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Doppler Spectrum of Sea Echo at 13.56 Mc./s.


THE Doppler frequency shift of radio waves reflected from the sea surface at 13.56 Mc./s. has been recorded and a portion of a typical record is shown in Fig. 1. Such records show the following unexpected features: (a) the frequency of the principal component (as initially obtained by measurement of the length of individual cycles on the record) is surprisingly constant at about 0.38 c./s., irrespective of wind conditions and state of the sea; (b) the records show that the range of frequencies present is small. Spectrum analysis of the records made with an Admiralty wave analyser1 confirmed these findings. Fig. 2 is a reproduction of a sample spectrum analysis which shows a large but narrow peak at a frequency of 0.38 c./s. accompanied by a smaller peak at a frequency of about 0.54 c./s.

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  1. 1

    Barber, N. F., Nature, 158, 329 (1946).

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  2. 2

    Russel, R. C. H., and MacMillan, D. H., “Waves and Tides”, 58 (Hutchinson, London, 1952).

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CROMBIE, D. Doppler Spectrum of Sea Echo at 13.56 Mc./s.. Nature 175, 681–682 (1955).

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