雷达对地成像技术多向演化趋势与规律分析

杨建宇

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雷达对地成像技术多向演化趋势与规律分析

    作者简介:
    杨建宇(1963–),电子科技大学教授,博士生导师,校科技委主任,国务院学位委员会信息与通信工程学科评议组成员,中国电子学会雷达分会副主任委员。主要研究方向为雷达前视成像、实孔径超分辨成像、双多基合成孔径雷达成像。获国家出版基金资助出版专著1部。获省部级奖6项、国家技术发明二等奖2项。E-mail: jyyang@uestc.edu.cn.
    通讯作者: 杨建宇 jyyang@uestc.edu.cn
  • 基金项目:

    国家自然科学基金重点项目(60632020),国家自然科学基金面上项目(61771113, 61671117)

  • 中图分类号: TN95

Multi-directional Evolution Trend and Law Analysis of Radar Ground Imaging Technology

    Corresponding author: YANG Jianyu, jyyang@uestc.edu.cn
  • Fund Project: The Key Program of the National Natural Science Foundation of China (60632020), The General Program of The National Natural Science Foundation of China (61771113, 61671117)

    CLC number: TN95

  • 摘要: 该文从成像结果表征、孔径流形、信号通道、系统形态、观测方向、处理方法、实现机理、目标识别等方面剖析了雷达对地成像技术的多向演化态势,并试图从宏观的视角和大的时间尺度,分析和认识雷达对地成像技术发展的内外因素和发展规律,推演预测未来发展方向,以期为把握雷达对地成像技术发展的时代脉络和宏观趋势、契合需求和引领创新、推动发展和促进应用,提供另类的观察视角和思维方式。
  • 图 1  GF-3星载全极化SAR图像[11]

    Figure 1.  GF-3 spaceborne fully polarized SAR image[11]

    图 2  用色彩表征视向形变量的SAR图像[13]

    Figure 2.  SAR image with color representation of line-of-sight deformation[13]

    图 3  用颜色表征地物散射方向性的SAR图像[14]

    Figure 3.  SAR image with color representation of ground scattering directivity[14]

    图 4  干涉SAR成像原理及维苏威火山成像结果[16]

    Figure 4.  InSAR imaging principle and imaging result of Vesuvius volcano[16]

    图 5  极化干涉SAR原理与地物三维成像结果[17]

    Figure 5.  Principle of Pol-InSAR and three-dimensional imaging result

    图 6  圣地亚国家实验室的视频SAR成像结果[19]

    Figure 6.  Video SAR imaging results of Sandia national laboratories[19]

    图 7  不同频段地物SAR图像差异的直观理解

    Figure 7.  Intuitive understanding of the differences between the SAR images of the ground objects in different frequency bands

    图 8  孔径流形的演变

    Figure 8.  Evolution of the aperture manifold

    图 9  圆周SAR与条带SAR成像结果对比[23]

    Figure 9.  Comparison of imaging results of circular SAR and stripmap SAR[23]

    图 10  圆周SAR试验情况[24]

    Figure 10.  Experiment of circular SAR[24]

    图 11  复杂机动轨迹SAR的示意图

    Figure 11.  Schematic diagrams of complex maneuvering SAR

    图 12  多航过层析SAR

    Figure 12.  Multi-pass tomographic SAR

    图 13  单平台多通道SAR示意图

    Figure 13.  Diagrams of single platform multi-channel SAR

    图 14  立体分布地物的三维成像[38]

    Figure 14.  Three-dimensional imaging of stereo distributed ground objects[38]

    图 15  建筑群的三维成像[25]

    Figure 15.  Three-dimensional imaging of buildings

    图 16  多通道SAR演进图

    Figure 16.  Multi-channel SAR evolution map

    图 17  双多基地SAR系统形态

    Figure 17.  Morphology of Bistatic/Multistatic SAR

    图 18  单双基SAR图像明暗关系差异[47]

    Figure 18.  Difference in light-dark relationship between monostatic and bistatic SAR images[47]

    图 19  聚束式双基SAR试验[48]

    Figure 19.  Experiment of spotlight bistatic SAR[48]

    图 20  机载双基侧视SAR试验[49]

    Figure 20.  Experiment of airborne bistatic side-looking SAR[49]

    图 21  星机双基侧视SAR试验[50]

    Figure 21.  Experiment of spaceborne/airborne bistatic side-looking SAR[50]

