Modelling of underwater light fields in turbid and eutrophic waters: application and validation with experimental data

Abstract. A reliable radiative transfer model is an essential and indispensable tool for understanding of the radiative transfer processes in homogenous and layered waters, analyzing measurements made by radiance sensors and developing remote sensing algorithms to derive meaningful physical quantities and biogeochemical variables in turbid and productive coastal waters. Existing radiative transfer models have been designed to be applicable to either homogenous waters or inhomogeneous waters. To overcome such constraints associated with these models, this study presents a radiative transfer model that treats a homogenous layer as a diffuse part and an inhomogeneous layer as a direct part in the water column and combines these two parts appropriately in order to generate more reliable underwater light field data such as upwelling radiance (Lu), downwelling irradiance (Ed) and upwelling irradiance (Eu). The diffuse model assumes the inherent optical properties (IOPs) to be vertically continuous and the light fields to exponentially decrease with the depth, whereas the direct part considers the water column to be vertically inhomogeneous (layer-by-layer phenomena) with the vertically varying phase function. The surface and bottom boundary conditions, source function due to chlorophyll and solar incident geometry are also included in the present RT model. The performance of this model is assessed in a variety of waters (clear, turbid and eutrophic) using the measured radiometric data. The present model shows an advantage in terms of producing accurate Lu, Ed and Eu profiles (in spatial domain) in different waters determined by both homogenous and inhomogeneous conditions. The feasibility of predicting these underwater light fields based on the remotely estimated IOP data is also examined using the present RT model. For this application, vertical profiles of the water constituents and IOPs are estimated by empirical models based on our in-situ data. The present RT model generates Lu, Ed and Eu spectra closely consistent with the measured data. These results lead to a conclusion that the present RT model is a viable alternative to existing RT models and has an important implication for remote sensing of optically complex waters.

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Modelling of underwater light fields in turbid and eutrophic waters: application and validation with experimental data

Ocean Science ◽

10.5194/os-11-33-2015 ◽

2015 ◽

Vol 11 (1) ◽

pp. 33-52 ◽

Cited By ~ 10

Author(s):

B. Sundarabalan ◽

P. Shanmugam

Keyword(s):

Remote Sensing ◽

Radiative Transfer ◽

Water Column ◽

Radiative Transfer Model ◽

Light Fields ◽

Transfer Model ◽

Inhomogeneous Layer ◽

Layer By Layer ◽

Direct Part ◽

Underwater Light

Abstract. A reliable radiative transfer (RT) model is an essential and indispensable tool for understanding the radiative transfer processes in homogenous and layered waters, analyzing measurements made by radiance sensors and developing remote-sensing algorithms to derive meaningful physical quantities and biogeochemical variables in turbid and productive coastal waters. Existing radiative transfer models have been designed to be applicable to either homogenous waters or inhomogeneous waters. To overcome such constraints associated with these models, this study presents a radiative transfer model that treats a homogenous layer as a diffuse part and an inhomogeneous layer as a direct part in the water column and combines these two parts appropriately in order to generate more reliable underwater light-field data such as upwelling radiance (Lu), downwelling irradiance (Ed) and upwelling irradiance (Eu). The diffuse model assumes the inherent optical properties (IOPs) to be vertically continuous and the light fields to exponentially decrease with depth, whereas the direct part considers the water column to be vertically inhomogeneous (layer-by-layer phenomena) with the vertically varying phase function. The surface and bottom boundary conditions, source function due to chlorophyll and solar incident geometry are also included in the present RT model. The performance of this model is assessed in a variety of waters (clear, turbid and eutrophic) using the measured radiometric data. The present model shows an advantage in terms of producing accurate Lu, Ed and Eu profiles (in spatial domain) in different waters determined by both homogenous and inhomogeneous conditions. The feasibility of predicting these underwater light fields based on the remotely estimated IOP data is also examined using the present RT model. For this application, vertical profiles of the water constituents and IOPs are estimated by empirical models based on our in situ data. The present RT model generates Lu, Ed and Eu spectra closely consistent with the measured data. These results lead to a conclusion that the present RT model is a viable alternative to existing RT models and has an important implication for remote sensing of optically complex waters.

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Error simulations of uncorrected NDVI and DCVI during remote sensing measurements from UAS

Miscellanea Geographica ◽

10.2478/mgrsd-2014-0017 ◽

2014 ◽

Vol 18 (2) ◽

pp. 35-45 ◽

Cited By ~ 2

Author(s):

Michał T. Chiliński ◽

Marek Ostrowski

Keyword(s):

Remote Sensing ◽

Radiative Transfer ◽

Vegetation Index ◽

Vegetation Mapping ◽

Radiative Transfer Model ◽

Unmanned Aerial Systems ◽

Transfer Model ◽

Digital Cameras ◽

Aerial Systems

Abstract Remote sensing from unmanned aerial systems (UAS) has been gaining popularity in the last few years. In the field of vegetation mapping, digital cameras converted to calculate vegetation index (DCVI) are one of the most popular sensors. This paper presents simulations using a radiative transfer model (libRadtran) of DCVI and NDVI results in an environment of possible UAS flight scenarios. The analysis of the results is focused on the comparison of atmosphere influence on both indices. The results revealed uncertainties in uncorrected DCVI measurements up to 25% at the altitude of 5 km, 5% at 1 km and around 1% at 0.15 km, which suggests that DCVI can be widely used on small UAS operating below 0.2 km.

