Mapping the Invisible: An Introduction to Satellite Oceanography
Earth observation (EO) satellites have become a key tool in studying the oceans as they are the only means of observing the oceans synoptically at high spatial resolution.
In contrast, traditional methods, using ships, buoys or moorings, can only provide data over limited regions of the ocean or with limited spatial resolution. However, they have the advantage that they can provide measurements below the ocean surface, whereas satellite sensors generally only provide data at the ocean surface.
Satellite sensors make use of different parts of the electromagnetic spectrum (visible, infrared, microwave) to make measurements, and can be either: passive – measuring radiation emitted from the sea surface; or active – transmitting signals to the sea surface and measuring the return signal that comes back. Visible and infrared sensor are limited by the presence of clouds, when the sea surface is obscured. In contrast, microwave sensors can “see” through clouds, but generally have poorer spatial resolution than that of visible and infrared ones.
The main longstanding measurements techniques that provide a wealth of data on the oceans are:
Sea Surface Temperature (SST) measurements using either infrared or passive microwave sensor. Typically, these have spatial resolutions of ~1km and ~25km, respectively
Ocean Colour measurements made in the visible part of the spectrum, which detect the presence of phytoplankton (microscopic algae) which contain chlorophyll and so change the colour of the waters from blue to green
Sea Surface Height, and hence currents via geostrophy, using radar altimetry – an active system that measures the travel time of a signal to and from the satellite to the sea surface
Other ocean parameters remotely sensed by satellites include Sea Ice, Significant Wave Height, Winds and Sea Surface Salinity.