D1 Satellite Remote Sensing

Topic

Satellite-based ocean remote sensing measures properties of the ocean surface and the underlying water column from orbital platforms using two types of sensors: passive sensors, which measure radiation emitted or reflected by the ocean itself, and active sensors, which emit their own signal and measure how it returns after interacting with the water. It is the only method capable of providing truly global coverage with update intervals ranging from days to weeks; consequently, it is essential for monitoring large-scale phenomena such as El Niño, ocean gyre circulation, phytoplankton blooms, and sea-level rise, making it the most comprehensive tool available today for monitoring the ocean as a whole.

Sea surface temperature is measured via thermal infrared radiometry: the ocean emits infrared radiation at a rate proportional to the fourth power of its temperature—as established by the Stefan-Boltzmann law—and the sensor measures this radiation as a "brightness temperature" across several distinct infrared channels. By comparing these channels, it is possible to correct for atmospheric absorption occurring between the ocean and the satellite. In clear waters, this technique achieves an accuracy of between one-tenth and five-tenths of a degree.

Satellite altimetry, in contrast, measures the distance between the satellite and the ocean surface rather than temperature, doing so with an accuracy of one to two centimeters using a radar operating in the Ku-band. This precise distance measurement allows for the calculation of sea-level height at any given point; furthermore, by analyzing how this height varies from point to point, the geostrophic relationship—the same balance between pressure gradient and Coriolis force found in the atmosphere—enables the calculation of ocean surface current fields. Ocean color—specifically, the reflectance of the ocean surface in visible light—is used to estimate the concentration of chlorophyll and other optically active constituents dissolved or suspended in the water; it is precisely this color variation that allows a satellite to distinguish, from space, areas of higher or lower phytoplankton concentration without ever touching the water.

Surface salinity, in turn, is measured via microwave radiometry at a frequency of approximately 1.4 gigahertz within the so-called L-band: microwave emission from the ocean depends on the water's dielectric temperature—an electrical property determined by salt content—and this chain of dependencies makes it possible to infer surface salinity from the microwave emissions detected by the satellite.

Finally, surface winds over the ocean are measured using scatterometers—instruments that transmit microwave pulses (in Ku-band or C-band) toward the sea surface and measure the energy reflected back by the small capillary waves generated by the wind itself; the stronger the wind, the rougher the surface becomes and the more energy is backscattered toward the satellite, allowing the intensity of the returned signal to be used to estimate wind speed. Thus, by combining infrared temperature, radar-measured height, visible color, microwave emission, and microwave backscatter, satellite-based ocean remote sensing reconstructs—channel by channel—an ocean environment that no ship or buoy could monitor with comparable coverage or frequency.

ID:1662