# Ocean Remote Sensing Techniques and Applications: A Review (Part II)
## 2 RS Applications in Ocean
### Sea Surface Temperature (SST)
#### 2.3.1 TIR Radiometer
Measuring SST using the SC method requires the sea surface emissivity and the atmospheric profiles, which can be obtained using the following equation [128; 139]:  
\[L_{\lambda}(\theta)\ =\ \tau_{\lambda}(\theta)\varepsilon_{\lambda}(\theta)B_{ \lambda}(T)+L_{\lambda}^{\dagger}(\theta)+\tau_{\lambda}(\theta)[1- \varepsilon_{\lambda}(\theta)]L_{\lambda}^{\downarrow}(\theta) \tag{2}\]  
where \(L_{\lambda}(\theta)\), \(L_{\lambda}^{\dagger}(\theta)\), and \(L_{\lambda}^{\downarrow}(\theta)\) refer to the sensor spectral radiance, the downward radiance of atmosphere, and Top Of Atmosphere (TOA) spectral radiance, respectively; \(\tau_{\lambda}(\theta)\) and  
Figure 5: The main satellites for SST measurements. \(\varepsilon_{\lambda}(\theta)\) are the atmospheric transmittance and the emissivity of the sea surface, respectively. The \(\varepsilon_{\lambda}(\theta)\) is computed using the geometric-optics models [140; 141] of the sea surface in the TIR atmospheric windows and the rest of the parameters are computed from the Radiative Transfer (RT) models. SST estimation using the SC method is negatively affected by uncertainties from the profile fed into the model and limitations in modeling water vapor absorption [128]. In fact, these methods can only be used for SST estimation when accurate atmospheric profiles are available. To cope with these uncertainties, MC methods that use the differential BTs measured in the two or more than two channels were proposed. Equation (3) provides the general formulation for an MC algorithm [98; 124]:  
\[SST\ \ aT_{i}+\gamma\big{(}T_{i}-T_{j}\big{)}+c \tag{3}\]  
in which \(T_{i}\) and \(T_{j}\) are the BTs measured in the two channels; c is an offset; and \(\gamma\) is the water vapor absorption coefficient. The MC methods have a high potential for SST estimation from all TIR radiometers with at least two thermal channels, and there is no need for accurate atmospheric profiles [142]. The coefficients in Equation (3) can be derived by regression analysis or RT simulations [98; 124; 129]. Since MC algorithms are not sufficiently accurate to represent the water vapor effects [98; 129], a group of other algorithms, called the nonlinear SST algorithms [143; 144] have been developed. These equations are mainly based on the BT values of the channels at the atmospheric windows (i.e., \(\lambda\) = 3.5-4.1 \(\upmu\)m and \(\lambda\) = 9.5-12.5 \(\upmu\)m) with correction terms of the effects of atmospheric moisture and satellite zenith angles [131]. Moreover, depending on the selected atmospheric windows, these equations can be divided into three categories of dual window (at \(\lambda\) = 3.7 and \(\lambda\) = 11 \(\upmu\)m), split window (at \(\lambda\) = 11 and \(\lambda\) = 12 \(\upmu\)m), and triple window (at \(\lambda\) = 3.7, \(\lambda\) = 11 \(\upmu\)m, and \(\lambda\) = 12 \(\upmu\)m) [129; 145]. TIR radiometers onboard LEO can produce global SSTs at a medium spatial resolution [99]. The number of thermal bands and the spatial resolution are the most important characteristics of the TIR radiometry for SST estimation. For example, the hyperspectral TIR radiometers with several narrow spectral bands are very useful for SST estimation [98]. The Atmospheric Infrared Sounder (AIRS) [146] deployed on the NASA satellite Aqua, and Infrared Atmospheric Sounding Interferometer (IASI) on the three Metopolar-orbiting satellites of the European Organization for the Exploitation of Meteorological Satellites (EUMETSAT) [147] are the examples of the hyperspectral TIR sensors which have been used for SST estimation. For example, Figure 6 demonstrates SST products generated from ASTER and MODIS around the Sendai Bay. As is clear, the thermal variations of coastal waters in Sendai Bay are more evident in the ASTER SST image compared to that of the MODIS, indicating the potential of ASTER data to produce high-resolution SST maps in the coastal areas. Landsat, ASTER, and MODIS are among the most important LEO TIR radiometers for SST estimation.