New approach for hybrid strip-map∕spotlight SAR data focusing

A new algorithm is presented for processing SAR data acquired in the hybrid strip-map/spotlight SAR configuration whose acquisition mode allows the generation of microwave images with an azimuth resolution better than that obtained in the strip-map case and with an imaged area larger than that achieved in the spotlight operation. The algorithm extends the focusing capability of conventional strip-map processing techniques to the hybrid SAR data; this result is achieved by generalising a recently proposed two-step focusing technique for spotlight SAR data processing, the spotlight mode being a particular case of the more general hybrid one. The key point of the procedure is the first filtering step which implements an azimuth convolution between the raw data and a chirp signal whose rate is selected, in the generalised approach, depending on the characteristics of the hybrid acquisition mode. Following this stage, standard strip-map processing procedures can be used to implement the second processing step, leading to fully focused SAR images. The algorithm, mainly oriented to spaceborne systems, is simple and efficient and allows the design of a generalised processing code suitable for data acquired in the hybrid strip-map/spotlight SAR configuration including the strip-map and spotlight modes as special cases. Experiments carried out on simulated data clarify the rationale of the method and confirm its validity.