Assessment of directional accuracy of GNSS-Acoustic measurement using a slackly moored buoy

Abstract

We assessed the positioning accuracy of GNSS-Acoustic (GNSS-A) measurement using a slackly moored buoy. A key feature of real-time buoy-based GNSS-A measurement is that positioning must be performed via single ranging from an arbitrary observation position and encompassing the drifting range of the buoy. In this study, slack-line mooring was employed to resist strong Kuroshio current of up to 5.5 knots in the Nankai Trough. During a year-long sea trial, the buoy drifted within a circle of~4000 m radius, which is much larger than the dimension of a seafloor transponder array. In the sea trial, more than 500 successful regular and on-demand ranging measurements were obtained at various observation positions. The results show that the horizontal positioning accuracy (2σ) of the array for single ranging was 46 cm when the buoy was inside the array and 97 cm when it was outside the array. These accuracies are comparable to those achieved through single ranging in traditional ship-based surveys at the same site. In addition to the distance dependency, we observed directivity in the accuracy, depending on the geometry between an observation point and seafloor transponders. To interpret the accuracy degradation and directivity as a function of observation position, we calculated error ellipses using a dilution-of-precision (DOP) analysis procedure for GNSS-A positioning. The error ellipses clearly illustrate that the variance in array positions is large in the line-of-sight direction from an observation point to the array center. We translated the error ellipses to positioning accuracies using the standard deviation of travel time residuals when an array position is estimated using all pings. The positioning accuracy resulting from the translation corresponded to that obtained using array positions in a buoy observation. The DOP analysis results and their translation into positioning accuracies enabled a detailed assessment of the directional accuracy of GNSS-A positioning at an arbitrary observation position, which is important for realtime measurement of seafloor movement in the event of a huge earthquake.

Development of a kinematic GNSS-Acoustic positioning method based on a state-space model

“…This type of the GNSS-A positioning, using individual pings, previously introduced as the classic GNSS-A positioning method in "Introduction" and often called "array positioning", can in principle be achieved using the kinematic GNSS-A positioning method. Note that observational data collected from "point surveys" (where the acoustic pings are transmitted from the center of the seafloor transponder array) are required for kinematic GNSS-A positioning because accuracy of the array displacements degrades away from the array center (e.g., Kido 2007;Imano et al 2015Imano et al , 2019. In the following sections, we introduce a kinematic GNSS-A positioning method based on array positioning where the array geometry is already determined.…”

“…We considered that this modeling error is related to the apparent shift in the point of the array center when some transponders fail to respond. Since the accuracy of the array positioning degrades away from the array center (e.g., Kido 2007;Imano et al 2015Imano et al , 2019, deviation of the vertical array displacements results in serious modeling errors due to the apparent shift in the point of the array center. This problem can be avoided by estimating the vertical array positions precisely.…”

“…Another issue that should be overcome for achieving the precise real-time GNSS-A positioning is how to keep the sea-surface platform position at the array center. Regardless of implementation of the EKF, the kinematic array positioning basically requires the point survey data because accuracy of the array displacements degrades away from the array center as explained in "Principles of the GNSS-A positioning method" (e.g., Kido 2007;Imano et al 2015Imano et al , 2019. In the past trials of the GNSS-A measurement using the moored buoy, the buoy occasionally moved out of the transponder array, and the accuracy of the positioning was degraded (Imano et al 2019).…”

“…Although GNSS-A observations are typically collected by campaign-style surveys using a research vessel as the sea surface platform, continuous GNSS-A observations have recently been developed using moored buoys (e.g., Imano et al 2015;Kido et al 2018;Kato et al 2018;Imano et al 2019) for an early warning system through instant offshore geodetic positioning. To support these efforts, we investigate a precise "kinematic" GNSS-A positioning method.…”

Sensitivity Analysis for Seafloor Geodetic Constraints on Coseismic Slip and Interseismic Slip-Deficit Distributions

“…Here, specific analysis has been performed on the data obtained by these methods after the observation. In recent years, the real-time measurement of OBP data using submarine cable networks (e.g., Mochizuki et al, 2018) and the quasi real-time analysis of GNSS-A data using marine platforms (e.g., Imano et al, 2019;Kinugasa et al, 2020) have been undergoing continuous development. Monitoring of vertical crustal deformation by the submarine cable network OBP system is expected to contribute to improving the accuracy of real-time estimation of coseismic slip distribution.…”