Unmanned aerial vehicles (UAVs) have become widely used since its low-priced, powerful function and easily deployment. It has been proofed that the rotation of rotor blades on UAVs introducing micro-Doppler signatures can be used to discriminate UAVs from other aircrafts or birds. Moreover, the micro-Doppler (m-D) always consists of multiple frequency components which are time-varying and induced by the vibration of the platform when UAVs have continuous motion. This paper focuses on extraction of the special frequency signals in UAVs, which would decompose m-D signals into a series of intrinsic mode functions. For multi-rotors UAV, the number of rotors and the length and rotational speed of each rotor are identified through k order Bessel function decomposition. Using the micro-Doppler modulation induced by the rotation of rotor blades, it would be an effective mechanism for discriminating the feature of the special UAVs.
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