经典雷达资料-第12章 地 物 回 波-5

海面冰层
冰层是非常复杂的介质。

观察者用许多不同的类别来描述冰层的特征。

而这些类别由冰层厚度、年代和形成过程决定[109]。

因此，人们不能用任何一种简单的方式来描述冰层的雷达回波，在这种意义上它和植被的散射类似。

从雷达观测的角度看，最重要的冰层类型是首年冰层（FY，1～2m厚），多年冰层（MY，大于2m厚），混合冰层（小于1m厚）。

与雪地相似，阳光融化和结冰温度之上的冰层所散射的微波，与正常结冰表层的散射相差很大。

在冬天，MY冰层的散射远大于FY冰层。

在夏天，MY冰层的σ0衰减到约等于FY 冰层的σ0。

图12.43[110]示出这种变化和典型的角度响应。

这些结果是在13.3GHz频率下测量得到的，但低至S波段频率，它们的结果也与此相似。

图12.44[91]示出不同冰层的σ0随频率的变化。

在海岸线上，与海岸紧紧相连的冰层位于海底之上，在这种情况下冰层可能是MY 类冰层。

灰色冰层是一种厚度小于FY类的冰层。

图12.43 当频率为13.9GHz时，海面冰层在夏季和冬季散射的比较（引自Gray等人[110]）
第12章地物回波
·460·
图12.44 不同类型海面冰层σ0的频率响应举例（引自Kim[53]）
Kim[53]提出一种理论来解释海面冰层σ0的大范围变化。

根据该理论和大量关于冰层特性的文献资料，图12.45[91]示出冬天FY和MY类冰层散射的变化范围。

很明显，较高频率在识别冰层类型上优于较低频率，并且当频率约低于5GHz时不能识别冰层的类型。

在L波段和更低的频率时，MY和FY类冰层的散射即使在冬天也相差无几。

这也意味着，较高频率的成像雷达在冬天（而非夏天）仅通过回波的强度就能轻易地区分冰层的类型。

这是前苏联和加拿大冰层监视系统的基础，其中前苏联采用Toros Ku波段侧视机载雷达（SLAR）[111]，加拿大采用改进型的X波段APS—94侧视机载雷达，最近加拿大采用STAR—1 X波段合成孔径雷达。

第12章地物回波·461·
图12.45 基于实测的首年冰层和多年冰层σ0的理论变化
变化范围由冰层特性的已知变化确定。

（引自Kim等人[91]）
大雪能掩盖冰层的散射，这和在地面上一样。

由于北极相对干燥，大部分冰层上有少量的雪，但雪有时会影响雷达对冰层的区分。

由于北极及其气象对地球非常重要，因而人们进行了大量关于海面冰层微波特性的实验。

在北极由于冬天夜晚很长，频繁的云层和不易接近性，因而微波遥感在监测北极冰层特性方面是必不可少的。

12.8 成像雷达判读
具有真实孔径或合成孔径的侧视高分辨力成像雷达能产生类似于航空照片的图像。

地面不同位置上的阴影和σ0性质的差异产生图像灰度变化，这与照片灰度变化很相似。

基于此，照片判读员（Photointerpreters）能容易地学会判断雷达图像。

但是，由于雷达图像是微波反射率的一种反映，而不是光学反射率，因而判读员必须知晓它们的差距，并且知道不同波长的雷达图像实际上是互补的。

此外，雷达图像的几何失真是指侧视雷达测距系统的失真，而航空照片则是俯视角测量系统的失真，这一点也是判读员必须知道的。

对雷达来说，在低入射余角时这种失真很小；但在低入射角时却很大。

而且，照片上不存在雷达图像中的斑点（Speckle）。

现代成像雷达采用数字记录技术，用胶片生成图像或直接用数字设备生成数字图像。

由于侧视雷达的特殊结构，它生成的是带状图像，因而输出胶片也是长条形的。

大多数照相机产生的照片是近似正方形的分离照片。

带状照片照相机和光学红外线扫描仪生成的带状照片
第12章 地 物 回 波
·462· 与雷达图像类似，但由于它们是测角装置而不是测距装置，因而具有不同的失真。

所有关于空中摄影的理论学科和应用学科，也可以适用于雷达图像。

这在多云的环境中特别有用，但因为雷达的性能与一天内的时间无关，因而在晴朗天空中，雷达图像也很有用。

此外，地面的雷达图像与可见光和红外线照片不同。

雷达已经应用于农业、林业、地质、水文、城市地理、区域性研究、海洋学及冰层测绘等。

衰落所产生的雷达图像斑点使图像的判读变得复杂。

这就意味着，有斑点的图像通常都须经平均处理。

平均处理有时由处理器完成，有时由判读员靠智力完成，在解读雷达图像时这一步是必需的。

单视合成孔径雷达图像亮度服从瑞利分布。

大多数合成孔径雷达处理器由于采用了平均处理，比如四像素综合，而牺牲了某些空间的分辨力。

发射比距离分辨力所需带宽更宽的脉冲，可在没有空间分辨力损失下达到要求的距离分辨力[112]，但是这将需要更大的发射功率。

适当的频率捷变可达到相同的效果。

对于空间分辨力和测量精度间的矛盾，人们通常采取折中的方案。

测量精度可用于定义灰度级分辨力（Gray-level resolution ）[113]。

然后，人们可用容量V 来考虑图像分辨力的问题。

g y a r r r V =
式中，r a 是航迹方向的分辨力；r y 是地面距离分辨力；r g 是灰度级分辨力。

相关的研究表明，图像的可解释性取决于V 的大小，并且在V 的三个元素间折中是可能的。

对判读员来说，当三个衰落的独立样本被平均后，图像得出的结果最佳。

若忽视了这种衰落（斑点），则在具体的应用中对空间分辨力会得出错误的结论。

单频、单极化雷达图像是很有用的。

但是使用多极化（特别是正交极化）和多频率可增强图像的价值。

不同的入射角适用于不同用途。

例如，土壤湿度的监测最好是采用5 GHz 左右的频率，并且入射余角在垂直方向左右20︒范围内。

但是，对植物的判读，在更高的频率和更大的入射角时效果更佳。

地物回波方面的文献众多，如果读者想了解更多内容，可参阅《遥感手册》[23]、《微波遥感》[21]，特别是其第三卷、第二卷第11章，以及本章12.7节列出的期刊。

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