雷达 外文翻译 外文文献 英文文献 雷达系统的介绍

Introduction to Radar Systems

雷达系统的介绍

美什科尔尼克

起止页码：1—20页 出版日期：2001年

出版单位：麦格劳希尔公司数字工程图书馆

第一章 雷达的简介和概要

1.1雷达的简介

雷达是一种检测和定位的反射物体电磁传感器。它的操作可归纳如下: ●雷达从天线辐射电磁波传播到空间。

●有些是截获反射对象的辐射能量通常称为目标由雷达定位距离。 ●截获目标许多方面是辐射能量。

●一些辐射(回声)能量回到并接收到雷达天线。

●经过放大接收器并在适当的信号处理后，判定在接收器输出是否目标回波信号的存

在。此时目标位置和可能的其他有关信息都应被获取。

一个普通的波形由雷达辐射一系列相对狭窄波形，如矩形脉冲。一个为中程雷达探测飞机可能被视为一个的持续时间1秒短脉冲（1微秒）;脉冲之间的时间可能是100万毫秒（所以脉冲重复频率波形1千赫）从雷达发射机峰值功率可能有100万瓦（1兆瓦），以及与这些数据中发射机平均功率为1千瓦。一个1千瓦的平均功率可能低于通常在一个“典型的”教室中电力照明功率。我们假设这个例子雷达可工作在微波频率的中间范围，如从2.7至2.9 GHz，这是一个典型的民用机场监控雷达频带。它的波长可能是大约10厘米（为简单起见四舍五入）。这种用合适的天线雷达可探测飞机外或多或少50至60海里范围。回声功率从一个目标雷达接收到变化可以有较大的范围数值，但我们随便假设的“典型”作说明用途，回波信号可能有可能10−13瓦的功率。如果辐射功率为106瓦（1兆瓦），在这个例子中雷达发射功率从一个目标比例的回波信号功率的为10–19瓦，或接收回声是比传输信号更少190分贝。这是一个传递信号的幅度和检测接收到的回波信号之间特别的差异。

一些雷达的探测目标范围是后面本垒板的投手土墩到棒球场的短距离（测量一个抛球速度），而其他雷达的工作范围可能是最近的行星那么大的距离。因此雷达可能是小到足以保持在一个足够大的手或手掌，大到占足球空间领域。

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雷达目标可能是飞机，船舶，或导弹，但雷达目标也可以是人，鸟类，昆虫，降水，晴空湍流，电离媒体，地面特性（植被，山，道路，河流，机场，建筑物，围墙，电力电线杆），海，冰，冰山，浮标，地下特性，流星，极光，航天器和行星。除了测量范围目标以及它的角方向，雷达还可以通过确定的时间范围与测量的变化率确定一个目标相对速度，或从径向速度转移的回波信号提取多普勒频率。如果该位置运动目标是衡量一段时间内跟踪或目标轨迹，可以发现其中的目标和方向的运行相对速度，可确定和作出预测的将来位置。正确设计的雷达可以确定目标的大小和形状，甚至可以能够识别另一种对象或类型。

基本的雷达组成。图1.1是一个非常基本的框图上展示子系统中经常出现的雷达。这里表示的这个发射器是作为一种功率放大器,其产生一个合适的特定工作波形使雷达来完成。它的平均功率可能小到毫瓦的功率和大如兆瓦特。(平均功率是一个的比雷达的峰值功率更好的体现。)大多数的雷达使用短脉冲波形这样一个可以用在时间共享单一的天线为发送和接收的基础

双工器的功能是允许一个单一的天线被用在保护敏感接收器关掉而发射机在通过直接接到回波信号而不是发射机。

天线设备能够应用传输能量到空间,然后收集到接收回声能源。它几乎是一个导向天线引导辐射能量到一个集中窄的波束以及允许测定目标的方向。天线上传输指令产生的窄波束通常对接收允许弱目标回波信号大面积收集。天线不仅集中传输能量和而且在接收回声能量上,但它也是作为一个空间滤波器来提供角分辨率和其它功能。

本地振荡器 混频器 放大 滤波 检测波 形 音频放 大 显示 低噪声放大器 双工器 功率放 大器 波形产 生器 图1.1框图是一个简单的采用在图上部作发射器和一个在图中下部作超外差接收机功率放大器雷达。

放大接收器接收信号弱的水平可以被检测到它的存在。由于噪声对雷达作出操作的能力和可靠的检测获得有关信息最终目标根本有限，采取以保护方式是接收器产生很少自身噪音。大多数雷达发现在微波频率噪声影响雷达性能通常是从接收第一阶段表现为低噪声放大器如这里图1.1。对于许多雷达应用中的检测是不需要的，从

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空间中雷达回波（称为杂波）接收器需要有一个足够大的动态范围以避免出现回声探测的混乱产生不利影响使移动目标方向接收器饱和。一个接收器的动态范围通常以分贝表示被定义为比例最大最小功率电平输入信号。最高信号水平可能需要设置接收机的反应是可以允许的（例如非线性的影响，信号功率和接收开始饱和）开始饱和的最低的信号可能是最小的探测信号。如果接收器是IF部分，这个信号处理器通常是可能会被描述为是一部分隔开的干扰信号，组成部分可能降低检测过程所需信号。信号处理还包括多普勒处理及最大化的信号与运动目标，杂波大于杂比接收器的噪声。信号处理包括匹配滤波器的输出信噪比的比值最大化。,并把来自另一个移动目标和杂乱回波信号分离开。这个检测由接收器输出决定，那么接收器输出超过一个预定的阈值时一个目标是存在的。如果阈值设置太低噪声过量可引起虚惊一场。如果阈值的设定过高会被发现一些目标可能错过了,否则会被发现。所以这个标准水平的决定是设置阈值的数值，它产生一个可接受的设定的平均值的假警报因为接收器的噪音。 这个检测确定后一个跟踪目标位置的轨迹测量即可以确定。这是一个数据处理例子。目标探测信息或跟踪由操作显出来,或检测信息可能被用于自动引导导弹目标或雷达的输出可能进一步的处理提供其他有关的情况目标。雷达管制确保各部分的雷达在一个协调与合作的方式操作。例如提供定时信号不同部分的雷达的要求。 雷达工程师已经是资源的时间让良好的多普勒加工、带宽范围、空间好让一个大型天线对能量有长范围性能和精确测量。外部因素影响雷达性能包括目标特征;外部噪音可能进入通过天线不必要的杂乱回波信号。从陆地、海洋、鸟或其它的电磁干扰雨天,由于散热器和传播的影响及地球表面的氛围。这些因素都提到要强调的是他们在雷达施工中出现会更加重要的设计。

雷达发射器。该雷达发射机必须不仅能够产生所需的高峰期，发现在最大范围内的平均预期的目标的功率，而且要产生一个适当的波形和特定应用所需的稳定的信号。发射器和振荡器放大器但后者通常提供更多的优势。

已经有许多种类的雷达功率源应用于雷达(第10章)。磁控管的功率振荡器一度非常流行但很少除了民用航海雷达（第22章）使用。由于磁控管的相对较低的平均功耗（一个或两个千瓦）和稳定性差，其他功率型通常更适合需要长期应用在大型杂波回波存在移动目标小范围的检测。磁控管功率振荡器是一个是所谓的交叉领域管的例子。还有一个相关的交叉领域放大器（CFA）的已使用在过去一段雷达，但它也经历了重要雷达应用的局限性，特别是对需要在移动目标检测的杂波。高功率速调管和行波管（TWT）是所谓的线性束管的例子。在雷达经常采用的高功率，既有适当的宽带宽以及良好的稳定需要的多普勒处理，并且都大受欢迎。

固态放大器，如晶体管也被用于雷达尤其是在相控阵。虽然个别晶体管功耗相对较低，许多辐射的天线阵列的每个单元可以利用多个晶体管来实现高功率雷达许多应用的要求。当固态晶体管放大器的使用时雷达设计师能够满足高占空比长脉冲在这些

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设备有操作，他们必须使用需要脉冲压缩，以及不同宽度的多种脉冲使在短期以及远距离探测。因此利用固态发射器会影响系统中其他部位的雷达系统。在毫米波段可以得到很高的功率无论是作为一个放大器回旋或振荡器。超高频和频率较低的雷达在这个分开控制真空管用来发挥很长一段时间, ,但是很少有应用在雷达低频率。 虽然并不是每个人都可能同意, 如果给一个选择一些雷达系统工程师会考虑速调管作为“候选人”——如果应用现代雷达是适合它的使用。

雷达天线。雷达的天线是连接到外部世界传播(第12章及第13章),有几个以下用途：(1)集中发射能量；也就是说它是指导和有一条狭窄的宽度模型;(2)收集从目标接到的回声能量;(3)提供了一种测量目标的角度方向;(4)提供了空间分辨率(或解决目标的角度);(5)允许所需的空间。天线可以是一种机械扫描抛物线反射镜,一个机械扫描平面阵,或机械扫描终端天线。它可以是一个电子扫描相控阵使用单一发射机一起工作配置空间的功率分配给每个天线单元或电子扫描相控阵天线单元，采用一个小固态“微型”雷达（也称为有源孔径相控阵）。每个天线都有其独特的优势和局限型。一般来说天线越大越好，但不可能有实际的其规模。

1.2雷达类型

虽然没有单一的方式来描述了雷达,在这里我们这么做的可能是最主要的特征相区别的某型雷达。

脉冲雷达。这是一个辐射几乎矩形脉冲重复系列的雷达。定义一个雷达没有人说

这样定义它可能被称为一个规范的形式雷达。

高分辨率雷达。这种雷达可获得高分辨率的范围、角、或多普勒速度坐标,但通

常意味着高分辨率雷达具有高距离分辨率。一些高分辨率雷达有范围的分数但这一公尺的可大可小几厘米。

脉冲压缩雷达。这是一个使用长脉冲内调制(通常频率或相位调制)获得的能量脉

冲的很长一段时间短脉冲的判定雷达。

连续波(CW型)的雷达。这个雷达使用了一个连续的正弦波。它几乎总是用于移

动目标检测或测量目标相对速度多普勒频移。

调频连续波雷达。这种雷达采用的频率连续的波形调制允许范围内的测量。 监视雷达。虽然字典定义监视可能不这样，监视雷达是一种能够检测出存在一个目标（如飞机或船），并确定其范围和角度位置。它也可以观察一段时间，以便为目标获得其轨道。

移动目标显示雷达（MTI）。这雷达是一个脉冲杂波中检测移动使用低脉冲重复频率（PRF），通常没有距离模糊的目标。它在多普勒域含糊而导致所谓的盲区的速度。

脉冲多普勒雷达。这种雷达有两种脉冲多普勒雷达可以采用类型高或中等脉冲重

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复频率脉冲雷达。它们都使用多普勒频移来提取运动目标的。高重复频率脉冲多普勒雷达没有含糊之处（盲区速度），但它确实有距离模糊。一个PRF脉冲多普勒雷达中无论在范围和多普勒含糊之处。

跟踪雷达。这是一个提供了跟踪或轨迹的目标雷达。跟踪雷达可以进一步划分为STT，ADT，TWS和相控阵，如下所述：

单目标跟踪（STT）雷达。此雷达轨迹上的数据速率提供足够高机动目标的精确跟踪一个目标。在抽取测时间为0.1秒（每秒10测量数据速率）可能是“典型的”这可能使用在单脉冲跟踪的角度准确地追踪信息的协调方法。

自动检测和跟踪（ADT）雷达。这是跟踪监视雷达进行。它可以通过使用目标位置的测量对天线的多种扫描获得的跟踪目标有很大的数目。它的数据率不如STT高。具体取决于应用子扫描时间可能范围从1到12秒。

跟踪边扫描（TWS）雷达。通常的雷达可以通过角度的狭窄区域的一个或两个方面的监测以便能提供更快速更新在有限的地区的观察角度所有的目标位置信息雷达。它被用于过去地面的雷达，指导飞机降落后，在某些类型的武器控制雷达，以及一些军事机载雷达。

相控阵跟踪雷达。一种电子扫描相控阵（几乎）可以“继续”跟踪数据率大多个目标。它可以同时提供更低的数据率由ADT类似的多个目标进行跟踪。 成像雷达。该雷达产生例如对地球表面的一部分，一两一个目标或一个场景，立体的形象。通常它是这些雷达的移动平台。

侧面机载雷达(SLAR)。该航空侧面成像雷达高解析度的范围和获得合适的角度分辨率采用细波束天线。

合成孔径雷达（SAR）。特区是在移动的车辆上使用的回波信号的相位信息相干成像雷达获取的是一个带有两个范围和横向距离现场的高分辨率的图像。远距离分辨率通常采用脉冲压缩获得。

逆合成孔径雷达（ISAR）。ISAR是一个连贯的成像雷达，它使用跨尺度范围和目标相对运动在多普勒域内允许有高分辨率获得。它可以在移动的车辆也可以是静止的。 武器控制雷达。该名称通常应用于单一目标跟踪对空袭的防御使用。 制导雷达。这通常是在导弹对目标的雷达使导弹的“自身”或导航本身。

天气（气象）观测。这种雷达探测，识别，测量降水率，风速和风向，以及观测其他天气情况。相干意味雷达信号的相位是作为雷达过程中重要的一部分。重要气象用途。这些可能是特殊的雷达或其他监视雷达的功能。

多普勒天气雷达。这是一个气象观测雷达，采用了多普勒频移通过天气移动影响，以确定大风造成的风向切变（当在不同的方向吹风），这可以表明如龙卷风或下击暴流危险天气情况风，以及其他气象等因素影响。

