Measurements of Electromagnetic Back-scattering from Known, Rough Surfaces

Measurements of backscattering of electromagnetic (EM) waves from rough surfaces have been performed in the past using microwaves scattered from moon, sea and terrain surfaces. Among others, we refer to the experimental work of Davies and Macfarlane, 1 Grant and Yaplee,2 Wiltse et al3 and Evans and Pettengill.4 More recently there has also been interest in backscattering of EM waves from rough overdense plasma surfaces, particularly near grazing angles of incidence. In order to relate the measurement of backscattering of EM waves from randomly-rough surfaces to the characteristics of the scattering surface, the statistical properties of such surfaces must be independently measured. The bulk of EM wave scatter measurements from randomlyrough surfaces with gentle slopes consists of those obtained from the sea. Although efforts have been made to specify the state of the sea by means of the prevailing winds at the time of the experiment, the statistical parameters of the rough sea surfaces and of the moon have been matters of conjecture based on many untested assumptions. On the other hand, a direct statistical study of the surface irregularities of the moon is impossible, at present, and very difficult for the sea. For these reasons, the use of randomly-rough surfaces specially prepared for a scatter experiment was desirable and furthermore, was necessary to test the range of validity of the available rough-surface scattering theories. Surface preparation for such scatter experiments at microwave frequencies is a formidable task because of the following requirements: the mean height correlation distance (or scale size) I of the surface should be much larger than the wave length X to correspond to most cases of physical interest, the beam diameter d must be much larger than I in order to make the scattering area a representative member of the statistical ensemble, and the largest dimension L of the scattering surface must be much larger than d so that all of the beam is intercepted even near grazing incidence.