MARS lander: Georeferencing landing and pop points of untethered ocean monitoring systems using fundamental physics

Abstract

Subsurface observations are crucial for understanding the ocean's role in Earth's climate and for refining climate models. However, existing aquatic monitoring systems that allow such insights remain complex and costly due to their high demands for deployment, sampling, and recapture. Since low-cost, easy-to-deploy deep-sea landers are scarce, and with the aim of facilitating more subsurface observations, this study provides a simple method for georeferencing small-sized untethered landers. Their underwater trajectories are modelled with fundamental physics, dead reckoning, lander geometry, and numerical simulations. Using free fall, upthrust, and ocean current dynamics, the proposed approach estimates their underwater trajectories, including landing (at the seabed) and pop (at the sea surface) points. The method relies on the lander's physical characteristics, including its vertical and horizontal cross-sectional areas, to calculate the drag force coefficients used to determine its trajectories during descent and ascent through the water column. Ocean currents' magnitudes are modelled using Ekman's exponential decay down to 90 m of the water column, while the depths until 900 m are modelled from prior ADCP surveys by varying ocean current headings with depth between −20 and 20°. Surface ocean and wind current headings are modelled with open datasets from satellite telemetry. Lander's velocity, displacement, and dive time to the landing and pop points, including the total radial excursion and uncertainty in heading, are analytically derived, numerically calculated, and empirically assessed a-posteriori until 90 m, yielding a ∼38 m radial excursion (40% error) against the obtained GNSS coordinates in field deployment, and 33° in heading uncertainty during a 138-s excursion. Additional random walk simulations are shown for full ocean depth obtaining radial excursion of 1038 m with 278 min total dive time. This approach is generalizable to any subsurface aquatic monitoring systems targeting deployments with diverse payloads from smaller sea vessels, not requiring cranes, radio, GNSS, or acoustic telemetry. Since it accounts for key nature factors, our method provides special benefits in planning and optimizing deployments. Additional discussion focuses on the method's practicality for full ocean depth deployments.