目标识别。在某些情况下可能必须认识到的目标类型被雷达观测（例如，是汽车而不

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是鸟），确认的特定类型的对象（是汽车而非一辆卡车，或是椋鸟而不是晶石），或从另一个认识（邮轮而不是一个目标油轮）。用于军事目的它通常被称为非共同目标识别（NCTR）雷达，相比识别系统的合作，如IFF（敌我识别）这不是一个雷达。当目标识别涉及一些自然环境的一部分，该雷达通常被称为远程（环境）雷达。 多功能雷达。如果上述雷达每个人看作是提供一些雷达的功能，那么多功能雷达的设计执行多个这样的功能之一，通常表现在时间上的一次功能共享的基础。

还有许多其他的方法来描述雷达其应用，包括土地，海洋，航空，星载，移动，可运输，空中交通管制，军事，穿透地面，超宽带远视距，仪器仪表，激光（或雷达），由频带在其经营（UHF频段，S频段等）等等。

1.3雷达的可用信息

目标检测除非有关于目标的信息以及获得价值不大。同样没有目标的探测的信息是没有意义的。

范围。与传统雷达最显著的区别特点是它能够通过测量信号传播所需的到目标的速度和返回雷达时间以确定目标范围。没有其他传感器可以测量的准确度在与远程雷达的距离远程目标相比（基本上是由对信息的传播速度远的距离精度有限）。在适度范围内精度可以是几厘米。为了测量范围部分时间标志必要介绍了传输波形。时间标记可以是一个短脉冲（1信号的幅度调制），但它也可以是独特的频率或相位调制。一个范围的测量精度取决于雷达信号带宽：更广泛的带宽，更大的准确性。因此带宽范围是准确的基本措施。

径向速度。一个目标径向速度是从一系列的变化率在一段时期。它也可以采取的多普勒频移测量。径向速度的精确测量需要时间。因此时间是基本参数描述径向速度测量的质量。

角度方向。一个目标的方向确定由角方向运行速度可判定，从它的轨道可以从在一段时期目标位置的雷达测量发现。一个确定的方向目标的方法是通过确定角度那里的回波信号从一个扫描天线的幅度是最大的。这通常需要一个具有细波束（高增益天线）天线。一种空中监视与旋转天线波束雷达决定以这种方式角度在一个目标角度也可以决定使用两个天线波束轻微地在角度上取代，比回波幅度每束使用。四波束需要获得两个方位角和仰角测量。在单脉冲跟踪雷达的第9章讨论的是一个很好的例子。角度的测量精度取决于电子天线的尺寸，即在给定波长的天线的大小。

大小和形状。如果雷达在范围或角度足够高的分辨率的能力，它可以提供在高分辨率的尺寸测量目标的程度。范围通常是在协调得到解决。在其交叉范围（考虑由天线波束宽度乘以范围），可以用很窄波束天线获得的。但是，一个天线波束角宽度是有限的，所以横向距离分辨率用这种方法获得的并不如距离分辨率好。很好的解决交叉范围尺寸可以通过采用频域,基于SAR(合成孔径雷达)或测量(逆合成孔径雷达系

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统),在第17章。应该有相对运动的目标和雷达为了获得交叉范围分辨率SAR和测量。在具有足够的分辨率和交叉范围的大小,不仅可以获得两个正交坐标系中,但目标形状有时可以分辨。

雷达带宽的重要性。带宽基本上代表了信息，因此这是非常重要的许多雷达应用。有两种带宽雷达用到的类型。一个是信号带宽这是由信号脉冲宽度确定的带宽或任何内部的信号调制。另一种是可调带宽。一般来说一个简单的脉冲信号带宽是正弦波的时间1/τ。(脉冲压缩波形,在第8章中讨论,就会有更大的带宽的相关关系,他们比脉冲宽度。) 大带宽是需要解决的一系列指标，以便准确测量范围为目标识别从目标类型及其他类型提供了一种有限的能力。远距离分辨率也可以是有用的减少就是所谓的跟踪测量雷达闪出的关于时间延迟（雷达范围之间的双向信号）的差异为基础的飞机高度,基于时延(范围内)之间的双向直射信号从雷达目标和双向表面散射信号从雷达目标表面(也称为多测),并在增加目标信号与杂波比率。在军事系统、远距离分辨率可用于计算的测量以及密集队形飞行的航空器来认识和对抗某些类型是不可靠方式。 可调式带宽的提供能力,以改变(频率)雷达信号的频率范围很宽的频谱。这可以被用来减少相互间的干扰雷达,在相同的频带运作,以及在试图让敌对的电子对抗有效。操作频率越高越容易获得广泛的信号和广泛的可调的带宽。

关于提供的带宽在雷达是由控制的频谱管理机构（在美国，联邦通信委员会，和国际上，国际电讯联盟）。在第二次世界大战中的雷达成功使用后，雷达被允许经营约三分之一的微波频谱的三分之一以上。这种频谱空间已经大大减少了同在“无线时代频谱出现很多商业用户”，要求的其他服务多年电磁波谱。因此雷达工程师面临越来越小的可用频谱空间和带宽分配，可用于许多雷达应用的成功至关重要。

信号信噪比。所有雷达测量的精确度以及目标的可靠的检测率E/No，其中E是接收到的信号总能量是由雷达处理。取决与No 每单位带宽接收器的噪声功率。因此E/No是雷达能力的重要指标。

加强与超过一个频率。可能有这样的雷达能够工作在超过一个频率重要的优势。频率敏捷通常是指在多个频率上使用脉冲对脉冲的基础。频率分集通常涉及多个频率在一个以上的雷达波段上有时使用相距甚远。频率多样性可能运行在每个频率同时或几乎同时。它被用于几乎所有的民用空中交通管制雷达。脉冲对脉冲频率变化，但是不符合使用要求的多普勒处理（探测移动目标的杂波），但频率多样性可以兼容。在两个灵活性和多样性行动比脉冲宽度τ固有的带宽有更大的频率范围

海拔零填充。一个雷达操作在一个单一频率可能导致波瓣结构的天线由于直接信号之间的干扰（雷达目标海拔模式）和表面散射信号（雷达由地球表面到目标）。由片状结构，我们的意思是将减少一些仰角（空值）和（表面）其他角度增加信号强度范围。其频率变化将改变仰角和表面的位置，这样通过在较宽的频率范围内工作，在海拔可以填写和雷达将不太可能失去一个目标回波信号。例如，以宽带为森得实验，

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已知的雷达测量结果可在850到1400兆赫，显示当只有一个频率使用，暂时现象扫描率（实验测得的单扫描探测概率）被发现要根据0.78特别的设置。当雷达在四个不同的操作远离频率，暂时现象扫描比率为0.98，由于频率集极显着增加。 增加的目标探测。一个复杂目标雷达散射截面如飞机部分可以有很大差异在频率的变化。在某些频率的雷达散射截面将是很小别的会很大。如果在一个单一的雷达工作频率错过了检测可能会导致小目标回波，因此通过在不同频率的检测截面会有所不同，可小或大，但一个成功的检测比如果只有一个频率的使用变得更加容易。这是一个原因几乎所有空中交通管制雷达操作与两个频率间隔足够宽的频率外以确保这一目标的回声相关，因此提高检测的可能性。

减少敌对对策的有效性。任何军事雷达要成功就一定可以预期的敌对对手采用对策以减少其有效性。如果操作只在一个频率比在很宽的频率范围困难得多。对噪声干扰，不断变化的不可预知的方式较广泛的频率干扰的频率会导致不得不分散在较宽的频率范围的功率，因此会减少敌对干扰雷达信号的带宽的信号强度。频率多样性在很宽的频带也变得更加困难（但不是不可能的拦截敌对接收器或反辐射导弹）来检测和定位雷达信号。

在雷达多普勒频移。多普勒频移变化的重要性认识到的雷达脉冲不久第二次世界大战后成为许多雷达的应用日益重要的因素。如果不存在现代雷达将少有兴趣或有多普勒效应。多普勒频移fd可以写为

fd =2vr /λ=（2vcosθ）/λ （1.1）

这里vr = vcosθ为目标的相对速度（相对于雷达）是米/秒，v是在米/秒目标相对速度，雷达波长λ是米，θ是角度目标之间的方向和雷达波束。为了约百分之三的准确性在赫兹多普勒频率约等于vr（kt）除以λ（米）。

多普勒频移被广泛用来区分固定杂波移动目标，如第2章通过5讨论。这种雷达被称为MTI（移动目标指示），AMTI（空降台MIT）和脉冲多普勒。所有现代空中运输控制雷达，所有重要的军事地面和机载空中监视雷达，和所有的军事空中战斗机雷达利用多普勒效应的优势。然而，在二战中使用，这些脉冲多普勒雷达的应用方面。在CW（连续波）雷达还采用了多普勒效应检测移动目标，因为它曾经是此连续波雷达的使用。在高频超视距雷达（第20章）不能完成它正从地球表面目标，大杂乱回波存在不使用的多普勒检测工作。

另一个重要的应用雷达，多普勒频移上的依赖是天气的观察，提及在本章前面在美国国家气象局的（第19章）新一代天气雷达，。

在特区专家组可以说在他们的多普勒频移（第17章）使用条件。机载多普勒导航雷达也是基于多普勒频移。在多普勒雷达中普遍地使用对雷达发射机的稳定性更高的要求，它增加了信号处理的复杂性，但这些要求愿意接受以实现多普勒提供的重大利益。还应提及的是多普勒频移，这是一个雷达可以测量速度的关键能力，如它被应

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用交警用于维持车辆的车速和其他速度测量。

1.4雷达方程

该雷达距离方程（或雷达方程简称）不仅符合估计作为一项特殊功能的雷达范围非常有用的目的，而且是非常有用作雷达系统设计的参考依据。该雷达方程的简单形式可以写成

在右边被写成三个因素的积，代表实际过程的发生。右边的第一个字母是在从一个雷达到从一个天线增益Gt到辐射功率Pt距离R的功率密度。分子中第二个因素σ是目标雷达散射截面。它具有单位面积（例如，平方米），是由在雷达的方向目标重回定向能源方式。在相关其返回路径，第二个字母归于前两个字母的积返回到雷达回波信号的分析。介绍了每单位功率面积返回雷达天线。请注意雷达目标散射截面σ是由这个方程定义。有效面积的一个Ae收集的回波部分功率Pr从接收天线返回雷达。如果最大的雷达范围，Rmax是发生在接收到的信号等于最小的雷达，Smin该雷达方程的简单形式检测信号的定义变得天线

一般而言，多数都用相同天线雷达发射和接收。从天线理论,有增益Gt之间的传输和对有效面积Ae的接收，这里G=4πAe/λ 2，其中λ是雷达信号波长。代入方程1.3提供了另外两个雷达方程有用的形式（这里没有显示）：一个只占其增益天线关系和其他只占其天线的有效面积。

该雷达方程的简单形式是有指导意义,因为它留下了很多东西但不是非常有用。最低检测信号，Smin受限于接收器的噪声，可以表示为

在这个表达式中，kToB是所谓的理想导体电阻，其中k =波尔兹曼常数，To标准温度热噪，B =接收带宽（通常如果超外差接收机范围）,kTo的积等于4×10−21瓦/赫兹。为了说明一个实际（非理想的）接收器，热噪声表达乘以噪声数字接收器的Fn，作为一个实际的接收机噪声出一个理想的接收器噪声进行界定。对于接收到的信号将检测，它也需要更大比接收器噪声（S / N）1。如果只需要一个脉冲存在这里信号与噪声比（S/ N）1就可以。条件用在这里它必须足够大以获得所需的不实探测（因噪音过接收阈值）和所需的检测阈值（可在各种雷达文章中找到）。但整体处理多个脉冲

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检测后雷达然后才能检测决定。我们假设雷达波形是一个重复的一系列矩形脉冲。这些脉冲被集成在一起检测（加在一起）来决定。为了说明这些外加的信号，该雷达方程的分子是乘以一个系数nEi（n），这里Ei（n）相加n脉冲是有效的。这个值也可以在标准文本找到。

功率Pt是一个雷达脉冲峰值功率。平均功率Pav是一个很好的探测雷达目标能力措施，所以有时到雷达使用Pt=Pav/fpτ，fp是其中的脉冲雷达重复频率方程以及τ是脉冲持续时间。地球的表面和地球大气可显着影响电磁波的传播和变化的范围和雷达能

4力。在雷达方程中，这些传播效应是由在雷达方程分子F 的因素，如在第26章讨论。

上述代入方程的雷达我们可以简单的形式

在上述公式，假定在其推导了Bτ≈1，这是普遍适用于雷达。因素Ls（大于1），称为系统的损失，若代入以考虑到损失可以发生在一个雷达许多方面。这一损失的因素可能相当大。如果系统的损失将被忽略，它可能导致的估计范围非常大的错误，在雷达方程预测。（损失从10分贝到20分贝，当所有的雷达系统损失的因素考虑在内可就是不一样了。）

方程1.5适用于雷达的观察目标到获得n个脉冲。更根本的说它适用于在一个雷达对目标的时间等于n /fp。例子是一个跟踪雷达连续观察一个目标的时间是to。然而这个等式需要修改的监视雷达的观察角度Ω与重新时间ts 。（空中交通可能有一个重新从第4至12时管制雷达），因此监视雷达的额外，它必须包括在某一时间ts角量Ω。重提时间ts保持是等于为（Ω/Ωo），其中to=n /fp和Ωo，该天线（立体角度）固体波束宽度，大约是有关天线增益G由G=4/Ωo。因此，n /fp在Eq.1.5替代它的是等于4πts/GΩ，该监视雷达方程求得

该雷达设计师在ts或再扫描时间的角度范围Ω，这是由工作的雷达确定后来控制。该雷达散射截面也由雷达来确定。如果需要大范围的监视雷达，雷达必须有该PavAe积的值。为此，一个监视雷达能力的常用指标是它的功率孔径积。请注意，雷达频率没有出现明确的监视雷达方程。但是在频率的选择将暗含在其他方面。