    图 22  国内首幅机载双基侧视SAR图像[51]

    Figure 22.  The first airborne bistatic side-looking SAR image in China[51]

    图 23  外辐射源双基SAR试验[52]

    Figure 23.  Experiment of passive bistatic SAR[52]

    图 24  机载双基前视SAR图像[61]

    Figure 24.  Airborne bistatic forward-looking SAR image[61]

    图 25  星机双基地后视SAR试验[63]

    Figure 25.  Experiment of spaceborne/airborne bistatic backward-looking SAR[63]

    图 26  合成孔径原理的4种不同解释

    Figure 26.  Four different interpretations of synthetic aperture principle

    图 27  扫描波束锐化技术的交汇船只分辨试验[82]

    Figure 27.  Resolving ships experiment of scanning beam sharpening[82]

    图 28  扫描波束锐化技术的面目标成像试验[82]

    Figure 28.  Surface target imaging experiment of scanning beam sharpening[82]

    图 29  电磁涡旋成像的可行性验证[96]

    Figure 29.  Feasibility verification of electromagnetic vortex imaging[96]

    图 30  支撑成长识别能力的主要机制

    Figure 30.  The main mechanisms that support growth recognition

    图 31  雷达对地成像技术发展的外部因素

    Figure 31.  External influencing factors for the development of radar ground imaging technology

    图 32  雷达对地成像技术发展的内部因素

    Figure 32.  Internal influencing factors for the development of radar ground imaging technology

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  • 收稿日期:  2019-11-19
  • 录用日期:  2019-12-20
  • 刊出日期:  2019-12-28

雷达对地成像技术多向演化趋势与规律分析

    通讯作者: 杨建宇 jyyang@uestc.edu.cn
    作者简介:
    杨建宇(1963–),电子科技大学教授,博士生导师,校科技委主任,国务院学位委员会信息与通信工程学科评议组成员,中国电子学会雷达分会副主任委员。主要研究方向为雷达前视成像、实孔径超分辨成像、双多基合成孔径雷达成像。获国家出版基金资助出版专著1部。获省部级奖6项、国家技术发明二等奖2项。E-mail: jyyang@uestc.edu.cn
  • 电子科技大学 成都 611731
基金项目:  国家自然科学基金重点项目(60632020),国家自然科学基金面上项目(61771113, 61671117)

摘要: 该文从成像结果表征、孔径流形、信号通道、系统形态、观测方向、处理方法、实现机理、目标识别等方面剖析了雷达对地成像技术的多向演化态势,并试图从宏观的视角和大的时间尺度,分析和认识雷达对地成像技术发展的内外因素和发展规律,推演预测未来发展方向,以期为把握雷达对地成像技术发展的时代脉络和宏观趋势、契合需求和引领创新、推动发展和促进应用,提供另类的观察视角和思维方式。

English Abstract

    • 上世纪六十年代前后,人类掌握了微波相位的控制和利用技术,因此,几乎同时产生了相控阵雷达、脉冲多普勒雷达和合成孔径雷达。至今它们依然是雷达的主流技术体制,而且处在持续的完善和演变过程之中,以期获得更高的性能和新的功能,去适应多元化的任务,满足不同应用场景的需求。

      自从1951年Carl Wiley提出合成孔径的最初概念后,以合成孔径为代表的雷达对地成像技术得到了很大的发展,并沿着多个方向演化。而且,这种演化仍在持续,整个雷达对地成像技术领域呈现出生机勃勃的多向演化态势。分析、认识和理解这种态势,探究其形成的内部因素、外部动因和相互作用关系,有利于把握雷达对地成像技术的发展脉络,认清发展规律,推演未来发展。

      对于雷达对地成像技术发展的态势,可以从不同的观察视角进行分析,从而得到不同侧面的认识和结论,以服务于不同的目的。这方面已经有一些很有价值的文献[14]可供参考和借鉴。如果立足于信息获取的方式、系统构成的形态、回波处理的方法、成像结果表征的方式和实际应用中的功效等方面,从地表及附着物信息的采录、归位、表征和提取全链条的宏观视角,去分析和梳理雷达对地成像技术的发展态势,并结合它与视觉感知的相似性去理解,还可以得出一些新的认识,并从中有所感悟,进而发现技术演化走向的规律性和必然性,利于承前启后,促进技术创新。

