Spotlight SAR data focusing based on a two-step processing approach

We present a new spotlight SAR data-focusing algorithm based on a two-step processing strategy that combines the advantages of two commonly adopted pr ocessing approaches: the efficiency of SPECAN algorithms and the precision of stripmap, focusing techniques. The first step of the proposed algorithm implements a linear and space-invariant azimuth filtering that is carried o ut via a deramping-based technique representing a simplified version of the SPECAN approach. This operation allows us to perform a bulk azimuth raw da ta compression and to achieve a pixel spacing smaller than (or equal to) th e expected azimuth resolution of the fully focused image. Thus, the azimuth spectral folding phenomenon, typically affecting the spotlight data, is ov ercome, and the space-variant characteristics of the stripmap, system trans fer function are preserved. Accordingly, the residual and precise focusing of the SAR data is achieved by applying a conventional stripmap processing procedure requiring a minor modification and implemented in the frequency d omain. The extension of the proposed technique to the case of high bandwidt h transmitted chirp signals is also discussed. Experiments carried out on real and simulated data confirm the validity of the presented approach, which is mainly focused on spaceborne systems.