正如监控雷达方程不同于常规雷达方程。 1.5式或1.2式是简单的雷达方程，雷达每一个特定的应用程序一般要采用雷达方程适合特定的应用程序。当从陆，海，或天气的雷达回波杂波大于接收器的噪声，雷达方程进行修改归因于被检测到而不是

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接收机噪声。它可能会发生一个雷达探测能力可能是有限或其覆盖一些地区的混乱，或被接收在其他地区的噪音。这会导致两种不同的雷达特性对噪声进行了优化设计和其它优化杂波设计。不同类型的雷达方程时当雷达能力被敌对干扰噪声不同类型的雷达方程时通常也要设计。

1.5雷达频率段名称表

它并不总是方便使用的确切数字频率范围在一个特定类型的雷达操作。由于许多军用雷达，准确的工作频率范围的雷达通常没有透露。因此，使用字母指定雷达工作频段非常有帮助。在IEEE（电气与电子工程师协会）正式规范了雷达波段命名，如表1.1概括。

看下图。国际电信联盟（国际电联）的分配的无线电（雷达）电磁频谱使用的特定部分在第三栏适用于国际电联第2区，包括显示北美和南美。略有差异发生在其他两个国际电联地区。因此，一个L波段雷达只能运行频率范围内，从1215兆赫到1400兆赫，即使在这个范围内，可能有。国际电联的表示他们一些乐队的使用，例如，与4.2和4.4 GHz频段预留（有少数例外）的机载雷达高度设计。目前没有正式的国际电信联盟在高频波段雷达的分配，但大多数高频电磁雷达与其他服务共享频率。该信波段的毫米波雷达名称是毫米，有几个频率分配给本地区的雷达波段，但他

们没有被列在这里。虽然官方国际电联毫米波介绍，从30至300 GHz，实际上就是， 表1.1 IEEE标准的雷达名称频带表

频带名称 频率特定范围 第2区国际电信联盟频率分配

特定频率范围

在Ka波段雷达技术是比对技术的W频技术非常的接近微波频率。毫米波雷达的频

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率通常被认为那些谁在这一领域的工作有一个下界40千兆赫，而不是“规定”降低30千兆赫的约束在技术方面的重大差别，应用识别百分比是毫米波雷达特征。微波都没有界定这个标准，但这个词一般适用于雷达，从超高频的Ka波段操作。理由是这些字母名称非雷达工程师确认是不容易，他们最初选择来形容雷达在第二次世界大战中使用的频段。保密是重要的，当时选择这样指定不同的波段且难以猜测的它们采用的频率。但是谁解决这些雷达很少有雷达波段发信息的使用问题。

另一频带已被用于描述电磁频谱，但他们并不适合雷达绝不可用于雷达。其中一个指定使用的A，B，C等，原本设计进行电子对抗演习。 IEEE标准前面提到的国家，这些“不符合雷达的做法不得用来形容雷达频段。”因此，可能是D-波段干扰器，但从未有D -波段雷达。

1.6 影响雷达频率工作频带

雷达可工作在高达2 MHz的低频率（略高于AM广播频段）和几百兆赫高（毫米波地区）。更常见的雷达频率可从约5 MHz至超过95千兆赫。这是一个很大的频率范围所以它应该可以预期。雷达技术能力和应用将差别很大的频率范围在其中视一个雷达操作而定。在特定频率波段雷达通常具有不同的功能和比其他频段雷达特征。一般来说，大范围更容易在较低频率实现的，因为它更容易获得高功率发射机和在较低的频率对大型天线。另一方面，在雷达频率更高更容易实现准确的测量范围和位置因为更高的频率提供更大的带宽（决定范围精度和距离分辨率）以及一个给定的物理尺寸天线的窄波束天线（决定角精度和角分辨率）。在下面的应用过程通常发现在不同的雷达波段进行了简要说明。然而相邻带的差异在实际中很少钝化，并在相邻频段重叠的特点是可能的。

高频（3至30兆赫）。在高频波段雷达的主要用途（第20章）是在远距离探测目标（名义上超过2000海里）通过利用高的电离层折射的是高频能量在地球表面的优势。业余无线电将此称为短波传播和使用它来长距离通信。这种高频雷达的目标可能是飞机，舰艇，导弹和弹道导弹，以及从海面本身提供了有关方向和风速资料，推动海洋回波。

甚高频（30至300兆赫）。在20世纪30年始发展的雷达，雷达在此频段的频率，因为这代表了无线电技术在当时的前沿。这是一个远程空中监视，或弹道导弹的探测具有良好的频率。在这些频率反射系数从地球表面散射可能非常大，特别是在水中它们之间的直接信号和表面性的结构干扰反射信号可以大幅度增加的甚高频雷达探测范围。有时候这种效果几乎可以成倍的增加VHF的范围。但是当有结构的干扰增加了范围可以有破坏性的干扰，范围由于在海拔平面降低了深空天线模式范围。同样破坏性的干扰导致较差的低空覆盖。在杂波的运动目标检测是一种更好的频率较低时雷达采用的多普勒频移的优势。因为多普勒模糊（造成盲区速度）的速度远远在

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低频少。甚高频雷达是无关雨的回波。但他们会受到多一次从流星电离和极光回声。飞机的雷达散射在VHF一节，这通常比在更高的频率雷达散射截面大。甚高频雷达常常成本较低比较相同的范围性能工作在更高的频率雷达。

虽然有对于远程监控雷达，甚高频有许多吸引人的优点但也有一些严重的局限性。在海拔和恶劣低空覆盖深空已经提到。分配在甚高频雷达可用的频谱宽度小距离分辨率通常很差。该天线定向带宽通常较广泛的微波频率，所以不存在分辨率低和角度的准确性。在VHF频段拥挤，如电视和FM广播重要的民事服务，从而进一步降低了频谱空间雷达可用性。外部噪声水平可以进入雷达天线是更高的频率比在微波用于甚高频。在VHF比微波频率也许在VHF雷达经营行政是取得这些拥挤的频率适合的频谱空间的困难。

它的局限性尽管如此，甚高频空中监控雷达广泛利用因为苏联是一个大国，而且成本较低的甚高频雷达使他们提供在该国大片空中监视有吸引力。据说他们制作了大量的一些非常大的规模和范围广甚高频空监控雷达且大多数很容易运输。有趣的是机载甚高频拦截雷达被广泛使用在第二次世界大战中的德国。例如，列支特士SN-2机载雷达运营的约60个不同的模式以超过100兆赫。在这样的频率雷达并没有受到所谓的对策条件（也称为窗口）。

超高频（300至1000兆赫）。对在VHF雷达地区活动的许多特点也适用于一些在超高频的程度。超高频是一种机载动目标显示（AMTI）在机载预警雷达（简称AEW）雷达，如第3章讨论了良好的频率。这也是一个为检测远程雷达的运作良好的频率和卫星和弹道导弹跟踪。在这个区间的上半部分都可以发现有远程舰载空中监视雷达和雷达（称为风廓线仪）测量的速度和风向。

探地雷达（GPR），在第21章讨论的是一个所谓的超宽带（UWB）雷达的例子。其广泛的信号带宽有时涵盖了高频和超高频波段。这种雷达的信号带宽可扩展，例如由50至500 MHz。宽阔的带宽是需要的以便获得良好的距离分辨率。较低的频率需要允许进入地面雷达能量传播。（即便如此，通过典型的土壤传播的损失是如此之高，一个简单的移动雷达的范围可能只有数米。）这样的范围是寻找埋电缆和电力管线以及适合对象。如果雷达是看到表面上设目标，但在表面同样的频率还需要有探地雷达。

L波段（1.0至2.0千兆赫）。这是长期运行范围（超出200海里的首选频率波段）空中监视雷达。该航路监视雷达（ARSR）远距离空中使用的交通管制是一个很好的例子。正如一位频率的上升，雨天效果的影响开始变得重要，因此雷达设计者可能担心减少L在雨天对更高的频率的影响。这些频段也被吸引卫星和对洲际弹道导弹防御远程探测。

S波段（2.0至4.0千兆赫）。机场监视雷达系统（ASR），可以监测范围内的机场地区空中交通是S波段。它的范围通常是50到60海里。如果一个3D雷达被使用（一个确定的范围，方位角和仰角），它可以实现在S波段。

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据说以前的远程监视可更好地执行在低频和目标位置的精确测量是在高频率地进行。如果只有在单个频段单雷达操作也可以使用则S波段是一个很好的应用。它有时也可以接受的作为一雷达执行两种功能选择C波段。在空中预警机空中监视雷达也于S波段运作。通常大多数雷达应用的最佳运作的特定频段在该雷达的性能最佳。然而，在机载空中例子监视雷达，预警发现S波段和美国海军的E2类在用超高频机载预警雷达。在这样一个不同的频率，尽管有人说这两个雷达具有相当的性能。（这是一种每次例外地申请最佳的频带。)

新一代天气雷达工作在S波段。这是一个良好的气象观测频率，因为较低的频率会产生由弱得多雷达回波信号（因为雨雷达回波变化作为第四权力的频率）和更高的频率会产生信号的衰减，因为它通过雨水传播也不会允许有降雨率的精确测量。有在较高频率天气雷达，但这些通常是短于新一代天气雷达范围，可能是一个更具体的天气比新一代天气雷达提供准确的气象雷达测量应用中使用。

C波段（4.0至8.0千兆赫）。这个波段在于S波段和X波段并且在性能两者之间。通常，S或X波段可能是可取的C波段的使用，虽然已经在过去的C波段重要的应用。

X波段（8?12.0千兆赫）。这是军事上的应用比较受欢迎的雷达波段。它广泛应用于军事机载雷达，以便执行拦截战斗机的作用及攻击（对地面目标），在第5章讨论这，SAR和ISAR成像雷达也广泛使用。 X波段是一个适合民间海上雷达频率，机载气象预警雷达，机载多普勒导航雷达，的速度表，导弹制导系统，有时在X波段。在X波段雷达是一种方便，并因此一般大小应用的兴趣，在流动性和重量轻，很长范围并不是一个很重要地要求。在频率可在X波段比较广泛，并能够获得在这一带比较小天线缩小波束是高清晰度应用的重要因素。由于X波段高频，雨有时是一个严重的因素在减少X性能波段系统。

Ku，K，和Ka波段（12.0至40千兆赫）。至于到更高的雷达频率，天线的物理尺寸减少，而在一般情况下更难以产生大的发射功率。因此在上述的X波段雷达的频率范围的性能一般不超过的X波段的。军用机载雷达被发现在Ku波段以及在X波段。这些频段是有吸引力的较小的雷达要一个不要求远程应用程序。机场表面检测设备（ASDE）一般认为在主要机场已在Ku波段，主要是因为它优于X波段的条件。关于控制塔的顶部在原来的K波段吸收线在22.2千兆赫这会导致衰减，可在一些应用中有严重问题吸收线。这是发现后发展的K—波段雷达在第二次世界大战期间后来开始实施。所以无论是Ku和Ka波段从雨水回波可以在这些频率的雷达能力。

毫米波雷达。尽管这一地区的频率在很大程度上是在毫米波雷达大多数兴趣，都在94千兆赫附近，有一个最低限度（称为一个窗口在大气衰减）。（窗口是低衰减相对于邻近地区的频率。在94千兆赫的窗口约等于整个微波频谱宽。如前所述），雷达在毫米波地区目的实践中一般在40千兆赫或甚至更高的频率开始。在毫米波雷达

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和环境影响技术的传播，不仅和微波雷达不同，但他们通常有更多。不像是可以用的微波。即使在清朗的空气传播毫米波雷达信号高度减弱。衰减变化对毫米波地区在94 GHz的窗口衰减实际上高于大气中水蒸汽的衰减，在22.2千兆赫奥波尔吸收线。一个在60 GHz的氧气吸收线方式衰减大约是每公里12分贝基本上排除其适用。雨衰减也可以在毫米波有地区的。

在毫米波雷达的兴趣主要是因为作为一个前沿的挑战是要探讨并投入生产使用。它的优点是它是一种采用高带宽信号的频谱（有大量的频谱空间）;雷达有宽范围分辨率并用小型波束天线定向频带;敌对军事雷达电子对抗难以采用及它可以更容易地截获低阈值的军用雷达比在这些频率要低。在过去毫米波发射机不可能平均功耗比数一百瓦特高，通常要少得多。在陀螺仪的研究进展（第10章）可以产生平均功率比传统毫米大功率要更大。因此高功率的供应并没有一个因为它曾经是毫米波。

激光雷达。激光能以光的频率可用功率和频谱的红外区。他们可以利用宽带（极短脉冲），并且可以有很窄定向频带。但是天线孔径远远小于在微波炉。在大气中衰减和雨水是非常高，在恶劣天气的表现却相当有限。接收机噪声决定，而不是热噪声量子效应。其他原因激光雷达只取得了有限的应用。

1.7雷达命名

军用电子设备包括雷达是由联合电子形式指定系统（JETDS），如美用标准MIL-STD - 196D确定。在表中部分组成的一栏和另外三栏选择以指示设备安装和它的用途。继三个字母是一个破折号和一个解说。该数字是按顺序分配该特定的字母。表1.2显示了已被用于雷达名称字母。

一个后缀（A、B、C、„)跟随原本的设计对每次设备修改原在互换性一直持续下去。括号中的字母V添加到指定显示变化系统（那些功能可通过增加设备检测，分组，部件，其中其它组合）。当指定由一个破折号之后，字母T和一个数字该设备是专为用途。除了美国这些名称也可以用于加拿大，澳大利亚，新西兰和英国。特殊的数字都保留在这些国家。可以在网上找到军用互联网找到MIL-STD-196D之下结果。

美国联邦航空局（FAA）的使用下列指定其空中交通控制雷达： ●ASR 机场监控雷达 ●ARSR 航路监控雷达 ●ASDE 机场地面检测设备 ●TDWR 多普勒天气雷达

