The transducer, or through-system, gain is a measure of amplification related to the receive sensitivity of the echosounder. Transducer gain is calculated from calibration data relative to a known standard. Simrad units come with factory default values, but these are not adequate for data processing. Biosonics units come with the factory calibration values stored in the transducer. For Biosonics units, the correction constant is the difference between this factory installed gain and the gain from a calibration.
Gain constants can be modified in post-processing software.
The Sa correction (dB) in Simrad units is needed to account for differences in TS and Sv calibrations. This value is calculated from calibration data relative to a known standard. The Simrad Sa correction is equivalent to the difference between the TS and Sv gains in the old EY500 systems. Similarly, a calibration offset can be specified for Biosonics units in post processing that may be different for Sa and TS data. For Biosonics units, we recommend using default values in the field and applying any needed adjustment as calibration offsets during post processing because only one offset can be specified during data collection.
Sa correction can be modified in post-processing software.
The equivalent beam angle and beam width are related (Equation 14 and 15). The values are supplied by the manufacturer or can be calculated if beam width is adjusted following calibration. The 3 dB, or half intensity angle is the angle between the half power points on either side of the main lobe, measured in degrees.
3 dB angles and EBA can be modified in post-processing software.
For 120 and 200 kHz transducers, we recommend that maximum output power is limited to 250 W (120 kHz) and 100 W (200 kHz) to avoid harmonic distortion (Korneliussen et al. 2008). See Korneliussen et al. (2008) for recommendations for Simrad transducers operating at other frequencies. Harmonic distortion results in two errors. First, the sound level does not increase proportionally with increasing input power. Second, the transducer beam pattern shifts toward a flatter, wider main lobe and increased side lobes. The combination of these two errors results in incorrect TS and integration values (Simrad 2002). Power output can be reduced on Biosonics transducer, but we do not have sufficient experience with changing those settings as most users apply the default setting.
The recorded Biosonics data are adjusted for reduced power output. Even so, if using reduced power output, calibration should be done with the same setting. Output power cannot be modified in post-processing software.