The use of Lagrangian platforms and of Autonomous Underwater Vehicles (AUVs) in oceanography has increased rapidly over the last decade along with the development of improved biological and chemical sensors. These vehicles provide new spatial and temporal scales for observational studies of the ocean. They offer a broad range of deployment and recovery capabilities that reduce the need of large research vessels. This is especially true for ice-covered Arctic ocean where surface navigation is only possible during the summer period. Moreover, safe underwater navigation in icy waters requires the capability of detecting sea ice on the surface (ice sheets). AUVs navigating in such conditions risk collisions, RF communication shadowing, and being trapped by ice keels. In this paper, an underwater sea-ice detection apparatus is described. The source is a polarized continuous wave (CW) diode-pumped solid-state laser (DPSS) module operating at 532 nm. The detector is composed of a polarizing beam splitter, which separates light of S and P polarization states and two photodetectors, one for each polarized component. Since sea-ice is a strong depolarizer, the ratio P/S is an indicator of the presence or absence of sea-ice. The system is capable of detecting sea-ice at a distance of 12m. This apparatus is designed to be used by free drifting profiling floats (e.g., Argo floats), buoyancy driven vehicles (e.g., sea gliders) and propeller-driven robots (e.g., Hugin class AUV).
We consider the characteristics of macroalgal (kelp) targets of a LiDAR capable of assessing algal 3D morphology and quantifying algal biomass via fluorescence or differential absorption. Spectral absorption, fluorescence emission, fluorescence efficiency, and temporal fluorescence induction dynamics of Arctic algae can differ by class due to variation in photopigment complement. Surface reflectance characteristics of macroalgae can vary by morphology and structure. In this, work, we present an investigation of fluorescence via excitation-emission spectra of Arctic macroalgal targets. Simulations using these optical characteristics will later guide us in optimizing LiDAR configuration and performance under various operating conditions.
The physical and biological properties of Arctic ice and coastal benthos remain poorly understood due to the difficulty of accessing these substrates in ice-covered waters. A LiDAR system deployed on an autonomous underwater vehicle (AUV) can interrogate these 3D surfaces for physical and biological properties simultaneously. Using our understanding of the absorption, inelastic scattering (fluorescent), and elastic scattering properties of photosynthetic micro- and macroalgae excited by lasers, we present results of in situ tank tests using a two-wavelength (473 nm, 532 nm) prototype to evaluate both fluorosensor and differential absorption (DIAL) approaches using reflectance standards and selected macroalgae as targets.
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