Methods of unwrapping phase ambiguity and selecting direct sounds in an ultra short baseline positioning system

This paper presents a novel method to unwrap phase ambiguity and an expert system to select the direct sound from several reflecting pulses in an USBL positioning system. Generally, in the crossed-shaped USBL array, unwrapping ambiguity in the long edge is based on the absence of ambiguity in the short edge, where the array spacing is less than half-wavelength. However, if the positioning system works at a high frequency, which results in the short-edge phase ambiguity, the classical method to unwrap long-edge ambiguity will fail. This paper proposes a sector searching method for this scenario. It utilizes all combinations of two edges to solve all possible target orientations, and takes the mode as the final result. Additionally, hydrophones receive multi-path reflections besides the direct sound in practical application. The expert system is particularly used for selecting the direct sound correctly, which seriously effects the positioning precision. It contains series of criteria to evaluate the qualit...