Radar Cross Section Knott.pdf
Latrisha Adan
Radar cross section (RCS) is a comparison of two radar signal strengths. One is the strength of the radar beam sweeping over a target, the other is the strength of the reflected echo sensed by the receiver. This book shows how the RCS gauge can be predicted for theoretical objects and how it can be measured for real targets. Predicting RCS is not easy, even for simple objects like spheres or cylinders, but this book explains the two exact forms of theory so well that even a novice will understand enough to make close predictions. Weapons systems developers are keenly interested in reducing the RCS of their platforms. The two most practical ways to reduce RCS are shaping and absorption. This book explains both in great detail, especially in the design, evaluation, and selection of radar absorbers. There is also great detail on the design and employment of indoor and outdoor test ranges for scale models or for full-scale targets (such as aircraft). In essence, this book covers everything you need to know about RCS, from what it is, how to predict and measure, and how to test targets (indoors and out), and how to beat it.
This book is an introduction to the rather broad field of the echo characteristics of radar targets. It is intended to acquaint engineers, scientists, and program managers with what may be a new and unfamiliar discipline, even though a great body of knowledge has existed since the widespread use of radar in World War II. Modern weapons systems often carry RCS performance specifications in addition to other, more conventional requirements, such as speed, weight, and payload. Integrating these specifications into a new or existing system requires that engineers of several disciplines interact with each other. Our intention is to improve that interaction by showing why certain electromagnetic design procedures or features are important in the overall system design.
The purpose of this chapter is to provide a brief survey of radar fundamentals that can be used to put the RCS and RCS reduction (RCSR) problems in context. Obviously, only some of the most basic points concerning radar can be covered in a single chapter, and the reader who seeks more detail can find it in any one of a number of books on the general topic of radar. Here, after a brief history, radar system fundamentals are covered, with a focus on how radars discriminate in their measurement space of range, angle, and doppler. The radar range equation is developed, and its implication for RCSR is examined. Detection theory is then briefly discussed, and, finally, electronic countermeasure (ECM) techniques are outlined, with an emphasis on the effects of RCS control on ECM effectiveness.
This chapter has presented an overview of electromagnetic scattering. We have seen that RCS is a measure of power scattered from the incident wave; that it is a function of the angular orientation and shape of the scattering body, frequency, and polarization of the transmitter and receiver. The scattered wave, of which RCS is a measure, is caused by reradiation of currents induced on the scattering body by the incident wave. The scattering process breaks into three natural regimes: the low-frequency or Rayleigh region, where the wavelength is much longer than the scattering body size and the scattering process is due to induced dipole moments where only gross size and shape of the body are of importance; the resonant region, where the wavelength is on the same order as the body size and the scattering process is due to surface waves (traveling, creeping, and edge) and optics; and the high-frequency optics region, where the wavelength is much smaller than the body and the scattering process is principally a summation of the returns from isolated, noninteracting scattering centers. Maxwell's equations tell us that EM waves are a combination of electric and magnetic fields that are perpendicular to each other and to the direction of propagation. When an EM wave is incident on a body, the boundary conditions on the fields require that surface currents flow. These currents, in turn, reradiate a scattered EM wave. The strengths of the reflected and transmitted waves for specular scattering are given by the Fresnel coefficients, which are functions of the incident polarization and material properties. Surface fields were shown to be characterized as surface electric and magnetic currents and charges. The formal expressions that then relate the surface source currents and charges to the scattered fields are then known as the Stratton-Chu equations, which are an alternate expression of Maxwell's equations. These expressions are integrals of source currents and charges over the scattering body.
The objective of this chapter is to review briefly the classical modal solutions for 2-D cylinders and spheres and then to examine the powerful numerical techniques used to solve Maxwell's equations as expressed by the Stratton-Chu integral formulations and the differential equation formulations. Exact solutions for practical geometries for scattering are rarely found. This is because the wave equation is solvable by historical analytical methods when the scattering geometry coincides with one of the few separable coordinate systems for which exact series solutions are available. Unfortunately, few practical geometries match the solutions available.
In this chapter we have discussed the more important of the high frequency methods for predicting the RCS of simple structures. The great utility of these theories is that in the high frequency region, the target elements scatter the incident wave independently of one another. This makes it possible to assemble a collection of relatively simple shapes, such as flat plates, cylinders, and spheroids to model a complicated target. The analytical high-frequency formulas are relatively easy to derive for specific geometries, and we only need to sum the individual contributions coherently to obtain the total RCS of the target.
In Chapter 3 the reader was acquainted with the electromagnetic basis of radar echoes and in Chapters 4 and 5 with ways the echo may be predicted. As sound as those prescriptions may be, however, they are merely collections of formulas and equations that may not help us understand the echoing properties of targets of interest. In this chapter we present examples of the characteristics of both simple and complex targets. By simple we mean metallic objects having elementary shapes, such as a sphere, a circular cylinder, or a flat plate. Simple targets may be arranged in a hierarchy according to the strength and frequency dependence of the echo, and this ordering gives us some insight into their relative importance. The hierarchy of scattering mechanisms is particularly useful when we approach the task of reducing the echo from complex targets. By complex we mean objects whose surfaces are not good conductors, whose composition is not uniform or homogeneous, whose profiles defy mathematical description, or combinations of all of these.
In this chapter we presented the four basic techniques for RCS reduction. Of the four, the use of shaping and radar absorbers are by far the most effective. As the design and use of RAM is covered in detail in Chapter 8, this chapter focuses on the fundamentals of shaping for RCSR. Shaping typically is available only for systems still in the design stage, because it can seldom be exploited for vehicles already in production. But, in those cases where shaping can be applied, it is the most effective method available for RCSR.
This chapter has cataloged typical types of RAM and discussed their performance characteristics. A wide range of commercial material can be obtained for RCSR application. Nevertheless, whether commercially available RAM is purchased or an effort is made to tailor a new design to the specific problem, RAM application will usually involve a trade-off between performance, cost, complexity, and ease of manufacture and maintenance. Only through knowledge of RAM performance, familiarity with the types of RAM available, and good engineering judgment will the best RCSR solution to a given problem be chosen.
The test and evaluation of absorbers and the materials we use in designing and constructing them depends on the information needed. To design absorbers, we need to know the intrinsic properties of the materials we hope to use, whether they already exist or we cook up new batches to test. The determination of those parameters generally requires the use of enclosed test systems in the form of transmission lines, cavities or interferometers, or admittance tunnels. We emerge from such tests with characterizations of bulk permeability and permittivity or, in the case of thin sheets, the sheet admittance or impedance. We noted that sheet materials are not easily measured in transmission-line systems, and in the case of bulk materials, we must ensure that the sample is reasonably homogeneous throughout the small sample we insert in our sample holder.
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