下面的数字表指定的特殊雷达模型（按时间顺序）。

美国气象局（NOAA）的气象雷达研制采用指定WSR。以下指定数量的雷达年度进入服务因此，WSR- 88D是新一代天气多普勒雷达，首先于1988年开始服役。字母D表明它是一个多普勒天气雷达。

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表1.2 JETDS设计表及雷达用途 安装地 设备类型 用途 A. 无人驾驶飞机 L. 电子设备 B. 轰炸

B. 水下移动潜艇 P. 雷达 D. .定向仪，侦察和监视 D. 无人号航母 S. 特别或组合 G. 消防监控 F. 固定地面 W. 武器装备（特有 N. 导航

的没有包括军备）

G. 一般地使用 Q. 特别联合 K. 两栖 R. 接收

M. 移动（地面） S. 检测/距离方向及搜索 P. 便携式 T. 发射

S. 水（船） W. 自动飞行或远程控制 T. 交通（地面） X. 鉴定和识别

U. 一般地使用 Y. 监视和控制（包括消防，空

中管制）

V. 车辆（地面） W. 水面和水下

Z. 无人飞机驾驶车

1.8过去的一些进展雷达

在20世纪一些简单的雷达在技术方面的重大进展有一定时间顺序但没有确切的顺序，如下所述：

●甚高频雷达上舰部署的发展，军事防空飞机前和二战期间。

●在微波磁控管的发明和技术的波导早在二战中的获得雷达应用，可以工作在微波频率使体积更小移动雷达可采用。

●在超过100种不同雷达开发的麻省理工学院辐射实验室在二战期间在其成立以来五年是提供基础模型微波雷达。 ●马库姆理论的雷达探测。

●发明和发展与波管的手册行波放大管提供高功率稳定性好。 ●多普勒频移来检测从杂乱回波存在更大的移动目标。 ●发展的雷达适合空中交通管制。 ●脉冲压缩。

●单脉冲跟踪雷达具有良好的跟踪精度和更好的耐高温性能比以前跟踪雷达有电子对抗性能。

●合成孔径雷达它提供了地面的图像。

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●机载MTI公司远距离（AMTI）在空气中存在的混乱空中监视。

●稳定的组件和子系统和超低副瓣天线，允许高重复频率脉冲多普勒雷达（预警）与大量无用的杂波抑制。

●高频超视距雷达，延长了飞机和船只检测的一个数量级范围。

●数字处理，20世纪70年代初以来这已经对改善雷达功能有常重大的影响。 ●自动检测和监视雷达的跟踪。 ●串行生产电子扫描相控阵雷达。

●逆合成孔径雷达（ISAR），提供了一个用于非目标识别船舶所需的目标图像。 ●多普勒天气雷达。

●空间雷达适合的行星，如金星的观察。 ●精确的复杂目标雷达散射截面计算机计算。

●多功能机载雷达的军事相对较小，重量轻适合在一个战斗机的鼻子和可以执行不同的空对空导弹和飞机的功能。

雷达它总是有争议的问题上有重大进步。其它的有不同的名单。不是每一个主要的雷达成就已被列入此列表。它本来可以更长也可能包括来自本书其他各章的多个实例，但列表表明类型的进步已为改善雷达能力的重要。

1.9雷达的应用

军事应用。由于的发明国防需要的20世纪30年代对抗重型军事轰炸机。军事需要雷达可能是其最重要的应用和其主要发展的主要来源，其中包括用于民用目的。

军事雷达主要用途已经从陆地空中，海上，防空工作。它并没有实际成功操作的防空雷达。在防空雷达用于远程空中监视，短期一系列低探测低空“弹出式”目标，武器控制导弹制导，非正式目标识别，和战斗损伤评估。在许多武器近炸也是一个雷达的例子。一个军事防空雷达十分成功方法是钱已经花的办法来消除其巨额款项效果的措施。这些措施包括电子对抗和电子战，抗辐射导弹雷达信号低截面飞机和船只。雷达还用于军事侦察在陆地或海上目标以及监视海面上空。

在战场上，雷达被要求执行空中侦察（包括飞机，直升机，导弹监视的功能，以及无人机），武器控制的空中拦截，地点敌对武器（迫击炮，大炮和火箭），入侵探测人员以及空中交通管制。

对于使用弹道导弹防御雷达感兴趣是因为弹道导弹的威胁都在50年代末出现。范围的时间越长高超音速的速度以及弹道导弹尺寸更小的目标使问题的挑战。因为那里对飞机的防守没有空间的杂波问题，但弹道导弹可以出现在一个外在目标混乱和存在，其他措施的大量存在攻击者可以启动飞行器携带弹头。基本的弹道导弹防御问题成为一个目标识别问题，而不是更多的探测和跟踪。在对弹道导弹的预警方法的需要导致了对不同类型的雷达用于执行这种功能。同样已部署雷达是探测和跟踪卫星的能力。

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与此相关的任务雷达不是是侦查和毒品贩运拦截。有几种类型的雷达，能够推动这方面的需要其中包括对超视距远程高频雷达。

遥感环境。这个类别的主要应用是气象观测雷达，如新一代天气雷达系统，它的输出往往在电视上看到的天气报告。还存在着垂直前瞻性风廓线，雷达是通过检测确定从晴空很弱的雷达回波风速和高度功能的方向。位于机场周围是机场多普勒天气雷达（多普勒天气雷达）系统是报下击暴流警报，它可能伴随严重的暴风雨天气的影响。通常有一个精心设计前侧雷达警告小型以及大型飞机在危险或不利的天气回避飞行。

另一个成功的遥感雷达的下视星载雷达测高仪，全球大地水平面（即平均海平面，这不是世界各地同一计算）以极高的精度。目前已在过去使用确定土壤湿度和评估农业作物状况雷达的用途，但这些都没有提供足够的精度。卫星或飞机成像雷达用来帮助船舶有效地航行北部冰海域，因为雷达可以告诉哪些类型的冰层更容易行驶。

空中交通管制。安全在现代空中旅行高度的部分原因是对雷达的有效，高效的成功应用和安全的空中交通管制。主要机场采用观察在机场附近的空中交通的机场监视雷达系统（ASR）。这种雷达还提供关于附近的天气信息，以便飞机可以舒服的天气路由左右。主要机场也有雷达观测要求和安全控制飞机和机场地面车辆交通机场地面检测设备（地面探测设备）。对空中交通控制途中从一个到另一个终端，远程航线监视雷达（ARSR）在世界各地。空中交通管制雷达信标系统（ATCRBS）不是雷达，而是采用一种合作制度以确定飞行中的飞机。它使用雷达等技术原本在军事论坛的（识别朋友或敌人）系统。

其他方面应用。雷达是一个非常重要的应用任何其他方法提供的信息都不可用，由成像雷达对金星探测行星表面，可以看到在不断所有云层掩盖了行星。世界中最广泛使用的和最昂贵的雷达一直是各地的渔船和船舶民间航海雷达安全航行。有些读者已经不怀疑高速公路使用CW多普勒雷达来测量车辆速度的。探地雷达已被用来寻找埋藏公用事业线路，来确定埋葬的对象定位和物品。考古学家已用它来确定从哪里开始寻找埋藏文物。雷达一直都有帮助的鸟类学家和昆虫学家更好地了解鸟类和昆虫的变动情况。人们还表明雷达可以探测气体渗流的往往是在地下的石油和天然气积层中发现。

1.10雷达系统的概念设计

目前雷达系统设计有各个方面。但是在一个新的雷达已不存在以前可以制造一个念的设计执行以指导实际的发展。概念设计是基于对雷达将满足客户或雷达用户的要求。一个概念设计努力的结果是提供一个清单雷达特征在雷达方程和相关的子系统中的一般特征（发射机，天线，接收机，信号处理，等等）可能雷达方程被用来采用。为确定各种权衡和选择的雷达系统设计，从而确定最合适的理念，以满足所需的必要的指导意义。本节简要概述了如何雷达系统工程师可能开始一个新的雷达概念设计。没有牢固的既定程序进行概念设计。每个雷达公司及每个雷达设计工程师开发自己的

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风格。一种雷达设计方法的概要是这里描述的是一种概念。

通用的方法。前提应该是至少有两个其中一个新的雷达系统可能对某些特定雷达应用程序方法的产生。一种方法是建立在发掘一些新发明新技术新设备或新知识的优势。在微波磁控管在第二次世界大战初期发明就是一个例子。磁控出现后雷达设计不同于它之前。另外也许更常见的雷达概念系统的设计方法是先有新的雷达必须这样做。检查可达到预期的能力，考虑各种方法每一种方法进行全面评估，然后选择一个最适合的业务范围和财政的需要施加的。简言之它可能包括以下步骤： ●需要或问题描述解决。这是从客户或用户雷达的观点。

●客户之间的相互作用和系统工程师。这是为探索权衡，而客户可能不知道的，这可能使客户能够更好地得到想要的是什么用途进行过多的费用或风险。不幸地是潜在用户之间的和雷达系统工程师互动并不总是做竞争采购。

●识别和探索可能的解决方案。这包括了解的优势和各种可能的解决办法。 ●选择最佳或接近最佳的解决方案。在许多工程领域，优化并不意味着以前是最好的，可能是无法承担的或在规定的时间实现的。优化这里所用是指在一定的假设一套最好的。工程往往涉及附近的实现最优而不是最佳。 选择首选的解决方案应基于一个明确的标准。

●详细说明所选择的办法。这是对雷达的特点条件和类型的子系统采用。 ●分析与建议设计的评价。这是验证所选方法是正确的。

正如一位通过这一进程的继续，人们可能达成一项“死胡同”，并重新开始，有时还不止一次。重新开始后在新的设计工作是不寻常。

人们不能制定用于执行一个雷达一套独特的设计指南。如果这是可能的，雷达的设计可以由电脑做到完全。由于缺乏完整的资料通常大多数工程设计需要在某些时候有判断和经验的设计工程师才能取得成功。

在概念设计雷达方程。该雷达方程是雷达系统的概念设计的基础。雷达方程的一些参数是由雷达需要做什么。其他人可能单方面做出决定的客户但应小心行事。客户通常应该有雷达原来状态性能目标，雷达操作环境其大小和重量的，使用该雷达信息放置以及有其他任何都要强加。从这个信息中雷达系统工程师确定是什么的目标范围和需要满足雷达用户的需要，角精度雷达散射截面以及天线扫描时间。例如天线增益的一些参数可能会影响到一个以上的需要或要求。例如一个特定的天线波束可能受到跟踪精度影响，附近的目标分辨率，可为特殊应用最大尺寸的天线以及对理想的雷达范围的需要选择雷达频率。该雷达的频率通常受到许多条件影响包括允许使用的工作频率。该雷达的频率可能是最后一个参数被选中后许多要求已经做出结果。

参考文献：

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1。 IEEE标准字典电气和电子条款，第4版。纽约：电机及电子学工程师联合会，1988。

2。米什科尔尼克湾林德，和K.米兹“Senrad：先进的宽带空中监视雷达，”电机及电子学工程师联合会。卷的AES - 37页。 1163年至1175年，2001年10月。 3。米什科尔尼克，雷达系统介绍，纽约：麦格劳希尔，2001年，图 2.6。 4。远东桑森，雷达的设计原理，纽约：麦格劳希尔，1991年，图 2.2。 5. 这表雷达频带名称来自IEEE标准，IEEE标准。 521-2002。

6。特定的无线电频率范围，可参考“催化裂化在线频率划分表”，47 CFR第§ 2.106。 7。“美国和加拿大的电子表演对抗，”美国海军OPNAVINST 3430.9B，1969年10月27日。类似的版本，由美国空军，AFC55-44签发;美国陆军河105-86;和海军陆战队，统筹处3430.1。

8。Zachepitsky，“甚高频（公用波段）由下诺夫哥罗德研究Radiotechnical研究所雷达，”电机及电子学工程师联合会AES公司系统杂志，第一卷15页.9月14日，2000年6月。

9。匿名，“AWACS与E2C战对峙，”电子战杂志，第31日，5月/ 1976年6月。 10。米什科尔尼克，D.海明威，和JP汉森，“雷达与石油和天然气矿床伴生天然气渗漏检测，”电机及电子学工程师联合会跨卷。地球观测卫星- 30页。 630-633，1992年5月。

数字工程图书馆@麦格劳希尔

Chapter 1

An Introduction and Overview of Radar

Merrill Skolnik

1.1 RADAR IN BRIEF

Radar is an electromagnetic sensor for the detection and location of reflecting objects. Its operation can be summarized as follows:

● The radar radiates electromagnetic energy from an antenna to propagate in space. ● Some of the radiated energy is intercepted by a reflecting object, usually called a target, located at a distance from the radar.

● The energy intercepted by the target is reradiated in many directions.

● Some of the reradiated (echo) energy is returned to and received by the radar antenna.

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● After amplification by a receiver and with the aid of proper signal processing, a decision is made at the output of the receiver as to whether or not a target echo signal is present. At that time, the target location and possibly other information about the target is acquired.

A common waveform radiated by a radar is a series of relatively narrow, rectangular-like pulses. An example of a waveform for a medium-range radar that detects aircraft might be described as a short pulse one millionth of a second in duration (one microsecond); the time between pulses might be one millisecond (so that the pulse repetition frequency is one kilohertz); the peak power from the radar transmitter might be one million watts (one megawatt); and with these numbers, the average power from the transmitter is one kilowatt. An average power of one kilowatt might be less than the power of the electric lighting usually found in a ―typical‖ classroom. We assume this example radar might operate in the middle of the microwave frequency range such as from 2.7 to 2.9 GHz, which is a typical frequency band for civil airport-surveillance radars. Its wavelength might be about 10 cm (rounding off, for simplicity). With the proper antenna such a radar might detect aircraft out to ranges of 50 to 60 nmi, more or less. The echo power received by a radar from a target can vary over a wide range of values, but we arbitrarily assume a ―typical‖ echo signal for illustrative purposes might have a power of perhaps 10−13watts. If the radiated power is 106 watts (one megawatt), the ratio of echo signal power from a target to the radar transmitter power in this example is 10–19, or the received echo is 190 dB less than the transmitted signal. That is quite a difference between the magnitude of the transmitted signal and a detectable received echo signal. Some radars have to detect targets at ranges as short as the distance from behind home plate to the pitcher’s mound in a baseball park (to measure the speed of a pitched ball), while other radars have to operate over distances as great as the distances to the nearest planets. Thus, a radar might be small enough to hold in the palm of one hand or large enough to occupy the space of many football fields.

