海军武器工程的介绍

Basic Radar Systems

工作原理

雷达is an acronym for Radio Detection and Ranging. The term "radio"
是指在所谓无线电中使用具有波长的电磁波
频谱的波浪部分，覆盖范围为10⁴公里至1厘米。雷达
systems typically use wavelengths on the order of 10 cm, corresponding to frequencies
oFabout 3 GHz. The detection and ranging part of the acronym is accomplished by
timing the delay between transmission of a pulse of radio energy and its subsequent
返回。如果时间延迟是dt，然后范围可以由简单确定
公式：

r=cdt/2

其中c = 3 x 10⁸m/s, the speed of light at which all electromagnetic waves propagate.
公式中两个因素来自于观察到雷达脉冲必须
travel to the target and back before detection, or twice the range.

Aradar pulse train is a type of amplitude modulation of the radar frequency
载波波，类似于在通信系统中调制载波的方式。
在这种情况下，信息信号非常简单：在
regular intervals. The common radar carrier modulation, known as the pulse train
如下所示。雷达的常见参数是指图1所定义的。

图2。

PW =脉冲宽度。PW有时间单位，通常在ms。PW是duration of the pulse. RT = rest time. RT is the interval between pulses. It is measured inms。prt =脉冲重复时间。PRT具有时间单位，通常在MS中表达。PRT是一个脉冲开始与另一个脉冲开始之间的间隔。PRT也等于总和PRT = PW+RT。prf =脉冲重复频率。PRF有时间单位^-1通常以Hz（1 Hz = 1/s）或每秒脉冲（PPS）表示。PRF是每秒发射的脉冲数，等于PRT的倒数。RF =射频。RF有时间单位^-1orHz and is commonly expressed in GHz or MHz. RF is the frequency of the carrier wave which is being modulated to form the pulse train.

Mechanization

Apractical radar system requires seven basic components as illustrated below:

图3

发射机。发射器会创建可发送的无线电波，并调节其形成脉冲序列。发射器还必须将信号放大到高功率水平以提供足够的范围。载波的来源可以是klystron，行动波管（TWT）或磁控杆。每个都有自己的特征和局限性。

2。receiver。接收者敏感freque的范围ncies being transmitted and provides amplification of the returned signal. In order to provide the greatest range, the receiver must be very sensitive without introducing excessive noise. The ability to discern a received signal from background noise depends on the signal-to-noise ratio (S/N).

这background noise is specified by an average value, called the noise-equivalent-power (NEP). This directly equates the noise to a detected power level so that it may be compared to the return. Using these definitions, the criterion for successful detection of a target is

p_r> (S/N) NEP,

在哪里p_r是返回信号的力量。由于这是确定雷达系统性能的大量数量_最小，被称为检测最小信号。

s_最小=（s/n）NEP

从s开始_最小, expressed in Watts, is usually a small number, it has proven useful to define the decibel equivalent, MDS, which stands for最小可识别信号。

MDS = 10 log（S_最小/1 MW）

使用分贝时，对数的括号内的数量必须是一个没有单位的数字。i MD的定义，此数字是分数S_最小/1 MW。提醒您，我们将特殊符号DBM用于MD的单位，其中“ M”代表1 MW。这是提到1 MW的分贝的速记，有时以DB // 1MW写入。

在接收器中，S/N设置了检测阈值，该阈值确定将显示的内容和将不会显示的内容。从理论上讲，如果S/N = 1，则仅以等于或大于背景噪声的功率返回。但是，噪声是一个统计过程，并且随机变化。NEP只是噪声的平均值。有时，噪声超过了接收器设定的阈值。由于这将显示并似乎是合法的目标，因此称为虚惊。If the SNR is set too high, then there will be few false alarms, but some actual targets may not be displayed known as a miss). If SNR is set too low, then there will be many false alarms, or a high false alarm rate (FAR).

somereceivers monitor the background and constantly adjust the SNR to maintain a constant false alarm rate, and therefore all called CFAR receivers.

somecommon receiver features are:

1.）脉冲积分。这receiver takes an average return strength over many pulses. Random events like noise will not occur in every pulse and therefore, when averaged, will have a reduced effect as compared to actual targets that will be in every pulse.

