# Ocean Remote Sensing Techniques and Applications: A Review (Part II)
## 2 RS Applications in Ocean
### Ocean Surface Salinity (OSS)  
OSS is defined as the amount of dissolved salt in ocean water, which affects the electrical conductivity of water [160], and is measured in Practical Salinity Units (PSU). The average ocean salinity is about 35 PSU, meaning that there are 35 g of salt in each liter of ocean water [160]. Since salinity is defined as the salt density in a water solution, it is affected by ecological processes which alter the amount of water or salt, such as ice formation or melting, evaporation or precipitation that can change the amount of fresh water in the solution, and river runoffs which enter salty matters into the ocean [161]. Moreover, global ocean circulations in both horizontal and vertical directions change the amount of OSS [160].  
OSS is an important parameter for oceanographic applications, such as ocean circulations and biogeochemical processes, and is widely used in ocean forecasting models [162]. OSS is also an important variable in understanding the amount of terrestrial substance delivered into the ocean [163], water density, carbonate chemistry near coasts and deep ocean waters, water acidification [164], optical properties, and algal blooms in coastal regions [163]. Additionally, a better understanding of OSS provides more profound knowledge of coastal water quality and hazards, marine pollution, ocean-atmosphere interactions [165], river discharge into the oceans and river-influenced regions [166]. Moreover, OSS is a key parameter in monitoring hurricanes, El Nino and La Nina forecasting, predicting terrestrial floods and droughts, understanding rainfall over the oceans, and forecasting ocean circulations [167].  
In situ ocean salinity measurements are mostly collected by the Array for Real-time Geostrophic Oceanography (ARGO) floats, moored buoys, ocean drifters, surface gliders, Thermo-Salino-Graph sensors, research vessels, marine mammals, and XCTD profilers [168, 169, 170, 171]. Continuous monitoring of global OSS was a difficult task until 2009 due to the low density of these in situ measurements and unreliable global models. However, launching the first RS system capable of OSS measurements in 2009 (i.e., Soil Moisture and Ocean Salinity (SMOS) microwave radiometer) brought new opportunities for various oceanographic applications. It should be noted that although in situ measurements do not suffice for mapping OSS [171], mainly because they are the only representative of one specific geographical point [172], they are usually required to calibrate, train, and validate the RS models. To this end, various physical parameters of the ocean, such as temperature, conductivity, and depth which provide salinity based on the electrical conductivity of the water can be used [160]. Moreover, in situ instruments are usually collated from lower than 1 m depth while satellite-based OSS values refer to a few centimeters on top of the ocean [170]. Therefore, this difference should be considered when in situ and RS data are jointly utilized in oceanographic models.  
Two types of RS systems can be mainly used for OSS estimation: optical and microwave radiometers operating in L-band. Table 4 provides these RS techniques along with their advantages and disadvantages. More information about the applications of each system for OSS estimation is provided in the following two subsections.