图2.19描述了这种情况对DTFT的影响。
Figure 2.19illustrates the resulting effect on the DTFT.
Figure 2.19. CPI内目标起伏对多普勒频谱的影响Effect of intra-CPI target fluctuations on Doppler spectrum.
图中的灰色谱线是一条非加窗的零频率正弦波，模拟了从静止的10×5米多散射体构成目标以恒定视线角度反射的20个脉冲回波。
The gray spectrum is thatof an unwindowed zero-frequency sinusoid, modeling the returns from 20 pulsesechoed from a stationary 10 × 5 msimulated many-scatterer target viewed at a constant aspect angle.
黑线是在使用相同目标和波形观察到的数据的频谱，但每个脉冲的视线角变化为0.7mrad。
The black line is thespectrum of data observed using the same target and waveform, but with theaspect angle changing 0.7 mrad per pulse.
20个脉冲的总角度变化为13.3 mrad，几乎是去相关间隔1.5 mrad的9倍。
The total anglechange over 20 pulses is then 13.3 mrad, nearly nine times the decorrelationinterval of 1.5 mrad.
因此，目标回波的幅度和相位都会显著起伏，从而使得旁瓣电平提高，目标能量在更大的频率范围内展宽。
The target echoamplitude and phase will then both fluctuate significantly, raising thesidelobes and smearing the target energy over a wider frequency range,effectively whitening the spectrum significantly.
2.3. 杂波
2.3. Clutter
在雷达中，杂波是指由体或面散射体反射回波而接收到的一部分信号。
In radar the termclutter refers to a component of the received signal due to echoes from volumeor surface scatterers.
这种散射体包括地球表面，如陆地和海洋；气象回波（例如雨云）；以及人为制造的杂波，例如所谓的空中散射体：箔条云，通常由轻便的条带反射材料制成。
Such scatterersinclude the earth’s surface, both terrain and sea; weather echoes (for example,rain clouds); and man-made distributed clutter, such as so-called chaff cloudsof airborne scatterers, typically made out of lightweight strips of reflectingmaterial.
有时候杂波回波是干扰，而有时候是所需要的信号。
Clutter echoes aresometimes interference and sometimes the desired signal.
例如，合成孔径成像雷达是用来对地表进行成像的，因此地形杂波是合成孔径雷达中的有用信号。
For instance,synthetic aperture imaging radars are designed to image the earth surface, thusthe terrain clutter is the target in a SAR.
对于试图探测地面运动车辆的机载或星载监视雷达，来自周围地形的杂波回波则是一种干扰信号。
For an airborne orspaceborne surveillance radar trying to detect moving vehicles on the ground,clutter echo from the surrounding terrain is an interference signal.
从信号处理的角度来看，主要关注的是如何对杂波回波进行建模。
From a signalprocessing point of view, the major concern is how to model clutter echoes.
与人造目标一样，杂波是一个复杂的目标，每个分辨率单元内都有许多散射体，因此杂波回波对雷达参数和相应的几何结构高度敏感。
As with man-madetargets, clutter is a complex target with many scatterers per resolution cellso that the echoes are highly sensitive to radar parameters and encountergeometry.
与复杂目标一样，杂波被建模为一个随机过程。
Like complex targets,clutter is therefore modeled as a random process.
除了时间相关性外，杂波还可以表现出空间相关性：来自相邻分辨率单元的反射样本可能是相关的。
In addition totemporal correlation, clutter can also exhibit spatial correlation: thereflectivity samples from adjacent resolution cells may be correlated.
——本文译自Mark A. Richards所著的《Fundamentals of Radar Signal Processing（Second edition）》
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