A two-step spotlight SAR data focusing approach

The authors present a new algorithm for spotlight SAR image generation based on a two-step focusing approach. The starting point of the procedure is an azimuth raw data filtering step which implements the (azimuth) convolution between the spotlight data and a chirp signal with a properly chosen constant chirp rate. This operation can be very efficiently implemented via a deramping-based technique and allows to achieve a bulk azimuth data compression and a pixel spacing smaller than (or equal to) the expected azimuth resolution of the final image. At this stage the spectral folding of the azimuth raw signal spectrum is avoided and the space-variant characteristics of the system transfer function are preserved; accordingly, the residual focusing of the data can be carried out via a conventional stripmap processing step implemented in frequency domain and requiring only a minor modification. The proposed algorithm is precise, efficient and particularly simple to be implemented. Presented results confirm the validity of the proposed approach