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Effect of OH emission on the temperature and wind measurements derived from limb-viewing observations of the 1.27 µm O<sub>2</sub> dayglow

Atmospheric Measurement Techniques ◽

10.5194/amt-13-1817-2020 ◽

2020 ◽

Vol 13 (4) ◽

pp. 1817-1824

Author(s):

Kuijun Wu ◽

Weiwei He ◽

Yutao Feng ◽

Yuanhui Xiong ◽

Faquan Li

Keyword(s):

Remote Sensing ◽

Radiative Transfer ◽

Spectral Region ◽

Emission Lines ◽

Radiative Transfer Model ◽

Transfer Model ◽

Altitude Range ◽

Band Emission ◽

Near Space ◽

Wind Measurements

Abstract. The O2(a1Δg) emission near 1.27 µm is well-suited for remote sensing of global wind and temperature in near-space by limb-viewing observations to its bright signal and extended altitude coverage. However, vibrational–rotational emission lines of the OH dayglow produced by the hydrogen–ozone reaction (H+O3→OH•+O2) overlap the infrared atmospheric band emission (a1Δg→X3Σg) of O2. The main goal of this paper is to discuss the effect of OH emission on the wind and temperature measurements derived from the 1.27 µm O2 dayglow limb-viewing observations. The O2 dayglow and OH dayglow spectrum over the spectral region and altitude range of interest is calculated by using the line-by-line radiative transfer model and the most recent photochemical model. The method of four-point sampling of the interferogram and sample results of measurement simulations are provided for both O2 dayglow and OH dayglow. It is apparent from the simulations that the presence of OH dayglow as an interfering species decreases the wind and temperature accuracy at all altitudes, but this effect can be reduced considerably by improving OH dayglow knowledge.

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Airborne Hyperspectral Imaging for Submerged Archaeological Mapping in Shallow Water Environments

Remote Sensing ◽

10.3390/rs11192237 ◽

2019 ◽

Vol 11 (19) ◽

pp. 2237 ◽

Cited By ~ 5

Author(s):

Alexandre Guyot ◽

Marc Lennon ◽

Nicolas Thomas ◽

Simon Gueguen ◽

Tristan Petit ◽

...

Keyword(s):

Remote Sensing ◽

Shallow Water ◽

Water Column ◽

Sea Level ◽

Hyperspectral Imagery ◽

Airborne Lidar ◽

Radiative Transfer Model ◽

Invasive Technique ◽

Transfer Model ◽

Water Bottom

Nearshore areas around the world contain a wide variety of archeological structures, including prehistoric remains submerged by sea level rise during the Holocene glacial retreat. While natural processes, such as erosion, rising sea level, and exceptional climatic events have always threatened the integrity of this submerged cultural heritage, the importance of protecting them is becoming increasingly critical with the expanding effects of global climate change and human activities. Aerial archaeology, as a non-invasive technique, contributes greatly to documentation of archaeological remains. In an underwater context, the difficulty of crossing the water column to reach the bottom and its potential archaeological information usually requires active remote-sensing technologies such as airborne LiDAR bathymetry or ship-borne acoustic soundings. More recently, airborne hyperspectral passive sensors have shown potential for accessing water-bottom information in shallow water environments. While hyperspectral imagery has been assessed in terrestrial continental archaeological contexts, this study brings new perspectives for documenting submerged archaeological structures using airborne hyperspectral remote sensing. Airborne hyperspectral data were recorded in the Visible Near Infra-Red (VNIR) spectral range (400–1000 nm) over the submerged megalithic site of Er Lannic (Morbihan, France). The method used to process these data included (i) visualization of submerged anomalous features using a minimum noise fraction transform, (ii) automatic detection of these features using Isolation Forest and the Reed–Xiaoli detector and (iii) morphological and spectral analysis of archaeological structures from water-depth and water-bottom reflectance derived from the inversion of a radiative transfer model of the water column. The results, compared to archaeological reference data collected from in-situ archaeological surveys, showed for the first time the potential of airborne hyperspectral imagery for archaeological mapping in complex shallow water environments.

