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The upcoming Earth Observation satellite Synthetic Aperture Radar (SAR) missions offer sensing capabilities that imply an increasing complexity of the image formation process. Among the major innovations, the ultra-high resolution Spotlight acquisition, performed exploiting long integration time with steering schemas with a wide range of angles obtained through innovative hardware design; more satellite agility is also essential to acquire wider areas even in nearcontiguous frames within a reduced time period, addressed by Spotlight squinted geometry obtained through both fast and precise attitude maneuvers and electronic antenna steering. It is worth stating that the acquisition of high resolution squinted spotlight scenes, especially in the case of terrains with strong topography variation, could result in a significant degradation of the SAR data focusing performance, mostly for the algorithms working in the Fourier-domain. Considering the challenging scenario described above, this article aims at presenting a new accurate and efficient Spotlight image formation algorithm that uses a focusing core implemented in the time-domain, where the precise accommodation of high topography variations can be easily handled working on pixel basis. The proposed solution has actually a hybrid design: the focusing of the signal in across-track direction uses a frequency-domain approach, while in the along-track direction the focusing is performed in the time-domain. The focusing core of the signal in the azimuth direction uses a Back-Projection approach, which allows to obtain excellent performances where the traditional algorithms in the frequency-domain can have severe limitations, but it will dramatically worsen the temporal performance making the processing algorithm generally not suitable for operational purposes. For this reason, a subaperture processing optimization has been proposed that guarantees significant efficiency improvements primarily for not squinted images and takes into account further considerations in order to obtain a comparable reduction of the computational complexity when applied to data representative of highly squinted geometry. The combination of the described techniques together with the increasing hardware computational power, will allow for a new generation of time-domain focusing algorithms to be implemented in the real world operational scenarios. The performance of the algorithm have been verified and validated mainly by simulation. The accurate Impulse Response Function (IRF) has been measured on a simulated dataset of very high resolution SAR X-Band Spotlight data with squinted and not squinted geometry, with point targets placed at different altitudes using different schemes of topography. The solution has finally been verified on real data from the COSMO-SkyMed (CSK) mission, acquired in the Spotlight Enhanced mode. These high quality X-Band SAR data allow verifying the behavior of the algorithm in terms of performance and stability of the focusing core in a real world scenario. The computational burden of the proposed solution vs its processing execution time has also been assessed.
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