第2.2.5节所有的统计模型都适用于从单个分辨率单元观察到的散射。
All of thestatistical models described in Sec. 2.2.5 applyto the scattering observed from a single resolution cell.
也就是说，这些模型表示通过多次测量同一物理空间区域观察到的RCS变化，例如在同一方向发射多个脉冲，并在每次发射后的相同延迟处测量接收功率。
That is, theyrepresent the variations in RCS observed by measuring the same region ofphysical space multiple times, for example by transmitting multiple pulses inthe same direction and measuring the received power at the same delay aftereach transmission.
式（2.74）模型的另一个用途是描述杂波反射率的空间变化。
Another use of theproduct model of Eq. (2.74) is to describe the spatial variation of clutterreflectivity.
如果雷达观察到的场景是非均匀的，那么在一个分辨率单元中观察到的RCS特性可能与另一个分辨率单元的特性有很大差异。
If the scene beingviewed by the radar is nonhomogenous, then the characteristics of the RCSobserved in one resolution cell might vary significantly from those of another.
例如，扫描雷达在海岸位置观测到的主要杂波可能是某个方向的城市区域，或者是另一个方向的海洋环境。
For example, thedominant clutter observed by a scanning radar at a coastal site might be anurban area in one look direction and the sea in another.
另一个例子是，当散射的雨滴仅占据扫描区域的一部分时，一些分辨率单元存在雨滴的散射回波，而其它分辨率单元却不存在。
Another exampleoccurs when scattered rain cells occupy only part of the scanned region, sothat some resolution cells contain rain while others are clear.
这种情况可以通过让模型中的缓慢去相关项x表示接收电压局部平均值的空间变化来建模。
This situation can bemodeled by letting the slowly decorrelating term x in the product modelrepresent spatial variations in the local mean of the received voltage.
如果x的PDF为对数正态分布，方差较大，且受x约束的ζ的PDF为γ分布（其中包括的瑞利分布是一种特殊情况），则乘积ζx的总体PDF为对数正态分布。
If the PDF of x islog-normal with a large variance and the PDF of ζ conditioned on x is gammadistributed (which includes Rayleigh as a special case), then the overall PDFof the product ζx has a log-normal distribution (Lewinski, 1983).
因此，以上模型表明，从一个分辨率单元到另一个分辨率单元的局部平均值的对数正态变化通常可以解释用于模拟地面杂波回波的对数正态变化。
Consequently, theproduct model implies that log-normal variations of the local mean from oneresolution cell to another could account for the log-normal variation oftenused to model ground clutter returns.
通过将随视线角变化的RCS建模为对数正态过程，类似的变量也可以用来证明目标RCS的对数正态模型是正确的。
A similar argumentcan be used to justify the log-normal model for target RCS by modeling thevariation of RCS with aspect angle as a log-normal process.
2.4. 噪声模型与信噪比
2.4. Noise Model andSignal-to-Noise Ratio
从目标或杂波接收的回波信号不可避免地与噪声竞争。
The echo signalreceived from a target or clutter inevitably competes with noise.
一般有两种噪声源：通过天线从外部接收的噪声源和雷达接收机本身产生的噪声源。
There are two sourcesof noise: that received through the antenna from external sources, and thatgenerated in the radar receiver itself.
外部噪声是雷达天线指向的强函数。
External noise is astrong function of the direction in which the radar antenna is pointed.
外部噪声的主要源头是太阳。
The primarycontributor is the sun.
如果天线指向夜空，并且没有微波源干扰，那么主要的外部噪声源就是星系（也叫宇宙）噪声。
If the antenna isdirected toward the night sky and there are no interfering microwave sources,the primary source is galactic (also called cosmic) noise.
内部噪声源包括由电阻损失引起的热噪声（也称为约翰逊噪声）、由电流的量子特性引起的散粒噪声和分配噪声，以及由导电和半导体器件中的表面泄漏效应引起的闪烁噪声。
Internal noise sourcesinclude thermal noise (also called Johnson noise) due to ohmic losses, shotnoise and partition noise due to the quantum nature of electric current, andflicker noise due to surface leakage effects in conducting and semiconductingdevices (Carlson, 1976).
在这些不同的噪声源中，热噪声通常占主导地位。
Of these varioussources, thermal noise is normally dominant.
统计和量子力学理论指出，电子电路中的热噪声电压是零均值的高斯随机过程。
The theories ofstatistical and quantum mechanics dictate that the thermal noise voltage in anelectronic circuit is a zero-mean Gaussian random process (Curlander andMcDonough, 1991).
——本文译自Mark A. Richards所著的《Fundamentals of Radar Signal Processing（Second edition）》
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