Fizik optik yöntemle radar kesit alanı hesabı
Radar cross section calculation with physic optic method
- Tez No: 142556
- Danışmanlar: YRD. DOÇ. DR. SELÇUK PAKER
- Tez Türü: Yüksek Lisans
- Konular: Elektrik ve Elektronik Mühendisliği, Electrical and Electronics Engineering
- Anahtar Kelimeler: Belirtilmemiş.
- Yıl: 2003
- Dil: Türkçe
- Üniversite: İstanbul Teknik Üniversitesi
- Enstitü: Fen Bilimleri Enstitüsü
- Ana Bilim Dalı: Elektronik-Haberleşme Eğitimi Ana Bilim Dalı
- Bilim Dalı: Belirtilmemiş.
- Sayfa Sayısı: 48
Özet
Radar kesit alanının hesaplanması, temel olarak hedeften dağılan elektrik alanın bulunması demektir. Gelen düzlemsel dalga tarafından hedefe endüklenen akım hesaplanabilirse, anten dizilerinde kullanılan aynı integraller dağılan bu elektrik alanın hesaplanmasında uygulanabilir. Bu çalışmada da, öncelikle radar kesit alam hakkında kısa bir tanımlama bilgisi verildikten sonra hesaplama yöntemleri üzerinde durulmuştur. Frekans ve zaman uzayında çözüm yapılabilen bu yöntemler, sıradan şekilli, üç boyutlu hedeflere uygulanabilen yöntemlerdir. Bunlar sırasıyla mikrodalga optik, moment metodu, sonlu farklar yöntemi ve fizik optik yöntemidir. Yöntemler hakkında kısaca bilgi verildikten sonra, çalışmanın asıl ana hattını oluşturan fizik optik yöntemi ile ilgili Matlab uygulamalarına yer verilip sonuçlar değerlendirilmiştir. Öncelikle basit yapıdaki geometrilerin fizik optik yöntemiyle radar kesit alanları incelenmiş, daha sonra bu basit şekillerden oluşan daha kompleks bir yapının fizik optik yöntemiyle radar kesit alanı hesaplattırılarak sonuçlar karşılaştırılmıştır. Fizik optik yöntemini cismi üçgenlere ayırmak suretiyle inceleyerek bu şekilde modellemenin getirdiği kolaylıklar ve sonuçların doğruluğu tam sonuçlarla karşılaştırılarak üçgenleme metodunun kullanılabilirliği tartışılmıştır. Cismi daha fazla sayıda üçgenlere ayırdıkça hesabın tam sonuca yakınsadığı rahatlıkla gözlemlenmiştir. Bu şekilde bir platformun inşa aşamasından önce bu metodla radar kesit alanının kolaylıkla hesaplanarak çıkan sonuca göre dizaynına karar verilebilmesi veya halihazırdaki platformların radar kesit alanlarım optimum seviyede kullanabilmelerine olanak sağlanmış olur. Fizik optik yöntemi, düzleme benzer yüzeyler için geçerli olduğundan hedef yüzeyinin modellenmesinde üçgen biçiminde düzlemsel elemanlar kullanılmıştır. RKA hesaplamalarında, gemi ve benzeri hedeflerin dalga boyuna göre çok büyük eğrilik yarıçaplı yüzeylere sahip olmalarından dolayı fizik optik yöntemi kullanılır. vııı
Özet (Çeviri)
Radar cross section is a measure of power scattered in a given direction when a target is illuminated by an incident wave. RCS has been defined to characterize the target characteristics and not the effects of transmitter power, receiver sensitivity, and the position of the transmitter or receiver distance. Another term for RCS is“echo area”. The definition of RCS can be stated as: Power reflected to receiver per unit solid angle r 2i RCS =Im J Incident power density / 4tc The IEEE dictionary of electrical and electronics terms [5] defines RCS as a measure of reflective strength of a target defined as 4tc times the ratio of the power per unit solid angle scattered in a specified direction to the power per unit area in a plane wave incident on the scatterer from a specified direction. More precisely, the scattered power is measured approaches infinity: a = lim 47tR2 E scat E1 [m2] Where Escat is the scattered electric field and Einc is the incident at the target. The unit of RCS most commonly used is decibels to relative to a square meter (dBm): a[dBm2]=10Log(a[m2]) The scattering characteristics of a target are strongly dependent on the frequency of the incident wave. There are three. frequency regions in which the RCS of a target is distinctly different. They are referred to as the 1) low-frequency,2) resonance and 3) high-frequency regions.. Low-frequency region. When the incident wavelength is much greater than the body size, scattering is called Rayleigh scattering.. Resonance region. When the incident wavelength is on the order of the body size, the phase of the incident field changes significantly over the length of the scattering body. This is also called Mie region. IX. High-frequency region. When the incident wavelength is much smaller than the body size. This is also called Optical region. Before presenting the different RCS calculation methods, it is important to understand the signifance of RCS prediction. Most radar systems use RCS as a means of discrimination. Therefore, accurate prediction of target RCS is critical in order to design and develop robust discrimination algorithms. Additionally, measuring and identifying the scattering centers (sources) for a given target aid in developing RCS reduction techniques. Another reason of lesser importance is that RCS calculations require broad and extensive technical knowledge, thus many scientists and scholars find the subject challenging and intellectually motivating. Two categories of RCS prediction methods are available: exact and approximate. Exact methods of RCS prediction are very complex even for simple shape objects. This is because they require solving either differential or integral equations that describe the scattered waves from an object under the proper set of boundary conditions. Such boundary conditions are governed by Maxwell's equations. Even when exact solutions are achievable, they are often difficult to interpret and to program using digital computers. Due to the difficulties associated with the exact RCS prediction, approximate methods become the viable alternative. The majority of the approximate methods are valid in the