# Ocean Remote Sensing Techniques and Applications: A Review (Part II)
## 2 RS Applications in Ocean
### Ocean Color (OC)
#### 2.5.1 Optical  
Remotely sensed OC measurements provide information about the spectrum of water reflectance and enable us to retrieve marine Inherent Optical Properties (IOPs). IOPs are the spectral absorption and scattering attributes associated with ocean water and its constituents [183]. OC is generally referred to as the spectrum of reflectance (called \(R_{rs}\)), which is computed as the ratio of water-leaving radiance to downwelling irradiance above the ocean surface [184]. The total radiance (\(L_{t}\)) from the TOA is also measured by optical RS systems. Considering the several radiances involved in the satellite OC measurements, there are generally two main approaches in the literature that define the relationship between the intended radiance and geophysical properties [185]. The first and most common approach is based on the fact that OC applications seek to measure the spectral distribution of water-leaving radiance (\(L_{w}\)). \(L_{w}\) illustrates photons emanating from absorption and scattering processes beneath the sea surface and emit into space [183; 186]. For example, a simple equation of different reflectance pertaining to satellite OC applications can be formulated as Equation (7) [187]. \[L_{t}=L_{atm}+L_{surf}^{TOA}+L_{w}^{TOA} \tag{7}\]  
where the superscript TOA demonstrates radiances reaching the TOA. The atmospheric contribution (\(L_{atm}\)) is known as the scattering caused by atmospheric gases and aerosols and multiple scattering interactions between them. The term total surface reflectance (\(L_{surf}^{TOA}\)) includes the reflection of sun glint and background sky radiance from the sea surface and the contribution of their radiance, which is reflected by surface whitecaps and foam [187]. All these correction terms must ultimately be subtracted from \(L_{t}\) to calculate \(L_{w}^{TOA}\)[188]. \(L_{w}^{TOA}\) can be then converted into \(R_{rs}\) after division by downwelling irradiance. Finally, the geophysical properties can be estimated by applying different algorithms to \(R_{rs}\). Regarding the second approach, \(L_{t}\) can be directly related to IOPs or representatives of biogeochemical properties without the obligation of implementing complicated atmo spheric corrections [183; 189]. Although atmospheric corrections are highly prone to be confounded by absorbing aerosols and optically complex waters, there are multiple non-conventional approaches that circumvent this prerequisite. For example, a combination of atmospheric and oceanographic methods can solve both atmospheric and marine variables at the same time. Generally, this method combines two models in which one model accounts for aerosol properties and the other one expresses water components via IOPs. Operation of the coupled atmosphere-ocean approaches is similar to semi-analytical inversion approaches. However, in these methods, the number of unknown variables in the coupled models is higher because both aerosol and marine expressions are involved. Additionally, coupled models normally need more spectral bands than conventional semi-analytical inversion approaches. By employing the existing RS OC data, coupled models use \(L_{t}\) from the visible and NIR bands by which the retrieval of aerosol and hydrosol variables converts to a classic inverse problem. The principal privilege of coupled atmosphere-ocean approaches is that they can better account for absorbing aerosols and intense NIR radiances. Nevertheless, the coupled models have inherent limitations due to their dependence on the general formulation of the aerosol and marine components, which has been historically challenging [183]. Total marine IOPs can be considered as the sum of the contributions of common component IOPs from different water constituents, namely phytoplankton, Total Suspended Matter (TSM), and CDOM. Investigations into TSM and CDOM concentrations from RS OC data and retrieval of main IOPs of OCh, which expresses phytoplankton abundance and physiology, have been widely performed using different arithmetic algorithms.