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Analysis of Sentinel-2 and RapidEye for Retrieval of Leaf Area Index in a Saltmarsh Using a Radiative Transfer Model

Remote Sensing ◽

10.3390/rs11060671 ◽

2019 ◽

Vol 11 (6) ◽

pp. 671 ◽

Cited By ~ 19

Author(s):

Roshanak Darvishzadeh ◽

Tiejun Wang ◽

Andrew Skidmore ◽

Anton Vrieling ◽

Brian O’Connor ◽

...

Keyword(s):

Remote Sensing ◽

Radiative Transfer ◽

Leaf Area Index ◽

Plant Traits ◽

Remote Sensing Data ◽

Radiative Transfer Model ◽

Transfer Model ◽

Area Index ◽

Field Campaigns ◽

Sentinel 2

The Sentinel satellite fleet of the Copernicus Programme offers new potential to map and monitor plant traits at fine spatial and temporal resolutions. Among these traits, leaf area index (LAI) is a crucial indicator of vegetation growth and an essential variable in biodiversity studies. Numerous studies have shown that the radiative transfer approach has been a successful method to retrieve LAI from remote-sensing data. However, the suitability and adaptability of this approach largely depend on the type of remote-sensing data, vegetation cover and the ecosystem studied. Saltmarshes are important wetland ecosystems threatened by sea level rise among other human- and animal-induced changes. Therefore, monitoring their vegetation status is crucial for their conservation, yet few LAI assessments exist for these ecosystems. In this study, the retrieval of LAI in a saltmarsh ecosystem is examined using Sentinel-2 and RapidEye data through inversion of the PROSAIL radiative transfer model. Field measurements of LAI and some other plant traits were obtained during two succeeding field campaigns in July 2015 and 2016 on the saltmarsh of Schiermonnikoog, a barrier island of the Netherlands. RapidEye (2015) and Sentinel-2 (2016) data were acquired concurrent to the time of the field campaigns. The broadly employed PROSAIL model was inverted using two look-up tables (LUTs) generated in the spectral band’s settings of the two sensors and in which each contained 500,000 records. Different solutions from the LUTs, as well as, different Sentinel-2 spectral subsets were considered to examine the LAI retrieval. Our results showed that generally the LAI retrieved from Sentinel-2 had higher accuracy compared to RapidEye-retrieved LAI. Utilising the mean of the first 10 best solutions from the LUTs resulted in higher R2 (0.51 and 0.59) and lower normalised root means square error (NRMSE) (0.24 and 0.16) for both RapidEye and Sentinel-2 data respectively. Among different Sentinel-2 spectral subsets, the one comprised of the four near-infrared (NIR) and shortwave infrared (SWIR) spectral bands resulted in higher estimation accuracy (R2 = 0.44, NRMSE = 0.21) in comparison to using other studied spectral subsets. The results demonstrated the feasibility of broadband multispectral sensors, particularly Sentinel-2 for retrieval of LAI in the saltmarsh ecosystem via inversion of PROSAIL. Our results highlight the importance of proper parameterisation of radiative transfer models and capacity of Sentinel-2 spectral range and resolution, with impending high-quality global observation aptitude, for retrieval of plant traits at a global scale.

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Radiometric Microwave Indices for Remote Sensing of Land Surfaces

Remote Sensing ◽

10.3390/rs10121859 ◽

2018 ◽

Vol 10 (12) ◽

pp. 1859 ◽

Cited By ~ 4

Author(s):

Simonetta Paloscia ◽

Paolo Pampaloni ◽

Emanuele Santi

Keyword(s):

Remote Sensing ◽

Radiative Transfer ◽

Snow Water Equivalent ◽

Hydrological Cycle ◽

Microwave Remote Sensing ◽

Dense Medium ◽

Single Frequency ◽

Radiative Transfer Model ◽

Transfer Model ◽

Land Surfaces

This work presents an overview of the potential of microwave indices obtained from multi-frequency/polarization radiometry in detecting the characteristics of land surfaces, in particular soil covered by vegetation or snow and agricultural bare soils. Experimental results obtained with ground-based radiometers on different types of natural surfaces by the Microwave Remote Sensing Group of IFAC-CNR starting from ‘80s, are summarized and interpreted by means of theoretical models. It has been pointed out that, with respect to single frequency/polarization observations, microwave indices revealed a higher sensitivity to some significant parameters, which characterize the hydrological cycle, namely: soil moisture, vegetation biomass and snow depth or snow water equivalent. Electromagnetic models have then been used for simulating brightness temperature and microwave indices from land surfaces. As per vegetation covered soils, the well-known tau-omega (τ-ω) model based on the radiative transfer theory has been used, whereas terrestrial snow cover has been simulated using a multi-layer dense-medium radiative transfer model (DMRT). On the basis of these results, operational inversion algorithms for the retrieval of those hydrological quantities have been successfully implemented using multi-channel data from the microwave radiometric sensors operating from satellite.

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