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單位代碼 0 2
學(xué) 號
分 類 號 TH
密 級
畢業(yè)設(shè)計(jì)
文獻(xiàn)翻譯
院（系）名稱
工學(xué)院機(jī)械系
專業(yè)名稱
機(jī)械設(shè)計(jì)制造及其自動化
學(xué)生姓名
指導(dǎo)教師
2012年 03 月 10 日
黃河科技學(xué)院畢業(yè)設(shè)計(jì)（文獻(xiàn)翻譯） 第12頁
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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.
● 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 civilairport-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?13 watts. 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, 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 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.
2 APPLICATIONS 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, no cooperative 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, anti radiation 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 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.
3 MECHANICAL DESIGN CONSIDERATIONS
Reflector mechanical design is a detailed discipline unto itself with a multitude of factors to consider. Furthermore, designs vary significantly depending upon many factors, including platform, reflector size, environment, frequency of operation, scan/FOV, and cost constraints. Although it is not within the scope of this chapter to address mechanical design in detail, a brief survey of design factors and considerations pro-vides some useful insights.
The platform, i.e., vehicle, or installation site is a significant driver for radar sensors in general, including the antenna. Platform is a generic term referring to the vehicle where the radar and antenna are installed. Typical radar platforms include pedestal (fixed site), ground vehicles, ships, airplanes, UAVs, and spacecraft/satellites. The following short section is devoted to platform impacts and some key associated design drivers. These include mass, volume (stowage/deployment), gimbals (precision
mechanical positioning systems), materials, and mechanical tolerances. Finally, there is a brief discussion of environmental design considerations and radomes.
The degree to which these five factors, mass, volume, stowage, deployment, and gimbaling drive the reflector design vary in accordance with the reflector system and the platform. However, mass and volume constraints generally have a significant impact on the reflector system design. Furthermore, some sort of stowage and deployment of the reflector is sometimes required, especially for larger reflectors. These considerations and constraints drive the choices of materials, structural designs, passive and active mechanisms, etc. It is beyond the scope of this chapter to address this topic in detail. However, it’s useful to show a couple examples for illustration.
Consider, first, a ground-based dual-reflector design with a 9-meter main reflector aperture. This reflector, shown in Figure 12.37, is used for an S-band meteorological radar application.48 The panelized aluminum reflector is mechanically scanned via use of a gimbal (not shown). The feed, a dual-polarized waveguide horn, is also shown in Figure 12.37. The structural design of this large reflector was a significant task driven by the need to maintain low reflector surface distortion (less than 50 mils) with severe wind and gravity-loading forces and thermal gradients.
The second example is a space-based deployable reflector. The mesh reflector, shown in both stowed and deployed configurations in Figure 12.38, is an offset reflector with a 12.25 meter circular projected aperture. This L-band design, developed by Northrop-Grumman Space Technologies Astro Aerospace group, has been successfully launched and deployed and is currently in use on several communication satellites. A total of five reflectors of aperture diameters 9 meters, 12 meters, and 12.25 meters have been flown. Studies have addressed the potential usage of this reflector technology for various space-based radar applications, including weather sensing/monitoring (NEXRAD)and planetary SAR mapping missions (lunar and Mars). Significant features of this reflector include its precise surface accuracy, high stiffness and stability, low mass, and reliable deployment. For example, for the reflector shown in Figure 12.38, an RMS surface accuracy of less than 50 mils from all error sources including in-orbit thermal gradients was achieved via prudent material choices and matching of the associated material coefficients of thermal expansion (CTEs). Pointing precision due to eclipse thermal snap has been measured in orbit at less than 0.01 degrees.
4 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.
● 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.
● Marcums 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 capabili-ties 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.
