Port and starboard ambiguity suppression for towed line array sonar is a long standing problem and unavoidable in practical application. Parallel twin-line array is a commonly used method to solve this problem by array configuration. Conventional beaming forming (CBF) usually uses delay-and-sum or delay-and-minus method. However, the restrain ratio is low with simple delay-and-sun or delay-and-minus method. Moreover, these methods can not at all distinguish the target between port and starboard for a certain frequencies or angles. 

Aiming at this problem, WANG Fupo, LI Shuqiu, LI Yu and HUANG Haining of Institute of Acoustics, Chinese Academy of Science conducted a series of studies to analyze why CBF can't get high port and starboard restrain ratio. They found that it's due to the difference of the port and starboard restrain ratio for signals at different frequencies from a certain angle.

The researchers produce a normalized bam output function for parallel twin-line array of uniform elements distribution, and find that the outputs from the conventional beam forming in a certain conjugate directions of arrival (DOA) are completely identical with that form the incident wave at some frequencies, thus it can not be distinguished between the port and starboard, though, at other frequencies the amplitude variation in the conjugate target direction can be predicted.

Based on this phenomenon, the researchers propose a method which can cancel the output from its conjugate direction by that from the direction of arrival. They first remove the frequencies at which port and starboard can't be distinguished for specified angels. Then, they compensate the amplitude and phase for outputs at other frequencies and angles to minimize the output at conjugate (mirror) target direction. Finally, they carry out simulation and sea trial. Both the results data show that this simple and practical method can better enhance the port and starboard restrain ratio, especially when the band is wide.