The use of sensors aboard a satellite that collect information about the ocean surface by measuring what is reflected from the water is surface is known as passive sensing. Two important measurements collected in this way are:
- Ocean Colour - used to determine the concentration of chlorophyll plant pigments and determine the amounts of phytoplankton in the water, a starting point for the ocean food chain.
- Sea Surface Temperature - the amount of heat given of by the ocean can be measured as an indication of sea surface temperature. This is done by monitoring thermal infrared wavelengths over the oceans.
The use of Instruments on board the satellite that transmit a signal which can bounce of the surface of the ocean and back to the satellite is known as active sensing. A wide range of applications of this method of collecting data have been developed including:
The ability to produce images of the earth's surface through clouds. An imaging radar works like a flash camera in that it provides its own light to illuminate an area on the ground and take a snapshot picture, but at radio wavelengths. A flash camera sends out a pulse of light (the flash) and records on film the light that is reflected back at it through the camera lens.
Instead of a camera lens and film, a radar uses an antenna and digital computer tapes to record its images. In a radar image, one can see only the light that was reflected back towards the radar antenna. In the case of imaging radar, the radar moves along a flight path and the area illuminated by the radar, or footprint, is moved along the surface building the image as it does so.
Sea Surface Levels - Topex - Poseidon and its recent replacement Jason -1 are a good example of this type of satellite. By actively sending out signals and bouncing them off the surface of the ocean scientists can accurately measure change in the surface height of the oceans over large distances. This information is very useful in measuring and predicting climate change due to events like the 'El Nino Southern Oscillation", as well as in determining patterns of water movement in the ocean.