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 雷達(dá)簡介
雷達(dá)是一種電磁傳感器，用來對反射性物體檢測和定位。其工作可歸納如下:
(1)雷達(dá)通過天線輻射電磁能量，使其在空中傳播。
(2) 部分輻射的能量被離雷達(dá)某個(gè)距離上的反射體(目標(biāo))截獲。
(3)目標(biāo)截獲的能量重新輻射到許多方向上。
(4) 一部分重新輻射的(因波)能量返回至雷達(dá)天線，并被雷達(dá)天線所接收。
(5) 在被接收機(jī)放大和合適的信號處理后，在接收機(jī)的輸出端做出目標(biāo)回波信號是否存在的判決。此時(shí)，目標(biāo)的位置和可能其他有關(guān)目標(biāo)的信息就得到了。
雷達(dá)輻射的一種常用波形是一串窄的類似矩形的脈沖。例如，中距離雷達(dá)用來探測飛機(jī)的波形可以描述為持續(xù) 1 微秒( vs) 的短脈沖:脈沖間隔可能為 lms (因此脈沖重復(fù)頻率為1 kHz); 雷達(dá)發(fā)射機(jī)峰值功率可能為 1 兆瓦C1MW); 由這些數(shù)據(jù)得出發(fā)射機(jī)的平均功率為 1千瓦C1kW) 0 lkW 的平均功率可能比一個(gè)"典型"教室中電燈的功率要小。我們假設(shè)這部作為例子的雷達(dá)工作在微波E頻段的中部，例如從 2.7-2.9GHz，這是民用機(jī)場監(jiān)視雷達(dá)的典型頻段。其波長約為 10cm (簡單起見取整數(shù))。使用合適的天線，這樣一部雷達(dá)可以探測
到距離③50-60n mile 左右的飛機(jī)。雷達(dá)從目標(biāo)接收到的回波功率可以在很寬范圍的值上變化，但為了示范的目的，我們?nèi)我饧僭O(shè)典型的回波信號可具有1O-13W 的功率。如果輻射的功率是 106W C1 MW) ，則此例中目標(biāo)回波信號功率與雷達(dá)發(fā)射機(jī)功率的比為 10-19 ，或接收回波比發(fā)射信號低 190dB 。這是發(fā)射信號和一個(gè)可檢測的接收回波信號幅度之間巨大的差別。
一些雷達(dá)需要在短到像棒球場上從本壘到投手間的距離上檢測目標(biāo)(為測量投出球的速度)，而其他雷達(dá)則需要在大到至最近的行星的距離上工作。因此，雷達(dá)可能小到足夠握在手中，或大到足夠占據(jù)許多個(gè)足球場的空間。
雷達(dá)目標(biāo)可能為飛機(jī)、艦船或者導(dǎo)彈:也可能為人、烏、昆蟲、降雨、晴空空氣端流、電離的媒質(zhì)、地表特征(植被、山脈、道路、河流、機(jī)場、建筑、圍墻、電線桿等)、海洋、冰層、冰山、浮標(biāo)、地下特征、流星、極光、宇宙飛船及行星。除了測量H 標(biāo)距離和角度方向之外，雷達(dá)通過確定距離隨時(shí)間的變化率或從回波的多普勒頻移申提取徑向速度來確定目標(biāo)的相對速度。如果在一段時(shí)間內(nèi)測量動目標(biāo)的位置，則可以得到目標(biāo)軌跡或航跡，從中可以判定目標(biāo)的絕對速度和運(yùn)動方向，于是可以對其未來的位置做出預(yù)測。合適設(shè)計(jì)的雷達(dá)可以判定目標(biāo)的尺寸和形狀，甚至可以識別不同類型的目標(biāo)。
2 雷達(dá)應(yīng)用
因?yàn)榉烙匦蛙娪棉Z炸機(jī)的需要，在 20 世紀(jì) 30 年代發(fā)明了雷達(dá)。對雷達(dá)軍事上的需要或許是雷達(dá)最重要的應(yīng)用及其主要進(jìn)展(包括民用雷達(dá)的進(jìn)展)的來源。
