Beamforming in SAR Satellites for Nadir Echo Suppression

Khosravi, Mahdi | 2019

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  1. Type of Document: Ph.D. Dissertation
  2. Language: Farsi
  3. Document No: 52744 (05)
  4. University: Sharif University of Technology
  5. Department: Electrical Engineering
  6. Advisor(s): Bastani, Mohammad Hassan; Fakharzadeh, Mohammad
  7. Abstract:
  8. In synthetic aperture radar (SAR) satellites, the nadir echo is defined as the backscattered signal from the point directly below the satellite on the earth surface. If the timing of the radar is such that the nadir echo arrives at the receiver while the receiver is turned on for receiving the desired signal, it could damage the receiver or disrupt the resulting SAR image, unless it is suppressed enough by proper design of antenna radiation pattern. In current SAR satellites, ”avoiding” this phenomenon is associated with two problems: 1) limiting the swath width, 2) imposing stringent restrictions on the selection of pulse repeatition frequency (PRF). Therefore, ”suppressing” the nadir echo, instead of avoiding it, not only provides the chance to image wider swaths, but also relaxes the restrictions on PRF selection. Both of these improvements increase the value of the products of the SAR satellite, however, the latter is more important than the former, since the restrictions on the selection of PRF, highly degrades the performance. Hence, relaxing these two restrictions will open a new horizon in front of SAR satellites and their products. It is predicted that future SAR satellites will have nadir echo suppression capability to provide products with a wide range of applications and attract large civil/defense customers. In this project, a method for suppression of nadir echo by generating nulls in the antenna radiation pattern (spatial filtering) is proposed. By exploiting Compressed Sensing (CS), the proposed method perturbs the weighting of a small number of elements to generate wide and deep nulls which makes it suitable for suppression of strong interferences with angular distribution. Simulation results show that the proposed method outperforms the existing methods in interference suppression by about 10dB to 20dB. Since, the goal of this work is nadir echo suppression , in the following, we will focus on evaluating the cababilities of the proposed method in suppression of nadir echo.In this project, it was shown that by applying the proposed method and relaxing the restrictions on the timing of the radar, swath width will be increased by about 25% in StripMap mode and the possibility to select arbitrary PRFs will be enhanced. It should be noted that large variations of PRF have many drawbacks in SAR satellites. In this sence, we showed that using the proposed method, one can decrease the PRF operating range in ScanSAR mode by about six times. Simulations showed that the proposed method is able to decrease Range Ambiguity to Signal Ratio (RASR) by about 10dB in comparison to the existing null steering methods and by about 20dB in comparison the the case where no null steering is performed. Furthermore, several simulations are executed to evaluate the performance of the proposed method in different scenarios such as different off-nadir angles, different land types, different acquision modes, etc. The results verify the performance of the proposed method under differnt practical situations. Since, the conventional formula for calculation of RASR neglects the nadir echo, we could not use it for evaluation of the proposed method. Hence, in this project, we derived a generalized formula for RASR and showed that it converges to the conventional formula at medium/high off-nadir angles, if the nadir echo is ignored
  9. Keywords:
  10. Synthetic Aperture Radar (SAR) ; Satellites ; Beamforming ; Null Steering ; Compressed Sensing Radar ; Phased Array ; Inverse Synthetic Aperture Radar ; Echo Cancellation

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