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目前,大气扰动场目标探测存在的主要问题为探测图像的退化问题,在探测空中动目标大气扰动场时,由于光线在传播过程中穿过的大气距离很长,大气中气溶胶颗粒对光线的吸收和散射作用的影响,图像会出现质量退化的现象。
相机对大气扰动场成像时,大气环境中的气溶胶颗粒对相机成像质量产生主要影响,主要表现在对光线的吸收和散射作用。根据Mie散射理论,景物的成像机制可以用入射光衰减模型来描述,可表示为:
$$ E = {E_\infty }(\lambda )\rho {{\rm{e}}^{ - \beta (\lambda )l}} $$ (1) 式中:
$ E $ 为探测器接收到的光能量;$ {E_\infty }(\lambda ) $ 为天空亮度;$\; \rho $ 为辐射度;$\; \beta (\lambda ) $ 为大气消光系数,能见度越小,大气消光系数越大;$ l $ 为光线在气溶胶颗粒中的传播距离。由此可以看出,大气消光系数和光线在气溶胶颗粒中的传播距离直接影响相机探测器接收到的能量,且探测器接收到的能量随大气消光系数和光线在气溶胶颗粒中的传播距离的增大成指数级衰减。当光线在气溶胶颗粒中的传播距离
$ l $ 越大时,对光线的吸收和散射作用越大,相机接收到的光能量越低,此时图像将存在对比度低,信噪比低,动态范围小等问题。针对该问题,目前采用图像退化补偿技术来对退化后的图像进行补偿,主要分为图像增强与图像去雾两类。
图像增强是通过对感兴趣的部分有针对性的技术处理,抑制“无用”信息、突出“有用”信息,从而提高图像对比度,增强图像的细节,从而进行退化补偿。主要有直方图均衡化、Retinex、同态滤波、小波变换、基于大气光调制传递函数的算法等四种算法,其中主要算法为直方图均衡化与Retinex算法。
图像复原通过图像退化先验知识,建立图像退化数学模型,从大气作用产生的结果出发沿着图像退化的逆过程,最终得到清晰的尽可能接近真实场景的图像。常用的图像复原算法有:Tan、Fattal、He、Tarel,其中He与Tarel算法是目前主要算法[104-105]。
虽然,目前针对图像退化问题国内外学者已经进行了相关研究,但大气扰动场目标探测的探测距离需求一般为100 km以上。根据公式(1),假设
$\; \beta (\lambda ) = 0.05 $ ,此时观测10 km目标与100 km目标探测器接收到的能量之差约为90倍,当$\; \beta (\lambda ) = 0.2 $ 时,差距将达到65000多倍,在此种远距离探测需求下,目前国内外尚未有在这种距离条件下的图像退化补偿算法的研究,故图像退化问题依然是现阶段大气扰动场目标探测技术存在的主要问题。 -
根据相关研究表明,等离子鞘套会影响雷达回波的强度,主要影响因素为等离子体频率
$ {\omega _p} $ 和等离子体碰撞频率$ \upsilon $ ,影响公式如下:$$ \alpha = \frac{{8.68\omega }}{{\sqrt 2 c}}{\left[ {\frac{{\omega _p^2}}{{{\omega ^2} + {\upsilon ^2}}} - 1 + \sqrt {{{\left(1 - \frac{{\omega _p^2}}{{{\omega ^2} + {\upsilon ^2}}}\right)}^2} + {{\left(\frac{\upsilon }{\omega }\frac{{\omega _p^2}}{{{\omega ^2} + {\upsilon ^2}}}\right)}^2}} } \right]^{{1 \mathord{\left/ {\vphantom {1 2}} \right. } 2}}} $$ (2) 式中:
$ \alpha $ 为衰减常数;c为光速;$ \omega $ 为雷达辐射频率。(1)当雷达工作频率大于等离子体频率时,等离子鞘套对雷达辐射电磁波的作用能力较弱,电磁波能够穿透等离子鞘套,仅产生轻微的反射和幅度衰减,在等离子体中的传播近似于自由空间传输,此时不能进行等离子鞘套的探测;
(2)当雷达辐射频率小于等离子体频率时,等离子鞘套表现出强烈的反射特性,对电磁波具有强烈的反射效应,此时可以进行等离子鞘套探测
(3)当雷达辐射频率等于等离子体频率时对雷达入射电磁波衰减达到最大;此时最不利于等离子鞘套的探测。
(4)在等离子鞘套复杂的流体特性下,理想情况下,等离子鞘套的RCS总体应呈现基本不变→减小→增大→减小→恢复的过程。
除此之外,根据代定冬[106]等人相关研究结果表明,现阶段采用雷达探测等离子鞘套具有如下问题:
(1)雷达探测等离子鞘套总是会出现探测目标丢失的现象;
(2)应答式、信标式雷达探测等离子鞘套极易产生通信中断,反射式雷达探测等离子鞘套误差较大;
(3)等离子鞘套在雷达探测中表现为两个航迹接近,但散射特性差异明显的目标形态。
由上述研究结果可知,现阶段研究对等离子鞘套对电磁波传播影响机理的不够明晰,雷达探测等离子鞘套尚存在丢失目标,误差大以及虚警等问题,并不能实现长时间高精度的探测。
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虽然目前国内外学者对天空偏振模式的分布特性、不同天气条件和影响因素下大气偏振模式的变化规律等方面进行了一系列研究,成果丰富。但是目前学者们对于天空偏振模式的研究大多数对于理论层面,主要进行仿真研究,实际大气环境影响因素众多,错综复杂,大气偏振模式会受到时间、地点、环境等诸多因素的影响,目前的研究并不足以构建一套完整的大气偏振模式的分布规律模型来支撑大气偏振模式目标探测技术的开展。
