NorthStar Navigation Multi-Function Unit M84 - Manual (Page 46)

Northstar M121/M84 Installation and Operation Manual 45
10-5 Sonar A-Scope window
The window shows the sonar history on the
left and the A-Scope window 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 9-6).
Setting gain and threshold
It is convenient to use the A-scope window
when adjusting gain and threshold manually.
Follow this procedure to adjust gain and
threshold for normal circumstances:
1 Switch to a sonar window (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
MENU
, select
Frequency and select 200 kHz or 50
kHz.
3 Press
MENU
, select Sonar splits then select
Split A-scope.
A
B
C
D
E
4 Press
ENTER
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
ESC
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 recognizing 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.