Foliage Penetrating Radar - FOPEN
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.
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.
One basic question is how to combine the technique of low-frequency SAR and the technique of detection of moving objects (GMTI) to produce signals which penetrate forest vegetation and at the same time permit detection of moving objects. The problem is especially the practical difficulty of providing on an airborne platform a sufficiently large radar antenna at low frequencies which has the same high directivity as a high-frequency narrow-lobe radar antenna so that the described methods for high-frequency SAR having the GMTI function can be used. The restricted physical space on board such a platform means essentially that low-frequency radar antennae are omnidirectional and have a low directly. The absence of directivity has two important consequences for a low-frequency SAR having the GMTI function, which mean that prior-art methods cannot be used.
First, the absence of directivity means a considerable problem of providing optimum performance for the GMTI function. The latter in fact requires that the directional sensitivity of the elements be as equal as possible, which is difficult to achieve if the directivity is low. The reason is that the antenna elements connect electromagnetically to the platform and thus the directional sensitivity changes. Consequently, the directional sensitivity is changed according to the exact position of the antenna element on the platform, the antenna elements having different direction properties which make the above-mentioned GMTI methods inefficient.
Second, the absence of directivity means that a high signal sensitivity and geometric resolution require optimal coherent signal processing of radar data for a long time of integration. The signal integration corresponds to the object being illuminated over a large aperture angle. The aperture angle is de facto so large that the signal-processing methods that are used for moving objects in high-frequency SAR with the GMTI function are not applicable. For exactly the same reasons, the signal processing of stationary objects in low-frequency SAR is different from that used in high-frequency SAR.
Detection of stationary objects in forests using low-frequency SAR requires an optimal geometric resolution to discriminate the objects from the background. This results, of course, in a SAR with a great fractional bandwidth. Apart from achieving the optimal resolution, also the static fluctuations of the background which originate from the speckle effect are reduced. The speckle effect arises when the resolution is much greater than the wavelength (small fractional bandwidth) and contains a plurality of scatterers. The waves backscattered by the scattererers interfere with each other, and the resulting return signal thus is considerably dependent on the observation angle in relation to the resolution cell. Normally the resolution cell contains many independent scatterers, which results in a random amplitude and phase between different resolution cells, so-called speckle.
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