather than just displaying everything from the surface to the bottom. Zooming increases screen resolution up to 100 times.

•It is difficult to get fish arches in shallow water as the transducer sonar beam is very narrow near the surface and fish do not stay within the beam long enough to display an arch. Several fish in shallow water tend to display as randomly stacked blocks of pixels.

•Wave motion may result in distorted fish arches.

4-3 Gain

Gain (sensitivity) controls the amount of detail displayed on the Explorer 435 and Explorer 438. Understanding how to set suitable Gain settings is important for optimum performance.

The Northstar fishfinder has two gain modes, Auto Gain and Manual Gain. Normally the best results are obtained in Manual Gain, but practice and experience are required to obtain the optimum settings for different conditions. Therefore, the use of Auto Gain is strongly recommended when learning to use the fishfinder or when travelling at speed.

•In Auto Gain, the gain adjusts automatically to compensate for water depth and clarity.

•In Manual Gain, the gain can be adjusted by the user to compensate for water depth and clarity.

High Gain settings may amplify the normal background noise until it appears as random pixels.

Changing between Auto and Manual

To change between Auto Gain and Manual Gain:

1.From any Sonar display, press .

2.Use the cursor key to select Auto or

Manual.

Adjusting Gain settings

1.From any Sonar display, press .

2.Use theorcursor keys to change the gain.

Note: The Gain mode automatically changes to Manual Gain if the gain setting is adjusted by the user.

Obtaining the best results

To obtain the best detection capability for both fish and bottom

we recommend the user adjusts the gain in the A-Scope display until the threshold line is just to the right of the unwanted noise:

Gain Threshold line

Unwanted signal

Gain line

20 Northstar Explorer 435/438 Installation and Operation Manual