As a critical technology for autonomous vehicles, high precise positioning is essential for automated container terminals to implement intelligent dispatching and to improve container transport efficiency. Because of the unstable performance of global positioning system (GPS) in some circumstances, an ultra wide band (UWB) positioning system is developed for autonomous trucks in an automated container terminal. In this paper, an asynchronous structure is adopted in the system, and a three-dimensional (3D) localization method is proposed. Other than a traditional UWB positioning system with a server, in this asynchronous system, positions are calculated in the vehicle. Therefore, propagation delays from the server to vehicles are eliminated, and the real-time performance can be significantly improved. Traditional 3D localization methods based on time difference of arrival (TDOA) are mostly invalid with anchors in the same plane. However, in order to guarantee anchors and tags in line of sight (LOS), anchors have to be installed in a vertical plane under the tire cranes. Given this problem, an improved method is presented, which overcomes the matrix singularity. Three hyperboloids can be drawn according to three TDOAs. Either two of them intersect to a plane curve, summing up to three planes in all. Two of them intersect in one line. The line and the hyperboloids intersect at two symmetrical points about the anchor-plane. Then positions of tags can be quickly confirmed as positional relations between trucks and the anchor-plane have been determined when anchors installed. Extensive experiments have been conducted to evaluate the performance in comparison to existing TDOA methods. As shown by experimental results, the proposed method performs better than traditional methods in location accuracy and stability.
