Passive Inverted UltraShort Baseline (piUSBL) Localization: An Experimental Evaluation of Accuracy The underwater environment poses significant challenges for accurate autonomous underwater vehicle (AUV) navigation. Electromagnetic (EM) waves rapidly attenuate due to absorption by water, thereby preventing the use of traditional EMbased positioning methods such as Global Positioning System (GPS) or visiblelight cameras. Consequently, underwater positioning is often performed using systems that operate in the hydroacoustic frequency range (leq 10 MHz). Recent work has demonstrated the efficacy of a novel acoustic positioning approach for multiAUV operations called passive inverted ultrashort baseline (piUSBL) localizationwith each vehicle equipped with a timesynchronized USBL array, oneway traveltime (OWTT) range and angle between the AUV and a single acoustic beacon enables multiAUV navigation relative to the beacon. In this work, a piUSBL system using a fivehydrophone pyramidal array implemented on a WAMV autonomous surface vehicle (ASV) was used to experimentally gather acoustic measurements and to compare the accuracy of piUSBL localization against groundtruth from a differential GPS unit. This paper provides a comprehensive analysis of the positioning accuracy of the system in a realworld environment, both prior to and after Bayesian filtering, using two independent acoustic beacons for validation. We demonstrate that piUSBL provides acoustic range and angle measurements with errors of about mupmsigma  0.03pm 1.49 mathrmm and mupmsigma  0.11pm 3.16circ respectively. These experimental results suggest that piUSBL localization can provide a highly accurate, inexpensive, and lowpower navigation solution for the next generation of miniature, lowcost underwater vehicle.

ModelAided Localization and Navigation for Underwater Gliders Using SingleBeacon TravelTime Differences An accurate motion model and reliable measurements are required for autonomous underwater vehicle localization and navigation in underwater environments. However, without a propeller, underwater gliders have limited maneuverability and carrying capacity, which brings difficulties for modeling and measuring. In this paper, an extended Kalman filter (EKF)based method, combining a modified kinematic model of underwater gliders with the traveltime differences between signals received from a single beacon, is proposed for estimating the glider positions in a predictupdate cycle. First, to accurately establish a motion model for underwater gliders moving in the ocean, we introduce two modification parameters, the attack and drift angles, into a kinematic model of underwater gliders, along with depthaveraged current velocities. The attack and drift angles are calculated based on the coefficients of hydrodynamic forces and the sensormeasured angle variation over time. Then, instead of satisfying synchronization requirements, the traveltime differences between signals received from a single beacon, multiplied by the sound speed, are taken as the measurements. To further reduce the EKF estimation error, the RauchTungStriebel (RTS) smoothing method is merged into the EKF system. The proposed method is tested in a virtual spatiotemporal environment from an ocean model. The experimental results show that the performance of the RTSEKF estimate is improved when compared with the motion model estimate, especially by 46% at the inflection point, at least in the particular study developed in this article.