Single target detection - split beam (method 1)

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.

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 an 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.):

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.

Note: You should also check the effects of Calibration settings.

Algorithm

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

The algorithm acts on 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:

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

where

is the mean of ai.

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:

3. Target_range - Pulse_Start_Normalized_PLDL * Transmitted_Pulse_Length
   
   to
    

4. Target_range - (Pulse_Start_Normalized_PLDL + PulseLength_Normalized_PLDL) * Transmitted_Pulse_Length

3. Other peaks are considered to indicate single targets.

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

Notes:

• See Single target pulse properties for illustrations on how target TS and normalized pulse lengths are determined.

• There is no limit on the number of possible target detections in a single ping than theoretically possible with the given number of samples.

• The number of targets detected by the Single beam (method 1) operator will always be greater than or equal to the number of targets detected by the Split beam (method 1) operator. This is because the Split beam (method 1) operator employs the same algorithm above, but the pulses must pass angle criteria as well before being accepted.

• The single target detection algorithm implemented in the EK500 has a constant Pulse length determination level of 6 dB, which is the default value in Echoview. That corresponds to half of the peak pressure. It might be argued that rather the half power points should be taken. In that case, the Pulse length determination level should be set to 3 dB.

• If you are comparing results with Simrad E telegrams please see Echoview and Simrad algorithms.

• Use of Sv input variables with this operator was deprecated in Echoview 4.30.

See also

About Single target detection
Single target detection algorithms
Echoview and Simrad algorithms
References
Simrad and Echoview single target detection terminology

Echoview Help file 5.1.25 for Echoview 5.1.45 Wednesday 1st  February 2012

Please report any errors or omissions to support@echoview.com.