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Simrad EK15
A pulse duration of 1 ms covers
1,5 meters in the water, and
this corresponds to a target
separation of approximately
75 cm. This is a typical pulse
duration that you may well use
down to 250 to 300 meters. If
you work in deeper waters use
a longer pulse duration, if it is
shallower, use a shorter pulse
duration. Try out different
values, and seek out the pulse
duration that provides you with
the clearest echo presentation
with minimum noise, but with
maximum fish detection and
separation.
The left vessel uses a long
pulse duration (C). As you can see, this causes the echoes from the two fishes (A) and
(B) to merge. The right vessel uses a shorter pulse duration, and the two fishes will
then appear as two separate echoes on the echogram. Thus, short pulses will provide
the best resolution and separation of individual fishes, but the echo sounder is more
sensitive to noise.
The speed of sound in water is approximately 1500 m/s. The length of a 1 mS sound
pulse will thus be approximately 1,5 meter. With the echo sounder you can then adjust
the sound pulse from 7,5 cm (0.05 mS) to 24 m (16 mS) depending of the operational
frequency. This is an important factor for the appearance of single fishes.
• When the vertical distance between to fishes, or the distance between a fish and the
bottom, is more than the distance covered by a half pulse duration, the echoes will be
presented as two separate echoes. The fish above the bottom will be identified.
• If the distance between two individual fishes, or the distance between a single fish
and the bottom, is less than the distance covered by a half pulse duration, the echo
will be presented as one echo. The echo from the fish close to the bottom will be
merged with the bottom echo.
All operational frequencies have different pulse durations. The difference between for
example a 50 kHz and a 38 kHz transducer is however not large:
• A 50 kHz transducer can be used with pulse durations from 0,12 mS to 2 mS
• A 38 kHz transducer can be used with pulse durations from 0,26 mS to 4 mS.
Basically, both these frequencies will provide you with the same detection ability. A
50 kHz transducer may provide better resolution in shallow waters, while the 38 kHz
transducer may provide longer range on deeper waters. On the 38 kHz transducer the
shortest pulse duration is 0,26 mS. This results in a 40 cm sound pulse and a 20 cm
fish separation.
280
Figure 1 Pulse duration principles
1
2
A
B
C
(CD010217-005)
1
2
A
B
D
351718/C
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