Simrad EY500, Instruction Manual

Page: 135 / 262

Theory of operation
P3403E/A
15
Side view
Top view
(CD470)
A
A
B
B
Figure 4 Split-beam principle.
Thus, determining fish size from received echo strength alone will not be
successful. However, knowing the beam pattern of the transducer and the
position of the fish within the beam it is possible to correct for the differences
in transducer gain and thereby obtaining the true target strength of the fish.
The beam pattern correction in the EY 500 is based upon a model which is
described in section "Installation, System Test and Calibration".
The split-beam measurement principle only works for echoes originating from
a single point target since the electrical phase will be random if echoes from
multiple individuals at different positions in the beam are received simulta-
neously.
It is essential therefore to distinguish single fish echoes from multiple fish
echoes, henceforth the single echo detection algorithm. The algorithm is based
on a number of criteria, and some of these can be set from the menu system:
* TS must exceed the min value parameter (default -50 dB).
* The normalized echo length must exceed the min echo length parameter
(default 0.8) where normalized echo length is the length between the 6 dB
points relative the peak value divided by the duration of the transmitted
pulse.