Synthetic Aperture Radar SAR
Synthetic Aperture Radar SAR is a known technique for two-dimensional high-resolution ground mapping. A platform, such as an aircraft or satellite, moves along a nominal straight path and illuminates a large ground area by means of an antenna. Short pulses, or alternatively long coded signals filtered by using pulse compression technique, are transmitted from the antenna and the return signal from the ground is received by the antenna and recorded along the straight path. By signal processing, high resolution is accomplished both along and transversely of the straight path. A condition for this is that the position of the antenna is known or can be calculated within a fraction of the wavelength and that the relative amplitude and phase of the transmitted and received radar signal are known. Moreover, the ground has to be invariable as the aircraft passes. The optimum geometric resolution that can be provided with SAR is determined by centre frequency and bandwidth of the transmitted signal and the aperture angle, over which the ground area is illuminated by the antenna, along the straight path.
The SAR technique has been applied in a very wide frequency range, about 20 MHz-100 GHz which corresponds to wavelengths of 3 mm-15 m. The choice of frequency determines largely which ground secures are to be reproduced since the backscattered return signal is affected above all by structures whose extent is of the wavelength size. Moreover, primarily the wavelength determines the capability of penetrating various ground layers, i.e. the penetration of the wave increases with a decreasing frequency. In connection with, for instance, vegetation, the attenuation is small for frequencies below 100 MHz and great for frequencies above 1 GHz. Thus the capability of penetrating vegetation decreases gradually with an increasing frequency, and a practical limit for detecting objects concealed in vegetation therefore is about 1 GHz. SAR systems which operate below and above 1 GHz, respectively, are referred to as low-frequency and high-frequency systems, respectively.
Static objects in forest terrain can be detected with low-frequency SAR, i.e. with a wavelength in the range 0.3-15 m. The low frequencies have the property of penetrating the vegetation layer with little attenuation and only causing a weak back-scattering from the coarse structures of the trees. Thus, static objects, such as stationary vehicles, can be detected also in thick forest by combining low frequencies with SAR technique which gives resolution of wavelength size. This has been scientifically demonstrated in a plurality of experiments in recent years.
Low-frequency SAR cannot detect objects that are moving. The high resolution of SAR arises by the imaging process using signals for a long time of integration. To enable sufficiently high resolution for detection, the radar must observe the object along a path which is of the same order as the distance to the object. This distance can be 20 km, i.e. for typical flying speeds the time of integration is about 100 s. During this time, an object must therefore be static within a fraction of the wavelength, i.e. the fraction of one meter. This fact makes is impossible in practice to detect moving camouflaged objects by using this technique. As a matter of fact, the speed of an object must be less than about 0.1 m/s for the object to be considered stationary.
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