TABLE OF CONTENTS The radiation integrals introduced in Chapter 2 provide a means of calculating the scattered field from a target once the induced current on the surface is known. Except for the simplest shapes, the current distribution on the target is not known accurately a priori. For electrically large bodies that are smooth and relatively flat, the physical optics approximation provides a good estimate at points far from discontinuities and shadow boundaries. A discontinuity is any abrupt change in the surface contour or composition. Examples are edges, steps, cracks, and material joints. It is expected that the smaller the discontinuity, the less impact it will have on RCS. This statement is true when the maximum RCS level of the target is used as a reference. However, small scattering sources tend to have broad patterns and, therefore, scattering from a discontinuity can dominate in directions in which other contributors are negligible.
For an accurate calculation of RCS, all significant current contributions must be included in the radiation integrals. For objects whose surfaces are metals (good conductors), the small skin depth justifies the use of an idealized surface current 
s. If the object has dielectric or magnetic regions, then polarization and magnetization currents may arise, respectively, in those materials. If so, then volume electric and magnetic m currents must be determined along with the surface currents.
In this chapter, integral and differential equations will be formulated from Maxwell s equations and the boundary conditions on the target.
