Satellite Remote Sensing of the Oceans
Synthetic Aperture Radar (SAR) imaging of the Earth is becoming increasingly popular due to the fact that these radars can be used regardless of the atmospheric conditions—they easily penetrate clouds, whereas clouds absorb almost all the sea surface infrared signals of infrared imaging instruments. The SAR gathers global information by emitting a beam of microwave radiation towards the sea surface of the world's oceans.
If the sea surface is smooth, i.e., calm conditions, the surface acts like a mirror reflecting the incident beam in a direction away from the satellite. If the sea surface is roughened by wind or currents, then some of this incident beam is reflected back to the satellite and is received by the SAR. Thus, the stronger the backscattered signal, the rougher the sea surface.
Marine surface pollution is something we see all too often on television and in the news. The enduring images of stricken sea birds and baby seals on oil-soaked beaches put a lot of public pressure on environmental agencies to monitor marine pollution and catch the culprits who may be illegally dumping oil/waste off our shores. There is work going on in our group related to the observation of sea surface slicks, both man-made and natural. This type of work is suited to “SAR” studies.
The SAR on board the ERS-2 satellite sends images of the ocean surface back to Earth receiving stations on a regular basis. The SAR images reveal a surprising amount of structure on the sea surface reflecting just how rough the sea actually is at the time the image is acquired. Where the sea surface is rough, the radar beam is strongly scattered back to the satellite antenna; and where it is smooth, the beam is strongly reflected from the sea surface away from the antenna.
Slicks have the effect of calming the sea surface and damping wave motion thereby resulting in most of the radar beam being reflected away from the antenna. On a SAR image, slicks typically appear as dark patches on the sea surface corresponding to calm conditions. Using IDL under Linux, sophisticated processing routines have been developed that reveal slick-like features on the sea surface and provide information about their position. Wind speeds can also be determined from the radar backscatter which helps to predict where the surface wind-driven currents may move the slick. The main problem with processing this data is the image sizes. The raw SAR images are 130MB in size and take a long time to process before they can be displayed on the screen. Linux has proved to be much more reliable at handling SAR data than MS Windows, and it is much faster too.
The scene in Figure 4 is a good SAR image of the Gulf of Thermaikos in the Aegean Sea, just off the coast of Greece. The image was taken by the ERS-2 SAR on 25 May 1996 at 20:43 GMT. A number of oceanographic features evident in this image which are of general interest. The numerous black swirls and bands across the water surface correspond to surface slicks. Much of the slick material comes from a river outflow at the edge of the city of Thessaloniki, seen at the top left in the image. This reverine material is concentrated around the top of the bay and is then distributed throughout the gulf by eddies, tides and wind-driven currents. Incidently, the dark square patches around the town are probably rice fields which give very little radar return signal. In the bottom right of the image is the edge of Mount Olympus, mythical home of the Greek gods.
Figure 5 shows the same area one year earlier, but this time there is very little of oceanographic interest in the image. The sea appears to be bright, which implies that most of the radar signal had been backscattered towards the SAR. This suggests that the sea surface is very rough (wind speeds greater than 10 meters per second), rough enough to break up any slick material and destroy the surface signatures of eddies and weak currents. Around some of the Eastern coastline are dark patches which correspond to areas of water which are sheltered from the effects of the wind by the mountains and are therefore calm, backscattering very little of the radar beam towards the SAR.
The full potential of SAR is yet to be realised, and the European Space Agency is keen to see the SAR data it produces fully used. Military applications include looking for surface vessels and the surface signatures of submarines, although here at SUDO we deal strictly with the oceanographic science.