The invention relates to a method and an apparatus for passive determination of target parameters by directionally selective reception of sound waves which are emitted or transmitted from a target, to be precise by means of an arrangement (24) of underwater sound sensors of a sonar receiving installation from estimated bearing angles, which are determined from estimated positions of the target, and bearing angles which are measured at the measurement point by the arrangement (24), wherein a bearing angle difference between measured and estimated bearing angles is iteratively minimized and, when the minimum is reached, the target parameters are used for an optimized solution for outputting target range, target position, target course, target range and/or target velocity and they are updated during each processing cycle in a series of successive processing cycles. In order to estimate the reliability of this optimized solution, during each processing cycle a multiplicity of different target tracks Z(i, j) are calculated from possible solutions for the target parameters to be determined, specifically an assumed target course C
est , an assumed target range R
est and/or an assumed target velocity V
est . Associated bearing angles B
est are assumed for each of these possible solutions, and the assumed bearing angles B
est are used to calculate a quality measure Q, Qinv along each of the assumed target tracks Z(i, j). In order to determine the reliability of the optimized solution, a reliability level is determined from the distribution of the quality measure Q, Qinv, as a function of at least one of the target parameters. The entire calculated solution scope for the various target parameters corresponding to the associated quality measure Q, Qinv is advantageously visualized in various diagrams.