2.）灵敏度时间控制（STC）。此功能降低了海洋国家回报的影响。在每个脉冲传输后，它立即在短时间内降低接收器的最小SNR。调整STC的效果是减少直接在发射机周围区域中显示屏上的杂物。STC的值越大，将去除混乱的发射器范围越大。但是，过多的STC将消除发射器附近的潜在回报。

3.）快速时间常数（FTC）。This feature is designed to reduce the effect of long duration returns that come from rain. This processing requires that strength of the return signal must change quickly over it duration. Since rain occurs over and extended area, it will produce a long, steady return. The FTC processing will
过滤这些返回从显示屏中出来。只有迅速上升和下降的脉冲才会显示出来。从技术上讲，FTC是区分，这意味着它决定了信号的变化速率，然后用来区分没有快速变化的脉冲。



3。power Supply。这power supply provides the electrical power for all the components. The largest consumer of power is the transmitter which may require several kW of average power. The actually power transmitted in the pulse may be much greater than 1 kW. The power supply only needs to be able to provide the average amount of power consumed, not the high power level during the actual
脉冲传输。例如，可以将能量存储在电容器库中，例如在剩下的时间内。然后，在传输时可以将存储的能量放入脉冲中，从而增加峰值功率。峰值功率和平均功率与称为“占空比” DC的数量相关。占空比是雷达实际传输的每个传输周期的比例。指的是图2中的脉冲火车，可以看出占空比是：

DC = PW / PRF

synchronizer。同步器协调范围确定的时间。

它调节发送脉冲的速率（即设置PRF）并重置时间安排
clock for range determination for each pulse. Signals from the synchronizer are sent

simultaneously to the transmitter, which sends a new pulse, and to the display,
重置返回扫描。

双工器。This is a switch which alternately connects the transmitter or receiver to the antenna. Its purpose is to protect the receiver from the high power output of the transmitter. During the transmission of an outgoing pulse, the duplexer will be aligned to the transmitter for the duration of the pulse, PW. After the pulse has been sent, the duplexer will align the antenna to the receiver. When the next pulse is sent, the duplexer will shift back to the transmitter. A duplexer is not required if the transmitted power is low.

Antenna。天线从发射器中取出雷达脉冲，并将其放入空气中。此外，天线必须将能量聚焦到明确的光束上，从而增加功率并允许确定目标方向。天线必须跟踪其自身的方向，这可以通过同步传播器来实现。还有一些天线系统不会物理移动，而是以电子方式转向（在这些情况下，雷达梁的方向已经知道先验).

天线的梁宽度是量度的量度
辐射能量中最强大的部分。出于我们的目的，主要部分
称为主叶，将是从垂直线的所有角度
不少于峰值功率的½，或分贝为-3 dB。梁宽是
the range of angles in the main lobe, so defined. Usually this is resolved into
感兴趣的平面，例如水平或垂直平面。天线会
具有单独的水平和垂直梁宽。对于雷达天线，
beam-width can be predicted from the dimension of the antenna in the plane of
兴趣

问=l/l

在哪里:
问is the beam-width in radians,
l是雷达的波长，
l是朝着感兴趣的方向（即宽度或高度）方向的天线的尺寸。

In the discussion of communications antennas, it was stated that the
beam-width for an antenna could be found using问=2l/l。soit appears that radar
天线作为通信天线具有一半的梁宽度。区别
is that radar antennas are used both to transmit and receive the signal. The
每个方向组合的干扰效应，具有还原的作用
梁宽。因此，当描述双向系统（如雷达）时
适合在梁宽度中降低梁宽度½倍
近似公式。


这directionalgain天线的量度是对光束的状况的量度
Focused in all angles. If we were restricted to a single plane, the directional
收益仅是比率2p/问。since the same power is distributed over a
较小的角度范围，方向增益表示功率的数量
in the beam is increased. In both angles, then directional gain would be given by:

G_dir=4p/q f

since there are 4p与所有方向相对应的Steradians（测量的固体角度，测量
in steradians, is defined to be the area of the beam front divided by the range
平方，因此非方向光束将覆盖4个面积pr²at distance R,
因此4psteradians).

Here we used:
问=横梁宽度（弧度）
F=垂直梁宽度（弧度）

sometimes directional gain is measured in decibels, namely 10 log (G_dir).
例如，水平梁宽为1.5的天线⁰（0.025弧度）和
vertical beam-width of 20^o（0.33弧度）将有：

directional gain(dB) = 10 log (4p/0。025 0.333) = 30.9 dB

Example: find the horizontal and vertical beam-width of the AN/SPS-49 long
range radar system, and the directional gain in dB. The antenna is 7.3 m wide
by 4.3 m tall, and operates at 900 MHz.

