Pictures of the Earth’s surface are becoming sharper, closer and more informative as imaging technology of ever-increasing sophistication becomes available.
America’s National Aeronautics and Space Administration (NASA) and Jet Propulsion Laboratory OPL) are leaders in imaging technology development for deployment on satellites and spacecraft, and their equipment is extensively tested in the atmosphere before being used in space. In 1996, a DC-8 equipped with NASA’s latest imaging equipment flew missions out of Christchurch for three days, collecting radar and optical data from a range of environments.
Among the instruments on board was AIRSAR, which stands for Airborne Synthetic Aperture Radar. Radar, itself an acronym for Radio Detection And Ranging, was developed to detect hostile aircraft, the first systems becoming operational during World War H. Its transformation into a tool for imaging the land has happened only in the past few decades.
Like all radar, SAR works by transmitting brief bursts of microwave-frequency electromagnetic radiation, then receiving and recording the signals that are reflected back, or backscattered—in this case from the landscape. Only a small fraction of the transmitted radiation is reflected back to the antennas—most is scattered in other directions.
From an imaging point of view, radar has the advantage over optical techniques that it is unaffected by light, so it can operate by night or day, through cloud, fog, smoke, rain or even volcanic ash.
In AIRSAR, some 1500 high-power pulses (each lasting only 10 to 50 millionths of a second) are fired per second, and the reflections from each pulse are received by the antennas before the next pulse is emitted. Although signal waves travel at the speed of light, the time that they take to return can be recorded with sufficient precision to give distance measurements accurate to a metre or two.
The longer the antenna (also known as an aperture) used to collect the reflected energy, the finer the resolution of the image. A very long antenna is impractical on an aircraft (or satellite), but SAR can mimic a very long antenna by combining the echoes received by the radar as the plane moves along its flight track. Because the radar is moving relative to the ground, the returning echoes are Doppler-shifted: the echo wavelength is slightly shorter from a target the plane is approaching, longer from one it is moving away from. These subtle variations, when combined with a knowledge of the plane’s precise position, allow SAR to obtain data which appear to have been recorded at the same instant with a very long antenna.
But there is more to AIRSAR’s wizardry. Radar waves are typically emitted from AIRSAR in a polarised form (with all the waves in the same plane) and in phase (with the peaks on every wave aligned). When the beam is reflected back from objects on the ground (rocks, trees, etc), the polarisation and phase will be disturbed, and these changes can be detected by the antennas and associated equipment, then used to provide insight into the ground cover. Different sorts of vegetation have distinct arrangements of their branches and leaves, and so give characteristic polarisation patterns. Pine forests, for instance, look different from beech or podocarp forest.
Radar beams of different wavelengths can provide additional information about the Earth’s surface and what is on it. This is because shorter wavelengths tend to image smaller objects, longer wavelengths larger. C-band wavelengths (5.6 cm) tend to reflect off the leaves of a tree, L-band wavelengths (24 cm) from twigs and branches, P-band (68 cm) from trunks. Stones on the ground will reflect differently from clay, volcanic ash from mud. All three wavelengths may be used simultaneously on AIRSAR, and all can be polarised. Echoes from each wavelength may be rendered in a different colour in a final image of the terrain.
In a further technique termed interferometric SAR, pairs of radar images, collected from two antennas widely spaced on the plane, are combined to derive accurate topographic information. Radar echoes of the same target area probed simultaneously from slightly different positions interfere with each other in ways that are determined by the terrain. Some signals will be reinforced, others cancelled out or attenuated. Untangling this complexity enables topographic maps to be produced from the data. A typical plot would be 12 km by 12 km, with accuracy to 10 m horizontally. In mountain areas, elevations are accurate to five metres; in flatter country, to within one metre.
The applications to which AIRSAR images can be put seem almost unlimited. Because different types of vegetation cover can be accurately distinguished, the mapping of land use and vegetation and forest cover is readily accomplished. Deforestation, erosion and damage to wetlands and the like can also be monitored. Soil moisture levels, biomass and even some idea of biodiversity (at least among flora) can be obtained. The speed and direction of ocean currents can be determined, while the smoothness produced by an oil slick on the ocean is readily spotted. Even rock types that vary in their roughness can be mapped, and coarse sediments can be differentiated from fine on a glacial outwash plain.
Longer wavelength microwaves do not penetrate just vegetation, but in dry conditions can also pierce a few metres of sand, revealing details of underlying geology and even archaeology. Erupting volcanoes shrouded in ash have been examined by AIRSAR to determine exactly where hazardous streams of molten lava are running. It may even be possible to analyse vegetation beneath the forest canopy.
AIRSAR can determine much, but NASA has yet another instrument in its imaging arsenal, MASTER, which stands for (take a deep breath!) Moderate resolution imaging spectrometer and Advanced Spaceborne Thermal Emission and Reflection radiometer). This instrument images the ground at 50 different wavelengths ranging from 0.4 microns, in the visible-near UV region of the electromagnetic spectrum, up to 13 microns, the region of long-wave infrared waves.
The infra-red region of the spectrum is invisible to human eyes, but longer wavelength infra-red is useful for imaging heat. Hence it is valuable for such applications as geothermal field analysis, detecting heat emission near coal mines and determining surface water temperatures.
Mid infra-red wavelengths can detect the presence of different minerals in rocks and soil where vegetation cover is scarce. Australian scientists would like to launch a satellite with this type of sensor for mineral exploration.
MASTER data is also useful for estimating the concentration of sulphur dioxide in volcanic plumes, for detecting soil moisture variations, the presence of shallow aquifers and also for some geological mapping. Unlike AIRSAR, MASTER depends upon a clear view of the ground, so is unusable in poor visibility.
Many New Zealand researchers are keen to see the NASA plane back in our skies. A further mission, codenamed Pacrim 2 and planned for March 2000, will take a broad swing through the Pacific and Asia. If sufficient local interest and funds are available, the aircraft will include New Zealand in its flight path.