Savaş gemilerinde radar saçılma yüzeyi hesabı

Abstract

ÖZET Günümüz teknolojisine bağlı olarak savaş gemilerinin hayatta kalabilme zorluklan sebebiyle, tasarımlarında `görünmezlik (stealth)` özelliği ön plana çıkarılmıştır. Bu amaçla gemi tesbitine olanak sağlayan `iz`lerin mümkün olduğunca azaltılması çalışmaları gündeme gelmiş ve bu konuda en çok `Radar Saçılma Yüzeyi (RCS) minimizasyonu` üzerine çalışmalar yapılmıştır. Bu çalışmada da, Türkiye'de ilk defa tasarımı yapılan bir savaş gemisinin iki değişik formu için RCS hesabı yapılmış ve gemi formunun RCS üzerindeki etkileri incelenmiştir. RCS hesaplarında, gemi ve benzeri hedeflerin dalga boyuna göre çok büyük eğrilik yançaplı yüzeylere sahip olmalarından dolayı fizik optik (FO) yaklaşıklığı kullanılmıştır. FO yaklaşıklığı, düzleme benzer yüzeyler için geçerli olduğundan hedef yüzeyinin modellenmesinde üçgen biçiminde düzlemsel elamanlar kullanılmıştır. Ayrıca bu yöntem, eğrilik yarıçaplarının dalga boyuna göre çok büyük olduğu ve kenar etkilerinin ihmal edilebileceği durumlarda gerçeğe yakın sonuçlar vermektedir. Su üstü hedeflerini sınıflama yeteğine sahip görüntüleme radarları için geliştirilmiş olan simülasyon yazılımı, FO yaklaşıklğı kullanılarak hareketli ve hareketsiz gemilerin değişik band aralıklarında (X,S,C), 0° - 180° bakış açısı aralığında ve 1° aralıklarla, hedef yüzeyini üçgenlere bölerek RCS hesabı yapacak şekilde güncellenmiş ve iki değişik korvet formu için kullanılmıştır. Bölüm 2. 1 'de korvet için amprik formülle bulunan ortalama RCS (38.8 dBm2) değeri, simülasyon yazılımı kullanılarak ve stealth özellikleri verilerek çıplak tekne ortalama RCS'inde 16.9 dBm2 gibi bir düşüş sağlanmıştır. Bununla beraber ikinci form için yapılan iyileştirmelerle, grafik üzerinde değişik bölgelerde 5 dBm2 kadar düşüşler olduğu görülmüştür. Sonuç olarak, güverte donanımlarının katılmadığı RCS hesabımızda, gemi formunun düz, geniş, sürekli ve eğimli yüzeylerden oluşturulmasının, gemi formundan doğacak RCS'in azaltılmasını olumlu yönde etkilediği görülmüştür.

