Abstract:  Combined positioning system consists of GPS, attitude sensor and USBL ( Ultra ?short Base Line) system. For ordinary USBL, target position is determined by the phase method. In order to avoid phase ambiguity,
the distance between two elements should be less than 0 ? 5 wave length, and thus the accuracy will not be very high. For the USBL in the combined system, position is determined by the time method, the distance between two
elements is larger than that of the ordinary U SBL. During positioning, the combined positioning system floats at the sea surface. A sound projector is mounted on the target beneath the sea surface. Wideband signal is transmitted by
the projector and received by the array elements. By maximum likelihood estimation, the time delay between transmitted and received signal can be obtained. Thanks to the large element?distance, wide signal bandwidth and
accurate time delay estimation, the accuracy of the positioning is much higher than that of the ordinary USBL. There are four elements in the USBL array. From four time delays, four ranges can quickly be got. The latitude,
longitude and depth of the target can be calculated by the method of least squares and the conversion of coordinatesystem. The position error simulation, time delay estimation simulation and results of the lake testing are present in the paper.