Increasing the Accuracy of Time-varied-gain in Digital Echosounders

Time-varied-gain (TVG) is indispensable feature in vertical echosounders used in fisheries research in order to compensate for transmission loss and make the echo level independent of a target range. In digital processing of data acquired from modern digital echosounders, the sample by sample compensation is applied, assuming that each echo envelope sample was received from the range determined by its time delay with reference to the sounding pulse. However, as the sounding pulse used for generation of acoustic wave lasts for a certain period of time, the compensation should be made rather on echo waveform basis than on sample basis. In the paper the new algorithm for digital TVG correction was introduced, which allows for more accurate echo level adjustment than conventional "20 log R" or "40 log R" formulae. The developed algorithm was investigated experimentally on sonar echoes obtained from the standard sphere targets. The results may explain the possible source of errors encountered in the field studies, especially in a shallow water.