The coupling among multiple coded orthogonal waveforms occupying the same frequency band may seriously affect the distributed terrain imaging of multiple-input and multiple-output synthetic aperture radar (MIMO-SAR). Based on the inter-pulse phase modulation among different transmitting waveforms, a range-Doppler decouple filtering method is presented to separate waveforms effectively. Finally, numerical experiments are provided to demonstrate the effectiveness of the proposed method.

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The occurrence of self-excited noise felt as squealing noise is a critical issue for an electrohydraulic servovalve that is an essential part of the hydraulic servocontrol system. Aiming to highlight the root causes of the self-excited noise, the effect of oil viscosity on the noise production inside a two-stage servovalve is investigated in this paper. The pressure pulsations' characteristics and noise characteristics are studied at three different oil viscosities experimentally by focusing on the flapper-nozzle pilot stage of a two-stage servovalve. The cavitation-induced and vortex-induced pressure pulsations' characteristics at upstream and downstream of the turbulent jet flow path are extracted and analyzed numerically by comparing with the experimental measured pressure pulsations and noise characteristics. The numerical simulations of transient cavitation shedding phenomenon are also validated by the experimental cavitation observations at different oil viscosities. Both numerical simulations and experimental cavitation observations explain that cavitation shedding phenomenon is intensified with the decreasing of oil viscosity. The small-scale vortex propagation with the characteristic of generating, growing, moving, and merging is numerically simulated. Thus, this study reveals that the oil viscosity affects the transient distribution of cavitation and small-scale vortex, which, in turn, enhances the pressure pulsation and noise. The noise characteristics achieve a good agreement with pressure pulsation characteristics showing that the squealing noise appears accompanied by the flow field resonance in the flapper-nozzle. The flow-acoustic resonance and resulting squealing noise possibly occurs when the amplitude of the pressure pulsations near the flapper is large enough inside a two-stage servovalve.

The unwanted coupling exists inevitably among multiple orthogonal waveforms in a same frequency area for multiple-input and multiple-output synthetic aperture radar (MIMO-SAR). In this paper, a new polarized MIMO-SAR model is established with two transmitting antennas and multiple receiving antennas at first. Then, a virtual polarization filter (VPF) is proposed to separate superposed returns caused by multiple transmitted waveforms based on detection on the polarized parameters via particle swarm optimizer (PSO). Compared with the conventional matched filter to separate the orthogonal waveforms, it is shown that the coupling noises can be significantly suppressed by the proposed VPF-based method and the orthogonality is not necessary among different transmitting waveforms. Finally, experimental data experiments are also provided to demonstrate the effectiveness of the proposed method.