【机械类毕业论文中英文对照文献翻译】雷达概述
【机械类毕业论文中英文对照文献翻译】雷达概述,机械类毕业论文中英文对照文献翻译,机械类,毕业论文,中英文,对照,对比,比照,文献,翻译,雷达,概述
黄河科技学院毕业设计(文献翻译) 第 4 页 毕业设计文献翻译 院(系)名称工学院机械系 专业名称机械设计制造及其自动化 学生姓名罗睿桢 指导教师王飞2012年 03 月 10 日 雷达概述雷达是利用无线电波来测定物体位置的无线电设备。英文radar原是“无线电探测与定位”的英文缩写,其基本任务是探测感兴趣的目标,测定有关目标的距离、方问、速度等状态参数。主要由天线、发射机、接收机(包括信号处理机)和显示器等部分组成。那么,雷达具体是怎么工作的呢,下面来做简单地介绍。雷达是通过比较发射信号和目标产生的反射信号来获取信息的。反射信号只能说明目标的存在,但仅仅知道这些是不够的,有时候还必须了解更多关于目标的信息。所以就要求雷达不仅能够提供目标的位置,还应该反映出更对的关于目标的信息。如目标物体的样式、方向、速度等。测量距离或范围也是雷达的一个重要功能。雷达主要是通过测定发射的信号和接收到的反射波的时间延迟来进行距离或是范围的测量。到目前为止,还没有其它的任何一种传感器能够像雷达一样测定到远距离目标的动态范围。一些专用的特殊雷达能够精确的测定千米以外的目标范围,其精确度可达厘米。小到交通测速大到测量临近星球的距离都可以用雷达来实现,雷达的应用广泛由此可见一斑。几乎所有雷达都带有定向天线。定向天线不仅为微弱信号提供传递增益和必要的接收孔径,而且它的窄波束宽度能够确定目标的方位。一个典型的雷达波束宽度一般为12度。波束宽度还决定了角坐标分辨率,但后者具有更高的准确度。对一个典型的雷达来说,出现波束分裂是很正常的。一些雷达还能更精准的检测角坐标的准确度,最好的追踪雷达甚至可能达到0.1毫拉德的均平方根误差。由于多普勒效应,雷达所发射的波束从一个移动的目标反射所产生的回波会产生电子频移多普勒效应用于测定相对速度。相对速度还可以通过变动率的范围来确定。追踪雷达通常就是用这种方法来确定目标的相对速度,而不是多普勒频移。但是,用来监测和追踪地球以外的目标,例如卫星和飞船的雷达通常直接利用多普勒频移来测量目标的相对速度,但是这一方法很少被用于飞行器的监视雷达。在飞行器监视雷达中,多普勒频移被用于区分所需的运动目标和不需要的固定杂波回波,如活动目标指示雷达。如果雷达可以从多个方位对目标进行观测,就可以确定它的形状。空间物体识别雷达就是能够提取目标形状信息的例子。另外一个例子就是能够描绘地形的合成孔径雷达。能够确定目标形状的雷达有时也称成像雷达。通过确定距离分辨力和角分辨力可以得到目标大小和形状。好的距离分辨率通常比同等的角分辨率更容易获得。在一些雷达应用中,如果监测分散式或是与雷达之间有相对运动的物体时,可以用多普勒频率分辨率代替角分辨率。这是因为一小部分的分散目标有不同的相对速度,所以测得的分辨率是不可用的。这一原理被用于合成孔径雷达地面测绘,逆SAR地面成像,空间物体识别,星体成像和根据入射角测量地表和海洋回波散射。目标探测雷达是一种专用监视雷达,通常与控射雷达紧密相连。这种雷达主要用于监控一个相当有限的扫描区域并且为射控雷达或者武器本身提供打击目标的指定数据。目标探测雷达既可以单独使用,又可以多功能雷达的模式使用。这两种方法都有人使用,就目前来说更趋向于多功能雷达,即雷达同时执行目标探测,武器引导和控制的功能(如爱国者相控阵雷达)。测高雷达具是一种经典的多功能雷达,目前仍被应用于许多空中防御系统。这种雷达的功能是通过二维监视雷达提供选中目标的高度基准。测高雷达一般工作在C频和D频波段,并且提供一定程度的与二维监视雷达相一致的ECCM频率分隔,通常工作在L波段和较低频率。典型的测高雷达(FPS-6) CAN可在四秒内旋转至方位角内的任一位置。然后雷达在高程点以每分钟二十到三十赫兹的频率摆动。方位波束宽度近似于三度,从而确立了二维监控雷达的越区转接精确度。使用两个测高雷达的空中防御系统以平均每分钟四十的频率摆动。峰点是通过测量目标仰角和范围,再根据三角形关系确定的。高度精确度是范围的函数,并且是以目标区间范围内每公里一到两米为标准的。在当今的空中防御系统中,由于目标密度过于密集因此需使用三维监视雷达。现代的三维监视雷达可在五到十秒内提供在它监视范围内的每一个目标的高度信息。三D雷达的使用同时也推动了平衡雷达频率(通常在S波段)的使用。应用于Hawk地空导弹系统的连续波搜索雷达也是目标探测雷达的一种。这种雷达用于搜索探测视野(零到四度高程范围)内的低空飞行器和导弹,但是在带有活动目标指示器的传统脉冲雷达中通常被地面杂波屏蔽。高效率发射机(X波段)用于使雷达多路径的影响最小,这种多路径会在低高度(htR4ifa)产生破坏性干扰。窄频带多普勒滤波器储库从杂波中提取目标信息并且允许检测射线的速率。频率调制对具有足够准确度的测量目标范围的传送波形有影响以便选择指定的目标跟踪雷达。顺时针方向行波的射频应用使得探测深嵌在地面杂乱回波中的目标成为可能。此外,这套系统也使得子过程杂波在100200dB的范围内是可见的。发射器渗漏是可能引起雷达性能降低,可以使用独立的接收和发射天线来减少渗漏。除此以外,空间碰撞技术,即把发射器载体的一个异相样品增添至接收器也可用来消除辐射渗漏。雷达是利用无线电波来测定物体位置的无线电设备。是“无线电探测与定位”的英文缩写,其基本任务是探测感兴趣的目标,测定有关目标的距离、方问、速度等状态参数。主要由天线、发射机、接收机(包括信号处理机)和显示器等部分组成。雷达工作时是通过发射机产生足够的电磁能量,经过收发转换开关传送给天线。天线将这些电磁能量辐射至大气中,集中在某一个很窄的方向上形成波束,向前传播。电磁波遇到波束内的目标后,将沿着各个方向产生反射,其中的一部分电磁能量反射回雷达的方向,被雷达天线获取。天线获取的能量经过收发转换开关送到接收机,形成雷达的回波信号。由于在传播过程中电磁波会随着传播距离而衰减,雷达回波信号非常微弱,几乎被噪声所淹没。接收机放大微弱的回波信号,经过信号处理机处理,提取出包含在回波中的信息,送到显示器,显示出目标的距离、方向、速度等。目前这种雷达所面临的挑战主要是具有敏锐洞察力的高速低空飞行的飞行器以及大量的低空飞行巡航导弹,运用弹出式和超低空飞行策略的直升机,在雷达视野内出现时间极短。缩短了雷达执行功能的可用时间尺度,哪怕几分之一秒都是重要的。考虑到它不仅能够探测目标,并且能在重电场和多目标环境中识别目标并产生一个轨道矢量,Hawk CWAR 面对这种威胁时扫描整个检测范围至少需要三秒的时间。雷达天线是自由空间传播和导行波(传输线)传播之间的换能器。其功能是在雷达发射过程中将辐射的能量集中成一字形能量束照亮目标所在的方向,从而可以发现目标。接收回波时天线收集目标所反射的波束中所含的能量,并传送到接收器。因此,雷达天线在雷达工作过程中担任着发射和接收波束的任务。在这种工作模式下,其主要目的是精确地确定目标的角方向。因此,高度精确的指令(窄)波束宽度是非常有必要的,它能够获得精准的测角精度。雷达天线的性能指标不仅仅是表现在波束的宽度,也和发射增益和有效接收孔径有关联,后两个参数彼此之间成正比关系,检测范围和测角精度也存在直接的关系。上述雷达天线的是集发射和接收功能于一体的,这种天线广泛应用于现代雷达系统。但也一些非民用的雷达使用的是两个功能独立的天线,也就是所谓的收发分属的雷达,这种雷达拥有两个相互独立的发射和接收的天线。雷达天线按照作用原理不同可分为两大类,光学天线和相控阵天线。光学天线即是基于光学原理的天线,包括两个分组:反射面天线和透镜天线。