| | |  | ASAR assists Antarctic rescue mission | |
Why is ASAR useful for assisting polar operations?
During the winter months, the hours of daylight at this Southern latitude are very limited. Currently there are only 2 hours when visibility is sufficient to allow a helicopter to make reconnaissance missions, to try and plot the best possible route through the weakest points in the ice.
The fact that the ASAR instrument can operate during the night as well as the day means that it is completely unaffected by the problem of restricted daylight. In addition, the Antarctic region is often covered by thick clouds. The ASAR radar system is able to penetrate cloud cover and reveal the surface below, which is not possible with optical satellite images (such as the images used by weather forecasters).
Using ASAR to plot a route through the ice
The winter ice belt in the waters surrounding the Antarctic continent is approximately 1100 km wide. However, the ice concentration and 'toughness' is not uniform and therefore some routes are much easier to traverse than others. Images from Envisat's ASAR instrument can assist in finding these easier routes. The three main kinds of ice conditions that will be found in the ice belt are 'old ice' (the most difficult to penetrate), new ice (easier) and open water (plain sailing). Often it is possible to use a SAR image to discriminate between these different conditions.
Old ice will have a rough surface caused by repeated compacting and buffeting. These processes also are responsible for the strength of ice that has survived for at least one summer. The rough surface will strongly reflect the radar signal; showing-up as bright regions in the ASAR image.
New ice will have a smoother surface compared to old ice and therefore will appear darker in an ASAR image; a large proportion of the radar signal is reflected away, in the opposite direction to the satellite.
For open water the situation is more complicated. If the weather is calm and there is little wind to roughen the surface of the sea, then, in a similar manner to new ice, the radar from the satellite will be reflected away from the satellite receiver, producing a dark signal. On the other hand, if the sea surface is roughened by high winds, then the radar backscatter can be even brighter than for old ice.
However, experienced ice experts can make use of many other clues in an ASAR image to help find the easiest passage through an ice field. One of the most important factors is an understanding of the way in which regions of open water are formed. These generally appear in narrow passageways called 'leads', which are formed by the motion and fracture of the ice cover. There are many leads visible in the ASAR images shown here. They appear as long dark features, indicating that the sea surface is relatively smooth. Finding these open water leads is one of the main aims of using ASAR data in these ice-infested regions.
Why is ASAR an "Advanced" SAR system?
One of the important advantages the Envisat satellite's ASAR instrument has compared to the "regular" SAR instrument onboard the earlier ERS satellites, is that it has the capability to take much larger images of the Earth's surface. ASAR can take images along very long tracks, typically thousands of km in length and 400 km in width. ERS's SAR systems are only able to image a swath width of 100 km. This quadrupling of the width of the image gives ice experts a greatly enhanced capability for plotting optimum routes and monitoring ice dynamics. Clearly it is much easier to navigate a long and hazardous journey when an image is available covering the full region to be crossed. Furthermore, the wider swath width available with ASAR means that any given point on the Earth's surface can be imaged more frequently than was possible with the ERS satellites. This in turn, means that more frequent updates can be made to ice maps using the ASAR system.
Last update: 1 August 2002 | |
