Objectives To mitigate the impact of acoustic transducers' coordinate errors on the positioning accuracy of undersea geodetic control points, an improved joint adjustment method incorporating between-buoy baseline constraints is proposed for global navigation satellite system/acoustic （GNSS/A) underwater precise positioning system integrated with a moored buoy observation system. This method treats both the surface buoy and the seafloor transponder as unknown points, using the mutual ranging information between GNSS buoys as constraints.

Methods First, the mutual ranging equation is formulated based on the baseline information between buoys, and the positions of the acoustic transducers provided by GNSS positioning are introduced as virtual observations. These are combined with underwater acoustic ranging data to form a joint functional model for solving the equations. Then, to balance the covariance matrices of buoy positions, baseline observations, and acoustic observations, variance component estimation is applied to improved joint adjustment process. This step accurately determines the weight matrices for the three types of observations. Finally, the proposed method is verified using simulation data.

Results Experimental results indicate that the proposed method significantly enhances the positioning accuracy of seafloor transponders. The positioning accuracy is improved by 17.5% to 46.3% compared with the conventional positioning method, and by 16.5% to 45.8% compared with the joint adjustment method.

Conclusions The simulation results further demonstrate that the proposed method outperforms the other two methods, showcasing its superior performance in undersea geodetic control point positioning.