波长，l=c/f = 0.33 m.

Given that L= 7.3 m, then
问=l/l = 0.33/7.3 = 0.045弧度，或
问=3⁰。

这antenna is 4.3 m tall, so a similar calculation gives
F=0。076 radians
F=4⁰。



这directional gain,
G_dir=4p/(0.045 0.076) = 3638.

以分贝表示
directional gain = 10 Log(3638)
=35.6 dB.

展示。这display unit may take a variety of forms but in general is designed to present the received information to an operator. The most basic display type is called an A-scan (amplitude vs. Time delay). The vertical axis is the strength of the return and the horizontal axis is the time delay, or range. The A-scan provides no information about the direction of the target.

图4

最常见的显示是PPI（计划位置指示器）。A-scan信息将转换为亮度，然后以与天线方向相同的相对方向显示。结果是对距离距离的情况的自上而下的视图。PPI可能是操作员最自然的显示，因此是最广泛使用的。在这两种情况下，同步器都为每个脉冲重置迹线，以使范围

information will begin at the origin.

图5

在此示例中，使用增加的STC抑制海上混乱将有所帮助。

雷达性能

AlloFthe parameters of the basic pulsed radar system will affect the
perFormance in some way. Here we find specific examples and quantify this dependence
在哪里possible.

脉冲宽度

脉搏的持续时间和沿径向方向的目标长度
determine the duration of the returned pulse. In most cases the length of the
return is usually very similar to the transmitted pulse. In the display unit, the
pulse (in time) will be converted into a pulse in distance. The range of values
从前沿到后边缘将在
范围到目标。以表面价值为准，能够准确测量范围
由脉冲宽度确定。

如果我们将测量范围内的不确定性指定为范围分辨率，则
r_res，那么它必须等于脉冲宽度的范围，即：

r_res=c PW/2

now, you may wonder why not just take the leading edge of the pulse as the
范围哪个可以精确地确定？问题是它是
几乎不可能创建完美的前沿。实际上，理想
脉冲真的看起来像：

图6

To create a perfectly formed pulse with a vertical leading edge would require an infinite bandwidth. In fact you may equate the bandwidth,b，发射器the minimum pulse width, PW by:

PW = 1/2b

鉴于这种见解，可以说范围可以确定不超过CPW/2或等效地确定范围是很合理的

r_res= C/4b

In fact, high resolution radar is often referred to as wide-band radar which you now see as equivalent statements. One term is referring to the time domain and the other the frequency domain. The duration of the pulse also affects the minimum range at which the radar system can detect. The outgoing pulse must physically clear the antenna before the return can be processed. Since this lasts for a time interval equal to the pulse width, PW, the minimum displayed range is then:

r_最小=c PW/2

可以在PPI显示器上看到最小范围效果作为饱和区域或空白区域
around the origin.

图7

增加脉冲宽度的同时保持其他参数也将影响占空比，从而影响平均功率。对于许多系统，希望保持平均功率固定。然后，必须与PW同时更改PRF，以使产品PW X PRF保持不变。例如，如果将脉冲宽度减少为½倍以改善分辨率，则PRF通常会加倍。

脉冲重复频率（PRF）

脉冲传输的频率会影响最大范围
displayed. Recall that the synchronizer resets the timing clock as each new pulse
传输。从遥远的目标返回，直到无法到达接收器
after the next pulse has been sent will not be displayed correctly. Since the timing
clock has been reset, they will be displayed as if the range where less than actual.
如果可能的话，那么范围信息将被认为是模棱两可的。
An operator would not know whether the range were the actual range or some greater
价值。

图8

这maximum actual range that can be detected and displayed without ambiguity, or最大明确范围，仅是对应于等于脉冲重复时间的时间间隔的范围。因此，最大明确范围

r_Unamb=c PRT/2 = c/(2PRF)

when a radar is scanning, it is necessary to control the scan rate so that a
足够数量的脉冲将以任何特定的方向传输
toguarantee reliable detection. If too few pulses are used, then it will more
很难区分错误目标。假目标可能存在
in one or two pulses but certainly not in ten or twenty in a row. Therefore to
保持较低的错误检测率，每种脉冲的数量
方向应保持较高，通常以上是十个。

