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Cold war bmews stations lunar echoes air traffic

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Cold war, BMEWS stations, lunar echoes Air traffic control Weather radar, Doppler radar, clear air turbulence and downdrafts Planetary radar (Venus, Mercury, asteroids, Mars to some degree) SAR and Earth surveys from aircraft, satellites, space shuttle GPR (archeology, geology, prospecting, construction, resource management) Ornithology and entomology Automotive radar, land mine detection, stud finders, baseball Sonar, Lidar, etc ... 7
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Tx Rx R P t P r Figure 1.1: RF communication link. 1.2 Motivation and outline Our objective is to understand how radars can be used to learn everything we can about the environment. It is useful to begin the study of radar, however, with an elementary question: how much of the power transmitted by a radar will be returned in the echoes from the target and received? The answer to this question, which culminates in the derivation of the so-called ‘radar equation,’ illustrates some important aspects of radar design and application. The derivation therefore serves as a vehicle for introducing basic ideas and terminology about radars. It also illustrates the central importance of antenna performance on transmission and reception. Much of the early material in this text will therefore have to address antenna theory, analysis, and design. We can sketch out the basic ideas behind radar systems with some common-sense reasoning. The analysis begins by considering a simple communications link involving transmitting and receiving stations and then considers what happens when a radar target is introduced to the problem. The relationships given below are defined rather than derived and serve as the basis for the material to follow in the text. More specific and detailed treatments will come later. The power budget for the simple communication link illustrated in Figure 1.1 is governed by: P rx = P tx G tx 4 πR 2 bracehtipupleft bracehtipdownrightbracehtipdownleft bracehtipupright P inc A eff with the received power P rx being proportional to the transmitted power P tx assuming that the media between the stations is linear. The incident power density falling on the receiving antenna P inc would just be P tx / 4 πR 2 if the illumination was isotropic. However, isotropic radiation cannot be achieved by any antenna and is seldom desirable, and the transmitting antenna is usually optimized to concentrate radiation in the direction of the receiver. The trans- mitting antenna gain G tx is a dimensionless quantity that represents the degree of concentration achieved. Rewriting the equation above provides a definition of gain or directivity, which we use interchangeably for the moment: G tx D = P inc P tx / 4 πR 2 = P inc P iso = P inc ( W/m 2 ) P avg ( W/m 2 ) where the denominator can be interpreted as either the power density that would fall on the receiving station if trans- mission was isotropic or the average power density on the sphere of radius R on which the receiver lies. So the gain
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