Simrad ES70 Operator's Manual page 116

Simrad ES70
The pulse duration can be adjusted according to the current depth and what kind of fish
you are looking for. The deeper you wish to see, the longer pulse duration should be
used. Remember that in ES70 echo sounder, the pulse duration and the bandwidth is
mutually dependant.
• Long pulse - lots of acoustic energy - narrow bandwidth - less sensitive for noise
from own vessel and environment
• Short pulse - less acoustic energy - wide bandwidth - more sensitive for noise from
own vessel and environment
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.
114
Figure 39 Pulse duration principles
1
2
A
B
C
(CD010217C)
1
2
A
B
D
343539/B