Abstract: The minimum distance between a passing projectile and a training (dummy) target is ascertained by means of four shock wave responsive transducers in the target, two of which are aligned with the target's propagation and the transit time differential of receiving the shock wave from the projectile as well as measured distances to the projectile's trajectory are used to establish a set of possible projectile trajectories, one of them being the real one; all of them being arranged in rotational symmetry to the line established by the two transducers. This information suffices already to determine the minimum distance of fly by.