Frequency Dependence of the Properties of Sea Echo

Abstract

The properties of sea echo---a radar echo associated with surface of the sea---have been measured at wave-lengths of 9.2, 3.2, and 1.25 cm, for grazing angles of incidence about 1\ifmmode^\circ\else\textdegree\fi{} and over a wide range of sea states. The measurements, which are given in terms of a suitably defined cross section per unit area of the sea, were obtained with three experimental truck-borne systems. Details of these systems, and of the measuring techniques, are described. The troublesome rapid fluctuations of the sea echo signals were eliminated by an electrical averaging scheme. A discussion of the possible errors in the determination of the absolute cross sections leads to an estimated uncertainty of \ifmmode\pm\else\textpm\fi{}2 db on 9.2 and 3.2 cm and \ifmmode\pm\else\textpm\fi{}4 db on 1.25 cm. The results have been interpreted in the light of possible theories of the scattering mechanism responsible for the echo. Assuming the scatterers to be spray drops small compared to $\ensuremath{\lambda}$, the wave-length dependence of the cross section should be between ${\ensuremath{\lambda}}^{\ensuremath{-}4}$ and ${\ensuremath{\lambda}}^{\ensuremath{-}8}$ whereas the observed variation is between ${\ensuremath{\lambda}}^{0}$ and ${\ensuremath{\lambda}}^{\ensuremath{-}4}$. While these results are in better accord with the hypothesis of scattering from irregularities on the surface, the observed large changes of the cross section with polarization seem explainable only by some form of the drop theory. A modification of the drop theory is proposed, which assumes the presence of drops of diameter of the order of $\ensuremath{\lambda}$. The consequences of such a theory are examined and found to be in rough agreement with experiment.