NorthStar Navigation EXPLORER 657 Sonar A-Scope display, Setting gain and threshold, Bottom type

9-5 Sonar A-Scope display

A

BE

D

The display shows the sonar history on the left and the A-Scope display on the right. The A-Scope shows:

A, B, C The strengths of echoes being received now from different depths - the longer the horizontal line the stronger the signal:

AUnwanted noise echoes.

BEchoes from fish and the bottom

CThe strongest echo, usually from the bottom

D A vertical line showing the threshold, the

weakest echo to display on the sonar history

B

A

C

EA vertical line showing the gain setting; echoes above this strength will display as the maximum signal strength

Use the A-Scope while adjusting the gain and threshold settings manually (see section 8-6).

Setting gain and threshold

It is convenient to use the A-scope display when adjusting gain and threshold manually. Follow this procedure to adjust gain and threshold for normal circumstances:

1Switch to a sonar display (see section 9). If

necessary, press or to adjust range so that the bottom is displayed.

2If necessary, select the sonar frequency to

adjust the settings for. Press , select

Frequency and select 200 kHz or 50 kHz.

3Press , select Sonar splits then select Split A-scope.

4Press to display the Gain window. To

adjust gain or threshold for a frequency, press or to select the setting to adjust, then press or to change the setting.

5Set threshold to zero.

6Adjust Gain so the peak of the strong signal from the bottom just touches the gain line.

7Adjust threshold so that it is just to the right of the noise.

8Press to close the gain window.

9If required, repeat these steps to adjust gain and threshold for the other frequency.

Note: Setting the gain higher will display more detail from weak echoes, like fish, but will loose detail from the strong echo from the bottom.

Fish recognition

The echo strengths shown on the A-scope can be useful in recognising the type of fish. Different species of fish have different sizes and shapes of swim bladders. The air in the swim bladder reflects the ultrasonic pulse, so the strength of the echo varies between fish species according to the size and shape of the swim bladder.

When catching fish from a school, note the species and the strength of the echo that it returns on the A-scope. Then, when that particular echo is seen again, it is likely to be the same fish species.

Bottom type

The shape of the echo strengths in the A-Scope can help you to recognise the type of bottom.

Northstar Explorer 657 Installation and Operation Manual 43