flying in the true direction of travel of the crests (i.e. 6, = 
@*), the true wave length of the waves will be observed. If not, 
some wave length greater than the true value will be observed as 
shown by equation (10.46). It is not possible to record a wave 
length shorter than the true value. Conversely, the wave number 
given by V, = 2r/L., will vary between zero and its true value, 
and it will not be greater than its true value. 
The problem is quite simple in this case if the pilot wishes 
to discover the true wave length of the waves below the aircraft 
in the fog. Many passes are made over the sea surface at various 
headings until a heading is found such that the length of the re- 
corded waves increases when either the aircraft is turned to the 
right or left. This minimum length is then the true wave length. 
By then flying very very slowly, or sending out a helicopter, 
the direction of wave travel could be determined by the Doppler 
effect. 
For the other simple cases discussed previously, similar 
techniques could be used and the resolution of five or six sinu- 
soidal waves of different periods and directions would not be too 
difficult a feat by ordinary techniques. However, the true sea 
surface is best represented by a Lebesgue-Stieltjes power integral 
over LaXGa euler and, as such, it is composed of an infinite 
number of infinitesimal sine waves traveling in all directions 
from (it is hoped) -1/2 to 1/2 with respect to the dominant dir- 
ection of the crests and with all possible spectral freouencies 
over a considerable range of the M axis. For these reasons, the 
determination of [Ay ( m ,e)]° is a complicated problem. 
200) 8 
