Port and starboard ambiguity suppression for towed line array sonar is a long standing problem and unavoidable in practical applications. Parallel twin-line array is a commonly used method to solve this problem by array configuration. Lots of works have been down on this problem. However, Conventional beam forming (CBF) usually uses delay-and-sum or delay-and-minus methods. The restrain ratio is low with simple delay-and-sum 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, the researches of Institute of Acoustics, Chinese Academy of Sciences conducted a series of studies, analyzed the reason why CBF cannot get high port and starboard restrain ratio and found it 's due to the difference of the port and starboard restrain ratio for signals at different frequencies from a certain angle.

They study a normalized beam 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 from 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, they propose a new method which can cancel the output from its conjugate direction by that from the direction of arrival. First, frequencies are removed at which port and starboard cannot be distinguished for specified angels. Then amplitude and phase are compensated for outputs at other frequencies and angles to minimize the output at conjugate (mirror) target direction. Finally, simulated bearing time histories with and without compensation are given for two moving targets under wide band Gaussian white noise background. Both the computer simulation and sea trial data show that this is a simple and practical method, can better enhance the port and starboard restrain ratio, especially when the band is wide.