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激光探测场是实现激光引信目标探测和抗干扰性能的重要决定因素,其参数中的探测视场角、激光脉冲宽度、光学基线等参数对性能的影响较大[9]。文中在其光学基线和探测能力一定的情况下,主要从激光脉冲宽度和单次探测视场角两个因素进行分析。
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实体目标表面散射和气溶胶介质散射存在物理学上的差异,实体表面为漫反射,其反射过程不影响回波信号的脉冲宽度。在系统带宽足够的情况下,探测系统检测到的回波幅度主要与目标漫反射系数相关;而气溶胶介质表现为Mie散射,激光在气溶胶介质中传输过程,经过与气溶胶粒子多次散射的空间调制,对激光回波功率的一维时间分布产生了影响[10]。
当激光引信激光发射系统产生高斯激光脉冲,工作于不连续的、具有边界的气溶胶环境内,根据气溶胶介质的脉冲特性,在通常的数值范围内,其后向散射过程中在激光接收系统输入端产生的气溶胶干扰光学信号包线变化特性具有如下形式[11]:
$$ {P}_{п}\left(H\right)={P}_{0}{A}_{\rm C}f\left(\theta \right){\sigma }_{\rm s}{\int }_{{H}_{0}}^{H}\begin{array}{c}\\ \end{array}\frac{{S}_{H}\left(R\right)}{{R}^{2}}{\rm e}^{-2{\sigma }_{\rm s}(R-{H}^{*})-{k}_{\rm m}(H-R)}{\rm d}R $$ (1) 式中:
${P}_{П}(H)$ 为激光接收系统输入端上气溶胶干扰光学信号辐射强度包线函数;${P}_{0}$ 为辐射源的窗口处光功率;${A}_{\rm C}$ 为激光接收系统入射光孔的面积;${f}\left({\theta}\right)$ 为气溶胶散射方向函数,$ {\theta }$ 为反射表面的法线与表面—激光接收系统方向(观察方向)的夹角;${\sigma }_{\rm s}$ 为气溶胶粒子的体积散射系数;$ {S}_{H}\left(R\right)=S\left(R\right)/{R}^{2}{\theta }^{2} $ 为接收、激光发射系统视场几何图形的重叠面积;$ R $ 为目标距离;$ {H}^{*} $ 为到气溶胶边缘轮廓的距离;$ H $ 为辐射的传播距离;$ {H}_{0} $ 为盲区深度;${k}_{\rm m}=2/c{\tau }_{\rm n},{\tau }_{\rm n}$ 为脉冲宽度。由公式(1)可以看出,气溶胶对脉冲激光的后向散射能量与脉冲宽度有关。图1为在相同气溶胶环境下不同脉冲宽度的散射特性,仿真了50 ns、30 ns、15 ns、10 ns和6 ns的激光脉冲分别以脉宽中心处于X轴0点为时间基准,经过相同气溶胶环境后的回波功率的对比曲线。从图中可以看出,不同脉冲宽度条件下的回波功率有明显区别,脉冲越窄,回波幅度越小。
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传统周视激光引信为保证对多类目标具有全向高检测概率,通常采用由4~8个探测象限连续拼接形成环形探测场。单个探测象限的激光发射组件和接收组件均具有较大的弧矢视场角。由于该视场角远大于远距目标最小截面形成的探测张角,在相同探测能力情况下,大视场角探测场的较多激光能量并不参与目标探测,而对于烟尘云雾等气溶胶介质环境,整个探测场的激光回波能量都会进入探测场,造成较强的后向散射干扰。基于上述问题,可将探测场进行视场细分,压缩单次探测的视场角,能有效抑制云雾、烟尘等气溶胶环境干扰回波。
根据主动激光探测原理,激光回波功率方程用下式表示:
$$ {{P}}_{\mathrm{R}}=\frac{{{P}}_{\mathrm{T}}}{{{R}}^{2}{{\varOmega }}_{\mathrm{T}}}\times \frac{(\mathrm{\rho }{{A}}_{\mathrm{T}})}{{{\varOmega }}_{\mathrm{R}}}\times \frac{{{A}}_{\mathrm{C}}}{{{R}}_{}^{2}}\times {\mathrm{\eta }}_{\mathrm{T}}{\mathrm{\eta }}_{\mathrm{R}}\times \mathrm{cos}\theta $$ (2) 式中:
${{P}}_{\mathrm{R}}$ 为接收到的回波能量;${{P}}_{\mathrm{T}}$ 为发射光能量;$ \;{\rho } $ 为目标反射率;$ {\mathrm{\eta }}_{\mathrm{T}}{\mathrm{\eta }}_{\mathrm{R}} $ 为往返传输系数;${{\varOmega }}_{\mathrm{T}}$ 为发射光束发散角;${{\varOmega }}_{\mathrm{R}}$ 为返回光束发散角($ \mathrm{\pi }\times \mathrm{\pi } $ );${{A}}_{\mathrm{T}}$ 为目标反射截面。目标反射截面
${{A}}_{\mathrm{T}}$ 为发射光斑在目标上的照射面积,其探测回波功率的计算以目标的最小激光截面来计算。当发射光束发散角为窄视场,其在目标距离处${{\varOmega }}_{\mathrm{T}}$ 内的投影面积小于目标形体面积时,其目标截面就是发射光斑的照射面积。当发射光束发散角为宽视场,其在目标距离内光斑投影大于目标截面时,其目标反射截面${{A}}_{\mathrm{C}}$ 为发射光斑在目标上的照射面积。根据公式(2)和几何关系,可得到相同探测能力下,宽视场与窄视场的激光发射能量关系,见公式(3):$$ {P}_{\rm T}^{W}=\frac{{P}_{\rm T}^{\rm N}\mathrm{\omega }{R}}{L} $$ (3) 式中:
