Simrad split-beam beam compensation

Simrad LOBE used by Simrad Ex60 data and Simrad EK80 narrowband data
Simrad LOBE used by Simrad EK80 wideband data

Simrad LOBE used by Simrad Ex60 data and Simrad EK80 narrowband data

Simrad split-beam two-way beam compensation in Echoview is based upon the LOBE algorithm employed by Simrad.

In split-beam systems the beam compensation equation can be written as function of beam position as follows:

TS(α,β) = TS(0,0) + B(α,β)

where:

α	is the minor-axis angle of the single target+ (degrees)
β	is the major-axis angle of the single target+ (degrees)
TS(0,0)	is the TS derived from received power measurements - valid at α=0, β=0 (dB re 1 m2) - see uncompensated_TS
TS(α,β)	is the TS predicted for a target at position α, β in the beam (dB re 1 m2) - see compensated_TS
B(α,β)	is the beam compensation function (dB). This is identical to CorrSimradSplitBeam

+Single target angles passed on by a Single target detection split beam method virtual variable.

If the transducer face is a circular disk, the theoretical beam shape is the first order Bessel function J1.

The beam shape function can be approximated by a polynomial however. In Echoview the same polynomial approximation is implemented as in Simrad's LOBE program. It is defined as follows:

B(α, β) = 6.0206 ( x2 + y2 - 0.18x2y2)

where:

x = 2α/BWa (-1 x 1)

y = 2β/BWb (-1 y 1)

-BWa/2 α BWa/2 and -BWb/2 β BWb/2

and

BWa is the minor axis beam width (degrees)

BWb is the major axis beam width (degrees)

This function is a polynomial approximation to the ideal Bessel function which is valid over the range 0 to 12dB only. If you wish to view or count targets with Beam Compensation values beyond that range, the accuracy of the calculated Beam Compensation cannot be guaranteed.

The table below contains an excerpt of Simrad EK60 Single Target Detection E1 Telegram data, with compensated and uncompensated TS values, angles, and Echoview-calculated Compensation values for comparison.

Simrad EK60 Compensated TS	Uncompensated TS	Minor Angle	Major Angle	Echoview-Calculated Compensated TS
-33.854	-45.754	-4.816	1.605	-33.851
-33.243	-42.767	-4.431	0.642	-33.247
-32.902	-44.814	-2.954	4.302	-32.905
-32.937	-37.394	-3.018	-0.514	-32.933
-32.525	-42.85	-2.055	4.302	-32.523
-32.478	-34.348	-1.926	0.449	-32.484
-32.784	-34.442	0	-1.862	-32.786

Simrad LOBE used by Simrad EK80 wideband data

Simrad LOBE beam compensation for Simrad EK80 pulse compressed data in Echoview is based upon the LOBE algorithm employed by Simrad. Echoview reads tabulated frequency dependent calibration values from the data file and evaluates values for transducer gain, beam width, angle offset and Simrad LOBE beam compensation as required for the Single target detection - wideband operator and the Wideband Frequency Response graph. See also Simrad EK80: Frequency dependent limitations.

The LOBE function is a polynomial approximation to the ideal Bessel function which is valid over the values of 0 to 12dB only. If you wish to view or count targets with Beam Compensation values beyond that range, the accuracy of the calculated Beam Compensation cannot be guaranteed.