Calibrating Multibeam Sonar by the Standard-Target Method: Extending Limited Measurements

Protocols have been developed for the calibration of multibeam sonars. These have been illustrated for a limited number of parameter values. Given the potentially large number of parameter values that could be involved in water-column measurements, there is a need to show how a calibration performed for a limited set of parameter values can be extended to the full range of available parameter values. This is done here principally for range measurement and range compensation by means of the time-varied-gain (TVG) function. Error analyses are performed, and algorithmic methods for correcting and extending limited calibration measurements are described. These are illustrated for a particular sonar, the Simrad SM2000/90-kHz multibeam echo sounder. Advantages of being able to perform a standard-target calibration at one or a few target ranges for just one or a very few gain functions are noted. However, this also requires determination of the sensitivity and saturation limits for the TVG functions. Attention is called to the nearfield problem; namely, performing a calibration in the transducer near- or farfield and making quantitative measurements in the respective far- or nearfield.