optical region, and each has its own strengths and limitations. Most approximate methods can predict RCS within few dBs of the truth. In general, such a variation is quite acceptable by radar engineers and designers. Approximate methods are usually the main source for predicting RCS of complex and extended targets such as aircrafts, ships, and missiles. When experimental results are available, they can be used to validate and verify the approximations. To predict the RCS of a target, several methods or approximations, have been devised such as, in high frequencies, optics region approximations. These approximations are widely being used because the targets, such as airplanes and ships, have to be treated in this region due to the large sizes of the targets with respect to the radar wavelength. The classical solution techniques are not discussed in this study because most of them are limited to one- or two-dimensional structures or simple three-dimensional shapes. The methods of interest in this study are those that can be applied to arbitrary three- dimensional targets. The methods most commonly encountered are physical optics, microwave optics (ray tracing), the method of moments, and finite difference methods. * Physical optics. One method of estimating the surface current induced on an arbitrary body is the physical optics (PO) approximation. On the portions of the body that are directly illuminated by the incident field, the induced current is simply proportional to the incident magnetic field intensity. On the shadowed portion of target, the current is set to zero. The current is then used in the radiation integrals to compute the scattered field far from the target. Physical optics is in a high-frequency approximation that gives best result for electrically large bodies. It is most accurate in the specular direction. Because PO abruptly sets the current to zero at a shadow boundary, the computed field values at the wide angles and in the shadow regions are in accurate. Furthermore, surface waves are not included. Physical optics can be used in either the time or frequency domains..Microwave optics. Ray-tracing methods that can be used to analyze electrically large targets of arbitrary shape are referred as microwave optics. The rules for ray-tracing in a simple medium (linear, homogeneous, and isotropic) are similar to reflection and refraction in optics. in addition, diffracted rays are allowed that originate from the scattering of the incident wave at edges, corners, and vertices. The formulas are derived on he basis of infinite frequency. This implies an electrically large target. Ray optics is frequently used in situations that»~severely violate this restriction and stili yields surprisingly good results. The majör disadvantage of ray tracing is the bookkeeping required for a complex target. it is used primarily in the frequency domain..Method of moments. The most common technique used to solve an integral equation is the method of moments (MM), integral equations are so named because the unknown quantity is in the integrand. in electromagnetics, integral equations are derived from Maxwell's equations and the boundary conditions. The unknown quantity can be an electric ör magnetic current (either volume ör surface). The method of moments reduced the integral equations to a set of simultaneous linear equations that can be solved using standart matrix algebra. The size of the matrix involved depends on the size of the body; current computer capabilities allow bodies on the order of 10 ör 20 wavelengths to be modeled..Most MM formulations require a discretization (segmentation) of the body. Therefore, they are compatible with finite element methods used in structural engineering, and the two are frequently used in tandem during the design of a platform. The method of moments can be used to solve both time- and frequency-domain integral equations..Finite difference methods. Finite differences are used to approximate the differential operators in Maxwell's equations in either the time ör frequency domain.As in the MM, the target must be discretized. Maxwell's equations and the boundary conditions are enforced on the surface of the target and at the boundaries of the discretization cells. This method has found extensive use in the computing the transient response of targets to various waveforms. Finite difference does not require the large matrices that the MM does because the solution is stepped in time throughout the scattering body. in this study, firstly the approximation of the physical optics explained with the simple geometrical objects like sphere, ellipsoid, truncated cone, circular flat plate, cylinder, rectangular flat plate and triangular flat plate. After laying down these expressions, we formed a more complex object which is made of by these simple objects and then gof the rcs solution of the total object. The matlab programme which is used, is a simple RCS prediction code based on the physical optics approximation. Scattering objects are approximated by arrays of triangels (facets) and superposition is used to compute the total RCS of the object. These four boundary conditions limited the study..The scattered fıeld of each triangle is computed as if it were isolated and other triangles were not present.Multiple reflections are not included xiEdge diffraction and surface waves are not included. Shadowing is only approximately included by considering a facet to be completely illuminated or completely shadowed by the incident wave ( shadows on one part of the object cast by other parts of the object are not included.) xn
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