Using satellites to observe the temperature of seawater near the surface of the ocean is probably the most mature application of ocean remote sensing. Observations are made with IR, which cannot "see" through clouds and with passive microwave which is not affected by clouds but has other trade-offs. SST sensors are aboard both polar-orbiting satellites and geostationary satellites.
Satellite altimeters use active radar to observe the surface height of the ocean which is not smooth or flat. Fluid hills and valleys deviate from a reference (mean geoid) height at the ocean surface. These vertical gradients are of interest for sea level rise, storm predictions, ocean currents, ecosystem ecology and other applications.
Radiation from the ocean surface of light in the visible wavelengths gives information about the color of the ocean. This "ocean color" (radiances) can be used to estimate chlorophyll concentration (the pigment in plants and phytoplankton responsible for photosynthesis and the dominant source of color in the open ocean) or the coefficients of light attenuation through the water column and other parameters (generally related to biological processes). In coastal areas, other biological compounds and minerals add complexity to interpretation. Clouds will block remotely sensed OC.
Sea Surface Winds (also known as Ocean Surface Vector Winds (OSVW) for some techniques)
Winds, both magnitude (speed) and direction over the ocean drive other physical and chemical processes and so are used to model dynamic earth/ocean/atmosphere coupled systems ocean and are used for marine weather forecasting. Different remote sensing techniques may be used for gathering information on ocean surface winds including active radar and passive microwave.
Spaceborne Synthetic Aperture Radar (SAR) imagery maps the surface microwave radar reflectivity at resolutions from a sub-meter to 100 m depending on the particular SAR satellite and mode. Since a radar provides its own illumination, imagery is independent of the time of day. At typical radar frequencies, SARs can image through clouds, so SARs are considered "all-weather" instruments. Several gephysical parameters can be derived from SAR including sea surface wind speed.
Both thermal and microwave sensors can be used to estimate sea ice. The thermal method has higher spatial resolution but is impacted by clouds (e.g., VIIRS). The microwave method is lower spatial resolution but not impacted by clouds (e.g. AMSR-2).
The salinity of seawater at the ocean surface can be remotely sensed using microwave frequencies. Currently, this technique is valid for open ocean measurements, while recognizing decreased sensitivity for colder water. Measurements within approximately 50 km of land are biased by land contamination and less accurate. Salinity is a defining parameter for ocean dynamics and can also serve as a proxy for certain biogeochemical processes.
Visible radiances can be combined to form images that look similar to a "photograph" of the earth and ocean from the satellite perspective.