毫不奇怪，由于可见光成像与人类视觉系统相同，所以照片对于人类来说更容易解释和分析。
Not surprisingly, the photograph is easierfor a human to interpret and analyze, since the imaging wavelengths (visiblelight) and phenomenology are the same as the human visual system.
相比之下，虽然雷达成像的效果也很明显，但只有单色调，提供的细节信息偏少，并显示出“斑点”纹理、一些似乎不自然的对比反转，同时存在一些缺失的特征，如跑道条纹等。
In contrast, the radar image, whileremarkable, is monochromatic, offers less detail, and exhibits a"speckled" texture, some seemingly unnatural contrast reversals, andsome missing features such as the runway stripes.
既然雷达成像具有这些缺点，为什么还能受到广泛关注呢？
Given these drawbacks, why is radar imagingof interest?
虽然雷达不能获得摄像系统的高分辨率或图像质量，但它具有两个强大的优点。
While radars do not obtain the resolutionor image quality of photographic systems, they have two powerful advantages.
首先，由于电磁波波长的优越传播特性，可以透过云层和恶劣天气对场景进行成像。
First, they can image a scene throughclouds and inclement weather due to the superior propagation of RF wavelengths.
其次，由于雷达不依赖太阳进行照明，因此可以在每天24小时都能够很好地成像；它们通过发射电磁脉冲提供自己独特的“光源”照射。
Second, they can image equally well 24hours a day since they do not rely on the sun for illumination; they providetheir own “light” via the transmitted pulse.
如果在漆黑的雨夜重复图1.22的示例，左边的SAR图像不会受到任何明显的影响，但是右边的光学图像将完全消失。
If the example of Fig. 1.22（译注：原文误为Fig. 1.21） were repeated in the middle of arainy night, the SAR image on the left would not be affected in any noticeableway, but the optical image on the right would disappear entirely.
为了获得高分辨率图像，雷达使用高带宽波形的组合来获得距离维度上的良好分辨率，并使用合成孔径雷达技术来获得横向距离维度上的良好分辨率。
To obtain fine-resolution imagery, radarsuse a combination of high-bandwidth waveforms to obtain good resolution in therange dimension and the synthetic aperture radar technique to obtain goodresolution in the cross-range dimension.
通过使用脉冲压缩波形（通常为线性调频）在保持足够信号能量的同时，获得期望的距离分辨率。
The desired range resolution is obtainedwhile maintaining adequate signal energy by using pulse compression waveforms,usually linear FM.
根据式（1.35），在足够大的带宽β上扫频，并使用匹配滤波器处理长脉冲可以提供非常好的距离分辨率。
A long pulse that is swept over a largeenough bandwidth β and processed using a matched filter can provide very goodrange resolution according to Eq. (1.35).
例如，用扫频150MHz的波形可以获得1m的距离分辨率。
For example, range resolution of 1 m can be obtained with a waveform sweptover 150 MHz.
根据具体应用，现代成像雷达的距离分辨率通常为30米或更高；许多系统具有10米或更高的分辨率，而一些高级系统的分辨率低于1米。
Depending on their applications, modernimaging radars usually have range resolution of 30 m or better; many systems have 10 m or better resolution, and some advancedsystems have resolution under 1 m.
对于传统的非成像雷达，称为实孔径雷达，横向距离分辨率由感兴趣范围内的天线波束宽度决定，即，Rθ3，如式(1.26)所示。
For a conventional nonimaging radar,referred to as a real aperture radar, the resolution in cross-range isdetermined by the width of the antenna beam at the range of interest and isgiven by Rθ3 as shown in Eq. (1.26).
窄波束天线的实际天线波束宽度通常为1°至3°，或大约17至52mrad。
Realistic antenna beamwidths fornarrow-beam antennas are typically 1° to 3°, or about 17 to 52 mrad.
即使在10公里的相对短的成像范围中，实孔径得到的横向距离分辨率将是170至520米，比典型的距离分辨率差得多，这样的横向距离分辨率太差而不能产生有用的图像。
Even at a relatively short imaging range of10 km, the cross-rangeresolution that results would be 170 to 520 m, much worse than typical range resolutions andtoo coarse to produce useful imagery.
通过使用SAR技术可以提高横向距离分辨率。
This poor cross-range resolution isovercome by using SAR techniques.
合成孔径技术是指通过使实际雷达天线相对于成像区域运动来合成非常大孔径的虚拟天线。
The synthetic aperture technique refers tothe concept of synthesizing the effect of a very large antenna by having theactual physical radar antenna move in relation to the area being imaged.
因此，SAR通常与运动的机载或天基雷达相关联，而不是与固定的地面雷达相关联。
Thus, SAR is most commonly associated withmoving airborne or space-based radars, rather than with fixed ground-basedradars.
图1.23描述了这个概念。
Figure 1.23 illustrates the concept.
——本文译自Mark A. Richards所著的《Fundamentals of Radar Signal Processing（Second edition）》
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