# BioSonics dual-beam beam compensation

Dual-beam beam compensation in Echoview is based upon the algorithm employed by BioSonics.

A dual beam echosounder transmits only on the narrow beam channel but receives on both narrow beam channel and wide beam channels. Therefore the total beam compensation for the wide beam channel is the sum (in the log domain) of the one-way narrow channel beam compensation (for transmission) and the one-way wide channel beam compensation (for reception). A description and basic equations for a dual-beam echosounder are discussed by Simmonds and MacLennan, p80.

In dual-beam systems the beam compensation equation is written as a function of two measured target strengths:

Bn - Bw = TSn - TSw

where

 Bn is the beam compensation for the narrow-beam signal (dB) Bw is the beam compensation for the wide-beam signal (dB) TSn is the TS derived from received power measurements on the narrow-beam transducer (dB re 1 m2) TSw is the TS derived from received power measurements on the wide-beam transducer (dB re 1 m2)   Echoview uses the value on the wide-beam echogram, at the same range as the peak TS for this target on the narrow-beam echogram. This is not necessarily the peak TS on the wide-beam echogram.

Note: For information about how Echoview selects the samples from which TSn and TSw are derived, see Single target detection (dual beam method 1) or Single target detection (dual beam method 2).

If the wide-beam drop-off is defined as:

where

 θn is the 3dB beam angle for the narrow-beam transducer (° ). This is the average of the Major axis 3dB angle and Minor axis 3dB angle on the Calibration page of the Variable Properties dialog box for the narrow beam variable. θw is the 3dB beam angle for the wide-beam transducer. This is the average of the Major axis 3dB angle and Minor axis 3dB angle on the Calibration page of the Variable Properties dialog box for the wide beam variable.

Note: Dw will be zero if θn equals θw.

and the expected wide-beam beam compensation in relation to the expected narrow-beam beam compensation is:

then the narrow-beam two-way beam compensation is calculated as:

If measurement error causes TSw to be greater than TSn, it is possible for this calculation to produce (unrealistic) negative estimates of Bn. The setting Over axis angle threshold on the Single Target Detection page of the Variable Properties dialog box is used to as follows:

if Bn < 0 and Bn > Over axis angle threshold then Bn = 0

if Bn < Over axis angle threshold then target is discarded (see Dual beam (method 1) or Dual beam (method 2) for more information)

The target strengths reported by Echoview for a dual-beam system are then defined as:

TS_uncomp = TSn

TS_comp    = TSn - 2Bn