Antarctica: satellites give the big picture
Satellites offer a great advantage over other means of observing the Antarctic region, its ice sheet, exposed land, sea ice cover, and ocean. They offer a unique ability to view very large remote areas, on a repetitive basis over long periods of time and almost at will. In this way they provide the "big picture" in both space and time for field programs that operate over limited areas or within short time frames.
Fixed installations such as Automatic Weather Stations provide long time series of observations at a number of discrete points. Field surveys by tractor traverse over the ice sheet, by aircraft, or by ship, collect data along discrete lines or over a network. The distance or area covered is limited by the duration of the field activity of weeks to months. A satellite can gather information for the whole study area in a matter of moments with a single image, or even for the whole region over a relatively short time.
Remote sensing by satellite uses measurements of radiation scattered or emitted from the surface to derive information about the surface, such as its shape, texture, temperature, movement and composition. Most satellite instruments carry no source of illumination, but depend on the sun for the source of radiation scattered from the surface. Other instruments measure the radiation emitted from the surface. A few instruments transmit their own signal and measure the radiation backscattered from the surface. These are various forms of "radar". Clouds, gases, and particles in the atmosphere also scatter, emit and absorb radiation. This affects what we see of the surface and complicates the interpretation of those measurements. Some instruments take advantage of these effects to measure the composition of the atmosphere. They map the ozone hole, and measure other trace gases important to the greenhouse effect.
Satellite imagery of Antarctica
The variety of satellite systems now available allows the measurement of many different parameters. Some examples of these for the snow and ice cover are surface elevation, movement, temperature, roughness, sea ice extent and sea ice concentration, ice sheet extent, abundance and size of icebergs, and snow melt. Some instruments measure parameters directly. For instance radar altimeters trace out a line vertically below the satellite measuring the elevation of the surface. By combining the observations from many orbits across the region, a map describing the shape and elevation of the surface is generated. A radiometer measuring the thermal infrared radiation emitted from the surface gives the surface temperature. It scans the surface in a series of lines across the orbit track and thus builds a temperature picture of the surface as the satellite moves along its orbit. The principles are simple but the practice is often difficult and much processing is required to arrive at the required product.
Imaging systems such as Landsat provide a wealth of information corresponding to the time the data is collected. There are many instruments that operate at different wavelengths and at different spatial resolutions from many kilometres down to metres. But darkness and frequent cloud cover limits the use of instruments operating at visible or infrared wavelengths. Satellites with Synthetic Aperture Radars (SAR), the new high-resolution imaging system, provide a powerful tool which can be used at anytime because they "see" through clouds. There is a compromise between resolution or detail and the area imaged at any time. Large areas are imaged by combining individual scenes from many orbits. From these images, the outer edge of the continental ice sheet is mapped and thus its change in extent over time. The velocity of glaciers is measured by tracking, in pairs of images, the displacement of crevasse patterns and other features visible on the surface of glaciers. Similarly the motion of the pack ice is given by the movement of the ice floes. Other systems with a coarse resolution but wide imaging swath cover the whole region in a day. These are used to monitor the broad-scale motion of the pack ice, calving and drift of icebergs, and so on.
Many parameters cannot be measured directly but are derived by combining data for different wavelengths. Sea ice concentration is obtained from measurements of microwave radiation emitted from the surface of the sea ice and water. New instruments offer the prospect of more and better measurements of parameters needed for the study of the ice. But the satellites cannot provide everything and measurements from field programs are needed to extend our knowledge of what is seen from the satellites and to validate the information that is collected.