AN ANTENNA PATTERN SYNTHESIS TECHNIQUE FOR SPACEBORNE SAR PERFORMANCE OPTIMIZATION

The antenna radiation pattern plays an important role in determining the performance achievable by a Synthetic Aperture Radar (SAR) system [1], [2]. Due to the presence of inevitable sidelobes in the antenna pattern and the need of transmitting a pulsed waveform for imaging purposes, the received signal contains undesired ambiguous signals superimposed to the useful terms. A quantitative measure of the SAR performance degradations, depending on these ambiguous contributions, can be evaluated by the ambiguity-to-signal ratio (ASR), defined as the power ratio between the ambiguous echoes and useful signal. This parameter expresses the capability of the antenna to reject signals returning from regions of the Earth's surface outside the swath. An antenna pattern design technique for spaceborne SAR's, which optimizes the signal-to-disturbance ratio, is presented in this paper. It takes into account the ground reflectivity (relative to the transmitted frequency) and the viewing geometry (altitude and off-nadir angle). The proposed technique makes use of the theory of adaptive arrays and takes advantage of a priori knowledge of the ambiguous echo power as a function of the geometry and average ground reflectivity. The optimized antenna weighting turns out to be complex, and asymmetrical patterns are generally obtained for a spaceborne SAR. The proposed technique allows the achievement of better performance compared to the current design values, especially at large off-nadir angles. In particular, it allows the system designer to relax the choice of the pulse repetition frequency (PRF), which is an over-constrained parameter, from the ASR constraint, kept under control by a proper design of the antenna radiation pattern.