軍用雷達(dá)的主要用途曾是地面、海面和機(jī)載空中防御。離開雷達(dá)，實(shí)施成功的空中防御是不切實(shí)際的。在空中防御中，雷達(dá)用來進(jìn)行遠(yuǎn)程監(jiān)視、低海拔"彈出"目標(biāo)的短距探測、武器控制、導(dǎo)彈制導(dǎo)、非合作目標(biāo)識別和戰(zhàn)斗損傷評估。許多武器中的近炸引信也是雷達(dá)的一個(gè)例子。對軍用防空雷達(dá)成功的一個(gè)極好度量是在反抗其有效性的方法上花費(fèi)的大量金錢。這包括電子對抗措施和其他方面的電子戰(zhàn)、尋的雷達(dá)信號的反輻射導(dǎo)彈及低截面積飛機(jī)和艦船。雷達(dá)在軍事上也用在對地面、海面的偵察及海洋監(jiān)視中。
在戰(zhàn)場上，要求雷達(dá)具有執(zhí)行空中監(jiān)視(包括對飛機(jī)、直升機(jī)、導(dǎo)彈和無人機(jī)的監(jiān)視)、空中攔截武器控制、敵方武器定位(迫擊炮、火炮和火箭)、入侵人員檢測和空中交通管制等任務(wù)的功能。
3 機(jī)械設(shè)計(jì)方面的考慮
反射面天線的機(jī)械設(shè)計(jì)是一門精細(xì)的學(xué)科，要考慮很多因素。依據(jù)諸如平臺、天線尺寸、工作環(huán)境、工作頻率、掃描IFOV 和成本等多種因素有多種不同的設(shè)計(jì)。限于篇幅，本節(jié)不對機(jī)械設(shè)計(jì)詳細(xì)敘述，僅對設(shè)計(jì)須考慮的因素做一概括，提供有用的深入了解。
載車或安裝場地等平臺通常是包括天線在內(nèi)的雷達(dá)傳感器的重要承載部件。平臺是載有雷達(dá)和天線的運(yùn)輸工具的統(tǒng)稱。典型的雷達(dá)平臺包括基座(固定站點(diǎn))、地面汽車、艦船、飛機(jī)、無人駕駛飛行器、太空船或衛(wèi)星等。下面一小段說明平臺的影響和確定設(shè)計(jì)的關(guān)鍵因素。這些因素包括平臺的重量、體積(折疊/展開)、精密機(jī)械指向系統(tǒng)、材料、機(jī)械公差等。最后，簡要討論了關(guān)于環(huán)境的設(shè)計(jì)上的考慮和天線罩。對于不同的反射面天線系統(tǒng)和安裝平臺，質(zhì)量、體積、折疊、展開和精密機(jī)械指向系統(tǒng)等五個(gè)因素對反射面天線設(shè)計(jì)的影響程度各不相同，必須靈活設(shè)計(jì)。但設(shè)計(jì)反射面天線系統(tǒng)時(shí)，質(zhì)量和體積通常是主要影響因素。此外，有時(shí)還需要考慮折疊和展開，特別是大型反射面。這些要求和約束影響到材料的選擇、結(jié)構(gòu)形式設(shè)計(jì)、機(jī)械自動控制或手動控制等。詳細(xì)論述這些專題已經(jīng)超出了本章的范疇。然而，舉兩個(gè)例子說明還是很有益的。
第一個(gè)例子是主反射面孔徑為 9m 的地基雙反射面天線，如圖 12 .3 7 所示。該反射面天線用于 S 頻段的氣象雷達(dá)I4810 鋁板鑲嵌制成的反射面夭線通過精密指向系統(tǒng)(圖中看不到)的轉(zhuǎn)動進(jìn)行機(jī)械掃描。圖 12 .3 7 中顯示了雙極化波導(dǎo)喇叭饋源。對于這樣大型的反射面天線，在大風(fēng)載、重力及熱量梯度的情況1'"，反射面天線要保持較低的表面變形(小于 50 密耳)的結(jié)構(gòu)設(shè)計(jì)是一項(xiàng)非常重要的工作。
