Radiative transfer models describing the interaction between matter and electromagnetic radiation serve as cornerstones for optical remote sensing. The radiative transfer theory provides the most logical linkage between observations and physical processes that generate signals in optical remote sensing. Radiative transfer modelling is therefore an integral part of remote sensing, since it provides the most efficient tool for accurate retrievals of Earth properties from satellite data. Radiative transfer models are used in a number of different applications such as sensor radiometric calibration, atmospheric correction and the modelling radiation processes in vegetation canopies.
Vegetation radiative transfer models (RTMs) study the relationship between leaf and canopy biophysical variables and reflectance, absorbance and scattering mechanisms. The infinite variability of vegetation structure complicates the modeling of RT in vegetation canopies. Numerous models of RT in vegetation canopies were developed in the second half of the last century. Models differ by the details accounted for and by the simplifications introduced in the description of canopy structure and photon–vegetation interactions. Gradual improvement in RTMs accuracy, yet in complexity too, have diversified RTMs from simple turbid medium RTMs towards advanced Monte Carlo RTMs that allow for explicit 3D representations of complex canopy architectures. This evolution has resulted in an increase in the computational requirements to run the model, which bears implications towards practical applications. When choosing an RTM, a trade-off between invertibility and realism has to be made: simpler models are easier to invert but less realistic, while advanced models more realistic but require a large amount of variables to be configured. The two most widely used models are the leaf model PROSPECT and Scattering by Arbitrary Inclined Leaves (SAIL) canopy model.
Atmosphere RTMs study the interaction of radiation with the atmosphere. The remotely-sensed signals at satellite or airborne platforms are combinations of surface and atmospheric contributions, with relative amounts varying across the two wavelength regions, depending on the condition of the atmosphere. The order of magnitude of atmosphere signals can be equal or larger than that of land or ocean surface signals that arise at the top of the atmosphere (TOA). In order to derive accurate sensor calibration and atmospheric correction, the contribution of the atmospheric constituents to the total retrieved signal must be understood and modelled. Atmospheric radiative transfer models simulate the radiative transfer interactions of light scattering, absorption and emission through the atmosphere. Some widely used atmospheric RTMs are 6SV, libRadtran, MODTRAN, and ATCOR.
Advances in radiative transfer modeling enhance our ability to detect and monitor changes in our planet through new methodologies and technical approaches to analyze and interpret measurements from air- and space-borne sensors.
Light has a key role for aquatic ecosystems, both in marine and freshwater. It penetrates underwater and interacts with dissolved and particulate water constituents, the optically active constituents (OACs). They absorb and scatter the light, giving water its characteristic colour and affect the light availability underwater. The three main OACs are phytoplankton, coloured dissolved organic matter (CDOM) and suspended particulate matter (SPM) and vary in time and space. Absorption and scattering represent the inherent optical properties (IOPs) of water and depend solely on the OACs present in the water. In addition, water bodies have apparent optical properties (AOPs) that depend both on OACs and the incident light field.
The chlorophyll in the phytoplankton absorbs blue and red wavelengths and reflects green. Therefore, the oceans appear blue-green depending on the concentration of phytoplankton. CDOM is primarily tannin-stained water released from decaying detritus. High CDOM concentrations appear yellow-green to brown. CDOM absorbs ultraviolet (UV) light in the surface waters which is harmful for phytoplankton but competes with phytoplankton for light. Inorganic suspended matter (ISM) is the suspended sediment in the water. It is a component of SPM and strongly scatters longer (red) wavelengths. High ISM concentrations give water a reddish-brown colour. Pure water, however, absorbs longer wavelength red light. As natural waters vary in their composition, oceanographers introduced ocean classification schemes based on the optical properties of water. The main differentiation is between Case 1 open ocean waters and Case 2 coastal waters. In open ocean waters, the optical properties are dominated by phytoplankton and covarying material. In coastal waters, optical properties are dominated by suspended sediments and CDOM that vary independently of phytoplankton.