Measuring radar cross section at short distance

Radar cross sections (RCS) must be measured at large distance. We exclude here the special near-field methods. The distance (R) must be so large that different RCS contributions are added in the same way as at infinite distance. This leads to the rule
R > 2D^{2}/\lambda
if the target cross range is confined within
\pm D/2
and a phase error of
\pi /4
is tolerated. It is shown, however, that if the RCS is measured as a function of an angle and is low-pass filtered with a cut off period of
\phi_{c}
(radians), the distance requirement could be changed to
R > 4D\phi^{-1}_{c}(1 - \lambda/2\phi_{c}D))
. All RCS measurements at such distances give approximately the same result after low-pass filtering. We assume that a set of isotropic point scatterers, accurately describes the target.