第二個(gè)例子是空間可展開反射面天線。該反射面天線系網(wǎng)狀，偏置饋電，圓投影孔徑直徑為 12.2 5m，折疊和展開兩種狀態(tài)如圖 12.3 8 所示[49] 。它是 North-Grumman 空間技術(shù)航天研發(fā)小組研制的 L 波段天線，己經(jīng)成功發(fā)射并展開，目前正用于多顆通信衛(wèi)星[50] 。共有五部天線在空中運(yùn)行，孔徑為 9m、 12m 和 12.25m。對于各種星載雷達(dá)，包括氣象測量/監(jiān)視雷達(dá)(NEXRAD) [51] 和行星 SAR 成像雷達(dá)系統(tǒng)(探月和火星)，人們對這種反射面天線的應(yīng)用潛力進(jìn)行了大量研究。這類反射面天線的重要特點(diǎn)是高表面精度、高強(qiáng)度、高穩(wěn)定性、輕質(zhì)量及可靠的展開性。例如，對于圖 12 .3 8 所示的反射面天線，在考慮所有誤差源條件下，包括軌道上的熱梯度[通過謹(jǐn)慎選擇材料及正常材料的熱膨脹系數(shù) (CTE)] 匹配，反射面表面的均方根誤差小于 50 密耳。經(jīng)過在軌測量，由于日照熱劇變引起的指向偏差小于 0.010 。
4 雷達(dá)過去的一些進(jìn)展
下面簡單列出了雷達(dá)技術(shù)和性能在 20 世紀(jì)中一些主要的進(jìn)展，按不很精確的年代順序排列，如下所示:
(1)第二次世界大戰(zhàn)之前和第二次世界大戰(zhàn)期間，開發(fā)為防空部署在地面、艦船和軍用飛機(jī)上的 VHF 雷達(dá)。
(2) 第二次世界大戰(zhàn)早期微波磁控管的發(fā)明和波導(dǎo)技術(shù)的應(yīng)用，以獲得能在微波頻段工作的雷達(dá)，從而可使用更小和機(jī)動性更強(qiáng)的雷達(dá)。
(3 )MIT 輻射實(shí)驗(yàn)室在第二次世界大戰(zhàn)期間存在的五年中開發(fā)了超過 100 種不同的雷達(dá)型號，為微波雷達(dá)奠定了基礎(chǔ)。
(4) Marcum 的雷達(dá)檢測理論。
(5) 速調(diào)管和行波管放大器的發(fā)明和發(fā)展，提供了穩(wěn)定性好的大功率源。
(6) 使用多普勒頻移來檢測淹沒于雜波中的移動目標(biāo)。
(7)適于空中交通管制的雷達(dá)的開發(fā)。
(8) 脈沖壓縮。
(9) 單脈沖跟蹤雷達(dá)有高的跟蹤精度，以及比以前的跟蹤雷達(dá)對電子對抗措施有更好的抵御能力。
(1 0) 合成孔徑雷達(dá)，對地面場景和地面上的物體成像。
(1 1) 機(jī)載動目標(biāo)顯示 (AMTI) ，用于在有雜波情況下遠(yuǎn)程機(jī)載空中監(jiān)視。
(12) 穩(wěn)定的元件、子系統(tǒng)和超低副瓣天線，使可大量抑制無用雜波的高 P盯脈沖多普勒雷達(dá) (AWACS) 成為可能。
( 13) 高頻超視距雷達(dá)，把飛機(jī)和艦船的探測距離擴(kuò)大了一個(gè)數(shù)量級。
(14) 數(shù)字處理，從 20 世紀(jì) 70 年代早期對雷達(dá)性能的改善有重大影響。
(1 5) 監(jiān)視雷達(dá)的自動檢測和跟蹤。
(16) 電掃描相控陣?yán)走_(dá)的批量生產(chǎn)。
(1 7) 逆合成孔徑雷達(dá) CISAR) ，提供目標(biāo)成像，如對艦船等非合作目標(biāo)識別需要的圖像。
(1 8) 多普勒氣象雷達(dá)。
(19) 太空雷達(dá)，適于對如金星等行星進(jìn)行觀測。
(20) 計(jì)算機(jī)對復(fù)雜目標(biāo)雷達(dá)截面積的精確計(jì)算。
(2 1)多功能機(jī)載軍用雷達(dá)，體積和質(zhì)量相對小，適于安裝在戰(zhàn)斗機(jī)前端，具有執(zhí)行大量不同的雪一空和空地任務(wù)的功能。
以上是對雷達(dá)過去一些主要發(fā)展的一點(diǎn)觀點(diǎn)。其他人或許有不同的看法。并非每種重大的雷達(dá)成就都包括在內(nèi)。如果包括本書其他章節(jié)的內(nèi)容，這個(gè)列表可能會更長并包含更多的例子。但是這個(gè)列表己足以顯示出對雷達(dá)性能改進(jìn)很重要的進(jìn)展類型。