Ultra-short baseline (USBL) underwater acoustic positioning systems utilizing narrow band signals are always confronted with the contradiction between phase-difference ambiguity and positioning accuracy.When the signal frequency is high

it is difficult to construct an array with an interval less than a half-wavelength which is always employed in traditional ambiguity resolution technique.Aiming at this problem

a phase-difference ambiguity resolution method based on the generalized maximum likelihood (GML) rule is proposed.Firstly

with vectors of the ambiguity integers initialized

the ambiguity resolution problem is formulated as problem of multiple composite hypothesis testing.Then decision can be made through the GML rule and direction of arrival is obtained.With inter-sensor spacing exceeding half the wavelength

the proposed method effectively extends the unambiguous array aperture and constraint on array configuration is easy to be met.Additionally

under full consideration of the statistical characteristics of observation data

accuracy of direction estimation approaches the Cramer-Rao bound.Simulation results verify the effectiveness of the proposed method.