${P}_{\rm T}^{\rm W}$ 为宽视场发射能量;${P}_{\rm T}^{\rm N}$ 为窄视场发射能量;L为目标截面长度;$ \mathrm{\omega } $ 为宽视场情况下的弧矢视场角。在目标最小截面L=0.6 m,探测距离R=9 m的条件下,当引信采用45°宽视场探测时,其弧矢视场角
$ \mathrm{\omega }=\mathrm{\pi }/4 $ ,两种系统发射功率之比${P}_{\rm T}^{\rm W}{/P}_{\rm T}^{\rm N}$ ≈11.8。在目标探测方面,采用宽视场探测需要的激光发射功率远大于窄视场。而气溶胶环境通常能包容整个发射视场,激光发射功率大必然导致背景后向散射能量大。图2是探测系统采用不同弧矢视场角回波波形曲线的仿真对比,其中y轴为归一化的回波功率。仿真过程采用相同发射功率和气溶胶干扰环境,采用弧矢视场角分别为2.5°、8°、20°、45°的情况下,目标的回波波形曲线对比情况。通过对比可得到如下结论:
(1)由于气溶胶环境能包容整个视场角,因此,当发射能量相同的情况下,不同弧矢视场角的气溶胶环境后向散射波形趋于一致,其回波特征与视场角相关性较小。
(2)不同弧矢视场角的目标回波差异很大,采用视场角2.5°的目标回波功率比视场角45°大10倍以上。窄视场探测在相同干扰环境中能获得更强的目标回波,利于后级信号处理对目标回波包络的识别。
Laser fuze anti-interference method based on array laser echo waveform feature recognition
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摘要: 针对激光引信易受云雾等气溶胶环境的干扰问题,提出了一种基于阵列激光波形特征识别的抗干扰方法。根据脉冲激光在气溶胶环境的散射特性,理论分析了发射脉冲宽度以及视场角对系统在干扰环境中目标识别的影响,仿真了目标处于干扰环境中的波形时域特征,并通过实验样机对仿真结果进行实测验证。基于回波波形特征,设计了一种窄视场阵列激光回波特征数字化识别的探测系统方案,并通过虚拟样机仿真技术获取了2°分辨率的目标和云雾的回波阵列数据。数据分析结果表明,由于目标形体和云雾弥散体的物理差异,目标回波阵列的能量方差极值及均值都大于云雾,通过设定帧内回波阵列的能量方差的阈值和帧间方差累计的方法能有效提升激光引信抗干扰性能。文中的仿真和实测结果都为基于阵列激光波形识别的抗干扰方法的有效性提供了理论和实验依据。Abstract: Aiming at the problem that the laser fuze is easily interfered by cloud, fog and other aerosol environment, an anti-interference method based on the array laser echo waveform feature recognition is proposed. According to the scattering theory of pulsed laser in aerosol environment, the influence of pulse width and field angle on target recognition is analyzed. The time domain characteristics of target waveform in interference environment are simulated, and the simulation results are verified. Based on the characteristics of echo waveform, a detection system scheme of narrow field array laser echo feature digital recognition is designed, and the target and cloud echo array data with 2° resolution are obtained by virtual prototype simulation technology. The results of analysis indicate that the extreme value and mean value of pulse amplitude variance of target echo array are larger than that of cloud, and the method based on array laser echo waveform feature can effectively improve the anti-interference performance. The simulation and measured results provide theoretical and experimental basis for the effectiveness of the anti-jamming method based on array laser waveform recognition.
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Key words:
- laser fuze /
- anti-interference /
- laser imaging /
- waveform feature recognition
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