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FEATURE REMOTE SENSING


The difference between mapping terrain topographically on land and bathymetrically under the sea is that light interacts with water much more than with air


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this occurs depends on the size, shape, and composition/refractive index of the particle,’ said Collister. ‘The depolarisation ratio refers to the ratio of light that is detected in a new orientation, to light that retains its original orientation. We separate light scattered back to the lidar into these two components using polarising filters.’ Because it does not rely on sunlight


like older colour-based methods, lidar can collect data during the day and at night, according to Schulien. Currently, to answer questions on a global scale, researchers use the Cloud-Aerosol Lidar with Orthogonal Polarisation (Caliop) sensor, a joint-venture between Nasa and the French space agency CNES. However, Schulien pointed out that Caliop – carried on the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation (Calipso) satellite – doesn’t measure Kd. She has therefore been using Nasa’s plane-mounted HSRL instrument, which has a more limited range in time and space, but provides more comprehensive data for oceanographers. ‘I can answer more specific questions at finer spatial scales,’ Schulien explained. From HSRL data, Schulien can get Kd and use it to look at a measure of light scattering from particles in the size range that includes phytoplankton, known as bbp. ‘I measured bbp and Kd in the water and demonstrated that HSRL was able to collect accurate measurements down to two optical depths,’ Schulien said, where optical depth is the variable ability of water to block light. ‘Then I used these


14 Electro Optics June 2021


parameters in a depth-resolved model to show that we can use lidar as a measure of phytoplankton primary production. The applications for using measures of light attenuation and phytoplankton biomass in the ocean are seemingly boundless. For example, bbp with respect to phytoplankton chlorophyll-a can vary physiologically as a response to light or nutrients.’ Schulien has also used lidar depolarisation measurements from HSRL to reveal changes in phytoplankton community composition related to changes in cell shape.


‘Only a few publications mention the lidar return on bubbles – the primary mechanism through which the atmosphere and ocean exchange heat, momentum and gas’


Illuminating bubbles Damien Josset, an oceanographer at the US Naval Research Laboratory’s Ocean Sciences division in Washington DC, uses lidar to study bubbles, an important component of ocean physics at high wind speed. ‘They are the primary mechanism through which the atmosphere and ocean exchange heat, momentum and gas,’ Josset said. ‘Only a few publications mention the lidar return on bubbles.’ The NRL team has therefore developed a prototype lidar specifically to study


bubbles with ‘a lot of promising features’, said Josset. They include a compact scanner/pointing system and dual visible/ infrared camera system to help show what the lidar is scanning, and extra feature identification capability. Depolarisation and the attenuated backscatter coefficient could both help observe bubble clouds in principle, Josset explained. Increase of attenuated backscatter profile when observing bubbles is clear in a controlled setup in a lab, he noted, ‘but in the field, we need polarisation to determine the presence of bubbles,’ Josset said. More complex lidar systems, or combining lidar with cameras, may be able to detect the presence of bubbles without using polarisation, he added. Josset first used his system in


the breaking wave tank in the NRL’s Laboratory for Autonomous Systems Research in Washington DC, and on the research vessel Sikuliaq in the Gulf of Alaska in 2019. More recently he confirmed that everything works well flying in the NRL’s UV-18 Twin Otter aircraft. ‘The airborne lidar is functional and it will perform very well after a few months of additional optimisation in the lab,’ said Josset. ‘The data will be an amazing resource to determine whitecap coverage fraction coincident with the lidar data from Nasa Calipso and echo sounder bubble vertical profiles. I can confirm, without ambiguity, that lidar is the perfect tool to study bubble clouds. It creates a strong depolarisation of the signal, and the scattering properties are such that attenuation is extremely limited. The lidar penetrates the whole depth of the cloud.’ All the various options for where the


lidars are mounted have advantages and limitations for this application, according to Josset. ‘The range of time and space scales covered by the different platforms offers complementarity,’ he explained. ‘Ground-based, underwater and shipborne lidar show the time evolution of the bubble clouds in a given area. The platform moves slowly, while the bubble clouds pass. Aerial and satellite lidar can provide global-scale observations at specific times, and there are typically large spatial differences between each lidar


@electrooptics | www.electrooptics.com


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