Single target detection - single beam (method 1)

This operator applies the algorithm implemented by Simrad in the EK500 echosounder to identify single targets from echogram data (Soule et al., 1995, 1996, 1997, Ona et al., 1999).

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

Variable properties

The operand must be a TS variable as input.

The following Single Target detection settings are used in the algorithm:

Parameter	Unit	Allowed range	Default value
TS threshold	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

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 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 outside 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

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.
The TS value must exceed the chosen TS threshold
The pulse length must be between the set limits, Minimum normalized pulse length and Maximum normalized pulse length (see below)

Pulse envelope determination

The pulse 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 that will pass the peak selection criteria above:

TS threshold - PLDL
TS threshold - PLDL/2
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.

Phase II: reject overlapping pulses

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

Pulses are screened in order, from low to high depth ranges.
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

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.

Analysis variable	Unit	Description
TS_uncomp	dB re 1m2	TS of the peak value
Target-range	m	Range of the first sample in the pulse envelope
Angle-minor axis	degrees	Not available for targets detected using this algorithm
Angle-major axis	degrees	Not available for targets detected using this algorithm

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 single 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.