到目前为止反射面天线仍然被广泛用于雷达系统,而透镜天线,虽然仍在一些通信和电子战(EW)中使用,但已经不再用于现代雷达系统;Information Available from RadarA radar obtains information about a target by comparing the received echo signal with the transmitted signal .The availability of an echo signal indicates the presence of a reflecting target ;but knowing a target is present is of litter use by itself. Something more must be known. Therefore, radar provides the location of the target as well as its presents. It can also provide information about the type of target. This is known as target classification.The time delay between the transmission of the radar signal and the radar signal and the receipt of an echo is a measure of the distance, or range, to the target. The range measurement is usually the most significant a radar makes. No other sensor has been able to compete with radar for determining the range to a distant target. Typical radar might be able to measure range to an accuracy of several hundred meters, but accuracies better than a fraction of a meter are practical. Radar ranges might be as short as that of the police traffic-speed- meter, or as long as the distances to the nearby planets. Almost all radars utilize directive antennas. A directive antenna not only provides the transmitting gain and receiving aperture needed for detecting weak signals, but its narrow beam width allows the targets direction to be determined. Typical radar might have a beam width of perhaps one or two degrees. The angular resolution is determined by the beam width, but the angular accuracy can be considerably better than the beam width. A ten to one beam splitting would not be unusual for typical radar. Some radar can measure angular accuracy considerable better than this. An rms error of 0.1 mrad is possible with the best tracking radars. The echo from a moving target produces a frequency shift due to the Doppler Effect, which is a measure of the relative velocity. Relative velocity also can be determined from the rate of change to range. Tracking radars often measure relative velocity in this manner rather than use the Doppler shift. However, radars for the surveillance and tracking of extraterrestrial targets, such as satellites and spacecraft, might employ the Doppler shift to measure directly the relative velocity, but it is seldom used for this purpose in aircraft-surveillance radars. Instead, aircraft-surveillance radars use the Doppler frequency shift to separate the desired moving targets from the undesired fixed clutter echoes, as in MTI radars.If the target can be viewed from many directions, its shape can be determined. Space object identification (SOI) radars are an example of those that extract target shape information. The synthetic aperture radar (SAR) which maps the terrain is another example .Radars that determines the shape of a target is sometimes called imaging radars.To obtain the target size or shape requires resolution in range and in angle. Good range resolution is generally easier to achieve than comparable resolution in angle. In some radar applications it is possible to utilize resolution in the Doppler frequency shift as a substitute for resolution in angle, if there is relative motion between the distributed target and the radar. Resolution is possible since element of the distributed target has a different relative velocity. This principle has been used in synthetic