# Ocean Remote Sensing Techniques and Applications: A Review (Part II)
## 2 RS Applications in Ocean
### Sea Ice (SI)  
SI is formed when the surface water of the ocean freezes. The main difference between SI and glaciers or icebergs is that SI forms from salty ocean water, while glaciers and icebergs form from fresh water and snow [57]. Generally, SI forms, grows, and melts exclusively in the ocean [57]. Although SI can cover up to about 30 million square kilometers of the Earth's surface [58], many people might never directly encounter SI in their lives because SI is found primarily in the Arctic and Antarctic regions [57; 58]. SI has direct and indirect effects on the climate, wildlife, and many human activities. Because of its bright surface, SI has a high surface albedo and reflects a significant portion of the sunlight into space because of its bright surface. The high surface albedo decreases the solar energy absorbed by SI and helps to keep the temperature of the polar regions low [59]. The warmer climate in the polar regions melts SI and decreases the bright surfaces' ability to reflect the received sunlight. Consequently, even a minor SI loss in the polar regions can lead to a global cycle of warming and melting [59]. Moreover, SI affects the thermohaline circulation by changing the water temperature, water salinity, and salt concentration [60]. SI also influences global atmospheric circulation by affecting the heat exchange between the ocean and the atmosphere [61]. Additionally, many animals that live in the Arctic and Antarctic, such as polar bears, penguins, and seals, depend on and are heavily affected by SI and its changes [62]. SI is also very crucial for human activities in the Arctic and Antarctic. For instance, indigenous people living in the Arctic depend on SI-covered areas for transportation, fishing, and hunting [63]. Finally, SI mapping and monitoring are essential for many industrial operations, including oil rigs, factories, safe ship navigation, and scientific research in polar regions [63; 64]. Despite its crucial role, gathering in situ data for SI studies is very difficult due to their remote locations, extreme climate, and changing nature. Scientists have previously used ships, submarines, buoys, and field camps to gather data for SI monitoring over relatively small regions [58]. These methods are costly and labor-intensive. However, RS provides various types of information from remote locations in broad areas and with suitable spatial and temporal resolutions. Consequently, RS techniques have become the primary data gathering methods for SI studies [65]. Various characteristics and physical parameters of SI,including extent [66; 67; 68], thickness [69; 70], drift and motion [6], lead [71], temperature [72], type [73], age [74], and snow cover [75] can be effectively derived from RS datasets. The reflected, emitted, or backscattered electromagnetic energy in the visible, Near Infrared (NIR), Thermal Infrared (TIR), and microwave parts of the electromagnetic spectrum can be measured by different RS systems to study SI. Different characteristics of SI (e.g., thickness, temperature, type, and age) can affect the electromagnetic wave received by RS systems and, thus, can be measured by these systems. Based on these characteristics, many studies have applied RS data to study SI [76; 77; 78; 79; 80; 81; 82; 83; 84; 85]. Moreover, imaging (e.g., geometry, imaging season, and weather conditions) and sensor (e.g., frequency, spatial resolution, and polarization) properties can affect electromagnetic energy and should be considered in SI mapping and monitoring using RS systems [76]. Table 2 summarizes different types of RS systems along with their advantages and limitations for SI studies. More details of the most commonly used RS systems for SI studies (i.e., optical, TIR radiometers, microwave radiometers, SAR, scatterometer, and altimeter) are also provided in the following subsections.