Drone Based Remote Sensing of Coastal Ocean Surface Currents

Ocean surface currents play a critical role in the coupled atmosphere-ocean system where they transport material, modulate the surface wave field, and enhance ocean mixing and the energy balance across the interface. In coastal areas, these currents may be more intense, characterized by large spatial gradients and enhanced by tidal motions. Surface currents are also the primary mechanism for the advection and distribution of material, such as pollution, in the coastal environment. Surface currents also hold significant implications for biogeochemical cycling and even maritime safety. For the U.S. Navy, surface currents impact all nearshore activity, including amphibious and subsurface operations, unmanned systems, diving, and other actions in the littoral zone. Thus, more effort is needed to understand these complex coastal currents and to provide precise, timely observations for validating and testing hydrodynamic simulations and prediction systems. For the Navy, predicting and exploiting inherently dynamic currents in this environment is crucial for minimizing risk and maximizing the effectiveness of operations. Presently, surface current observations typically come from widely used acoustic current meters (e.g., ADCP’s), Lagrangian floats, or remote sensing, including space-borne or land-based radar systems. Acoustic meters are widely used from both moorings and mobile platforms and provide key hydrographic measurements. However, these systems have natural operational limits, are in situ to the environment being sensed, and have significant challenges to resolving near-surface current shear which is critical to understanding coastal transport and wave field modulation. Lagrangian floats are useful but have limited applicability in a wide range of conditions. Remote sensing methods may have limited coverage and challenges in complex coastal domains. For the Navy, there is a critical need for timely measurement of surface currents that is not limited by in situ deployments and can provide observations outside of the scope of remote sensing platforms. The U.S. Navy routinely operates Unmanned Aerial Systems (UASs) that conduct remote sensing using a myriad of sensors, often including optical full motion video imagery. Given the advent and prevalence of these UAS platforms for passive scene visualization, including in the marine coastal domain, we ask the question: can quantitative ocean surface current data be extracted from overhead video imagery? Recently, drone-based remote sensing of surface waves has been used to estimate surface current fields in complex littoral zones. Streßer et al. present a novel algorithm to process video imagery of surface gravity waves to estimate the current field in a riverine environment. Lund et al. expanded on this work by refining the data processing to capture the impacts of platform motion more accurately, and successfully applied this method to quantify tidal currents through a coastal waterway. The fundamental premise of the method exploits the linear dispersion relationship of a two-dimensional wavefield that exhibits a Doppler frequency shift because of the presence of an underlying current field. Both authors presented their techniques using video of wind waves over a quasi-channelized flow. Here, we present the findings of a research study aimed to test these methods using a novel field data set collected from a coastal domain with an arbitrary current field. The project focuses on a field experiment in Monterey Bay, CA planned for three weeks in October and November 2024. A climatological analysis of HF radar-derived currents in the study area reveals predominantly alongshore currents with some cross-shore variation influenced by tides and mesoscale wind variability. Data acquisition for the project will be accomplished using a six-rotor heavy-lift drone, chosen for its flight endurance of nearly 50 minutes and wind resistance. Planned data collection missions focus on 3-5 km cross-shore transects to hover stations that can nominally last up to 30 minutes. The long dwell time will enable time series analysis of temporal variations in the current field. The drone will carry one of two gimbaled camera payloads: a high-resolution 20-megapixel optical camera, or a 2-megapixel optical/infrared imaging system. Flight missions over the three weeks are planned to collect video imagery capturing variations in tidal stage, diurnal effects, synoptic winds, and sea state conditions. A bottom-mounted ADCP and collocated moored wave buoy will be deployed to collect upper ocean bulk currents and directional wave spectra that will be used to validate measurements derived from the video imagery after processing. This presentation will summarize the field test outcomes and highlight preliminary analyses of the video data collected and derived surface current fields. The focus will be on validation with in situ sensors and evaluating the Streßer et al. and Lund et al. algorithms in an arbitrary current field environment. This work aims to develop drone-based surface current mapping as a key tool in coastal environmental sensing and improve our understanding of these processes in the littoral zone. Works Referenced: Lund, B., and Coauthors, 2021: UAS Current Mapping: A Wave-Based Heading and Position Correction. J Atmos Ocean Technol, 38, https://doi.org/10.1175/jtech-d-20-0123.1. Streßer, M., R. Carrasco, and J. Horstmann, 2017: Video-Based Estimation of Surface Currents Using a Low-Cost Quadcopter. IEEE Geoscience and Remote Sensing Letters, 14, https://doi.org/10.1109/LGRS.2017.2749120.