Abstract

Through decades of efforts and practices, we have achieved great progress in understanding ocean biology and biogeochemistry through satellite measurements of ocean (water) color, or passive remote sensing. These include detailed global maps of the distribution of surface phytoplankton, the production of newly formed particulate organic matter through photosynthesis (i.e., primary production), as well as the change and feedback of phytoplankton in a changing climate, to name a few. However, these results are still far from a full account of ocean biology and biogeochemistry, where we want more detailed information of phytoplankton (e.g., types and sizes), as well as information in the vertical dimension. For such, we are happy to see new developments in ocean optics and ocean color remote sensing. These include, but certainly are not limited to, hyperspectral sensors, measurements via polarized setups, as well as ocean lidar systems. In particular, through pumping laser light into deeper ocean, lidar has demonstrated great potential to fill the gap of passive ocean color remote sensing. These developments in technology are providing exciting new findings where breakthroughs in ocean biogeochemistry are on the horizon. Thus, we organized this feature issue in Applied Optics to summarize a few recent developments and achievements, where readers and the community can easily capture progress on both fronts, as well as the potential and advantages of the fusion of passive and active optical sensing. Specifically, this issue contains 12 papers describing research in both active and passive optical remote sensing of aquatic environment. They are still limited in number and subject, but are expected to stimulate the ocean color community with findings relevant for satellite applications.

© 2020 Optical Society of America

Active papers:

There are six contributions in active remote sensing, lidar systems and retrieval methodology. Churnside and Shaw were invited to provide an overview of lidar remote sensing of the aquatic environment. This paper describes the optical properties of sea water relevant to lidar performance and the transfer of a polarized laser through a highly scattering and absorbing media. They also analyzed lidar system design and examples of measurements from shipborne and airborne lidars.

Realizing that blue wavelength lidar would penetrate much deeper than a green laser in the open ocean, Ma et al. describe a blue–green laser system at wavelengths of 486 nm and 532 nm with peak power of 2 MW, and apply it in an airborne oceanic lidar. The laser source inherits the advantages of solid state Nd: YAG lasers, and uses a UV pumped optical parameter oscillator to generate blue light at one of the Fraunhofer lines. The field experiment in the clear sea water in the South China Sea shows promising penetration depth. For applications by wider communities, Lu et al. present a portable handheld fluorescence detect system to measure the water quality of a freshwater ecosystem. The system uses a CW laser as the excitation light source and can detect the water quality as a short-range remote sensing apparatus.

Retrieval methodologies and applications of aquatic lidar are always a focus of remote sensing technology, to this front Shen et al. discuss the instrument response function on the retrieval of water properties from lidar measurements, and suggest that short pulse lasers and fast detectors are of priority for a practical remote sensing system. Further, Liu et al. present an iterative retrieval method for the derivation of the profile of lidar attenuation coefficient from airborne measurements. In this approach, the backscatter-to-attenuation ratio is determined through a bio-optical model for an elastic scattering lidar.

As a new application of oceanic lidar, Dolin et al. present algorithms for deriving the spectral-energy characteristics of ocean internal wave from the power fluctuation of lidar signal. The correlation between the biological optics and physical oceanography indicates the interesting coupling of marine ecological system and kinetic process in the upper ocean.

Passive papers:

On the passive side there are also six papers covering recent achievements on a variety of topics embracing from primary-production and photo-acclimation models to drone-based spectral camera for aquatic sensing. Sathyendranath was invited to discuss and reconcile photo-acclimation and primary-production models, where Sathyendranath et al. show the spectral effects in light penetration in the ocean, and present results to accommodate the spectral sensitivity of the initial slope of the photosynthesis-irradiance curve, making the model fully compatible with spectrally resolved models of photosynthesis in the ocean. They also present better assignment of the chlorophyll-to-carbon ratio in primary-production models, which could be an important avenue for reducing model uncertainties and for improving the usefulness of satellite-based primary production calculations in climate research.

Regarding the scattering properties of marine particles, Hu et al. address the void of particle backscattering coefficient at 180° and its relationship with the backscattering coefficient through analysis of measured scattering phase functions, indicating that the extrapolated backscattering at 180° from neighboring scattering angles is sensitive to shape and internal structure of particles.

Remote sensing of floating algae is in high demand, Cui et al. introduce a method for macroalgae mapping from geostationary satellite images benefitting the high frequency observations. Specifically, the method reconstructs a high-spatial resolution image by integrating the high-frequency components contained in a sequence of geostationary low-spatial resolution images of the same region. Evaluations performed with GF-4 satellite images show that the spatial resolution is increased up to 25 m. The method is thus expected to improve the ability to distinguish macroalgae patches and to precisely delineate their boundaries.

Vicarious calibration of a satellite system is required for accurate remote sensing of aquatic properties, Bulgarelli and Zibordi analyze the adjacency effects of a small island for such a task. The analysis presented at representative visible and near-infrared center-wavelengths shows remarkably different adjacency radiance patterns up to several kilometres from an island as a function of the illumination and sensor viewing geometries. Results indicate that adjacency effects could not be universally quantified on the sole basis of the atmospheric and marine optical properties. Additionally, estimates of adjacency effects from a specific region cannot be confidently assumed representative for the effects in regions exhibiting different albedo and coastline, as well as resulting from diverse illumination and viewing geometries.

Instrumentation is always a hot topic in aquatic remote sensing. On this front, Gilerson et al. present a novel above-water snapshot hyperspectral imager used in combination with three differently oriented polarizers to determine the spectra of polarized radiation in terms of Stokes vector components. Measurements indicate appreciable agreement with simulations performed using a vector radiative transfer (VRT) code. Results from hyperspectral polarized measurements and VRT simulations indicate potential for multi-angular retrieval of the ratio of the beam attenuation to absorption coefficient in addition to the other parameters like absorption and backscattering coefficients. Meanwhile, Shea and Laney have developed a simulation framework for evaluating lightweight spectral cameras in drone-based aquatic sensing applications and explore its use in two realistic remote sensing scenarios. Such a simulation framework can provide insight into the use of existing camera systems, as well as aspects of optical design or hardware that would lead to an improved accuracy in aerial applications over natural water bodies. Additionally, it would be useful in addressing the complex trade-offs between optical design and deployment strategies.

Finally, we would like to thank all the contributors, reviewers, and editorial staff for their time and effort for this feature issue.

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