Conditions for calibration
Before conducting a calibration important issues to consider are:
The calibration should be conducted in the same environmental conditions (water temperature and salinity) as are experienced during the survey.
Water depths must be sufficient to exceed near-field limitations and/or system limitations for the echosounder frequencies to be calibrated.
Calibrations must be conducted before the survey begins to establish proper echosounder operation, and after or near the end of the survey to ensure no significant changes have occurred. Additional calibrations during the survey are valuable for maintaining system performance and ensuring high-quality data.
Calibrations must be conducted with the same pulse durations, transmit powers, and bandwidths used during the survey. A relatively fast ping rate such 5 pps is often used to increase the number of sphere observations.
If multiple frequencies will be operating simultaneously during collection, you may want to run all frequencies while calibrating each single one to include any effects from the other units. This is also useful to determine if there is acoustical interference between systems. Some manufacturers recommend calibrating each frequency separately.
The standard target method for calibrating echosounders is used to calibrate the overall acoustical system (combined transmit and receive echosounder components, transducer, transducer cable, and the electrical supply) to an absolute standard. Thus the calibrations reflect an integration of the echosounder, transducer, and shipboard electrical system. If any component of this system changes (e.g., the shipboard electrical system, transducer cable length) during the survey, the echosounder must be recalibrated (NMFS SOP).