aperture radars for ground mapping, inverse SAR for SOI and the imaging of planets, and in the scatterometter for measuring the ground or sea echo as a function of incidence angle. Target Acquisition RadarsTarget acquisition radars are a special form of surveillance radar, generally associated with weapon control radar. The function of this type of radar is to search a relatively limited surveillance volume and obtain target designation data for the weapon control radar, or in some cases for the weapon itself.The target acquisition radar can either be independent radar or a mode of multifunction radar. There are proponents of either type, but the current trend is towards multifunction where the radar performs both target acquisition and w weapon guidance and control functions (e.g. the patriot phased array radar) A classic form of this type of radar is the height-finding radars still in use in many currently operational air-defense systems. The function of these radars is to provide altitude data on selected targets by way of associate 2-D surveillance radars. Height-finding radars typically work at radar C-band or S-band frequencies and provide a degree of ECCM frequency diversity in conjunction with their associated 2-D surveillance radars, which typically work at L band and lower frequencies.Typical height-finding radar (FPS-6) CAN slew in azimuth to any target within four seconds. It then nods in elevation at a rate of 20 to 30 nods per minute. The azimuth beam width is on the order of three degrees, which establishes the hand-off accuracy of the 2-D surveillance radar. An average of 40 heights per minute is typical for an air defense system employing two height-finding radars. The height is estimated by measuring the targets elevation angle and range, and then solving the triangular relationship to determine height. The height accuracy is a function of range and is the order of 1 to 2m per km of target range.In modern air defense system, the expected target density is such that 3-D surveillance radars are employed. Modern 3-D surveillance radar can supply target height information for every target within its surveillance volume within a typical five to ten second frame time. The use of a 3-D radar forces a compromise radar frequency (usually in S band) to be employed. The height-finding accuracy improves with increasing frequency, which dictates a minimum S-band frequency for a reasonable vertical aperture, while the 2-D surveillance requirement generally favors L-band frequency, which is compatible with solid-state transmitter operation, by using an extended vertical aperture.Another example of a target acquisition radar is the continuous-wave acquisition radar (CWAR) used in the Hawk missile system. The function of this radar is to search the horizon (zero to four degree elevation coverage) to detect low-flying aircraft or missiles, which would normally be screened by ground clutter in a conventional pulse-type radar with MTI.A high transmitter frequency (X band) is used to minimize the effect of radar multipath, which can cause a destructive interference at low altitudes (htR4ifa). A bank of narrowband Doppler filters extracts the target from the clutter and allows a determination of its radial velocity. A frequency modulation (FM) is imposed