Classification of helicopters by the L/N-quotient

It is highly important that a ground-based anti-aircraft defence radar is able to detect a hovering helicopter, which has for example been hidden by a forested area, and then suddenly appears. The task is to alert and to measure position. Most ground-based surveillance radars operate with short integration times and low unambiguous velocity, which most likely will complicate classification of helicopters. Airborne radar systems have better possibilities to classify helicopters due to better Doppler resolution and coverage. The task is here to distinguish helicopters from other moving targets and perhaps even classify them into different types. This fact is confirmed by simulations, where classification methods have been tested on recorded signals from an airborne surveillance radar, Linde (1995). In this study a method for classification is presented, which is based on an estimate of the quotient of the number (N) of rotor blades and their length (L), Edman and Rotander (1993).