1.2. 雷达基本功能
1.2. BasicRadar Functions
大多数雷达的主要用途是探测、跟踪或成像。
Most uses of radar can be classified asdetection, tracking, or imaging.
本书将介绍以上这三个方面，以及实现这些任务所必需的信号采集和干扰抑制技术。
This text addresses all three, as well asthe techniques of signal acquisition and interference reduction necessary toperform these tasks.
雷达中最基本的问题是目标或物理现象的探测。
The most fundamental problem in radar isdetection of an object or physical phenomenon.
这需要确定雷达接收机在某时刻的输出信号中是否包含目标回波或仅仅只有噪声。
This requires determining whether thereceiver output at a given time represents the echo from a reflecting object oronly noise.
通过比较接收机输出幅度A(t)（其中t表示时间）与门限T(t)的大小，可以实现目标检测判决，其中门限T(t)可以在雷达设计时设定为先验知识，或者根据雷达接收数据进行自适应计算；本书第六章将会讲述为什么这种检测技术是适用的。
Detection decisions are usually made bycomparing the amplitude A(t) of the receiver output (where t represents time)to a threshold T(t), which may be set a priori in the radar design or may becomputed adaptively from the radar data; in Chap. 6 it will be seen why thisdetection technique is appropriate.
雷达发射脉冲经过传播距离R后遇到目标返回，因此发射脉冲的总运动距离为2R，所花费的总时间为2R/c；如果在某个时刻t0出现A(t) > T(t)，那么该时刻对应的距离上则认为有目标存在，即
The time required for a pulse to propagatea distance R and return, thus traveling a total distance 2R, is just 2R/c;thus, if A(t) > T(t) at some time delay t0 after a pulse istransmitted, it is assumed that a target is present at range
其中c表示光速。
一旦检测到目标，我们可能希望跟踪该目标的位置或速度。
Once an object has been detected, it may bedesirable to track its location or velocity.
如图1.1所示，单基地雷达在球坐标系中测量目标位置，其原点为雷达天线的相位中心。（注意：在阅读一本书之前请认真理解该书的坐标系定义原则！！！）
A monostatic radar naturally measuresposition in a spherical coordinate system with its origin at the radarantenna’s phase center, as shown in Fig. 1.1.
图1.1 用于雷达测量的球面坐标系Spherical coordinatesystem for radar measurements
在这个球面坐标系中，天线观测方向有时也称为视轴方向，定义为沿+x轴方向。
In this coordinate system, the antenna lookdirection, sometimes called the boresight direction, is along the +x axis.
图中的θ角称为方位角，ϕ角称为俯仰角。
The angle θ is called azimuth angle, while ϕis called elevation angle.
距离R的定义如式(1.1)所示。
Range R to the object follows directly fromthe elapsed time from transmission to detection as just described.
俯仰角ϕ和方位角θ由天线方向决定，因为目标通常必须位于天线主波束的检测中。
Elevation and azimuth angle ϕ and θ aredetermined from the antenna orientation, since the target must normally be inthe antenna main beam to be detected.
雷达通过测量目标回波的多普勒频移来估计速度。
Velocity is estimated by measuring theDoppler shift of the target echoes.
多普勒频移仅提供径向速度分量，根据一系列位置和径向速度的测量可推断出目标的三维动力学特性。
Doppler shift provides only the radialvelocity component, but a series of measurements of position and radialvelocity can be used to infer target dynamics in all three dimensions.
大多数人都熟悉雷达屏幕上目标的“闪烁”运动，所以检测和跟踪是最常见的与雷达相关的功能。
Because most people are familiar with theidea of following the movement of a “blip” on the radar screen,detection and tracking are the functions most commonly associated with radar.
然而，越来越多的雷达被用于产生空间上的二维图像。
Increasingly, however, radars are beingused to generate two-dimensional images of an area.
这些图像信息可以被分析用于情报和监视目的、用于地理测绘，或者用于分析诸如测绘、土地利用、冰层覆盖、毁林监测等地球资源问题。
Such images can be analyzed forintelligence and surveillance purposes, for topology mapping, or for analysisof earth resources issues such as mapping, land use, ice cover analysis,deforestation monitoring, and so forth.
——本文译自Mark A. Richards所著的《Fundamentals of Radar Signal Processing（Second edition）》
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