      本文将从8个主要的侧面去分析雷达对地成像技术的发展态势,探究发展过程中内外因素的相互作用,归纳总结发展演进的规律,并据此推演未来发展方向。

    • 雷达对地成像技术虽然已经历了六十多年的发展,但受到新的需求牵引和相关技术进步的推动,在它的实现机理、系统形态、技术体制、处理方法、成像效果乃至信息提取等方面,依然在发生着深刻的变化,呈现出生机勃勃和纷繁复杂的演化态势。只有立足于信息获取与表征的宏观视角,才能够分析归纳出其发展脉络和勾勒出清晰的演化图景。以下将从8个方面剖析雷达对地成像技术的不同演化形态,并推演和预测未来发展方向。

      (1) 成像结果表征由单色向彩色、由平面向立体、由静态向动态演化,将进一步向兼具彩色、立体和动态表征能力的方向演进。

      合成孔径雷达(Synthetic Aperture Radar, SAR)图像通常以单色灰度图像形式出现,它对应着单一工作频段、单一极化组合、特定观测视角和单次观测航过。多波段或多极化SAR[58]利用共平台安装的多个频段或多个极化通道,共视观测同一地域,感知地物对不同波段、不同极化的散射特性差异,从而获得彩色图像,其中的颜色用于表征地物除散射强度以外的其它信息。与人眼用色彩感来表征物体对不同波长光波的散射强度类似,相比于单色图像,彩色图像能够更有效地呈现地物特征的差异,凸显出单色图像无法表达的信息,利于地物分类与识别。例如,2016年发射升空的我国GF-3遥感卫星,搭载了中国科学院电子学研究所牵头研制的我国首部星载全极化SAR[9,10],所获得的彩色图像用不同颜色表征地物的不同极化属性[11,12],使不同类型的地表能够更容易辨识和区分,如图1所示。

      图  1  GF-3星载全极化SAR图像[11]

      Figure 1.  GF-3 spaceborne fully polarized SAR image[11]

      单波段和单极化SAR利用多视角或多时相的方式观测同一区域,能够感知地物散射的方向性和时变性,也可以获得彩色图像。例如,利用欧空局ERS-1/2 遥感卫星获取的苏州地区SAR数据,中国科学院遥感应用研究所通过差分干涉处理得到的彩色图像,可直观地反映两年间该地区不同地域的沉降情况差异[13],如图2所示;中国科学院电子学研究所利用不同观测方向的SAR图像进行融合,得到的彩色图像,可以直观地反映地物散射的方向性特征,如图3所示[14]

      图  2  用色彩表征视向形变量的SAR图像[13]

      Figure 2.  SAR image with color representation of line-of-sight deformation[13]

      图  3  用颜色表征地物散射方向性的SAR图像[14]

      Figure 3.  SAR image with color representation of ground scattering directivity[14]

      立体图像可提供平面图像缺失的目标高程信息,更加准确地反映地表及附着物的形貌,利于提升地物的辨识度。获取地表立体图像的基本途径是采用切航迹多通道SAR技术,以构成切航迹干涉效应或形成切航迹孔径。例如,干涉合成孔径雷达(Interferometric Synthetic Aperture Radar, InSAR)[15]在切航迹方向安装有两个以上有一定间距的接收通道,可以感知通道间的回波相位差,并利用通道间相位差与地表高程的定量关系,反演出地表高程信息,如图4所示[16]

      图  4  干涉SAR成像原理及维苏威火山成像结果[16]

      Figure 4.  InSAR imaging principle and imaging result of Vesuvius volcano[16]

      但是,InSAR只能够获得各距离-方位分辨单元的高程信息,对同一分辨单元中不同高程的多个散射体并没有分辨能力。因此,InSAR所获取的立体图像并不是真三维的,所以,有时也通俗地称为“二维半”。而且,在InSAR中,地表的高程突变会造成相位解缠出现错误,从而造成地表高程测量值出现差错。此外,InSAR采用下斜视观测方式,容易受到遮挡和阴影的影响,对地表起伏的适应性存在明显的局限性。所以,InSAR并不适用于高起伏和陡变地表的立体成像。

      极化干涉SAR(Polarized Interferometic SAR, Pol-InSAR)是一种相近原理的立体成像技术,它可以利用树冠和地表的极化特性差异,在所获得的立体图像中,反映出平坦和起伏地表上植被的错落生长形态,如图5所示[17]

