电离层回波谱展宽机理分析及频谱锐化方法

Analysis of broadening mechanism of ionospheric echo spectrum and spectrum sharpening method

高频天波探测设备在执行早期预警和海态遥感等任务时必须依靠电离层作为传播媒质,而电离层具有时变、不稳定的特性,会改变经过它传播的高频电磁波的特征,导致回波频谱展宽,严重影响了对目标的探测和海态参数的反演.从色散效应、相位污染和多模传播等方面详细分析了回波谱展宽的原因和机理,利用多层准抛物线电离层模型讨论了避免多模传播的选频措施.针对在实际中较难解决的相位污染问题,提出了一种不用估计回波瞬时频率的污染校正方法.该方法利用了信号子空间与信号导频矢量张成空间的一致性原理,能够较准确地估计出相位污染项,实测数据处理表明新方法能够使展宽的回波谱得到有效锐化.

High-frequency sky wave detection equipment must rely on the ionosphere as the propagation medium inthe early warning and sea state remote sensing tasks.The ionosphere is time-varying and unstable,which willchange the characteristics of the high-frequency electromagnetic wave propagating through it,resulting in thebroadening of the echo spectrum,thus seriously affecting the detection of targets and the inversion of sea stateparameters.The reason and mechanism of the echo spectrum expansion are analyzed in detail from thedispersion effect,phase contamination and multimode propagation.The bandwidth of the dispersion effect isdifferent from that of the high frequency detection equipment.When the bandwidth of the sky wave equipmentis 3 30 MHz,the bandwidths of the dispersion effect are 41.6 57.4 kHz and 0.17 10.8 MHz.The multi-quasi-parabolic ionospheric model is used to discuss the frequency selection measures to avoid multimode propagation.The modulation process of ionospheric contamination to echo is studied theoretically.It is shown that the non-linear phase contamination will cause the energy of echo to diffuse in frequency domain and to be unable toaccumulate.To solve the problem of phase contamination which is difficult to solve in practice,a contaminationcorrection method without estimating the instantaneous frequency of the echo is proposed.In the method theconsistency principle of signal subspace and signal frequency vector expansion space is used,and therefore thephase contamination term can be well estimated.Based on the real data,the contamination correction resultsfrom the proposed method,phase gradient autofocus method,maximum entropy spectral analysis method andtime-frequency processing method are given.The results show that the new method is a better method and caneffectively sharpen the broadened echo spectrum.