Experimental Design

Overview

Global Navigation Satellite System - Acoustic (GNSS-A) is a seafloor geodetic tool roughly analogous to GNSS. Standard GNSS is unable to position points on the seafloor because the ocean is opaque to the satellite radar used to communicate with GNSS receivers. To address this, the GNSS-A technique involves deploying two sets of instruments: acoustic transponders that rest on the seafloor and a sea surface platform with a transducer and standard GNSS. The surface platform utilizes acoustics to position the seafloor transponder relative to itself and GNSS to anchor the entire system (seafloor transponder and surface platform) in a terrestrial reference frame such as ITRF.

Experimental Setup

A GNSS-A site consists of an array of seafloor transponders. These transponders rest on the seafloor in a “listen” mode, replying with an acoustic pulse when they receive a like pulse with a specific address. This allows a user to measure the two-way travel time (TWTT) between a surface platform and a seafloor transponder. Because the TWTT is sensitive to the speed of sound in seawater, a value that may change due to instantaneous oceanographic sources such as internal gravity waves, a single transponder may not be positioned to much better than ~10 cm due to oceanographic signals. This may be mitigated by deploying acoustic transponders in arrays of 3 or more and averaging the apparent transponder positions in space and time, which allows us to position the geometric center of a transponder array to ~1 cm.

The sea surface platform may be a wave glider, research vessel, or buoy. GNATSS assumes that the surface platform surveys the acoustic transponders from the center of the array and interrogates each transponder simultaneously at regular intervals, typically every 15-20 seconds. Surveying from the array center yields robust array positions but less information about sound speed variations in the water column. Thus, we typically recommend surveying for 3-5 days at a single site and averaging the array position over this time period to mitigate oceanographic signals.