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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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單位代碼 0 2
學(xué) 號 080105037
分 類 號 TH6
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 18 日
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 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 frontierof radio technology at that time. It is a good frequency for long range air surveillanceor 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 deepnulls 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 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.8 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 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 60nmi. 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 that both radars have comparable performance. 9 (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 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 fre-quency 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 frontierto be explored and put to productive use. Its good features are that it is a great place foremploying wide bandwidth signals (there is plenty of spectrum space); radars can havehigh range-resolution and narrow beamwidths with small antennas; hostile electroniccountermeasures 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.
1.6 工作頻率對雷達(dá)的影響
雷達(dá)已在低至 3hz (剛好高于 AM 廣播頻段)的頻率上工作過，也在高至數(shù)百 GHz(毫米波段)頻率上工作過。雷達(dá)更常用的頻帶可能為 5陽也~95GHz 以上，這是一個(gè)巨大的頻率范圍，所以應(yīng)該可以預(yù)期的是雷達(dá)技術(shù)、性能及應(yīng)用會顯著依賴于雷達(dá)工作的頻段而變化。不同頻段的雷達(dá)通常具有不同的性能和特性?！?，在低頻段易于獲得遠(yuǎn)程性能，因?yàn)樵诘皖l易于獲得大功率發(fā)射機(jī)和物理上巨大的天線。另一方面，在更高的雷達(dá)頻率上，容易完成距離和位置的精確測量，因?yàn)楦叩念l率能提供更寬的帶寬(它決定距離精度和分辨率)，以及在給定夭線物理尺寸時(shí)更窄的波束(它決定角精度和角分辨率)。下面簡要介紹不同波段的雷達(dá)應(yīng)用。然而相鄰波段的區(qū)別在實(shí)踐中沒有顯著差別，在特性上可能會有重疊。高頻(HF. 3—30MHz)
HF頻段的主要用途是被雷達(dá)用來探測遠(yuǎn)程目標(biāo)(標(biāo)稱可達(dá)到 2000n mile) ，方法是利用高頻電磁波能量被遠(yuǎn)離地表的電離層折射的特性。無線電愛好者稱這為短波傳播并用它來在遠(yuǎn)距離上通信。 HF 雷達(dá)的目標(biāo)可能是飛機(jī)、艦船和彈道導(dǎo)彈，以及來自海面本身的回波(可提供驅(qū)動海面的風(fēng)向及風(fēng)速的信息)。
甚高頻（VHF30—300MHZ）
20 世紀(jì) 30 年代開發(fā)的大多數(shù)早期雷達(dá)都工作在該頻段，因?yàn)樵诋?dāng)時(shí)這些頻率代表無線電技術(shù)的前沿。它對遠(yuǎn)程空中監(jiān)視和探測彈道導(dǎo)彈是很好的頻率。在這些頻率上，地球表面特別是水面散射的反射系數(shù)會非常大，所以直達(dá)信號和面反射信號之間的相長干涉會顯著增大 VHF 雷達(dá)的作用距離。然而，當(dāng)有這種效應(yīng)使作用距離翻倍時(shí)，會有伴隨而來的相消干涉減少作用距離，這是由于在某些仰角上，天線方向圖有深的零點(diǎn)。同樣，相消干涉會導(dǎo)致低空上差的覆蓋。雷達(dá)利用多普勒頻移探測雜波中的動目標(biāo)時(shí)在低頻上經(jīng)常會更好，因?yàn)槎嗥绽漳：?導(dǎo)致盲速)在低頻段要少得多。 VHF 雷達(dá)不受雨雜波困擾，但受來自流星的電離和極光的多次時(shí)間折疊回波的影響。在 VHF 頻段，飛機(jī)的雷達(dá)截面積一般比在更高的頻率上大。 VHF 雷達(dá)在獲得同樣的距離性能時(shí)比工作在更高頻段上的雷達(dá)花費(fèi)要少。盡管甚高頻雷達(dá)對遠(yuǎn)程監(jiān)視有許多誘人的優(yōu)點(diǎn)，但也有很多嚴(yán)重的局限。俯仰上的深零點(diǎn)及差的低空覆蓋之前已經(jīng)提到了。分配給 VHF 雷達(dá)的可用頻譜寬度很窄，因此距離分辨率經(jīng)常很差。天線波束寬度通常比微波頻段的寬，因此角精度和分辨率也差。 VHF 頻段中擁擠著許多重要的民用服務(wù)，如電視和調(diào)頻廣播，這進(jìn)一步減少了雷達(dá)可用的頻譜空間。