同时,目前国内外学者尚未开展空中动目标飞行对大气偏振模式的影响的相关研究,大气偏振模式目标探测现阶段仅存在原理可行性基础,尚无技术机理研究支撑[108-112]。
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尾流目标探测存在的主要问题主要是尾流散射较弱,散射机理尚不完全清楚,现有雷达即使高灵敏度雷达也很难实现远距离探测。
在NASA进行的实验中,X波段雷达实验没有取得预想的成功,高旁瓣杂波成为影响尾流检测性能的重要不利因素[113],而且,实验还表明,晴空和潮湿大气中尾流在多个多普勒频率具有谱值,尾流是一种多普勒扩展目标。
为了满足实际应用需求,必须利用尾流特性增大尾流探测距离,这方面的理论研究成果较少。
根据相关研究表明,尾流探测技术已经趋于成熟,已进行多次飞机尾流探测试验,但基于目前的尾流探测试验结果显示,尾流的信号较弱,最高SNR也仅有约10 dB,难以远距离尾流探测,目前公开的最远距离尾流探测试验距离仅为15 km[113],尚无远距离尾流探测试验报道,与实现空中动目标探测的探测距离需求(百公里级)具有较大差距[48-114]。
Review of air moving target detection technology under environmental disturbance
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摘要: 空中动目标如空中隐身动目标以及高速飞行器对世界各国的国家安全产生了严重威胁,对空中隐身动目标与高速飞行器的探测问题也是世界各国目标探测技术发展的热点方向。文中阐述了一种对空中动目标具有较好探测能力的新型目标探测技术——空中动目标环境扰动探测技术,介绍了现阶段主要研究的四种空中动目标环境扰动现象及其探测方法,即大气扰动场、等离子鞘套、大气偏振模式以及尾流,研究了上述四种环境扰动探测技术的探测原理以及发展现状,分析现阶段四种环境扰动空中动目标探测技术存在的问题,最后对四种技术未来的发展方向进行展望并进行总结。Abstract: Air moving targets such as stealth moving targets and high-speed aircraft have caused serious threats to the national security of all countries in the world, the detection of stealth moving targets and high-speed aircraft in the air is also a hotspot in the development of target detection technology around the world. This article describes a new type of target detection technology with better detection capabilities for air moving targets—environmental disturbance detection technology for moving targets in the air, and introduces the four main researches on airborne moving target environmental disturbance phenomena and their detection methods at this stage. That is, atmospheric disturbance field, plasma sheath, atmospheric polarization mode and wake, the detection principles and development status of the above four environmental disturbance detection technologies are studied, and the problems existing in the four environmental disturbance detection technologies in the air at the present stage are analyzed. Finally, the future development directions of the four technologies are prospected and summarized.
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Key words:
- air moving target /
- environmental disturbance /
- target detection
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