Estimation of Micro-Doppler Parameters With Combined Null Space Pursuit Methods for the Identification of LSS UAVs

Micro-Doppler (m-D) effect, induced by the rotation of rotor blades, supplies a differentiable characteristic to address the problem for the identification of low, slow, and small unmanned aerial vehicles (LSS UAVs). However, the primary challenge for the estimation of m-D parameters is how to separate weak rotation signal from Doppler signal and other interferences. Theoretically, null space pursuit (NSP) is an operator-based signal decomposition approach to decompose a signal into additive subcomponents. The premise of NSP is that two separated components are orthogonal. However, due to the different modulation models, rotation signal, Doppler signal, or other interferences could not satisfy the condition. Moreover, the traditional multiorder differential operator is not suitable for the decomposition of m-D signal. In this article, a backprojection strategy with instantaneous orthogonal NSP (BPIO-NSP) is proposed to distill Doppler signal, and then, micro-Doppler NSP (MD-NSP) is jointly developed to separate rotation signal for the identification of LSS UAVs. First, the decomposed component after NSP is applied to the short-time Fourier transform (STFT) to find the segments with instantaneous nonorthogonal property. Second, the backprojection strategy is developed in BPIO-NSP to acquire the instantaneous orthogonal data, so as to adjust the decomposed Doppler signal with high accuracy. Finally, customized operator is specially constructed in MD-NSP to achieve the required rotation signal from the residue. Simulation results verify the theoretical analysis, and measured data of the fun and UAV detection experiments suggest that the proposed methods could be served for the application of the identification of LSS UAVs.