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.
COSMO-SkyMed di Seconda Generazione (CSG) will ensure operational continuity to the currently operating "first generation" CSK constellation. The CSG constellation will consist of two satellites in Low Earth Orbit equipped with an X-band Synthetic Aperture Radar (SAR). The CSG program is managed and co-financed by the Italian Space Agency (ASI) and the Italian Ministry of Defence. A wider portfolio of Spotlight imaging modes is offered with respect to CSK, among which there are new Spotlight civilian sub-metric modes. Furthermore, products acquired with squinted attitude (i.e. mean pitch angle during the acquisition different by zero) or with multi-swath approach could be used to serve more requests in a defined area, that otherwise could not be served because of conflict for time gap violation. Upgraded Spotlight focusing algorithms have been developed in order to correctly manage side-effects on the SAR Impulse Response Function (IRF) image quality depending on the long integration time and squinted geometry. The paper presents the new Spotlight processing algorithm enhancements providing details about design, implementation and verification results. Image quality and processor time performances have been assessed by simulated data representative of the various operational Spotlight CSG acquisition modes.
We present the implementation of a procedure to adapt an Asymmetric Wiener Filtering (AWF) methodology aimed to detect and discard ghost signal due to azimuth ambiguities in SAR images to the case for X-band Cosmo Sky Med (CSK) images in the framework of SEASAFE (Slick Emissions And Ship Automatic Features Extraction) project, developed at the Department of Science and Technology Innovation of the University of Piemonte Orientale, Alessandria, Italy. SAR is a useful tool to daily and nightly monitoring of the sea surface in all weather conditions. SEASAFE project is a software platform developed in IDL language able to process data in C- Land X-band SAR images with enhanced algorithm modules for land masking, sea pollution (oil spills) and ship detection; wind and wave evaluation are also available. In this contest, the need to individuate and discard false alarms is a critical requirement. The azimuth ambiguity is one of the main causes that generate false alarm in the ship detection procedure. Many methods to face with this problem were proposed and presented in recent literature. After a review of different approach to this problem, we describe the procedure to adapt the AWF approach presented in [1,2] to the case of X-band CSK images by implementing a selective blocks approach.
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