对于脉搏重复率高的系统（频率），雷达梁
can be repositioned more rapidly and therefore scan more quickly. Conversely, if the
prfis lowered the scan rate needs to be reduced. For simple scans it is easy to
问uantify the number of pulses that will be returned from any particular target. Let
trepresent thedwell time, which is the duration that the target remains in the
每次扫描过程中雷达的光束。脉冲的数量，n，目标是
在停留期间暴露于：

n=tprf

wemay rearrange this equation to make a requirement on the dwell time for a particular scan

t_最小=n_最小/prf

soit is easy to see that high pulse repetition rates require smaller dwell times. For a continuous circular scan, for example, the dwell time is related to the rotation rate and the beam-width.

t=问/w

在哪里q =梁宽度[度] W =旋转速率[度/秒]，该速率将在几秒钟内提供停留时间。可以将这些关系组合在一起，从而给出以下等式which the maximum scan rate may be determined for a minimum number of pulses per scan:

w_MAX=问prf/n

雷达Frequency

Finally, the frequency of the radio carrier wave will also have some
影响雷达梁传播的方式。在极端频率的雷达横梁下
会在大气中折射，并可能被“管道”捕获
范围。在高极端，雷达梁的表现很像可见光和
travel in very straight lines. Very high frequency radar beams will suffer high
lossesand are not suitable for long range systems.

频率也会影响梁宽度。对于相同的天线大小，
a low frequency radar will have a larger beam-width than a high frequency one.
In order to keep the beam-width constant, a low frequency radar will need a large
antenna.

这oretical Maximum Range Equation

Aradar receiver can detect a target if the return is of sufficient strength.
让我们指定可以检测到的最低返回信号_最小, which should
have units of Watts, W. The size and ability of a target to reflect radar energy can
besummarized into a single term,s，称为雷达横截面，有单位
m²。If absolutely all of the incident radar energy on the target were reflected
同样在所有方向上，雷达横截面将等于目标的
cross-sectional area as seen by the transmitter. In practice, some energy is absorbed
并且反射能量并非在各个方向上平均分布。因此，
雷达横截面很难估算，通常由
measurement.

鉴于这些新数量，我们可以为雷达功率构建一个简单的模型
that returns to the receiver:

p_r=p_tG1/4pr²s1/4pr²A_e

该等式中的术语已被分组，以说明从传输到收集的序列。这是详细的序列：

g =rG_dir

发射器伸出峰值功率P_tinto the antenna, which focuses it into a beam with gain G. The power gain is similar to the directional gain, G_dir，除了它还必须包括从发射器到天线的损失。这些损失由单个术语总结以提高效率，r。这reFore

雷达能量在所有方向均匀地散布。因此，随着面积的增加，每单位面积的功率必须降低。由于能量分布在球的表面上，因此1/4pr²accounts for the reduction.

这radar energy is collected by the surface of the target and reflected. The radar cross sections这两个过程都说明了这两个过程。

反射的能量就像传输能量一样传播。

接收天线收集与其有效区域成比例的能量，称为天线的光圈A_e。这还包括接收过程中的损失，直到信号到达接收器。因此，下标“ E”以“有效”。有效的光圈与物理孔径有关，a，通过在功率增益中使用的效率术语相同，给定符号r。以便

A_e=rA

我们检测到的标准仅仅是接收的功率P_r必须
exceed the minimum, S_最小。since the received power decreases with range, the maximum
detection range will occur when the received power is equal to the minimum , i.e.
p_r=s_最小。如果您解决该范围，则获得了最大理论的方程式
radar range:

该方程式的最重要特征也许是第四根依赖性。这样做的实际含义是，必须大大提高输出功率，以适度提高性能。例如，为了使范围增加一倍，必须增加16倍的传输能力。您还应注意，检测的最小功率水平，_最小，取决于噪声水平。实际上，该数量不断变化，以在容易受到噪声和低灵敏度的高灵敏度之间达到完美的平衡，这可能会限制雷达检测目标的能力。示例：在给定以下数据时，找到AN/SPS-49雷达的最大范围

Antenna Size = 7.3 m wide by 4.3 m tall
效率= 80％
peak power = 360 kW
Cross section = 1 m²
s_最小=1 10^-12w


从上一个示例中我们知道，定向天线的增益，

G_dir=4p/QF=4p/(.05 x .07) = 3430

这power gain,
g =rG_dir

g =2744.

同样的, the effective aperture,
A_e=rA=.8（7.3 x 4.3）

A_e= 25.1 m²。

因此范围是， 或者

r=112 km.