通過天線進(jìn)入雷達(dá)的外部噪聲電平在 VHF 頻段比微波頻段高。工作在 VHF 頻段，雷達(dá)的主要局限可能是在這個(gè)擁擠的頻段中獲得合適頻譜空間的困難。盡管有局限， VHF 對空監(jiān)視雷達(dá)在蘇聯(lián)曾廣泛使用，因?yàn)樘K聯(lián)國土廣大，而 VHF 雷達(dá)的低廉，對提供疆域這么廣闊國家的空中監(jiān)視很有吸引力[8] 。據(jù)說蘇聯(lián)生產(chǎn)了大量的 VHF 對空監(jiān)視雷達(dá)一一一些有著非常巨大的只寸和遠(yuǎn)的作用距離，但多數(shù)是可容易運(yùn)輸?shù)?。有意思的?VHF 機(jī)載攔截雷達(dá)曾在第二次世界大戰(zhàn)中被德國廣泛使用。例如， Lichtenstein SN-2 機(jī)載雷達(dá)在不同型號中工作在 60~100MHz 上。在這些頻率上的雷達(dá)不受稱為輔條(也稱為窗口)的對抗措施的影響。
超高頻(UHF.300MHZ—1GHZ)
工作在甚高頻雷達(dá)的許多特點(diǎn)在一定程度上也適合于超高頻。 UHF 特別適合于機(jī)載預(yù)警雷達(dá)系統(tǒng) (AEW) 中的機(jī)載 AMTI (動目標(biāo)檢測)雷達(dá)(參見第 3 章)，也適于探測和跟蹤衛(wèi)星和彈道導(dǎo)彈的遠(yuǎn)程雷達(dá)。在這個(gè)波段的上段可找到遠(yuǎn)程艦載對空監(jiān)視雷達(dá)和測量風(fēng)速及風(fēng)向的雷達(dá)(稱為風(fēng)靡線雷達(dá))。地面穿透雷達(dá) CGPR) ，是所謂超寬帶 (UWB) 雷達(dá)的例子，參見第 21 章。它寬的信號帶寬有時(shí)同時(shí)覆蓋 VHF 和 UHF 波段。這種雷達(dá)的信號帶寬可能從 50MHz延伸到 500MHz。寬的帶寬對獲得好的距離分辨率是需要的。低頻率對允許雷達(dá)能量穿透地面?zhèn)鞑ナ切枰?盡管如此，在典型土壤中傳播衰減迅速，因而簡單的機(jī)動 GPR 作用距離可能僅有幾米)。這個(gè)距離適合定位掩埋在地F的電線、管線和其他物體。如果雷達(dá)要發(fā)現(xiàn)位于地表但被樹木遮蓋的目標(biāo)，也需要同 GPR 所用類似的頻段。
Ku，K和Ka波段(14.0—40.0GHZ)
在更高的雷達(dá)頻率上，天線物理尺寸減小，一般更難產(chǎn)生大的發(fā)射機(jī)功率。因此， X 波段之上頻段的雷達(dá)的距離性能一般不如 X 波段的雷達(dá)。軍用機(jī)載雷達(dá)有 X 波段的，也有 Ku波段的。對必須要有小的尺寸而不需要遠(yuǎn)距離的雷達(dá)應(yīng)用，這些頻段具有吸引力。機(jī)場表面探測設(shè)備 CASDE) ，通常在大型機(jī)場控制塔的頂端可以找到，工作在 Ku 波段，主要因?yàn)樗?X 波段有更好的分辨率。在原先的 K 波段中，在 22.2GHz處有一條水蒸氣吸收線，這導(dǎo)致的衰減在一些應(yīng)用中是個(gè)嚴(yán)重的問題。這個(gè)問題當(dāng) K 波段雷達(dá)在第二次世界大戰(zhàn)期間研制開始以后被發(fā)現(xiàn)，這就是后來引入 Ku 和 Ka 波段的原因。雨雜波會限制該波段雷達(dá)的性能。
毫米波波段
盡管這個(gè)頻段很寬，多數(shù)毫米波雷達(dá)感興趣的頻率位于 94GHz附近，此處的大氣衰減有一個(gè)極小值(稱為窗口，是指相對于其附近的頻率衰減小的區(qū)域， 94GHz附近的窗口和整個(gè)微波頻段一樣寬)。如上面所提到的，對雷達(dá)的目的，毫米波范圍實(shí)際上一般從 40GHz甚至更高的頻率開始。毫米波雷達(dá)的技術(shù)和環(huán)境的傳播效應(yīng)不僅不同于微波雷達(dá)，而且通常有更多的限制。不同于微波波段雷達(dá)所經(jīng)歷的衰減，毫米波雷達(dá)信號即使在潔凈的空氣中傳播也會有很大的衰減。衰減在毫米波段是變化的。 94GHz 窗口中的衰減實(shí)際上比大氣 22.2GHz處的水蒸氣吸收線還要高。在 60GHz 氧氣吸收線處的單程衰減約為 12扭曲，基本上排除了雷達(dá)在其鄰近頻率的應(yīng)用。雨的衰減對毫米波波段也是一種限制。
對毫米波雷達(dá)感興趣的主要原因是因?yàn)樗鳛檠芯亢陀谐晒膽?yīng)用的前沿帶來的挑戰(zhàn)。它的好的特點(diǎn)在于它是采用寬帶寬信號的極好場所(有大量的頻譜空間);雷達(dá)可使用小的天線得到高距離分辨率和窄波束:敵方難以對軍用雷達(dá)使用電子對抗措施:它使位于這些頻率的軍用雷達(dá)比低頻率的雷達(dá)有低的被截獲概率。在過去，毫米波發(fā)射機(jī)平均功率無法超過數(shù)百瓦一一通常要低得多?；匦苌系倪M(jìn)展(參見第 10 章)使得可以產(chǎn)生比傳統(tǒng)的毫米波功率源大幾個(gè)數(shù)量級的平均功率。因此，獲得大功率不再成為限制。
激光雷達(dá)
激光器在頻譜的光學(xué)和紅外區(qū)可以產(chǎn)生可用的功率。它可使用寬帶寬(極短脈沖)并具有非常窄的波束寬度，而天線孔徑比微波段的小很多。大氣和雨的衰減非常高，因此在惡劣天氣中的性能十分有限。接收機(jī)噪聲由量子效應(yīng)而不是熱噪聲決定。由于幾種原因，激光雷達(dá)的應(yīng)用有限。
1.8 雷達(dá)過去的一些進(jìn)展
(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。
(10) 合成孔徑雷達(dá)，對地面場景和地面上的物體成像。
(11) 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。
(15) 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)。
(17) 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。
(18) 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。
(21)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。