I. Introduction
An increasingly demanded application during peace keeping missions under war conditions is noncooperative target classification of vehicles, where the unexpected appearance of friendly or threatening vehicles must be considered. A typical example is a helicopter armed or equipped for humanitarian tasks [1]. Consequently, helicopter classification is a very important matter. To perform the classification, inverse synthetic aperture radar (ISAR) images [1]–[5] and radar echoes [6]–[10] can be used.