      图  5  极化干涉SAR原理与地物三维成像结果[17]

      Figure 5.  Principle of Pol-InSAR and three-dimensional imaging result

      在毫米波和太赫兹等高频段,或雷达平台相对于目标高转角率运动时,只需要较短的合成孔径时间,即可实现对地成像,因此,可形成高帧率(例如5帧/秒)的SAR图像序列,连续播放时可以形成类似视频的动态效果,能够表现地表及附着物的短时变化动态,所以也称为视频SAR[18]。它是当今合成孔径雷达技术领域最引人注目的发展方向之一,也是合成孔径这只微波眼完全可预见的必然演化走向。国内外有不少单位在从事相关研究工作[1820]。例如,美国圣地亚国家实验室(Sandia National Laboratories)获得的视频SAR成像结果[19],如图6所示。可以观察到,在若干间隔帧的图像之间,路面上汽车阴影出现明显的位置(图6中箭头所指)移动。视频SAR的这种特性使它可用于对地面目标运动情况的实时监控。

      图  6  圣地亚国家实验室的视频SAR成像结果[19]

      Figure 6.  Video SAR imaging results of Sandia national laboratories[19]

      其实,如果淡化波动性的影响,且从定性和通俗的角度来讲,合成孔径雷达成像技术,好比给我们头顶上配个微波矿灯(发射微波的天线),在一片黑暗中,去照向我们要看的地域,还合成一只微波眼睛来。用这只另类的眼睛来观察世界,看到的是一个由无数尺寸和朝向不同的小镜面组成的起起伏伏的破碎大镜面,如图7(a)图7(b)所示。这些不那么平整的小镜面,间或反射出我们头顶那盏矿灯的影像(亮点)。这些亮点能否勾勒出地物的形貌,就要看这只微波眼所处的频段了,还要看这些小镜面的朝向比例和组合关系了。在较低的频段,世界和万物并不是那么破碎,有更多较大尺度的小镜面,如图7(a)所示,我们只能星星点点地看到头顶上那盏矿灯的影像,很难勾勒出地物的轮廓形貌,即便我们的微波眼有足够大的孔径和足够高的清晰度,也很难辨识出地物来,如图7(c)所示。

      图  7  不同频段地物SAR图像差异的直观理解

      Figure 7.  Intuitive understanding of the differences between the SAR images of the ground objects in different frequency bands

      在更高的频段,会有更多较小尺度的小镜面,如图7(b)所示,即使分辨率与低频段相同,微波眼看到的情形也会得到改善,更加接近光学眼看到的景象,地物也更易辨识,如图7(d)所示。而且,在较低的频段,路上汽车的影子不像图6中那么可见,因为这时候路面是大块的镜面,斜看时并不会反射出那盏矿灯的影像,道路与影子是一样的黑色,没什么差异,自然显不出影子来;而高频段就不一样了,路面成了破碎粗糙的镜面,即使斜看,也可见到那盏矿灯无数克隆的影像,密密麻麻地分布在粗粒铺装的高速公路上[19],也不算稀疏地分布在细粒铺装的高速公路上,所以,路亮影黑,在微波眼特别是太赫兹眼里的物虚影实[22]效应中,就成了追影检测的基础。

      实际上,如果知道合成孔径雷达技术是在制造微波眼,那么由单色、平面和静态到彩色、立体和动态的演化就是必然的了。而且,在较长时间段内对起伏地表形变及植被生长变化的持续观测中,全极化干涉SAR已经同时具备了彩色、立体和(长时)动态的表征能力,而即将出现的太赫兹全极化干涉SAR也将同时具备彩色、立体和(短时)动态的表征能力。

      (2) 孔径流形由直线状演化出曲线状和面状,并向多曲线交织的立体状演进。

      孔径流形是指雷达收发通道在平台运动过程中所形成的轨迹形态,合成孔径雷达技术发展到现在,已演化出多种不同的孔径流形,这种演化趋势还将持续,如图8所示,其中红色圆点代表承载平台上所有发射通道,蓝色圆点则对应所有接收通道。

