694 



BARBER AND TUCKER 



[chap. 19 



It is possible that radar methods can be used to reveal the directional 

 spectrum of waves at sea (Barber, 1959). A radio wave travelling across the sea 

 surface is reflected by the waves as if they were a complicated diffraction 

 grating. The echo which returns to the transmitter is produced only by that 

 wave train which happens to have a wavelength equal to half the radio wave- 

 length and happens to lie with its crest lines at right angles to the direction of 

 radio transmission. It should be possible therefore to examine all the com- 

 ponent wave trains in the sea by sweeping the radar frequency and by sending 

 the wave successively in different directions. There are likely, however, to be 



SWELL 



Fig. 14. A directional power spectrum of stornj waves. (After Pierson, 1960.) This was got 

 by calculation from a stereophotograph of a large area of sea from 1000-m height and 

 is thought to approximate to a fully developed sea under a wind of Beaufort force 5. 

 Direction from the centre of the plot is the direction of wave travel. Radial distance is 

 proportional to wave number, but a scale of wavelength is provided. One may imagine 

 this plot to have been divided up into a large number of small regions, each presenting 

 an equal area on the diagram and each denoting an incoherent wave with a certain 

 mean wavelength and mean direction. The contours then join regions whose waves 

 have equal power (mean square amplitude). The numbers marked on the contours are 

 proportional to their power. 



considerable practical difficulties in directing radio waves of the requisite 

 wavelengths between, perhaps, 30 and 300 m (10 to 1 mc/s). 



It has also been pointed out that towed electrodes such as have been used to 

 investigate ocean streams (von Arx, 1950) are sensitive to wave motions and 

 might be used to reveal wave directions in the open sea, but no experimental 

 tests have yet been done. Nor have proposals yet matured of using an air- 

 borne radio altimeter to record the profile of the sea along a number of courses. 



An alternative attack is to use a number of wave-detecting instruments set 

 out in a suitably spaced pattern. This is analogous to the spaced arrays of 



