Single Target Detection dialog box
The Single Target Detection parameters are used to control the operational settings for the detection of single targets. In order to detect single fish correctly, these parameters must be defined to suit the target characteristics. The chosen settings do not have any effect on the raw data you save during the survey.
How to open
The dialog box is opened from the Calibration program menu bar.
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Description
Several specific parameters are available for studies of single fish. In order to detect single fish correctly, these parameters should be defined to suit the target characteristics.
Note
The settings you make in the Single Target Detection dialog box have no effect on the raw data you record. The settings are only used during the real-time presentation of the data.
During calibration, the settings are used to maximize the detection of the calibration sphere, and suppress other echoes in the calibration layer.
When the Single Target Detection dialog box is opened, the availability of the functions reflects the current operational settings are read. Certain parameters will therefore be unavailable.
Details
Min. Threshold (dB)
This setting applies to both CW and FM operation.
The target strength (TS) for a single target must exceed this threshold (in dB) to be accepted. Use this function to define a "filter" value.
Minimum Echo Length / Maximum Echo Length
Some single targets are so close in range that they are overlapping. These will give you a longer echo than the length of the transmitted pulse. It is important that such multiple targets are excluded. By using the echo length values, you can define the maximum and minimum length of the echo compared to the transmitted pulse. If the echo is too long or too short, it will be excluded.
Example
If you set Minimum Echo Length to 0.8, all echoes shorter than 0.8 times the length of the transmitted pulse will be deleted.
If you set Maximum Echo Length to 1.8, all echoes longer than 1.8 times the length of the transmitted pulse will be deleted.
Maximum Phase Deviation
Several single targets occurring at the same range will give you echoes in different parts of the beam’s cross section. To remove the bad targets, the angle (phase) between the samples in the echo are measured. If the angle is too large, the echoes are deleted.
If the angle at a given range is too large, this indicates multiple targets.
Average electrical phase jitter between samples inside an echo from a single target must not exceed the maximum phase deviation setting where maximum phase deviation is set in units of phase steps (128 phase steps = 180 electrical degrees).
Recommended setting is 2 to 3 for normal conditions. For weak echoes in noisy conditions you should allow for more jitter (4 to 10). If the angle at a given range is too large, this indicates multiple targets.
Maximum Gain Compensation
This setting applies to both CW and FM operation.
Not all single targets are located in the centre of the beam. Targets located off centre will offer weaker echoes due to the beam properties. The ME70 automatically compensates for this using a mathematical model, and you can manually control the effect of this algorithm by defining a maximum gain value.
Using the 3 dB setting all echoes from within the nominal beam width of the transducer will be accepted. By reducing the value, you will only accept echoes that appear closer to the centre of the beam. Reducing the value of this parameter will effectively narrow the beam opening angles for single target detections, but will normally improve the accuracy of the target strength values for the detected single targets.
The correction value returned from the transducer gain model must not exceed the maximum gain compensation setting. (This is the one-way maximum gaincompensation. The two-way maximum compensation will be 12 dB). All single targets outside the angle corresponding to the chosen gain compensation are skipped. Thus you can reduce the sample volume (beam angle) by choosing a lower value for maximum gain compensation.
Minimum Echo Spacing
This setting only applies to CW operation.
This parameter defines the minimum distance between two single echoes when you are using CW pulses. If they are too close, the echoes are skipped.
The distance is defined as a relation to the length of the transmitted signal. Selecting 1 means that the minimum spacing corresponds to the physical distance covered by the transmit pulse. Increasing the value will require the targets to be further separated, but can improve the accuracy of the target strength values.
Tip
Overlapping targets will not be identified with this function. Use the Minimum Echo Length, Maximum Echo Length and Maximum CW Phase Deviation to handle these.