2.5 干扰
2.5. Jamming
干扰是指敌方发射机对雷达系统的有意干扰。
Jamming refers tointentional interference directed at the radar system from a hostile emitter.
干扰是电子对抗（ECM）或电子攻击（EA）中的一种。
Jamming is an exampleof electronic countermeasures (ECM) or electronic attack (EA).
如前所述，大多数雷达信号处理的目的是提高接收数据的SIR，从而提高检测、跟踪和成像性能。
As noted earlier, thepurpose of most radar signal processing is to improve the SIR of the data so asto improve detection, tracking, and imaging performance.
许多干扰技术的目的恰恰相反：为了降低SIR，使雷达性能下降。
The purpose of manyjamming techniques is just the opposite: to reduce the SIR so that the radarperformance is degraded.
最基本的干扰形式是简单的噪声干扰。
The most basic formof jamming is simple noise jamming.
敌方发射机将放大的噪声波形对准受干扰的雷达进行辐射，实质上提高了接收机的噪声水平。
A hostile emitterdirects an amplified noise waveform at the victim radar, essentially increasingthe noise level out of the receiver.
如果噪声功率谱填满整个雷达接收机带宽，那么接收机输出端的噪声将如同其它白噪声过程一样出现，并可以用相同的方式建模。
If the noise powerspectrum fills the entire radar receiver bandwidth, then the noise out of thereceiver will appear like any other white noise process and is modeled in thesame way.
更先进的噪声干扰形式还使用各种振幅和频率调制。
More advanced formsof noise jamming use various amplitude and frequency modulations.
其它的干扰技术可能不采用噪声，而是使用模拟目标回波的波形，欺骗雷达探测并跟踪不存在的目标。
Instead of noise,other jamming techniques use waveforms designed to mimic target echoes and foolthe radar into detecting and tracking nonexistent targets.
即使是对ECM的有限讨论也超出了本书的范围，因为该主题的内容和公开文献中可出版的资料数量有限。
Even a limiteddiscussion of ECM is outside the scope of this text, due both to the breadth ofthe topic and the limited amount of material publishable in the openliterature.
读者可参考Lothes等人（1990）关于雷达干扰信号的论述。
The reader isreferred to Lothes et al. (1990) for a good general reference on jammingsignals in radar.
2.6 频率模型：多普勒频移
2.6. FrequencyModels: The Doppler Shift
2.6.1多普勒频移
2.6.1. Doppler Shift
如果雷达和散射体彼此都不处于静止状态，由于多普勒效应，雷达接收到的回波频率Fr将与发射频率Ft不同。
If the radar andscatterer are not at rest with respect to each other, the frequency Frof the received echo will differ from the transmitted frequency Ftdue to the Doppler effect.
多普勒频移可用于在较强杂波情况下检测运动目标的回波，或显著提高横向距离分辨率。
Doppler shifts can beused to advantage to detect echoes from moving targets in the presence of muchstronger echoes from clutter or to drastically improve cross-range resolution.
未补偿的多普勒频移也会产生有害影响，特别是某些类型的波形会导致灵敏度损失。
Uncompensated Dopplershifts can also have harmful effects, particularly a loss of sensitivity forsome types of waveforms.
因此，多普勒频移的表征和测量是雷达研究的一个重要课题。
Thus,characterization and measurement of Doppler shifts is an important topic inradar.
该波形被目标反射的时变距离表示为R(t)。
The waveform isreflected from a perfectly conducting target at an arbitrarily time-varyingrange R(t).
例如，恒定距离目标的R(t)为固定不变的R米，而恒定速度目标的距离为R(t) = R – vt米。
For instance, aconstant-range target would have R(t) equal to a fixed R meters, while aconstant-velocity target would have R(t) = R – vt meters.
——本文译自Mark A. Richards所著的《Fundamentals of Radar Signal Processing（Second edition）》
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