Figure 2.18. 三次扫描或CPI，每次扫描包含10个采样样本，样本功率为单位均值的Swerling 4模型Three scans or CPIs, each having10 samples of a unit mean Swerling 4 power sequence

在这两种情况下，绘制了具有单位均值Swerling雷达散射截面的单点散射体的接收功率，同时假设在雷达三次扫描或CPI中的每一次都获得10个采样样本。

In both cases, thereceived power from a single point scatterer having a unit mean Swerling RCS isplotted, and in both it is assumed that 10 samples are obtained on each ofthree scans or CPIs of the radar.

图2.17为采样的Swerling 1序列（指数PDF，完全相关）。

Figure 2.17 is asample Swerling 1 (exponential PDF, fully correlated) series.

作为对比，图2.18描述了Swerling 4的情况（四自由度chi-square PDF，完全去相关），其中每个单独的样本之间是相互独立的。

In contrast, Fig.2.18 illustrates a Swerling 4 case (fourth-degree chi-square PDF, fullydecorrelated) in which each individual sample is independent of the others.

2.2.8. 目标起伏对多普勒频谱的影响

2.2.8. Effect of Target Fluctuations on Doppler Spectrum

雷达信号处理中的一种常见操作是计算一个CPI在特定距离门数据的离散时间傅立叶变换（DTFT）。

A common operation inradar signal processing is computing the discrete-time Fourier transform (DTFT)of the data in a particular range bin for one CPI.

DTFT是一种对测量数据的相干积累，通常在足够短的CPI上，目标回波的RCS和幅度不会显著地去相关。

The DTFT is acoherent combination of measurements, usually over a sufficiently short CPIthat the target echo RCS and thus amplitude do not decorrelate significantly.

如第4章所示，恒定速度目标在一个CPI内的一系列样本将形成一条离散时间的正弦曲线。

As will be seen inChap. 4, the series of samples within a CPI for a constant-velocity target willform a discrete-time sinusoid.

因此，目标DTFT的通用模型是一个混叠sinc函数（也称为asinc、dsinc（数字sinc）或Dirichlet函数），其主瓣能量集中在某个适当的频率上，副瓣峰值低于主瓣峰值13.2dB，并在远离主瓣的频率区域范围内不断衰减。

Thus, the usual modelfor the DTFT of a target is an aliased sinc function [also called an asinc,dsinc (digital sinc), or Dirichlet function] with its mainlobe centered at theappropriate frequency and with sidelobes that peak 13.2 dB below the mainlobepeak and decay at frequencies further from the mainlobe.

如果在CPI内存在明显的RCS起伏，目标数据的幅度和相位将在CPI中发生变化，因此DTFT的输入不再是具有恒定复振幅的离散正弦波。

In cases where thereare significant RCS fluctuations within the CPI, the amplitude and phase of thetarget data will vary within the CPI, so that the input to the DTFT is nolonger a discrete sinusoid with a constant complex amplitude.

——本文译自Mark A. Richards所著的《Fundamentals of Radar Signal Processing（Second edition）》

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