This paper proposed a straight-line survey scheme for misalignment calibration of a USBL (Ultra Short Baseline) positioning system. Under the scheme of straight-line survey, USBL positioning errors arising from heading, pitch, and roll misalignments are derived, respectively. The effect of each misalignment angle and how the differences can be used to calibrate each misalignment angle in turn are presented. An iterative algorithm that takes advantage of the geometry of position errors resulting from angular misalignments is then proposed for estimating alignment errors. The performance of the iterative algorithm is evaluated through simulation and field experiment. The simulation and experimental results have demonstrated the effectiveness of the iterative scheme in finding alignment errors. The proposed algorithm yields a very rapid convergence of the solution series; usually the estimates obtained in the first iteration approximate to true values, and only few iterations are necessary to achieve fairly accurate solutions.
Precise positioning is always essential for successful navigation of underwater vehicles (Whitcomb et al., 1999; Jouffroy and Opderbecke, 2007). Although the performance of USBL (Ultra Short Baseline) systems is quite effective, it subjects to various potential sources of error including alignment, travel times, and angle measurements (Philip, 2003a). With regard to the alignment error, it usually occurs when the USBL transceiver cannot be assembled with the attitude sensors as a single unit. Therefore, alignment calibration is always a problem needed to be solved for precision navigation (McEwen et al., 2005; Kinsey and Whitcomb, 2007). Misalignment calibration of USBL positioning has been an objective of study for a long time, and several methods have been proposed for identifying alignment errors. Usually, those proposed methods are performed using a fixed seabed transponder as a reference point. Opderbecke (1997) applied an iterative adaptive procedure to identify angular misalignments between attitude and USBL sensors.
