ISAR Mode for COSMO Skymed System with Super-Resolution Options: ICOSSOP Project
Bacci, Alessio1; Berizzi, Fabrizio1; Fico, Flavio2; Giusti, Elisa1; Lombardo, Pierfrancesco2; Martorella, Marco1; Pastina, Debora2; Rossi, Rainer Maria2
1University of Pisa, Department of Information Engineering, ITALY; 2University of Rome "La Sapienza", ITALY

The aim of this paper is to describe the activities related to the project "ISAR mode for COSMO Skymed System with Super-resolution Options (ICOSSOP)" funded by the Italian Space Agency (ASI) under the framework of the first Announcement of Opportunity (AO) for the Exploitation of COMO Skymed data. The project took two years and was carried out by two Italian Universities, namely the University of Pisa (Department of Information Engineering, Radar Laboratory), as principal investigator, and the University of Rome "La Sapienza" (Department DIET). The team was composed of full and associate professors, researchers and PhD students, who worked actively for the success of the project, under the careful supervision of the ASI. The project has proposed an innovative use of COSMO-Skymed data and specifically the development of an ISAR mode with super-resolution options for COSMO-SkyMed system. The goal was to reconstruct high resolution and well focussed ISAR images of moving targets extracted from a SAR scene. Several experimental activities have been led with international research institutions. The success of the project has also been confirmed by the publication of several high quality scientific papers[1], [2], [3], [5].

SAR systems, such as COSMO-SkyMed system, were originally employed in earth observation applications. Nevertheless, the ability to form high-resolution images has also rapidly made SAR systems very attractive for both military and homeland security applications. In such applications, radar imaging of man-made non-cooperative targets (NCTRI) becomes the main interest rather than the observation of natural phenomena.

Nevertheless SAR processors are based on the assumption that the illuminated area is static during the synthetic aperture formation. As a consequence of such an assumption, standard SAR techniques are typically unable to focus moving targets while forming a focussed image of the static scene. This leads to blurred and displaced images of any object that is not static during the synthetic aperture formation.

In this study the problem was addressed as a problem of non-cooperative target imaging and a solution based on the use of Inverse Synthetic Aperture Radar (ISAR) processing was proposed to solve this problem. ISAR techniques, in fact, do not base their functioning on the assumption that the target is static during the synthetic aperture formation, but instead they entirely exploit the target's own motions to form the synthetic aperture. Taking into account the advantages of the COSMO-SkyMed system (wide areas monitoring, short revisit time, etc...), the problem was tackled starting from formed SAR images.

The overall system concept is sketched in Figure 1. Figure 2-(b) shows an ISAR image obtained by applying the proposed method to the defocused target's SAR image shown in Figure 2-(a).


Figure 1: functional block of the proposed algorithm.


Figure 2: (a) SAR image of a moving target (b) refocused ISAR image.
In order to apply ISAR techniques to SAR images, moving targets must be detected first. The detection step was accomplished via a 2D CA-CFAR based technique. Once the target is detected, a sub-image containing the target is first selected and then back-transformed into a data domain suitable for the application of ISAR processing. The obtained data are then fed to the ISAR processor that produces a refocused image of the target. Moreover, in order to enhance the ISAR image quality, a number of signal and image processing techniques have been developed. Such techniques can be classified as denoising/despeckling techniques based on Compressive Sensing theory, super-resolution techniques (Super Spatially Variant Apodization SSVA and 2D SR SPECAN), and scattering centres extraction techniques (CLEAN technique).

It must be pointed out that, although the focus in our study was on the space-borne imaging of maritime targets, the overall concept could also be applied to ground and air moving targets as well as to other radar platforms, such as manned or unmanned airborne radar systems[4]. In the full paper details concerning the whole proposed algorithm will be provided.

The effectiveness of the proposed method was proven by using archived COSMO-SkyMed spotlight SAR data of maritime targets. Moreover, to properly evaluate the performance of the proposed method, measurement campaigns with cooperative targets have also been carried out.

Specifically a measurement campaign was conducted in collaboration with the Council for Scientific and Industrial Research (CSIR-South Africa). COSMO-SkyMed measurements were taken in False Bay, near Cape Town, South Africa in October 2010. The scenario was a typical coastal maritime surveillance. The target, shown in Figure 3, was a small sailing ship equipped with a GSP system to continuously monitor its position and a differential GPS system to measure its own rotational motions induced by the sea waves.


Figure 3: cooperative targets.
Another controlled experiment was carried out by the University of Pisa in November 2011 and January 2012. The target, shown in Figure 3, was a man-made rotating target composed of 4 quadrihedrals, placed at the extremes of a platform with different orientations. The maximum dimension of the platform was and its speed of rotation was set manually within the range . The target was located at latitude 43.72 and longitude 10.38. All the collected data has been used to validate the effectiveness of the proposed processing and to evaluate the performances. Results will be shown in the final paper.

References
[1] Martorella, M.; Giusti, E.; Berizzi, F.; Bacci, A.; Dalle Mese, E.; , "ISAR based techniques for refocusing non-cooperative targets in SAR images," Radar, Sonar & Navigation, IET , vol.6, no.5, pp.332-340, June 2012
[2] Martorella, M.; Berizzi, F.; Pastina, D.; Lombardo, P.; , "Exploitation of Cosmo SkyMed SAR images for maritime traffic surveillance," Radar Conference (RADAR), 2011 IEEE , vol., no., pp.113-117, 23-27 May 2011
[3] Martorella, M.; Berizzi, F.; Giusti, E.; Bacci, A.; , "Refocussing of moving targets in SAR images based on inversion mapping and ISAR processing," Radar Conference (RADAR), 2011 IEEE , vol., no., pp.068-072, 23-27 May 2011
[4] Martorella, Marco; Giusti, Elisa; Bacci, Alessio; Berizzi, Fabrizio; Meta, Adriano; , "Non-cooperative maritime target imaging with an FMCW SAR system," Synthetic Aperture Radar, 2012. EUSAR. 9th European Conference on , vol., no., pp.127-130, 23-26 April 2012
[5] Martorella, M.; Berizzi, F.; Pastina, D.; Lombardo, P.; , "Maritime non-cooperative target imaging with COSMO SkyMed data," Geoscience and Remote Sensing Symposium (IGARSS), 2012 IEEE International , vol., no., pp.3564-3567, 22-27 July 2012