polarity and the signal which must be applied to them to obtain by choice the two beams fore aft or starboard port these groups numbered 1 to 8 are identified in the figure by a distinct frame therefore to obtain the successive emission of the two beams fore aft and starboard port electrical signals corresponding to frequencies f1 or f2 with phases defined by a b or a 39 b 39 according to the definition seen in the case of the one dimensional system are applied to the transducers in a succession of four pulses corresponding to the distribution of frequencies and polarities given in the table of fig1 thus pulse 1 will correspond to a frequency f1 whose signals a will be applied to the transducers belonging to groups 1 2 5 and 6 and the signals b to the transducers corresponding to groups 3 4 7 and 8 the second pulse is at the frequency f2 a 39 and b 39 and is applied to the transducers according to the distribution of the second column of the table and so on etc a fore beam at the frequency f1 an aft beam at the frequency f2 a starboard beam at the frequency f1 and a port beam at the frequency f2 are therefore obtained successively in this manner as in the one dimensional case these beams are emitted successively within a short time span and the system then awaits the reception of the various echoes.