【導(dǎo)讀】雷達是一種檢測和定位的反射物體電磁傳感器。它的操作可歸納如下:. 此時目標(biāo)位置和可能的其他有關(guān)信息都應(yīng)被獲取。一個普通的波形由雷達輻射一系列相對狹窄波形，如矩形脈沖。（1兆瓦），以及與這些數(shù)據(jù)中發(fā)射機平均功率為1千瓦。能低于通常在一個“典型的”教室中電力照明功率。微波頻率的中間范圍，如從至GHz，這是一個典型的民用機場監(jiān)控雷達頻帶。它的波長可能是大約10厘米。飛機外或多或少50至60海里范圍。的范圍數(shù)值，但我們隨便假設(shè)的“典型”作說明用途，回波信號可能有可能10?傳遞信號的幅度和檢測接收到的回波信號之間特別的差異。一個目標(biāo)相對速度，或從徑向速度轉(zhuǎn)移的回波信號提取多普勒頻率。度，可確定和作出預(yù)測的將來位置?；镜睦走_組成。處理還包括多普勒處理及最大化的信號與運動目標(biāo)，雜波大于雜比接收器的噪聲。

　　

【正文】 e energy intercepted by the target is reradiated in many directions. 21 ● 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 mon waveform radiated by a radar is a series of relatively narrow, rectangularlike pulses. An example of a waveform for a mediumrange 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 to GHz, which is a typical frequency band for civil airportsurveillance 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 plas. 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 powerline poles), sea, ice, icebergs, buoys, underground features, meteors, aurora, spacecraft, and plas. 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 22 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. Basic Parts of a Radar. Figure 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 timeshared 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. 23 FIGURE 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