4.1 Introduction

The analysis of electromagnetic problems in the time domain has become more common in recent decades. In this approach, the differential or integral equations for the particular problem under consideration are typically solved numerically using a short time-duration waveform as excitation. By far the most common of these techniques is the so-called finite difference time-domain (FDTD) method.1 ^,2 There are other useful time-domain techniques such as time-domain physical optics (TDPO),3 the finite integration technique (FIT), and time-domain integral equations (TDIE),4 6 but FDTD is the most common and, consequently, it is the focus of the material in this chapter.

In the FDTD method, the target is first discretized in a convenient grid coordinate system. The differential operators in Maxwell s equations are approximated by finite differences. The selected waveform illuminates the target, and the fields at the grid nodes are computed at discrete time steps n ? t , where n is an integer. This process is referred to marching in time. As described in the following sections, the field at a particular node at time t can be determined from the fields at the same and adjacent nodes at the previous time step.

In some cases, the time-domain scattered field provides the desired target information. An example is the resolution of two individual scatterers on a complex target (see Chapter 8 for further discussion). If radar cross section (RCS) is the quantity of interest, however,...