Radar targets might be aircraft, ships, or missiles; but radar targets can also be people, birds, insects, precipitation, clear air turbulence, ionized media, land features (vegetation, mountains, roads, rivers, airfields, buildings, fences, and power-line poles), sea, ice, icebergs, buoys, underground features, meteors, aurora, spacecraft, and planets. In addition to measuring the range to a target as well as its angular direction, a radar can also find the relative velocity of a target either by determining the rate of change of the range measurement with time or by extracting the radial velocity from the doppler frequency shift of the echo signal. If the location of a

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moving target is measured over a period of time, the track, or trajectory, of the target can be found from which the absolute velocity of the target and its direction of travel can be determined and a prediction can be made as to its future location. Properly designed radars can determine the size and shape of a target and might even be able to recognize one type or class of target from another.

Basic Parts of a Radar. Figure 1.1 is a very elementary basic block diagram showing the subsystems usually found in a radar. The transmitter, which is shown here as a power amplifier, generates a suitable waveform for the particular job the radar is to perform. It might have an average power as small as milliwatts or as large as megawatts. (The average power is a far better indication of the capability of a radar’s performance than is its peak power.) Most radars use a short pulse waveform so that a single antenna can be used on a time-shared basis for both transmitting and receiving.

The function of the duplexer is to allow a single antenna to be used by protecting the sensitive receiver from burning out while the transmitter is on and by directing the received echo signal to the receiver rather than to the transmitter.

The antenna is the device that allows the transmitted energy to be propagated into space and then collects the echo energy on receive. It is almost always a directive antenna, one that directs the radiated energy into a narrow beam to concentrate the power as well as to allow the determination of the direction to the target. An antenna that produces a narrow directive beam on transmit usually has a large area on receive to allow the collection of weak echo signals from the target. The antenna not only concentrates the energy on transmit and collects the echo energy on receive, but it also acts as a spatial filter to provide angle resolution and other capabilities.

In radar, range is the term generally used to mean distance from the radar to the target. Range is also used here in some of its other dictionary definitions.

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FIGURE 1.1 Block diagram of a simple radar employing a power amplifier as the

transmitter in the upper portion of the figure and a superheterodyne receiver in the lower portion of the figure .

The receiver amplifies the weak received signal to a level where its presence can be detected. Because noise is the ultimate limitation on the ability of a radar to make a reliable detection decision and extract information about the target, care is taken to insure that the receiver produces very little noise of its own. At the microwave frequencies, where most radars are found, the noise that affects radar performance is usually from the first stage of the receiver, shown here in Figure 1.1 as a low-noise amplifier. For many radar applications where the limitation to detection is the unwanted radar echoes from the environment (called clutter), the receiver needs to have a large enough dynamic range so as to avoid having the clutter echoes adversely affect detection of wanted moving targets by causing the receiver to saturate. The dynamic range of a receiver, usually expressed in decibels, is defined as the ratio of the maximum to the minimum signal input power levels over which the receiver can operate with some specified performance. The maximum signal level might be set by the nonlinear effects of the receiver response that can be tolerated (for example, the signal power at which the receiver begins to saturate), and the minimum signal might be the minimum detectable signal. The signal processor, which is often in the IF portion of the receiver, might be described as being the part of the receiver that separates the desired signal from the undesired signals that can degrade the detection process. Signal processing includes the matched filter that maximizes the output signal-to-noise ratio. Signal processing also includes the doppler processing that maximizes the signal-to-clutter ratio of a moving target when clutter is larger than

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receiver noise, and it separates one moving target from other moving targets or from clutter echoes. The detection decision is made at the output of the receiver, so a target is declared to be present when the receiver output exceeds a predetermined threshold. If the threshold is set too low, the receiver noise can cause excessive false alarms. If the threshold is set too high, detections of some targets might be missed that would otherwise have been detected. The criterion for determining the level of the decision threshold is to set the threshold so it produces an acceptable predetermined average rate of false alarms due to receiver noise.

After the detection decision is made, the track of a target can be determined, where a track is the locus of target locations measured over time. This is an example of data processing . The processed target detection information or its track might be displayed to an operator; or the detection information might be used to automatically guide a missile to a target; or the radar output might be further processed to provide other information about the nature of the target. The radar control insures that the various parts of a radar operate in a coordinated and cooperative manner, as, for example, providing timing signals to various parts of the radar as required.

The radar engineer has as resources time that allows good doppler processing, bandwidth for good range resolution, space that allows a large antenna, and energy for long range performance and accurate measurements. External factors affecting radar performance include the target characteristics; external noise that might enter via the antenna; unwanted clutter echoes from land, sea, birds, or rain; interference from other electromagnetic radiators; and propagation effects due to the earth’s surface and atmosphere. These factors are mentioned to emphasize that they can be highly important in the design and application of a radar.

Radar Transmitters. The radar transmitter must not only be able to generate the peak and average powers required to detect the desired targets at the maximum range, but also has to generate a signal with the proper waveform and the stability needed for the particular application. Transmitters may be oscillators or amplifiers, but the latter usually offer more advantages.

There have been many types of radar power sources used in radar (Chapter 10). The magnetron power oscillator was at one time very popular, but it is seldom used except for civil marine radar (Chapter 22). Because of the magnetron’s relatively low average power (one or two kilowatts) and poor stability, other power sources are usually more appropriate for applications requiring long-range detection of small moving targets in the presence of large clutter echoes. The magnetron power oscillator is an example of

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what is called a crossed-field tube. There is also a related crossed-field amplifier (CFA) that has been used in some radars in the past, but it also suffers limitations for important radar applications, especially for those requiring detection of moving targets in clutter. The high-power klystron and the traveling wave tube (TWT) are examples of what are called linear beam tubes. At the high powers often employed by radars, both tubes have suitably wide bandwidths as well as good stability as needed for doppler processing, and both have been popular.

The solid-state amplifier, such as the transistor, has also been used in radar, especially in phased arrays. Although an individual transistor has relatively low power, each of the many radiating elements of an array antenna can utilize multiple transistors to achieve the high power needed for many radar applications. When solid-state transistor amplifiers are used, the radar designer has to be able to accommodate the high duty cycle at which these devices have to operate, the long pulses they must use that require pulse compression, and the multiple pulses of different widths to allow detection at short as well as long range. Thus the use of solid-state transmitters can have an effect on other parts of the radar system. At millimeter wavelengths very high power can be obtained with the gyrotron, either as an amplifier or as an oscillator. The gridcontrol vacuum tube was used to good advantage for a long time in UHF and lower frequency radars, but there has been less interest in the lower frequencies for radar.

Although not everyone might agree, some radar system engineers—if given a choice—would consider the klystron amplifier as the prime candidate for a high- power modern radar if the application were suitable for its use.

Radar Antennas. The antenna is what connects the radar to the outside world on transmit; that is, it is directive and has a narrow beamwidth; (2) collects the received echo energy from the target; (3) provides a measurement of the angular direction to the target; (4) provides spatial resolution to resolve (or separate) targets in angle; and (5) allows the desired volume of space to be observed. The antenna can be a mechanically scanned parabolic reflector, a mechanically scanned planar phased array, or a mechanically scanned end-fire antenna. It can be an electronically scanned phased array using a single transmitter with either a corporate feed or a space-feed configuration to distribute the power to each antenna element or an electronically scanned phased array employing at each antenna element a small solid-state ―miniature‖ radar (also called an active aperture phased array). Each type of antenna has its particular advantages and limitations. Generally, the larger the antenna the better, but there can be practical constraints that limit its size.

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1.2 TYPES OF RADARS

Although there is no single way to characterize a radar, here we do so by means of what might be the major feature that distinguishes one type of radar from another.

Pulse radar. This is a radar that radiates a repetitive series of almost-rectangular pulses. It might be called the canonical form of a radar, the one usually thought of as a radar when nothing else is said to define a radar.

High-resolution radar. High resolution can be obtained in the range, angle, or

doppler velocity coordinates, but high resolution usually implies that the radar has high range resolution. Some high-resolution radars have range resolutions in terms of fractions of a meter, but it can be as small as a few centimeters.

Pulse compression radar. This is a radar that uses a long pulse with internal

modulation (usually frequency or phase modulation) to obtain the energy of a long pulse with the resolution of a short pulse.

Continuous wave (CW) radar. This radar employs a continuous sine wave. It almost

always uses the doppler frequency shift for detecting moving targets or for measuring the relative velocity of a target.

FM-CW radar. This CW radar uses frequency modulation of the waveform to allow

a range measurement.

Surveillance radar. Although a dictionary might not define surveillance this way, a

surveillance radar is one that detects the presence of a target (such as an aircraft or a ship) and determines its location in range and angle. It can also observe the target over a period of time so as to obtain its track.

Moving target indication (MTI). This is a pulse radar that detects moving targets in

clutter by using a low pulse repetition frequency (PRF) that usually has no range ambiguities. It does have ambiguities in the doppler domain that result in so-called blind speeds.

Pulse doppler radar. There are two types of pulse doppler radars that employ either a

high or medium PRF pulse radar. They both use the doppler frequency shift to extract moving targets in clutter. A high PRF pulse doppler radar has no ambiguities (blind speeds) in doppler, but it does have range ambiguities. A medium PRF pulse doppler radar has ambiguities in both range and doppler.

Tracking radar. This is a radar that provides the track, or trajectory, of a target. Tracking radars can be further delineated as STT, ADT, TWS, and phased array trackers as described below:

Single Target Tracker (STT). Tracks a single target at a data rate high enough to

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provide accurate tracking of a maneuvering target. A revisit time of 0.1s (data rate of 10 measurements per second) might be ―typical.‖ It might employ the monopulse tracking method for accurate tracking information in the angle coordinate.

Automatic detection and tracking (ADT). This is tracking performed by a

surveillance radar. It can have a very large number of targets in track by using the measurements of target locations obtained over multiple scans of the antenna. Its data rate is not as high as the STT. Revisit times might range from one to 12 seconds, depending on the application.

Track-while-scan (TWS). Usually a radar that provides surveillance over a narrow

region of angle in one or two dimensions, so as to provide at a rapid update rate location information on all targets within a limited angular region of observation. It has been used in the past for ground-based radars that guide aircraft to a landing, in some types of weapon-control radars, and in some military airborne radars.

Phased array tracker. An electronically scanned phased array can (almost)

―continuously‖ track more than one target at a high data rate. It can also simultaneously provide the lower data rate tracking of multiple targets similar to that performed by ADT.

Imaging radar. This radar produces a two-dimensional image of a target or a scene, such as a portion of the surface of the earth and what is on it. These radars usually are on moving platforms.

Sidelooking airborne radar (SLAR). This airborne sidelooking imaging radar provides high resolution in range and obtains suitable resolution in angle by using a narrow beamwidth antenna.

Synthetic aperture radar (SAR). SAR is a coherent imaging radar on a moving vehicle that uses the phase information of the echo signal to obtain an image of a scene with high resolution in both range and cross-range. High range resolution is often obtained using pulse compression. Inverse synthetic aperture radar (ISAR). ISAR is a coherent imaging radar that uses high resolution in range and the relative motion of the target to obtain high resolution in the doppler domain that allows resolution in the cross-range dimension to be obtained. It can be on a moving vehicle or it can be stationary.

Weapon control radar. This name is usually applied to a singletarget tracker used for defending against air attack.

Guidance radar. This is usually a radar on a missile that allows the missile to

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―home in,‖ or guide itself, to a target.

Weather (meteorological) observation. Such radars detect, recognize, and measure precipitation rate, wind speed and direction,and observe other weather effects .  Coherent implies that the phase of the radar signal is used as an important part of the radar process. important for meteorological purposes. These may be special radars or another function of surveillance radars.

 Doppler weather radar. This is a weather observation radar that employs the doppler frequency shift caused by moving weather effects to determine the wind; the wind shear (when the wind blows in different directions), which can indicate a dangerous weather condition such as a tornado or a downburst of wind; as well as other meteorological effects.

Target recognition. In some cases, it might be important to recognize the type of target being observed by radar (e.g., an automobile rather than a bird), or to recognize the particular type of target ( an automobile rather than a truck, or a starling rather than a spar- row), or to recognize one class of target from another (a cruise ship rather than a tanker). When used for military purposes, it is usually called a noncooperative target recognition (NCTR) radar, as compared to a cooperative recognition system such as IFF (identification friend or foe), which is not a radar. When target recognition involves some part of the natural environment, the radar is usually known as a remote sensing (of the environment) radar.

Multifunction radar. If each of the above radars were thought of as providing some radar function, then a multifunction radar is one designed to perform more than one such function—usually performing one function at a time on a time-shared basis.

There are many other ways to describe radars, including land, sea, airborne, spaceborne, mobile, transportable, air-traffic control, military, ground-penetrating, ultra-wideband, over the horizon, instrumentation, laser (or lidar), by the frequency band at which they operate (UHF, L, S, and so on), by their application, and so forth.

1.3 INFORMATION AVAILABLE FROM A RADAR

Detection of targets is of little value unless some information about the target is obtained as well. Likewise, target information without target detection is meaningless.

