## 4 Conclusions  
The pivotal role of oceans in regulating climate patterns, transportation, provision of human food, and economic growth is beyond any dispute. To this end, RS systems have been widely used to tackle the issues with the traditional in situ or shipborne approaches. In the first part of this review paper, 12 different RS systems, which are frequently utilized for ocean studies, and six oceanographic applications of RS systems (i.e., OSW, OSC, OWH, SL, OT, and SD) were discussed. RS of OSW depends on the emissive and reflective properties of the ocean surface. Microwave radiometer, GNSS-R, SAR, scatterometers, and HF radar are the systems widely used for OSW estimation. Microwave radiometers provide large-scale coverage and are suitable for high wind speeds. However, they have coarse spatial resolution and suffer from low accuracy for estimating OSW direction in low wind speeds. Higher spatial and temporal resolutions and less sensitivity to atmospheric attenuation are the major privileges of GNSS-R systems for OSW estimation. SAR systems providing high spatial resolution are applicable at both low and high wind speeds. However, speckle noise issue and challenging preprocessing steps are the main drawbacks of SAR systems for OSW studies. Scatterometers provide acceptable efficiency in low wind speeds. They can be used for the entire globe at the cost of coarse spatial resolution. HF radars can provide appropriate accuracy at diverse wind speeds, but OSW data are only available at specific coastal regions where the HF radar has been installed. There are many RS systems to directly/indirectly estimate OSC. Among these systems, HF and marine radar (i.e., X-band/C-band) provide a more accurate estimation of coastal OSC and its variations by real-time observations. However, they need costly maintenance and are not applicable for global OSC estimations. Therefore, the altimeter satellites have been commonly used to provide geostrophic OSC at a global scale with a high temporal frequency. However, the altimetry measurements are negatively affected by weather conditions and atmospheric variations and need corrections before being used in the OSC estimation. Different RS systems, such as GNSS-R, SAR, altimeter, and marine radar, have been utilized for OWH estimation. GNSS-R systems have high temporal and spatial resolutions but have relatively low accuracy for OWH estimation. Although SAR data are less affected by the atmosphere, they are more affected by ocean surface substances. Altimeters also suffer from low spatial and temporal resolutions, as well as sensitivity to wind and wave directions. Although HF radars lack sufficient data availability, they provide accurate measurements of OWH and are operational in all-weather and day/night conditions. Marine radars provide a better SNR ratio and are cost-effective, but they can only be used for local scales. Both active and passive RS systems can also measure SL and its variations. GNSS-R is often employed for global and regional SL monitoring. However, obtaining high accuracy in SL estimation with such systems requires long-term GNSS-R observations. Moreover, altimeters have been widely used for global SL mapping with a relatively coarse spatial resolution. Gravimetry satellites are used for measuring SL changes and distribution of ocean mass in very coarse spatial resolutions. RS systems can be utilized to investigate various aspects of OT, such as tidal flats, tidal channels, tidal currents, OTL, and tidal wetlands. Optical, GNSS-R, SAR, altimeter, and LiDAR systems have been used for OT studies. Optical systems are not good at estimating water height changes but provide a wide range of spectral and spatial resolutions. The NRT data of GNSS-R are also important resources for OT studies. SAR systems are useful for all OT applications because they estimate ocean surface topographic changes with a high accuracy level. Altimeters benefit from multilook processing, but they are applicable only to tidal channels and tidal flats.