A Tool for Planning of High Resolution / Wide Coverage Imaging Radars

The hybrid stripmap/spotlight operating mode for Synthetic Aperture Radar (SAR) system is able to generate microwave images with an azimuth resolution better than that achieved in the stripmap configuration, and a ground coverage better than the one of the spotlight configuration, thanks to the radar antenna beam steering about a point farther away from the radar than the area being illuminated. In the last years the attention of remote sensing scientific community for the hybrid SAR mode is increased and, consequently, the subject of design, processing and data interpretation is gaining a growing interest. SAR spaceborne sensors operating in the hybrid mode are still under design, as SAR 2000 in the Cosmo/Skymed project or TerraSAR-X, while airborne ones are already available as the wide band sensor SAR/MTI PAMIR. Consequently, a hybrid SAR raw signal simulator is strongly required, especially when real raw data are not yet available, to test processing algorithms and help mission planning. In addition, to analyse the effects of processing errors and to verify the impact of different system design choices on the final image for different kinds of imaged scenes, an extended scene SAR raw signal simulator is very useful and it is what we present in this paper. Sommario La modalità di funzionamento ibrida stripmap/spotlight per Radar ad Apertura Sintetica (SAR) [1] è in grado di generare immagini nelle microonde con una risoluzione in azimut migliore di quella raggiunta dalla modalità stripmap ed una copertura a terra maggiore di quella garantita dalla geometria spotlight. La rotazione del fascio dell’antenna radar attorno ad un punto più distante dal radar dell’area che si vuole illuminare è l’espediente che conferisce alla modalità ibrida le caratteristiche descritte di risoluzione e copertura in azimut. Tali proprietà hanno suscitato notevoli interessi nella comunità scientifica che infatti ha visto crescere, negli ultimi anni, gli studi di progettazione, elaborazione e di interpretazione dati relativi a tale modalità. Alcuni sensori SAR per lo spazio operanti in questa modalità sono tuttora in fase di progettazione, come SAR 2000 del progetto Cosmo/Skymed [2]o TerraSAR-X, mentre da aereo è già disponibile, ad esempio, il sensore a larga banda SAR/MTI PAMIR. Si sente come urgente a questo punto, specialmente se dati grezzi reali non sono ancora disponibili, la realizzazione di un simulatore di segnali grezzi SAR ibridi in grado di supportare la progettazione di sistemi SAR ibridi e valutarne i relativi algoritmi di elaborazione ed è quanto presentiamo in questo lavoro. Introduction Hybrid stripmap/spotlight configuration is a new mode in which a Synthetic Aperture Radar (SAR) system can image an area over the ground. In the hybrid acquisition the radar antenna beam is steered about a point farther away from the radar than the area being illuminated, see Figure 1. That is why it results to be ‘hybrid’ between the well-known stripmap mode, in which the radar antenna is pointed along a fixed direction with respect to the platform flight, and the spotlight configuration in which the radar antenna beam is steered during the overall acquisition time. Such a system allows the generation of microwave images with an azimuth resolution better than that achieved in the stripmap configuration, and a ground coverage better than the one of the spotlight configuration. In many cases, this is of paramount importance because a flexible operational mode as hybrid one allows to look at a wide range of scenarios every time with the best ‘eye’, that is with the most right couple of values of azimuth resolution and ground coverage. In order to support the planning of SAR hybrid systems currently under design [1] or to test processing procedures proposed, in the last years, for hybrid mode [2-3] we need of a SAR raw signal simulator, being real raw data not yet available. A frequency domain approach, being time and memory saving, would be highly desirable when extended scenes are considered. While efficient extended scene SAR simulators, based on a frequency domain approach, have been presented for the stripmap and spotlight operational modes [4-5], no one, to the best of our knowledge, is currently available for the hybrid geometry. In this work, the definition of a new transfer function for the hybrid case and its analytical evaluation via an asymptotic expansion are crucial steps for our aim. After showing that in the hybrid case a 2D Fourier domain approach is not viable, we demonstrate that a 1D range Fourier domain approach, followed by 1D azimuth time domain integration, is possible when some approximations, usually valid in the actual cases, are accepted. Theoretical statements are followed by a simulation example, relevant to an actual extended scene, which confirm the effectiveness of the simulator here presented. The hybrid stripmap/spotlight mode In order to evaluate the SAR transfer function for the hybrid configuration we need to first evaluate the corresponding raw signal. At this aim we have to introduce the factor [2-3]