Range. Probably the most distinguishing feature of a conventional radar is its ability to determine the range to a target by measuring the time it takes for the radar signal to propagate at the speed of light out to the target and back to the radar. No other sensor can measure the distance to a remote target at long range with the accuracy of radar (basically limited at long ranges by the accuracy of the knowledge of the velocity of

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propagation). At modest ranges, the precision can be a few centimeters. To measure range, some sort of timing mark must be introduced on the transmitted waveform. A timing mark can be a short pulse (an amplitude modulation of the signal), but it can also be a distinctive modulation of the frequency or phase. The accuracy of a range measurement depends on the radar signal bandwidth: the wider the bandwidth, the greater the accuracy. Thus bandwidth is the basic measure of range accuracy.

Radial Velocity. The radial velocity of a target is obtained from the rate of change of range over a period of time. It can also be obtained from the measurement of the doppler frequency shift. An accurate measurement of radial velocity requires time. Hence time is the basic parameter describing the quality of a radial velocity measurement. The speed of a moving target and its direction of travel can be obtained from its track, which can be found from the radar measurements of the target location over a period of time.

Angular Direction. One method for determining the direction to a target is by determining the angle where the magnitude of the echo signal from a scanning antenna is maximum. This usually requires an antenna with a narrow beamwidth (a high-gain antenna). An air-surveillance radar with a rotating antenna beam determines angle in this manner. The angle to a target in one angular dimension can also be determined by using two antenna beams, slightly displaced in angle, and comparing the echo amplitude received in each beam. Four beams are needed to obtain the angle measurement in both azimuth and elevation. The monopulse tracking radar discussed in Chapter 9 is a good example. The accuracy of an angle measurement depends on the electrical size of the antenna; i.e, the size of the antenna given in wavelengths.

Size and Shape. If the radar has sufficient resolution capability in range or angle, it can provide a measurement of the target extent in the dimension of high resolution. Range is usually the coordinate where resolution is obtained. Resolution in cross range (given by the range multiplied by the antenna beamwidth) can be obtained with very narrow beamwidth antennas. However, the angular width of an antenna beam is limited, so the cross-range resolution obtained by this method is not as good as the range resolution. Very good resolution in the cross-range dimension can be obtained by employing the doppler frequency domain, based on SAR (synthetic aperture radar) or ISAR (inverse synthetic aperture radar systems), as discussed in Chapter 17. There needs to be relative motion between the target and the radar in order to obtain the cross-range resolution by SAR or ISAR. With sufficient resolution in both range and cross-range, not only can the size be obtained in two orthogonal coordinates, but the target shape can sometimes be discerned.

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The Importance of Bandwidth in Radar. Bandwidth basically represents information; hence, it is very important in many radar applications. There are two types of bandwidth encountered in radar. One is the signal bandwidth, which is the bandwidth determined by the signal pulse width or by any internal modulation of the signal. The other is tunable bandwidth. Generally, the signal bandwidth of a simple pulse of sine wave of duration τ is 1/τ. (Pulse compression waveforms, discussed in Chapter 8, can have much greater bandwidth than the reciprocal of their pulse width.) Large band-width is needed for resolving targets in range, for accurate measurement of range to a target, and for providing a limited capability to recognize one type of target from another. High range resolution also can be useful for reducing the degrading effects of what is known as glint in a tracking radar, for measuring the altitude of an aircraft based on the difference in time delay (range) between the two-way direct signal from radar to target and the two-way surface-scattered signal from radar to surface to target (also called multipath height finding), and in increasing the target-signal-to-clutter ratio. In military systems, high range resolution may be employed for counting the number of aircraft flying in close formation and for recognizing and thwarting some types of deception countermeasures.

Tunable bandwidth offers the ability to change (tune) the radar signal frequency over a wide range of the available spectrum. This can be used for reducing mutual interference among radars that operate in the same frequency band, as well as in attempting to make hostile electronic countermeasures less effective. The higher the operating frequency the easier it is to obtain wide signal and wide tunable bandwidth.

A limitation on the availability of bandwidth in a radar is the control of the spectrum by government regulating agencies (in the United States, the Federal Communication Commission, and internationally, the International Telecommunications Union). After the success of radar in World War II, radar was allowed to operate over about one- third of the microwave spectrum. This spectrum space has been reduced considerably over the years with the advent of many commercial users of the spectrum in the age of ―wireless‖ and other services requiring the electromagnetic spectrum. Thus, the radar engineer is increasingly experiencing smaller available spectrum space and bandwidth allocations that can be vital for the success of many radar applications.

Signal-to-Noise Ratio. The accuracy of all radar measurements, as well as the reliable detection of targets depends on the ratio E/No , where E is the total energy of the received signal that is processed by the radar and No is the noise power per unit bandwidth of the receiver. Thus E/No is an important measure of the capability of a radar.

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Operation with More Than One Frequency. There may be important benefits when a radar is able to operate at more than one frequency. Frequency agility usually refers to the use of multiple frequencies on a pulse-to-pulse basis. Frequency diversity usually relates to the use of multiple frequencies that are widely separated, sometimes in more than one radar band. Frequency diversity might operate at each frequency simultaneously or almost simultaneously. It has been used in almost all civilian air-traffic control radars. Pulse-to-pulse frequency agility, however, is not compatible with the use of doppler processing (to detect moving targets in clutter), but frequency diversity can be compatible. The frequency range in both agility and in diversity operations is much greater than the inherent bandwidth of a pulse of width t.

Elevation Null Filling. Operation of a radar at a single frequency can result in a lobed structure to the elevation pattern of an antenna due to the interference between the direct signal (radar to target) and the surface-scattered signal (radar to earth’s surface to target). By a lobed structure, we mean that there will be reduced coverage at some elevation angles (nulls) and increased signal strength at other angles (lobes). A change in frequency will change the location of the nulls and lobes so that by operating over a wide frequency range, the nulls in elevation can be filled in, and the radar will be less likely to lose a target echo signal. For example, measurements with a wideband experimental radar known as Senrad, which could operate from 850 to 1400 MHz, showed that when only a single frequency was used, the blip-scan ratio (the experimentally measured single-scan probability of detection) was found to be 0.78 under a particular set of observations. When the radar operated at four different widely separated frequencies, the blip-scan ratio was 0.98—a highly significant increase due to frequency diversity.

Increased Target Detectability. The radar cross section of a complex target such as an aircraft can vary greatly with a change in frequency. At some frequencies, the radar cross section will be small and at others it will be large. If a radar operates at a single frequency, it might result in a small target echo and, therefore, a missed detection. By operating at a number of different frequencies, the cross section will vary and can be small or large; but a successful detection becomes more likely than if only a single frequency were used. This is one reason that almost all air-traffic control radars operate with two frequencies spaced wide enough apart in frequency to insure that target echoes are decorrelated and, therefore, increase the likelihood of detection.

Reduced Effectiveness of Hostile Countermeasures. Any military radar that is suc- cessful can expect a hostile adversary to employ countermeasures to reduce its effectiveness.Operating over a wide range of frequencies makes countermeasures

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more difficult than if operation is at only one frequency. Against noise jamming, changing frequency in an unpredictable manner over a wide range of frequencies causes the jammer to have to spread its power over a wide frequency range and will, therefore, reduce the hostile jamming signal strength over the bandwidth of the radar signal. Frequency diversity over a wide band also makes it more difficult (but not impossible) for a hostile intercept receiver or an antiradiation missile to detect and locate a radar signal.

The Doppler Shift in Radar. The importance of the doppler frequency shift began to be appreciated for pulse radar shortly after World War II and became an increasingly important factor in many radar applications. Modern radar would be much less interesting or useful if the doppler effect didn’t exist. The doppler frequency shift fd can be written as

fd =2vr/λ = (2 vr cosθ)/ λ (1.1)

where vr = v cos θ is the relative velocity of the target (relative to the radar) in m/s, v is the absolute velocity of the target in m/s, λ is the radar wavelength in m, and θ is the angle between the target’s direction and the radar beam. To an accuracy of about 3 percent, the doppler frequency in hertz is approximately equal to vr (kt) divided by λ (m).

The doppler frequency shift is widely used to separate moving targets from stationary clutter, as discussed in Chapters 2 through 5. Such radars are known as MTI (moving target indication), AMTI (airborne MTI), and pulse doppler. All modern airtraffic control radars, all important military ground-based and airborne air-surveillance radars, and all military airborne fighter radars take advantage of the doppler effect. Yet in WWII, none of these pulse radar applications used doppler. The CW (continuous wave) radar also employs the doppler effect for detecting moving targets, but CW radar for this purpose is not as popular as it once was. The HF OTH radar (Chapter 20) could not do its job of detecting moving targets in the presence of large clutter echoes from the earth’s surface without the use of doppler.

Another significant application of radar that depends on the doppler shift is observation of the weather, as in the Nexrad radars of the U.S. National Weather Service (Chapter 19) mentioned earlier in this chapter.

Both the SAR and ISAR can be described in terms of their use of the doppler frequency shift (Chapter 17). The airborne doppler navigator radar is also based on the doppler shift. The use of doppler in a radar generally places greater demands on the stability of the radar transmitter, and it increases the complexity of the signal processing; yet these requirements are willingly accepted in order to achieve the significant benefits

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offered by doppler. It should also be mentioned that the doppler shift is the key capability of a radar that can measure speed, as by its diligent use by traffic police for maintaining vehicle speed limits and in other velocity measuring applications.

1.4 THE RADER EQUATION

The radar range equation (or radar equation, for short) not only serves the very useful purpose of estimating the range of a radar as a function of the radar characteristics, but also is quite useful as a guide for designing a radar system. The simple form of the radar equation may be written as

The right-hand side has been written as the product of three factors to represent the physical processes that take place. The first factor on the right is the power density at a distance R from a radar that radiates a power Pt from an antenna of gain Gt . The numerator,σ, of the second factor is the radar cross section of the target. It has the unit of area (for example, square meters) and is a measure of the energy redirected by the target back in the direction of the radar. The denominator of the second factor accounts for the divergence of the echo signal on its return path back to the radar. The product of the first two factors represents the power per unit area returned to the radar antenna. Note that the radar cross section of a target,σ , is defined by this equation. The receiving antenna of effective area collects a portion Pr of the echo power returned to the radar. If the maximum radar range, Rmax, is defined as occurring when the received signal is equal to the minimum detectable signal of the radar, Smin, the simple form of the radar equation becomes

Generally, most radars use the same antenna for both transmitting and receiving. From antenna theory, there is a relation between the gain Gt of the antenna on transmit and its effective area Ae on receive, which is G t = 4π A e /λ 2 , where λ is the wavelength of the radar signal. Substituting this into Eq. 1.3 provides two other useful forms of the radar equation (not shown here): one that represents the antenna only by its gain and the other that represents the antenna only by its effective area. The simple form of the radar equation is instructive, but not very useful since it leaves out many things. The minimum detectable signal, Smin, is limited by receiver noise and can be expressed as

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In this expression, kToB is the so-called thermal noise from an ideal ohmic conductor, where k = Boltzmann’s constant, To is the standard temperature of 290 K, and B = receiver bandwidth (usually that of the IF stage of the superheterodyne receiver). The product kTo is equal to 4 × 10−21 W/Hz. To account for the additional noise introduced by a practical (nonideal) receiver, the thermal noise expression is multiplied by the noise figure F n of the receiver, defined as the noise out of a practical receiver to the noise out of an ideal receiver. For a received signal to be detectable, it has to be larger than the receiver noise by a factor denoted here as (S/N)1. This value of signal-to-noise ratio (S/N)1 is that required if only one pulse is present. It has to be large enough to obtain the required probability of false alarm (due to noise crossing the receiver threshold) and the required probability of detection (as can be found in various radar texts3,4). Radars, however, generally process more than one pulse before making a detection decision. We assume the radar waveform is a repetitive series of rectangular- like pulses. These pulses are integrated (added together) before a detection decision is made. To account for these added signals, the numerator of the radar equation is multiplied by a factor nEi (n), where Ei (n) is the efficiency in adding together n pulses. This value can also be found in standard texts. The power Pt is the peak power of a radar pulse. The average power, Pav , is a better measure of the ability of a radar to detect targets, so it is sometimes inserted into the radar equation using Pt = Pav /fp , where fp is the pulse repetition frequency of the pulse radar and t is the pulse duration. The surface of the earth and the earth’s atmosphere can drastically affect the propagation of electromagnetic waves and change the coverage and capabilities of a radar. In the radar equation, these propagation effects are accounted for by a factor F4 in the numerator of the radar equation, as discussed in Chapter 26. With the above substituted into the simple form of the radar equation we get

In the above equation, it was assumed in its derivation that Bt≈1, which is generally applicable in radar. A factor Ls (greater than unity), called the system losses, has been inserted to account for the many ways that loss can occur in a radar. This loss factor can be quite large. If the system loss is ignored, it might result in a very large error in the

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estimated range predicted by the radar equation. (A loss of from 10 dB to may be 20 dB is not unusual when all radar system loss factors are taken into account.)

Equation 1.5 applies for a radar that observes a target long enough to receive n pulses. More fundamentally, it applies for a radar where the time on target to is equal to n/fp . An example is a tracking radar that continuously observes a single target for a time to . This equation, however, needs to be modified for a surveillance radar that observes an angular volume Ω with a revisit time ts . (Air traffic control radars might have a revisit time of from 4 to 12 s.) Thus, a surveillance radar has the additional constraint that it must cover an angular volume Ω in a given time ts.The revisittime ts is equal to to(Ω/Ωo), where ts = n/fp and Ωo, the solid beamwidth of the antenna (steradians), is approximately related to the antenna gain G by G = 4π/Ωo. Therefore, when n/fp in Eq.1.5 is replaced with its equal 4πt /GΩ, the radar equation for a surveillance radar is obtained as

The radar designer has little control over the revisit time ts or the angular coverage Ω , which are determined mainly by the job the radar has to perform. The radar cross section also is determined by the radar application. If a large range is required of a surveillance

radar, the radar must have the necessary value of the product PavAe. For this reason, a common measure of the capability of a surveillance radar is its power-aperture product . Note that the radar frequency does not appear explicitly in the surveillance radar equation. The choice of frequency, however, will enter implicitly in other ways.

