Echosounders may be calibrated by suspending a solid metal sphere in the acoustic beam and adjusting the system gain so that the measured target strength (TS) equals the theoretical value for the sphere. The suspension apparatus (i.e. net bag or loop of line glued into a hole, and control lines) may appreciably scatter sound, potentially affecting the sphere-echo intensity and phase, and hence the calibration uncertainty. Here, we characterize the effects of conventional suspension materials and configurations on measurements of sphere TS and recommend approaches to mitigate the observed effects on echosounder calibrations. We show that the effects of suspension lines on both calibration accuracy and precision change with line type and increase with line diameter and acoustic frequencies above similar to 100kHz. Compared to commonly used polyamide (nylon) line, ultra-high-molecular-weight polyethylene (PE) line has an acoustic impedance closer to that of water and therefore has a lessened effect on measurements of sphere TS. The effects may be mitigated in a practical manner using a 0.38-mm diameter, multifilament, PE line for the suspension.

A prototype broadband echo sounder has been used for measuring zooplankton and fish in a new collaborative project between the Institute of Marine Research and Kongsberg Maritime-Simrad. The prototype system used is a Simrad broadband system which includes a transceiver operating from 10–500 kHz. We have mainly used the transceiver together with four separate Simrad ESXX-7CD standard pressure resistant transducers, the ES70-7CD, ES120-7CD, ES200-7CD, and ES333-7CD, covering the band from 50 to 450 kHz. The research has so far concentrated on data output formats, calibration methods, and measurements from single targets in ex situ and in situ situations. Examples of recorded reflected spectra for selected calibration and biological targets will be shown and the potential for improved aquatic ecosystem assessment discussed.

The new Simrad scientific multibeam systems, the MS70 sonar and the ME70 echosounder, each transmit over many electronically formed beams with centre frequencies spanning from 70 to 120 kHz. Calibrations of these systems are therefore more complex than for conventional split-beam echosounder systems. Two large tungsten-carbide spheres (75 and 84 mm diameter) were designed and manufactured to facilitate accurate field calibrations over the entire operational bandwidth. These are heavy and therefore stable when suspended beneath a ship, and have target strengths much larger than those of biological targets potentially within the measurement volume. This paper presents procedures for calibrating each system in the field and the results from two such experiments. Detailed inspections of the results for individual beams indicate that minor adjustments in the described procedures might further improve the reported calibration accuracy.

The new multi-beam sonar, Simrad MS70, can transmit over the frequency band 70-120 kHz, and calibration of each individual beam becomes slightly more difficult than for conventional split beam systems. Two new tungsten carbide spheres have been specially designed and manufactured to enable an accurate calibration over the specified frequency band. They are also heavy enough to stabilize the calibration rig, and have sufficiently high target strength for discrimination against biological targets within the measurement volume during sea calibrations. Calibration results from the first trials of the new spheres will be shown. This includes tests made with the sonar itself to verify that the backscattering of the spheres was correct and according to the numerical computations. A procedure for standard calibration of the MS70 multibeam system is suggested and tested. Additionally, results from detailed inspections of individual beams in the 500-beam array of the MS70 sonar will be shown and discussed.