Single target detection - split beam (method 1) algorithm

This operator applies a superset of the Single beam (method 1) algorithm (Soule et al., 1995, 1996, 1997, Ona et al., 1999), adding available angle data and beam compensation estimates to the peak selection criteria.

See Single target pulse properties for illustrations on how target TS and normalized pulse lengths are determined. The page also offers an illustration of a single target pulse with respect to single beam detection settings.

Refer to Single Target detection algorithms for information about the method that best suits your data.

Variable properties

The first operand must be a TS variable.

The second operand must be angular position, complex angular position or pulse compressed complex angular position variable that corresponds to the TS data.

The following Single Target detection settings are used in the algorithm (which is a superset of the single beam (method 1) algorithm, additions being indicated in red.):

Parameter

Unit

Allowed range

Default value

Beam compensation model

-

Simrad LOBE
BioSonics
HTI*
Furuno

Simrad LOBE

TS threshold (see note)

dB re 1m2

-120 - 20

-50

Pulse length determination level (PLDL)

dB re 1m2

0.01 - 30

6

Minimum normalized pulse length

-

0.01 - 10

0.7

Maximum normalized pulse length

-

0.01 - 10

1.5

Maximum beam compensation

dB

0 - 35

4.0

Maximum standard deviation of minor-axis angles

°

0 - 45

0.6

Maximum standard deviation of major-axis angles

°

0 - 45

0.6

See Beam compensation for details on the Beam compensation models.

The maximum standard deviation settings for minor-axis and major-axis angles are expressed in mechanical degrees.

*HTI beam compensation offers additional settings, refer to HTI split-beam beam compensation for more information.

The single target detection parameters are set on the Single Target Detection page of the Variable Properties dialog box. Lines can also be selected for excluding targets above or below a line. Apart from limiting the target detection range, exclusions will also speed up processing, since less data will then be screened for single targets.

Notes:

Algorithm

The algorithm is a superset of the single beam (method 1) algorithm. Additions are indicated in red.

The algorithm acts on (uncompensated) TS data on a ping by ping basis.

The algorithm begins by removing data for which no targets need to be determined (i.e. data above and below the exclusions lines) and then processes the data in two main phases:

Phase I: determine all TS peaks that may indicate single targets

In the first stage the algorithm detects all peak values that could indicate the presence of a single target. In order for a TS value to be retained as a peak value it must satisfy following peak selection criteria. The criteria are applied in this order, and only samples that pass one criterion are considered in the next.

Peak selection criteria

  1. The TS value must be a local maximum. If the local maximum consists of more than one sample with the same TS value, then the first sample in this sequence is used.
  2. The TS value must exceed the chosen TS threshold.
  3. The beam compensation value must not exceed the Maximum beam compensation (see Beam compensation).
  4. The pulse length must be between the set limits, Minimum normalized pulse length and Maximum normalized pulse length (see below).
  5. The standard deviation of the angles (minor and major axis) of all samples within the pulse envelope must not exceed the Maximum standard deviation of angles allowed (see below)

Pulse envelope determination

The pulse length is determined as the distance between the first and last samples within the pulse envelope. The envelope consists of all those samples surrounding the peak which are above both (peak TS - PLDL) and a threshold determined as follows:

If the chosen TS threshold is...

Then Echoview uses...

less than (peak TS - PLDL)

the chosen TS threshold

greater than (peak TS - PLDL)

the lowest of the following three thresholds which will pass the peak selection criteria above:

  • TS threshold - PLDL - 2 x Maximum beam compensation

  • TS threshold - PLDL/2 - Maximum beam compensation

  • TS threshold

The pulse length (for the peak selection criteria) is determined as the distance between the first and last samples within the pulse envelope.

The standard deviation of the angles within the envelope is calculated for the minor-axis and major-axis angles independently as follows:

Let αi be the n angles under consideration. The standard deviation is:

where

is the mean of αi.

Phase II: reject overlapping pulses

Based on the set of peaks obtained in phase I, single targets are determined as follows:

  1. Pulses are screened in order, from low to high depth ranges.
  2. If a pulse overlaps with an earlier pulse, the pulse with the lower (peak) Target Strength is rejected. The pulse range used to detect overlap is as defined at the Pulse Detection Determination Level:

Target_range - Pulse_Start_Normalized_PLDL * Transmitted_Pulse_Length

to
 
Target_range - (Pulse_Start_Normalized_PLDL + PulseLength_Normalized_PLDL) * Transmitted_Pulse_Length

  1. Other peaks are considered to indicate single targets.
  2. Each detected single target's TS value is identical to the uncompensated TS.
  3. Compensated TS is calculated using the selected Beam compensation model.
  4. TS_comp = TS_uncomp + Corr

Where:

TS_uncomp
= Uncompensated TS of the target
TS_comp
= Compenstated TS of the target
Corr
=

TS_uncomp corrected by the selected Beam compensation model.

Simrad split beam beam compensation model TS_comp = TS_uncomp + CorrSimradSplitBeam
BioSonics split beam beam compensation model TS_comp = TS_uncomp + CorrBioSonicsSplitBeam
HTI split beam beam compensation model TS_comp = TS_uncomp + CorrHTISplitBeam

Single target properties

The table below describes how algorithm specific single target properties are calculated. See About analysis variables for a complete list of single target properties

Analysis variable

Unit

Description

TS_uncomp

dB re 1m2

TS of the peak sample. The TS value originates from the input operand (and is not calculated from power under the operator). Beam compensation has not been applied to this analysis variable.

TS_comp dB re 1m2 TS of the peak sample with selected Beam compensation applied.

Target_range

m

Range of the first sample in the pulse envelope

Angle_minor_axis

degrees

Minor-axis angle of the peak sample

Angle_major_axis

degrees

Major-axis angle of the peak sample

Notes:

See also

About Single target detection
Single target detection algorithms

Echoview and Simrad algorithms

References

Simrad and Echoview single target detection terminology
Single target pulse properties