synthetic aperture radar Ordinary radar works by measuring the precise time between the transmission of a radio pulse and the return of its echo from a distant object. In this way, a very precise distance (range) measurement can be achieved, but the angular (azimuthal) resolution is limited by the physical size (aperture) of the antenna. Synthetic aperture radar (
SAR) overcomes this limitation by combining the radar echoes received at several small fixed antennae or one small moving antenna to simulate a single antenna with a large effective aperture. The latter method allows SAR to be used as a detailed imaging tool from a small platform such as an aeroplane or satellite. The image must be reconstituted by combining the radar returns some time after measurement, since each pixel in the image depends on several sets of returns as the antenna progresses along track. The reconstitution method relies on the Doppler effect: echoes from points ahead of the platform are shifted to higher frequencies, whereas returns from points behind the platform are shifted to lower frequencies. Because microwave SAR is mostly insensitive to cloud cover and does not require solar illumination, SAR instruments have been placed on several space-borne systems, including ERS-1, ERS-2, RADARSAT, the Space Shuttle, and the now-defunct SEASAT and JERS-1. SAR has been used by the Magellan space mission to map the surface of Venus, which is hidden at optical frequencies by the dense atmosphere.
A further development of SAR is
interferometric SAR (InSAR). SAR images contain information about the amplitude and phase of the radar return from each pixel. If two images of an area are taken with the same SAR system, the difference in the phase of a pixel from one image to the next will depend on the ground deformation in that pixel. There may also be a phase change dependent on the ground elevation, because of the stereoscopic effect that arises if the images are not taken from exactly the same track. In the known absence of ground movement, or if the images are taken simultaneously, this latter effect can be used to construct
digital elevation models (
DEMs) of wide areas. If the ground elevation is known sufficiently accurately for its effect to be removed, surface movements can be detected which are of magnitudes somewhat smaller than the wavelength used to form the radar pulse. In the case of ERS-1 and ERS-2, which have a radar wavelength of 56 mm, movements of a few millimetres can be resolved. This level of sensitivity makes InSAR a readily feasible method of observing landslides, glacier movements, earthquakes, and deformation associated with volcanic eruptions. Sensitivity is limited by seasonal and climatic effects: the radar return is affected by groundwater movement and the growth of vegetation, and to a lesser extent by atmospheric variability. Under optimum (desert) conditions, it may be possible to use InSAR to monitor the still smaller deformational signals associated with inter-seismic and post-seismic fault movements.
peter clarke
Bibliography
Curlander, J. C. and and McDonough, R. N. (1991) Synthetic aperture radar: systems and signal processing. Wiley Interscience, New York.