Just as the radar equation for a surveillance radar is different from the conventional radar equation of Eq. 1.5 or the simple radar equation of Eq.1.2, each particular application of a radar generally has to employ a radar equation tailored to that specific application. When the radar echoes from land, sea, or weather clutter are greater than the receiver noise, the radar equation has to be modified to account for clutter being the limitation to detection rather than receiver noise. It can happen that the detection capability of a radar might be limited by clutter in some regions of its coverage and be limited by receiver noise in other regions. This can result in two different sets of radar characteristics, one optimized for noise and the other optimized for clutter; and compromises usually have to be made in radar design. A different type of radar equation is also required when the radar capability is limited by hostile noise jamming.

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1.5 RADAR FREQUENCY LETTER-BAND NOMENCLATURE

It is not always convenient to use the exact numerical frequency range over which a particular type of radar operates. With many military radars, the exact operating frequency range of a radar is usually not disclosed. Thus, the use of letters to designate radar operating bands has been very helpful. The IEEE (Institute of Electrical and Electronic Engineers) has officially standardized the radar letter-band nomenclature, as summarized in Table 1.1.

Comments on the table. The International Telecommunications Union (ITU) assigns specific portions of the electromagnetic spectrum for radiolocation (radar) use as shown in the third column, which applies to ITU Region 2 that includes North and South America. Slight differences occur in the other two ITU Regions. Thus an L-band radar can only operate within the frequency range from 1215 MHz to 1400 MHz, and even within this range, there may be restrictions. Some of the indicated ITU bands are restricted in their usage; for example, the band between 4.2 and 4.4 GHz is reserved

(with few exceptions) for airborne radar altimeters. There are no official ITU allocations

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for radar in the HF band, but most HF radars share frequencies with other electromagnetic services. The letter-band designation for millimeter wave radars is mm, and there are several frequency bands allocated to radar in this region, but they have not been listed here. Although the official ITU description of millimeter waves is from 30 to 300 GHz, in reality, the technology of radars at Ka band is much closer to the technology of microwave frequencies than to the technology of W band. The millimeter wave radar frequencies are often considered by those who work in this field to have a lower bound of 40 GHz rather than the ―legal‖ lower bound of 30 GHz in recognition of the significant difference in technology and applications that is characteristic of millimeter wave radar. Microwaves have not been defined in this standard, but this term generally applies to radars that operate from UHF to K band. The reason that these letter designations might a not be easy for the non-radar engineer to recognize is that they were originally selected to describe the radar bands used in World War II. Secrecy was important at that time so the letters selected to designate the various bands made it hard to guess the frequencies to which they apply. Those who work around radar, however, seldom have a problem with the usage of the radar letter bands.

Other letter bands have been used for describing the electromagnetic spectrum; but they are not suitable for radar and should never be used for radar. One such designation uses the letters A, B, C, etc., originally devised for conducting electronic countermeasure 7 exercises. The IEEE Standard mentioned previously states that these ―are not consistent with radar practice and shall not be used to describe radar-frequency bands.‖ Thus, there may be D-band jammers, but never D-band radars.

1.6 EFFECT OF OPERATING FREQUENCY ON RADAR

Radars have been operated at frequencies as low as 2 MHz (just above the AM broadcast band) and as high as several hundred GHz (millimeter wave region). More usually, radar frequencies might be from about 5 MHz to over 95 GHz. This is a very large extent of frequencies, so it should be expected that radar technology, capabilities, and applications will vary considerably depending on the frequency range at which a radar operates. Radars at a particular frequency band usually have different capabilities and characteristics than radars in other frequency bands. Generally, long range is easier to achieve at the lower frequencies because it is easier to obtain high-power transmitters and physically large antennas at the lower frequencies. On the other hand, at the higher radar frequencies, it is easier to achieve accurate measurements of range and location because the higher frequencies provide wider bandwidth (which

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determines range accuracy and range resolution) as well as narrower beam antennas for a given physical size antenna (which determines angle accuracy and angle resolution). In the following, the applications usually found in the various radar bands are briefly indicated. The differences between adjacent bands, however, are seldom sharp in practice, and overlap in characteristics between adjacent bands is likely.

HF (3 to 30 MHz). The major use of the HF band for radar (Chapter 20) is to detect targets at long ranges (nominally out to 2000 nmi) by taking advantage of the refraction of HF energy by the ionosphere that lies high above the surface of the earth. Radio amateurs refer to this as short-wave propagation and use it to communicate over long distances. The targets for such HF radars might be aircraft, ships, and ballistic missiles, as well as the echo from the sea surface itself that provides information about the direction and speed of the winds that drive the sea.

VHF (30 to 300 MHz). At the beginning of radar development in the 1930s, radars were in this frequency band because these frequencies represented the frontier of radio technology at that time. It is a good frequency for long range air surveillance or detection of ballistic missiles. At these frequencies, the reflection coefficient on scattering from the earth’s surface can be very large, especially over water, so the constructive interference between the direct signal and the surface-reflected signal can increase significantly the range of a VHF radar. Sometimes this effect can almost double the radar’s range. However, when there is constructive interference that increases the range, there can be destructive interference that decreases the range due to the deep nulls in the antenna pattern in the elevation plane. Likewise, the destructive interference can result in poor low-altitude coverage. Detection of moving targets in clutter is often better at the lower frequencies when the radar takes advantage of the doppler frequency shift because doppler ambiguities (that cause blind speeds) are far fewer at low frequencies. VHF radars are not bothered by echoes from rain, but they can be affected by multiple-time-around echoes from meteor ionization and aurora. The radar cross section of aircraft at VHF is generally larger than the radar cross section at higher frequencies. VHF radars frequently cost less compared to radars with the same range performance that operate at higher frequencies.

Although there are many attractive advantages of VHF radars for long-range surveillance, they also have some serious limitations. Deep nulls in elevation and poor low-altitude coverage have been mentioned. The available spectral widths assigned to radar at VHF are small so range resolution is often poor. The antenna beamwidths are usually wider than at microwave frequencies, so there is poor resolution and accuracy in angle. The VHF band is crowded with important civilian services such as TV and FM

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broadcast, further reducing the availability of spectrum space for radar. External noise levels that can enter the radar via the antenna are higher at VHF than at microwave frequencies. Perhaps the chief limitation of operating radars at VHF is the difficulty of obtaining suitable spectrum space at these crowded frequencies.

In spite of its limitations, the VHF air surveillance radar was widely used by the Soviet Union because it was a large country, and the lower cost of VHF radars made them attractive for providing air surveillance over the large expanse of that country. They have said they produced a large number of VHF air-surveillance radars—some were of very large size and long range, and most were readily transportable. It is interesting to note that VHF airborne intercept radars were widely used by the Germans in World War II. For example, the Lichtenstein SN-2 airborne radar operated from about 60 to over 100 MHz in various models. Radars at such frequencies were not affected by the countermeasure called chaff (also known as window).

UHF (300 to 1000 MHz). Many of the characteristics of radar operating in the VHF region also apply to some extent at UHF. UHF is a good frequency for Airborne Moving Target Indication (AMTI) radar in an Airborne Early Warning Radar (AEW), as discussed in Chapter 3. It is also a good frequency for the operation of long-range radars for the detection and tracking of satellites and ballistic missiles. At the upper portion of this band there can be found long-range shipboard air-surveillance radars and radars (called wind profilers) that measure the speed and direction of the wind.

Ground Penetrating Radar (GPR), discussed in Chapter 21, is an example of what is called an ultrawideband (UWB) radar. Its wide signal bandwidth sometimes covers both the VHF and UHF bands. Such a radar’s signal bandwidth might extend, for instance, from 50 to 500 MHz. A wide bandwidth is needed in order to obtain good range resolution. The lower frequencies are needed to allow the propagation of radar energy into the ground. (Even so, the loss in propagating through typical soil is so high that the ranges of a simple mobile GPR might be only a few meters.) Such ranges are suitable for locating buried power lines and pipe lines, as well as buried objects. If a radar is to see targets located on the surface but within foliage, similar frequencies are needed as for the GPR.

L band (1.0 to 2.0 GHz). This is the preferred frequency band for the operation of long-range (out to 200 nmi) air-surveillance radars. The Air Route Surveillance Radar (ARSR) used for long range air-traffic control is a good example. As one goes up in frequency, the effect of rain on performance begins to become significant, so the radar designer might have to worry about reducing the effect of rain at L-band and

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higher frequencies. This frequency band has also been attractive for the long-range detection of satellites and defense against intercontinental ballistic missiles.

S band (2.0 to 4.0 GHz). The Airport Surveillance Radar (ASR) that monitors

air traffic within the region of an airport is at S band. Its range is typically 50 to 60 nmi. If a 3D radar is wanted (one that determines range, azimuth angle, and elevation angle), it can be achieved at S band.

It was said previously that long-range surveillance is better performed at low frequencies and the accurate measurement of target location is better performed at high frequencies. If only a single radar operating within a single frequency band can be used, then S band is a good compromise. It is also sometimes acceptable to use C band as the choice for a radar that performs both functions. The AWACS airborne air-surveillance radar also operates at S band. Usually, most radar applications are best operated in a particular frequency band at which the radar’s performance is optimum. However, in the example of airborne air-surveillance radars, AWACS is found at S band and the U.S. Navy’s E2 AEW radar at UHF. In spite of such a difference in frequency, it has been said 9 that both radars have comparable performance. (This is an exception to the observation about there being an optimum frequency band for each application.)

The Nexrad weather radar operates at S band. It is a good frequency for the observation of weather because a lower frequency would produce a much weaker radar echo signal from rain (since the radar echo from rain varies as the fourth power of the frequency), and a higher frequency would produce attenuation of the signal as it propagates through the rain and would not allow an accurate measurement of rainfall rate. There are weather radars at higher frequencies, but these are usually of shorter range than Nexrad and might be used for a more specific weather radar application than the accurate meteorological measurements provided by Nexrad.

C band (4.0 to 8.0 GHz). This band lies between S and X bands and has properties in between the two. Often, either S or X band might be preferred to the use of C band, although there have been important applications in the past for C band. X band (8 to 12.0 GHz). This is a relatively popular radar band for military applications. It is widely used in military airborne radars for performing the roles of interceptor, fighter, and attack (of ground targets), as discussed in Chapter 5. It is also popular for imaging radars based on SAR and ISAR. X band is a suitable frequency for civil marine radars, airborne weather avoidance radar, airborne doppler navigation radars, and the police speed meter. Missile guidance systems are sometimes at X band. Radars at X band are generally of a convenient size and are, therefore, of interest for applications

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where mobility and light weight are important and very long range is not a major requirement. The relatively wide range of frequencies available at X band and the ability to obtain narrow beamwidths with relatively small antennas in this band are important considerations for high-resolution applications. Because of the high frequency of X band, rain can sometimes be a serious factor in reducing the performance of X-band systems.

Ku , K, and Ka Bands (12.0 to 40 GHz). As one goes to higher radar

frequency, the physical size of antennas decrease, and in general, it is more difficult to generate large transmitter power. Thus, the range performance of radars at frequencies above X band is generally less than that of X band. Military airborne radars are found at Ku band as well as at X band. These frequency bands are attractive when a radar of smaller size has to be used for an application not requiring long range. The Airport Surface Detection Equipment (ASDE) generally found on top of the control tower at major airports has been at Ku band, primarily because of its better resolution than X band. In the original K band, there is a water-vapor absorption line at 22.2 GHz, which causes attenuation that can be a serious problem in some applications. This was discovered after the development of K-band radars began during World War II, which is why both Ku and Ka bands were later introduced. The radar echo from rain can limit the capability of radars at these frequencies.

Millimeter Wave Radar. Although this frequency region is of large extent, most of the interest in millimeter wave radar has been in the vicinity of 94 GHz where there is a minimum (called a window) in the atmospheric attenuation. (A window is a region of low attenuation relative to adjacent frequencies. The window at 94 GHz is about as wide as the entire microwave spectrum.) As mentioned previously, for radar purposes, the millimeter wave region, in practice, generally starts at 40 GHz or even at higher frequencies. The technology of millimeter wave radars and the propagation effects of the environment are not only different from microwave radars, but they are usually much more restricting. Unlike what is experienced at microwaves, the millimeter radar signal can be highly attenuated even when propagating in the clear atmosphere. Attenuation varies over the millimeter wave region. The attenuation in the 94 GHz window is actually higher than the attenuation of the atmospheric water-vapor absorption line at 22.2 GHz. The one-way attenuation in the oxygen absorption line at 60 GHz is about 12 dB per km, which essentially precludes its application. Attenuation in rain can also be a limitation in the millimeter wave region. Interest in millimeter radar has been mainly because of its challenges as a frontier to be explored and put to productive use. Its good features are that it is a great place for

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employing wide bandwidth signals (there is plenty of spectrum space); radars can have high range-resolution and narrow beamwidths with small antennas; hostile electronic countermeasures to military radars are difficult to employ; and it is easier to have a military radar with low probability of intercept at these frequencies than at lower frequencies. In the past, millimeter wave transmitters were not capable of an average power more than a few hundred watts—and were usually much less. Advances in gyrotrons (Chapter 10) can produce average power many orders of magnitude greater than more conventional millimeter-wave power sources. Thus, availability of high power is not a limitation as it once was.

Laser Radar. Lasers can produce usable power at optical frequencies and in

the infrared region of the spectrum. They can utilize wide bandwidth (very short pulses) and can have very narrow beamwidths. Antenna apertures, however, are much smaller than at microwaves. Attenuation in the atmosphere and rain is very high, and performance in bad weather is quite limited. Receiver noise is determined by quantum effects rather than thermal noise. For several reasons, laser radar has had only limited application.

