Observations of coherent backscattering of hectometric radiowaves from along the sea surface

Douglass D. Crombie and James M. Watts

Abstract: Observations of coherent backscatter from the surface of the sea, at range s up to 200 km, using a slightly modified ionosonde are described. The range of frequencies used was between 2.6 and 8.5 MHz corresponding to sea wave lengths of 58 -17m and periods of 6. 1 -3.3 sec. Lower frequencies could be used to observe sea waves of longer periods. The echoes show a Doppler shift. Both positive and negative Doppler shifts have been observed as well as some echoes without Doppler shift. When present the Doppler shift was proportional to the square root of the radio frequency, as expected, on theoretical grounds. Observations show that the amplitude of the echoes can depend on the direction from which they are received. It is estimated that it may be possible to observe sea scatter regularly to distances as great as 400 km. If this proves to be the case, such observations might be of economic value to the shipping industries. Fishing boats and coastal shipping could use them to avoid areas of rough water.

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