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43Northstar Explorer 657 Installation and Operation Manual
9-5 Sonar A-Scope display
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:
A Unwanted noise echoes.
B Echoes from fish and the bottom
C The strongest echo, usually from the
bottom
D A vertical line showing the threshold, the
weakest echo to display on the sonar history
E A 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).
A
C
E
D
A
B
B
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:
1 Switch to a sonar display (see section 9). If
necessary, press
or to adjust range
so that the bottom is displayed.
2 If necessary, select the sonar frequency to
adjust the settings for. Press
, select
Frequency and select 200 kHz or 50
kHz.
3 Press
, select Sonar splits then select
Split A-scope.
4 Press
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.
5 Set threshold to zero.
6 Adjust Gain so the peak of the strong signal
from the bottom just touches the gain line.
7 Adjust threshold so that it is just to the right
of the noise.
8 Press
to close the gain window.
9 If 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.