1.7 RADAR NOMENCLATURE

Military electronic equipment, including radar, is identified by the Joint Electronics Type

Designation System (JETDS), as described in U.S. Military Standard MIL-STD-196D. The letter portion of the designation consists of the letters AN, a slant bar, and three additional letters appropriately selected to indicate where the equipment is installed, the type of equipment, and its purpose. Following the three letters are a dash and a numeral. The numeral is assigned in sequence for that particular combination of letters. Table 1.2 shows the letters that have been used for radar designations.

A suffix letter (A, B, C,…) follows the original designation for each modification of the equipment where interchangeability has been maintained. The letter V in parentheses added to the designation indicates variable systems (those whose functions may be varied through the addition or deletion of sets, groups, units, or combinations thereof). When the designation is followed by a dash, the letter T, and a number, the equipment is designed for training. In addition to the United States, these designations can also be used by Canada, Australia, New Zealand, and the United Kingdom. Special blocks of numbers are reserved for these countries. Further information can be found on the Internet under MIL-STD-196D.

The U.S. Federal Aviation Agency (FAA) uses the following to designate their airtraffic control radars:

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● ASR Airport Surveillance Radar ● ARSR Air Route Surveillance Radar ● ASDE Airport Surface Detection Equipment ● TDWR Terminal Doppler Weather Radar

The numeral following the letter designation indicates the particular radar model (in sequence).

Weather radars developed by the U. S. Weather Service (NOAA) employ the designation WSR. The number following the designation is the year the radar went into service. Thus WSR-88D is the Nexrad doppler radar that first entered service in 1988. The letter D indicates it is a doppler weather radar.

1.8 SOME PAST ADVANCES IN RADAR

A brief listing of some of the major advances in technology and capability of radar in the twentieth century is given, in somewhat chronological but not exact order, as follows:

● The development of VHF radar for deployment on surface, ship, and aircraft for military air defense prior to and during World War II.

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● The invention of the microwave magnetron and the application of waveguide technology early in WWII to obtain radars that could operate at microwave frequencies so that smaller and more mobile radars could be employed.

● The more than 100 different radar models developed at the MIT Radiation Laboratory in its five years of existence during WWII that provided the foundation for microwave radar.

● Marcum’s theory of radar detection.

● The invention and development of the klystron and TWT amplifier tubes that provided high power with good stability. ● The use of the doppler frequency shift to detect moving targets in the presence of much larger echoes from clutter.

● The development of radars suitable for air-traffic control. ● Pulse compression.

● Monopulse tracking radar with good tracking accuracy and better resistance to electronic countermeasures than prior tracking radars.

● Synthetic aperture radar, which provided images of the ground and what is on it. ● Airborne MTI (AMTI) for long-range airborne air surveillance in the presence of clutter.

● Stable components and subsystems and ultralow sidelobe antennas that allowed high-PRF pulse doppler radar (AWACS) with large rejection of unwanted clutter. ● HF over-the-horizon radar that extended the range of detection of aircraft and ships by an order of magnitude.

● Digital processing, which has had a very major effect on improving radar capabilities ever since the early 1970s.

● Automatic detection and tracking for surveillance radars. ● Serial production of electronically scanned phased array radars.

● Inverse synthetic aperture radar (ISAR) that provided an image of a target as needed for noncooperative target recognition of ships. ● Doppler weather radar.

● Space radars suitable for the observation of planets such as Venus. ● Accurate computer calculation of the radar cross section of complex targets.

● Multifunction airborne military radar that are relatively small and lightweight that fit in the nose of a fighter aircraft and can perform a large number of different air-to-air and air-to-ground functions.

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It is always a matter of opinion what the major advances in radar have been. Others might have a different list. Not every major radar accomplishment has been included in this listing. It could have been much longer and could have included multiple examples from each of the other chapters in this book, but this listing is sufficient to indicate the type of advances that have been important for improved radar capabilities.

1.9 APPLIATIONS OF RADAR

Military Applications. Radar was invented in the 1930s because of the need for defense against heavy military bomber aircraft. The military need for radar has probably been its most important application and the source of most of its major developments, including those for civilian purposes.

The chief use of military radar has been for air defense operating from land, sea, or air. It has not been practical to perform successful air defense without radar. In air defense, radar is used for long-range air surveillance, short-range detection of low- altitude ―pop-up‖ targets, weapon control, missile guidance, noncooperative target recognition, and battle damage assessment. The proximity fuze in many weapons is also an example of a radar. An excellent measure of the success of radar for military air defense is the large amounts of money that have been spent on methods to counter its effectiveness. These include electronic countermeasures and other aspects of electronic warfare, antiradiation missiles to home on radar signals, and low cross-section aircraft and ships. Radar is also used by the military for reconnaissance, targeting over land or sea, as well as surveillance over the sea.

On the battlefield, radar is asked to perform the functions of air surveillance (including surveillance of aircraft, helicopters, missiles, and unmanned airborne vehicles), control of weapons to an air intercept, hostile weapons location (mortars, artillery, and rockets), detection of intruding personnel, and control of air traffic.

The use of radar for ballistic missile defense has been of interest ever since the threat of ballistic missiles arose in the late 1950s. The longer ranges, high supersonic speeds, and the smaller target size of ballistic missiles make the problem challenging. There is no natural clutter problem in space as there is for defense against aircraft, but ballistic missiles can appear in the presence of a large number of extraneous confusion targets and other countermeasures that an attacker can launch to accompany the reentry vehicle carrying a warhead. The basic ballistic missile defense problem becomes more of a target recognition problem rather than detection and tracking. The need for warning of the approach of ballistic missiles has resulted in a number of different types of radars

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for performing such a function. Similarly, radars have been deployed that are capable of detecting and tracking satellites.

A related task for radar that is not military is the detection and interception of drug traffic. There are several types of radars that can contribute to this need, including the long-range HF over-the-horizon radar.

Remote Sensing of the Environment. The major application in this category

has been weather observation radar such as the Nexrad system whose output is often seen on the television weather report. There also exist vertical-looking wind-profiler radars that determine wind speed and direction as a function of altitude, by detecting the very weak radar echo from the clear air. Located around airports are the Terminal Doppler Weather Radar (TDWR) systems that warn of dangerous wind shear produced by the weather effect known as the downburst, which can accompany severe storms. There is usually a specially designed weather avoidance radar in the nose of small as well as large aircraft to warn of dangerous or uncomfortable weather in flight.

Another successful remote-sensing radar was the downward-looking spaceborne altimeter radar that measured worldwide the geoid (the mean sea level, which is not the same all over the world), with exceptionally high accuracy. There have been attempts in the past to use radar for determining soil moisture and for assessing the status of agriculture crops, but these have not provided sufficient accuracy. Imaging adars in satellites or aircraft have been used to help ships efficiently navigate northern seas coated with ice because radar can tell which types of ice are easier for a ship to penetrate.

Air-Traffic Control. The high degree of safety in modern air travel is due in part to the successful applications of radar for the effective, efficient, and safe control of air traffic. Major airports employ an Airport Surveillance Radar (ASR) for observing the air traffic in the vicinity of the airport. Such radars also provide information about nearby weather so aircraft can be routed around uncomfortable weather. Major airports also have a radar called Airport Surface Detection Equipment (ASDE) for observing and safely controlling aircraft and airport vehicle traffic on the ground. For control of air traffic en route from one terminal to another, long-range Air Route Surveillance Radars (ARSR) are found worldwide. The Air Traffic Control Radar Beacon System (ATCRBS) is not a radar but is a cooperative system used to identify aircraft in flight. It uses radar-like technology and was originally based on the military IFF (Identification Friend or Foe) system.

Other Applications. A highly significant application of radar that provided information not available by any other method, was the exploration of the

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surface of the planet Venus by an imaging radar that could see under the ever-present clouds that mask the planet. One of the widest used and least expensive of radars has been the civil marine radar found throughout the world for the safe navigation of boats and ships. Some readers have undoubtedly been confronted by the highway police using the CW doppler radar to measure the speed of a vehicle. Ground penetrating radar has been used to find buried utility lines, as well as by the police for locating buried objects and bodies. Archeologists have used it to determine where to begin to look for buried artifacts. Radar has been helpful to both the ornithologist and entomologist for better understanding the movements of birds and insects. It has also been demonstrated that radar can detect the gas seepage that is often found over underground oil and gas deposits.

1.10 CONCEPTUAL RADAR SYSTEM DESIG

There are various aspects to radar system design. But before a new radar that has not existed previously can be manufactured, a conceptual design has to be performed to guide the actual development. A conceptual design is based on the requirements for the radar that will satisfy the customer or user of the radar. The result of a conceptual design effort is to provide a list of the radar characteristics as found in the radar equation and related equations and the general characteristics of the subsystems (transmitter, antenna, receiver, signal processing, and so forth) that might be employed. The radar equation is used as an important guide for determining the various tradeoffs and options available to the radar system designer so as to determine a suitable concept to meet the desired need. This section briefly summarizes how a radar systems engineer might begin to approach the conceptual design of a new radar. There are no firmly established procedures to carry out a conceptual design. Every radar company and every radar design engineer develops his or her own style. What is described here is a brief summary of one approach to conceptual radar design.

General Guideline. It should be mentioned that there are at least two ways by which a new radar system might be produced for some particular radar application. One method is based on exploiting the advantages of some new invention, new technique, new device, or new knowledge. The invention of the microwave magnetron early in World War II is an example. After the magnetron appeared, radar design was different from what it had been before. The other, and probably more common method for conceptual radar system design, is to start with what the new radar has to do, examine the various approaches available to achieve the desired capability, carefully evaluate each approach, and then select the one that best meets the needs within the operational and

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fiscal constraints imposed. In brief, it might consist of the following steps:

● Description of the need or problem to be solved. This is from the viewpoint of the customer or the user of the radar.

● Interaction between the customer and the systems engineer. This is for the purpose of exploring the tradeoffs, which the customer might not be aware of, that might allow the customer to better obtain what is wanted with- out excessive cost or risk. Unfortunately, interaction between the potential user of the radar and the radar systems engineer is not always done in competitive procurements.

● Identification and exploration of possible solutions. This includes understanding the advantages and limitations of the various possible solutions.

● Selection of the optimum or near optimum solution. In many engineering endeavors, optimum does not mean the best since the best might not be affordable or achievable in the required time. Optimum, as used here, means the best under a given set of assumptions. Engineering often involves achieving a near-optimum, not the optimum. Selecting the preferred solution should be based on a well-defined criterion.

● Detailed description of the selected approach. This is in terms of the characteristics of the radar and the type of subsystems to be employed.

● Analysis and evaluation of the proposed design. This is to verify the correctness of the selected approach.

As one proceeds through this process, one might reach a ―dead end‖ and have to start over—sometimes more than once. Having to start over is not unusual during a new design effort.

One cannot devise a unique set of guidelines for performing the design of a radar. If that were possible, radar design could be done entirely by computer. Because of the usual lack of complete information, most engineering design requires, at some point, the judgment and experience of the design engineer in order to succeed.

The Radar Equation in Conceptual Design. The radar equation is the basis

for conceptual radar system design. Some parameters of the radar equation are determined by what the radar is required to do. Others may be decided upon unilaterally by the customer—but that should be done with caution. The customer usually should be the one who states the nature of the radar target, the environment in which the radar is to operate, restrictions on size and weight, the use to which the radar information is to be put, and any other constraints that have to be imposed. From this information, the radar systems engineer determines what is the radar cross section of the target, the range and angle accuracies needed to meet the radar user’s needs, as well as the antenna revisit time.

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Some parameters, such as antenna gain, might be affected by more than one need or requirement. For instance, a particular antenna beamwidth might be influenced by the tracking accuracy, resolution of nearby targets, the maximum size the antenna can be for a particular application, the need for a desired radar range, and the choice of radar frequency. The radar frequency is usually affected by many things, including the availability of allowed frequencies at which to operate. The radar frequency might be the last parameter of the radar to be selected—after many other compromises have been made.

REFERENCES

1. IEEE Standard Dictionary of Electrical and Electronic Terms, 4th Ed. New York: IEEE, 1988.

2. M. I. Skolnik, G. Linde, and K. Meads, ―Senrad: an advanced wideband air-surveillance radar,‖

IEEE Trans., vol. AES-37, pp. 1163–1175, October 2001.

3. M. I. Skolnik, Introduction to Radar Systems, New York: McGraw-Hill, 2001, Fig. 2.6. 4. F. E. Nathanson, Radar Design Principles, New York: McGraw-Hill, 1991, Fig. 2.2. 5.This table has been derived from IEEE Standard Letter Designations for Radar-Frequency Bands, IEEE Std. 521-2002.

6. Specific radiolocation frequency ranges may be found in the ―FCC Online Table of Frequency Allocations,‖ 47 C.F.R. § 2.106.

7.―Performing electronic countermeasures in the United States and Canada,‖ U.S. Navy OPNAVINST 3430.9B, October 27, 1969. Similar versions issued by the U.S. Air Force, AFR 55-44; U.S. Army, AR 105-86; and U.S. Marine Corps, MCO 3430.1.

8. A. Zachepitsky, ―VHF (metric band) radars from Nizhny Novgorod Research Radiotechnical Institute,‖ IEEE AES Systems Magazine, vol. 15, pp. 9–14, June 2000.

9. Anonymous, ―AWACS vs. E2C battle a standoff,‖ EW Magazine, p. 31, May/June 1976.

10. M. Skolnik, D. Hemenway, and J. P. Hansen, ―Radar detection of gas seepage associated with oil and gas deposits,‖ IEEE Trans, vol. GRS-30, pp. 630–633, May 1992. Downloaded from Digital Engineering Library @ McGraw-Hill

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