the radar scattering characteristics ,f each source of return and in which parts of the antenna pattern t'ey may be found. Clutter signals are due to direct terra~in reflections that occur at the same effective range as the target, and multipath signals are due to indirect reflections from the terrain between the target and . . •

Chaff is a radar countermeasure used to interfere with ground radar detection or missile target detection by disrupting the radar. As radar is essential in modern electronic warfare, chaff is also essentially used in many military vehicles. A large amount of chaffs in the form of a thin and long wire is contained in a cylindrical or rectangular cartridge and is launched to form a chaff cloud to disturb the radar. It is virtually impossible to examine the chaff cloud analytically because it is released at once into the air in large quantities. Therefore, for chaff cloud analysis, an approximation method or statistical characteristics are used, or real data is measured directly through experiments. The study of chaff consists of two parts: chaff cloud modeling, which determines the dynamic characteristics of chaff, and estimation of electromagnetic scattering properties, which determines the signal characteristics of the chaff. This review paper focuses on the techniques used in chaff cloud modeling and the estimation of electromagnetic scattering properties. We categorized the techniques by statistical, numerical, and empirical methods. In chaff cloud modeling, studies for the purpose of scattering analysis generally use typical distribution, whereas if chaff cloud modeling itself is the purpose, it is aimed at realizing a realistic chaff cloud by referring to experimental data. For estimation of electromagnetic scattering properties, a conventional method that simply multiplies the average radar cross-section of chaff element to obtain the total radar cross-section of the chaff cloud is used, and a method that allows rapid calculation while considering the distribution of the chaff cloud is being studied.INDEX TERMS Chaff, chaff cloud modeling, electromagnetic scattering properties estimation, radar crosssection, review.

Chaff is a passive jammer widely used to disrupt radar or radio-frequency sensors. A mass of chaff fibers dispersed in the air is commonly referred to as a chaff cloud. It is nearly impossible to numerically simulate in real-time the enormous amount of chaff fibers composing the chaff cloud. In this paper, we model the behavior of numerically estimated chaff clouds as probability density functions (PDFs) and apply approximation techniques to estimate the radar cross-section (RCS) of the chaff cloud in real time. To model the aerodynamics of the chaff cloud, we represented the combination of PDFs as functions of time and wind speed. The applied approximation techniques—vector radiative transfer and generalized equivalent conductor method—showed a computation time that cannot be achieved by low-frequency methods such as the method of moments or finite-difference time-domain. Moreover, the dynamic RCS results of the approximation techniques showed a similar trend to those of other studies simulating similar situations. The proposed scheme is effective for real-time chaff cloud simulation, and the modeled dynamics and estimated dynamic RCSs can be a standard baseline for developing new analysis methods for chaff clouds. In the future, the proposed scheme will extend to more chaff fibers and more diverse environmental parameters.