SUMMARY Since its invention during World War II, radar has played a crucial role in both military and civilian systems. In the civilian sector, radar is used for various aspects of navigation such as terrain avoidance, air traffic control, weather avoidance and altimeters. In addition to these functions, radars on military platforms, such as planes, ships and satellites, must perform reconnaissance, surveillance and attack roles. Millitary missions that encounter an adversary's radar are most effectively performed when detection is avoided. Consequently, the reduction of radar cross section (RCS) has received high priority in the design of all new platforms. Since revelation of the stealth technology to the public in the early 1970s, the term `stealth` has been associated with `invisible to radar`. Radar is one of several sensors that is considered in the design of a low-observable platform. Stealthy targets are not completely invisible to radar. To be undetectable, it is only necessary that a target's RCS be low enough for its echo return to be below the detection thresold of the radar. In order to provide stealth designs, one must know the RCS concept and the techniquies to be used in its prediction. 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 Incident power density / 4ix The IEEE dictionary of electrical and electronics terms [5] defines RCS as a measure of reflective strength of a target defined as 4n times the ratio of the power per unit solid angle scattered in a specified direction to the power per unit area in a plane ? XHlwave incident on the scatterer from an specified direction. More precisely, the scattered power is measured approaches infinity: E.vca,2 a = lim4nr2 - [m2/ r_>c0 re inc where Escat is the scattered electric field and Einc is the incident at the target. The unit of RCS most commonly used is decibels relative to a square meter (dBm): a, dBm = 10. log ( 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 regimes.. 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.. High-frequency region. When the incident wavelength is much smaller than the body size. This is also called Optical region. To predict the RCS of a target, several methods or approximations, have been devised such as, in high frequencies, optics region approximations. These approximation are wiedly 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 - XIV -. 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 (L > 10). 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 arbitratry 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 the basis of infinite frequency ( X, -* 0 ). This implies an electrically large target. Ray optics is frequently used in situations that severely violate this restriction and still yields surprisingly good results. The major disadvantage of ray tracing is the bookkeeping required for a comlex 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 or magnetic current (either volume or surface). The method of moments reduced the integral equations to a set of simultaneous linear equations that can be solved using standard matrix algebra. The size of matrix involved depends on the size of the body; current computer capabilities allow bodies on the order of 10 or 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 or 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. - XV -Another way to find the value of RCS of naval ships is to use a simple empirical expression [24]. a = 52fl/2D3/2 Where <j = radar cross section in square meters, f = radar frequency in megahertz, and D is the ship's ( full load ) displacement in kilotons. This expression was derived from measurements made at X,S and L bands and for naval ships ranging from 2000 to 17,000 tons. Although it is probably valid outside this size and frequency range, it does not apply elevation angles other than grazing incidence. An important problem often encountered in many military and civilian applications is how to reduce the radar return echo from the target to a specified minimum. In certain military applications, for obvious reasons, it is necessary to reduce the RCS of targets like aircraft, missiles and naval ships. Radar cross section reduction techniques generally fall into one of four categories: 1) target shaping 2) material selection and coating 3) passive cancellation 4) active cancellation.. Target shaping. The shape of target is modified to redirect the scattered energy from one angular region of interest in space to another region of little or no interest. It is usually best to keep large surfaces as flat and smooth as possible so that the specular flash is confined to a very narrow angular region. For moving objects like a ship or other vehicle to avoid dihedral angle corners with interior angles of 90° and so that the edges are smoothed. A mast can be designed with polygonal cross section, which would give it an RCS lower than one with a circular cross section. When using foam columns for support, a square cross section can be used for low RCS. Generally, RCS reduction by shaping the target would assume some significance in situations where orienting the target to reduce the probability of its detection by radar is possible. RCS reduction of target by modifying its shape has the following limitations: 1) Normally, reduction is achieved in a limited angular region and at the cost of increased RCS in other regions. 2) The method becomes more effective at higher frequencies; in fact, at low radar frequencies shaping becomes almost ineffective. 3) After the contribution due to a major scattering center is reduced, there often appears a tendency of a previously unimportant scattering area of the body to contribute significantly. 4) Often, the principal radii of curvature of the modified shape may dominate the nature of specular return.. Material selection and coating. In this category the target is coated with lossy material, known as the radar absorbing material (RAM). The lossy material XVI.absorbs, in the form of heat, a considerable amount of incident energy and thereby reduces the RCS. For narrow bandswidths, a single coating of RAM may suffice, but broad bandwidth generally require multiple coating. Although the term RAM suggests the selection of materials that absorb and attenuate waves, it actually has come to encompass a wide range of techniques and materials, including coating and composites. Most modern materials are classified as composites. Passive cancellation. In this category, the target is loaded at selected points with passive impedances to reduce the overall cross section. The method relies on phase cancellations in certain directions and, hence, it is severely limited bandwidth. The target with the scattering element is called the loaded body, as opposed to the bare target, which is the unloaded body. Consequently, thie method is also known as impedance loading Active cancellation. The active cancellation is similar to the passive cancellation but uses active elements to achieve the necessary phase cancellation. Active cancellation is the extension of passive cancellation to handle dynamic threat scenarios. In the context used here, it does not include deception, that is, techniques for modifying and retransmitting a signal. In this study, PO approximation has been used to predict of the RCS of naval ships. Triangular patching method has been used to represent the target due to both its flexibility and easy analytical calculations of the scattering field from a patch. In optical regime, the interactions between the patches can be ignored and the total RCS of the target can be obtained by summing the contribution of each patch coherently. In this study, a computer code which was used a computationally efficient algorithm called repeated trials method, was used to predict the RCS' s of complex targets. The calculation was performed 1° intervals, t=0 and for X, S and C bands and results were presented in both cartesian and polar coordinates. In addition to this, the calculations were repeated 100 times for each aspect angle so that it can be easily obtain the probability graphics. As a result, there was no value bigger than 38 dBm in calculations. - XVll -