      图  8  孔径流形的演变

      Figure 8.  Evolution of the aperture manifold

      不同的孔径流形会明显地改变雷达获取信息的方式,从而使回波规律和成像处理产生大的变化。相对于最初的直线状孔径流形,曲线状孔径流形需要适应平台的机动飞行,却能够形成更大的或二维的观测视角变化,还能够从不同的方向观测地物,从而获得更加丰富的地物信息,例如,曲线SAR和圆周SAR。而面状孔径流形的主要作用是能够获得对地立体成像能力,如层析SAR。而接收通道与发射通道分离运动形成的流形,却可以获得地物的双基散射信息,而不是传统的后向散射信息,例如双基SAR和多基SAR。

      圆周SAR通过其环形孔径获得多视角观测能力和高程信息获取能力,后者又有利于起伏地表上地物的回波聚焦,从而改善图像质量。例如,中国科学院电子学研究所获得的圆周SAR图像[23],如图9所示,比条带SAR图像和较小转角的SAR图像包含有更丰富的地物信息,使地物形貌更加完整,如图9(d)所示;甚至能明显观察到条带SAR图像中不易查觉的输电线,如图9(b)中箭头所指。

      图  9  圆周SAR与条带SAR成像结果对比[23]

      Figure 9.  Comparison of imaging results of circular SAR and stripmap SAR[23]

      圆周SAR利于在等照射强度的条件下获得大的观测视角变化,不仅能够获得更高分辨率的SAR图像,还能够将不同观测视角所得图像进行融合,增强地物的可辨识性。例如,国防科技大学获得的圆周SAR多视角融合图像中[24],十字路口四角各类地物的可见性、清晰度和轮廓完整性,都明显优于普通条带SAR,如图10所示。

      图  10  圆周SAR试验情况[24]

      Figure 10.  Experiment of circular SAR[24]

      曲线SAR的目的,是要实现平台机动时的对地成像。复杂的机动飞行方式能够获取地物的多视角图像,从而增强雷达对地物的辨识能力,甚至可以使运动的雷达形成类似蝙蝠机动飞行时的环境三维实时感知能力,如图11所示。这是因为,从成像的物理学原理上看,只要在机动过程中能够产生足够的观测视角变化,就能够获得相应的分辨能力。当然,要在实际应用中做到这一点,还需要有强大的飞行控制能力、足够的测姿定位系统(Position and Orientation System, POS)设备精度、敏捷精准的波束指向控制能力、与飞行方式相适应的空间采样技术和高精度的实时处理能力作为支撑。

      图  11  复杂机动轨迹SAR的示意图

      Figure 11.  Schematic diagrams of complex maneuvering SAR

      多航过层析SAR属于面状孔径流形,如图8(e)所示。它利用多次飞行所得的复图像进行相干处理,能够形成高程方向的分辨能力,从而获得目标场景的立体图像。例如,德国DLR利用层析SAR原理,实现了建筑群的三维成像[25],如图12所示。其中的高程分辨采用了压缩感知处理方法,来解决层析向多航迹稀疏性对高程分辨的不利影响。

      图  12  多航过层析SAR

      Figure 12.  Multi-pass tomographic SAR

      不同的孔径流形可以引入更大和更多样的观测视角变化,从而获得更高维度的信息,或新的成像能力,可以用于解决不同应用场景的特殊问题。因此,对新的孔径流形的探索从未停止,未来很可能向复杂多轨迹交织的立体时变流形演进,以期获得更灵活和更强的对地成像能力,如图8(h)所示。

      (3) 信号通道从最初的单通道向多通道演化,通道构型由线状向交叉线状和曲面状拓展。

      广义的多通道雷达成像技术,也包含利用多路T/R组件来构建电扫天线的技术,例如,数字阵列SAR[26,27]和MIMO-SAR[28]等。这类技术可用来增强成像雷达天线的波束主副瓣赋形能力[29],或波束指向稳定和精准控制能力,从而使成像雷达具备多任务、多功能、低截获和抗干扰等能力。

      狭义的多通道雷达成像技术是指,能够利用多个发射或接收通道,来获取同一个地域的回波信号,从而获取更加丰富的地表散射信息。多通道的重要作用和价值在于,由于新增通道向着沿航迹、垂直切航迹和水平切航迹方向延伸,对地成像雷达由最初的单通道SAR分别演化出了合成孔径雷达-地面动目标指示(Synthetic Aperture Radar- Ground Moving Target Indication, SAR-GMTI)[30]、干涉SAR[15]和阵列SAR[31],从而获得了前所未有的成像动目标检测、起伏地表立体成像和地表及上方立体成像能力,如图13所示。此外,沿航迹多通道技术也