on the transmitted waveform to measure range to the target with sufficient accuracy to select targets for designation to the tracking radar.The CW frequency operation provides the ability to detect targets that are heavily imbedded in ground clutter. Sub clutter visibilities in the 100-200 dB regions are feasible with this type of system. Transmitter leakage is a problem which may reduce performance. Separate receive and transmit antennas are used to reduce leakage, in addition to a nulling technique whereby an out-of-phase sample of the transmitter carrier is injected into the receiver to cancel the radiated leakage.The current threat facing this type of radar consists of high-speed/low-level penetrating aircraft, large numbers of low-flying cruise missiles, and helicopters using pop-up and nap-of-earth tactics causing them to be in view for only a short time. This compresses the time scale available for the acquisition radar to perform its function to the point where even fractions of a second are important. The minimum three-second time period it takes the Hawk CWAR to search the surveillance volume is marginal for this kind of threat, considering that it must not only detect the target, but also identify it and generate a track vector possibly in the presence of a heavy ECM and multi-target environment.Radar SubsystemsThe basic role of the radar antenna is to provide a transducer between the free-space propagation and the guided-wave propagation of electromagnetic waves. The specific function of the antenna during transmission is to concentrate the radiated energy into a shaped directive beam which illuminates the targets in a desired direction. During reception the antenna collects the energy contained in the reflected target echo signals and delivers it to the receiver. Thus the radar antenna is used to fulfill reciprocal but related roles during its transmit and receive modes. In both of these modes or roles, its primary purpose is to accurately determine the angular direction of the target. For this purpose, a highly directive (narrow) beam width is needed, not only to achieve angular accuracy but also to resolve targets close to one another. This important feature of a radar antenna is expressed quantitatively in terms not only of the beam width but also of transmit gain and effective receiving aperture. These latter two parameters are proportional to one another and are directly related to the detection range and angular accuracy.The above functional description of radar antennas implies that a signal antenna is used for both transmitting and receiving. Although this holds true for most radar systems, there are exceptions: some monocratic radars use separate antennas for the two functions; and, of course, biostatic radars must, by definition, have separate transmit and receive antennas.Radar antennas can be classified into two broad categories, optical antennas and array antennas. The optical category, as the name implies, comprises antennas based on optical principles and includes two subgroups, namely, reflector antennas and lens antennas. Reflector antennas are still widely used for radar, whereas lens antennas, although still used in some communication and electronic warfare (EW) application, are no longer used in modern radar systems.
收藏