Abstract: In this paper, the ability of likelihood ratio method to in the classification of underwater spherical targets with same diameters has been investigated. The formulas for probabilities of correct classification and incorrect classification for various threshold are developed based on statistic theory. It is shown that these probabilities depend on the mean value and variance of test function. Both of them can be determined by the quadrature samples of target echoes.
Three target pairs consistied of four spherical targets were used in experiment. They include a solid steel sphere, a hollow Steel sphere, a hollow aluminium sphere and a solid cement sphere, all have the same diameter （19cm）. The results show that the performance indsices of classification （i.e. the area under the ROC curve） for these three target pairs are different and are functions of S/N ratio. As an example, for one pair of them the performance index increases 0.0268 on average as the S/N ratio increases IdB when the S/N ratio takes the value from 0 dB to 5dB. While the average increasing rate becomes 0.0212/dB when the S/N ratio takes from 5 dB to 10 dB.