Single target detection - split beam (method 2)

This operator applies a superset of the Single beam (method 2) algorithm 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; it is left to you to select the angular position data that corresponds to the selected TS data.

The following Single Target detection settings are used in the algorithm (which is a superset of the single beam (method 2) 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.

Notes:

• You should also check the effects of Calibration settings.
• TVG (and any TVG range correction) are applied after targets are detected. The TVG range correction algorithm applied is specified on the Calibration page of the operator. It is initally inherited from the Calibration page of the operand.

Algorithm

The algorithm is a superset of the single beam (method 2) algorithm. Additions are indicated in red.

The algorithm acts on power data on a ping by ping basis. It calculates power data from TS values using the following formula:

Where:

TSi = target strength of sample i
Ri = the corrected range of the sample defined as Ri = ri - at
a = absorption coefficient (dB/m)

Where:

ri= range of sample i
a = TVG range correction offset specified on the Calibration page of the operator. The TVG range correction algorithm is also specified on this page. (See note above).
t = sample thickness

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 power 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 power 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 beam compensation value must not exceed the Maximum beam compensation (see Beam compensation)

3. The pulse length (n)must be between the set limits, Minimum normalized pulse length and Maximum normalized pulse length (see below).

4. 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 samples surrounding the peak sample (p)which are above (peak power - Pulse length determination level).

The pulse length (for the peak selection criteria) is determined as the distance between the first sample (m)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. The target range is calculated as:

   

   Note: the term ct/4 is the equivalent of applying a Simrad TVG range correction of 2 samples for Ex60 series echosounders. For TS data derived from power measurements you can also apply this correction to the TS operand directly (on the Calibration page of the Variable Properties dialog box) but you should avoid applying it twice (in the operand and the choice of operator - the method 1 operators do not apply it). If your TS data already has a TVG range correction applied to it and you know (or can calculate) what it is, you can remove it with this operator (on the Calibration page of the operator).

3. The target TS value is calculated as:
   
   TS = Pp + 40log(r) + 2ar

3. If the target TS value is less than the TS minimum threshold, the target is rejected.

4. If two target pulses overlap, the one with the lowest TS value is rejected.

Where:
 

n = pulse length

m = first sample in the pulse envelope

p = peak sample in the pulse envelope

ri = uncorrected range of sample i

r = corrected range of single target (see above)

Pi = power at sample i (see above)

Pp = power at peak sample in envelope

c = sound speed (m/s)

t = transmitted pulse duration (s)
a = absorption coefficient (dB/m)

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 2) operator will always be greater than or equal to the number of targets detected by the Split beam (method 2) operator. This is because the Split beam (method 2) operator employs the same algorithm above, but the pulses must pass angle criteria as well before being accepted.

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

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.