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.