# Ocean Remote Sensing Techniques and Applications: A Review (Part II)
## 2 RS Applications in Ocean
### Ocean Surface Salinity (OSS)
#### 2.4.2 Microwave Radiometer  
Microwave radiometers can estimate OSS by measuring ocean BT [161]. The dielectric constant in open water is determined using microwave frequency and electrical conductivity. The ocean surface emissivity is a function of the dielectric constant and the state of the surface roughness. In principle, OSS can be estimated from BT observations [178]. The emissivity is the linking quantity between BT and OSS [160] and depends on multiple parameters. Multiple factors affect the spaceborne BT measurements from microwave radiometers, consequently decreasing the accuracy of the retrieved OSS. For instance, land contamination in large ground pixels (pixel sizes of about \(\sim\)50 km) and antenna orientation (due to the existence of side lobes) decrease the OSS accuracy [171]. Furthermore, SI contamination occurring in high latitudes could affect OSS estimation [162]. On the other hand, BT values derived from microwave radiometers are less accurate in cold waters (polar regions) due to the reduced sensitivity of L-band measurements [162]. Moreover, several variables, such as Radio Frequency Interferences (RFI), solar and galactic radiations, ionosphere Faraday rotation, surface roughness, and atmospheric effects should be precisely modeled to obtain an accurate OSS product using microwave radiometer data [161]. It is also worth noting that some error patterns have not been fully modeled. For instance, SMOS is affected by seasonal biases, differences between ascending and descending passes, and some systematic sources of RFI [179]. Finally, it should be noted that differences in OSS estimation from SMOS, Soil Moisture Active/Passive (SMAP), and Aquarius are expected because these missions use different dielectric constants, surface roughness correction models [180], minimization equations, filtering criteria, and debiasing techniques [181]. There are currently three main microwave radiometers that are capable of measuring OSS: SMOS, SMAP, and Aquarius. SMOS is known as ESA's Water Mission, launched \begin{table}
\begin{tabular}{c c c c} \hline \hline
**RS System** & **RS System** & **Advantage** & **Disadvantage** \\
**(Passive/Active)** & **(Type)** & & \\ \hline \multirow{4}{*}{Passive} & Optical & higher spatial resolution, simple & Only available during \\  & & resolution, simple & daytime in cloud-free conditions \\ \cline{1-1}  & Microwave & All-weather & Low spatial resolution, \\ \cline{1-1}  & radiometers & observation, faster & Affected by land \\ \cline{1-1}  & & global coverage & contamination \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Different RS systems for OSS estimation along with their advantages and disadvantages. on November 2, 2009. It was designed to improve understanding of Earth's water cycle and land moisture for hydrological cycles. SAC-D/Aquarius was an international project between NASA and Argentina National Space Activities Commission (Comision Nacional de Actividades Espaciales--CONAE). The mission objectives were to study OSS variations to understand better water cycle changes and ocean circulation and their influence on climate. The overall objective of the SMAP mission was to monitor global soil moisture. SMAP includes an L-band radiometer and an L-band radar both of which operate at multiple polarizations at the frequencies of 1.41 GHz and 1.29 GHz, respectively. Although SMAP was primarily designed to measure soil moisture, its radiometer data have been used for OSS estimation. Figure 8 illustrates a sample of SMAP observations which was processed into higher level products (8-day averaged). The uncertainty of this product increases by increasing latitudes towards north and south poles due to the SI contamination. Many studies have been so far conducted to retrieve OSS from the SMOS, Aquarius, and SMAP radiometers. For example, [PERSON] et al. (2019) compared OSS data from these microwave radiometers with in situ measurements and analyzed the causes of differences.