OBSERVATION OF COHERENT BACKSCATTER OF 2-10 MHZ RADIO SURFACE WAVES FROM SEA

Radio waves propagating along the surface of the sea are known to be selectively scattered backwards by components of the sea waves which have lengths of half the radio wavelength, and which are traveling radially to the transmitter and receiver. The scattered signals have a Doppler shift which is characteristic of the radio frequency used. An account is given of preliminary experiments designed to investigate some of the characteristics of ground wave signals scattered from the surface of the sea. The observations were made in the Gulf of Mexico using frequencies between ∼2 and 10 MHz. Thus the wavelengths of the sea waves observed were between 75 m and 15 m. The experiments verified that the Doppler shifts of the scattered signals were as expected from theory. Observational ranges of 200 km were obtained, but there is evidence that the range is smaller at night, probably because of tropospheric influences an the radio wave propagation. Two antannae having moderate directivity and oriented at right angles were used to demonstrate that waves traveling in different directions could be distinguished. © 1968.