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.
The main lobe and the side lobes of strong scatterers are sometimes clearly visible in SAR images. Sidelobe reduction is of particular importance when imaging scenes contain objects such as ships and buildings having very large radar cross sections. Amplitude weighting is usually used to suppress sidelobes of the images at the expense of broadening of mainlobe, loss of resolution and degradation of SAR images. The Spatial Variant Apodization (SVA) is an Adaptive SideLobe Reduction (ASLR) technique that provides high effective suppression of sidelobes without broadening mainlobe. In this paper, we apply SVA to process COSMO-SkyMed (CSK) StripMap and Spotlight X-band data and compare the images with the standard products obtained via Hamming window processing. Different test sites have been selected in Italy, Argentina, California and Germany where corner reflectors are installed. Experimental results show clearly the resolution improvement (20%) while sidelobe kept to a low level when SVA processing is applied compared with Hamming windowing one. Then SVA technique is applied to Interferometric SAR image processing (INSAR) using a CSK StripMap interferometric tandem-like data pair acquired on East-California. The interferometric coherence of image pair obtained without sidelobe reduction (SCS_U) and with sidelobe reduction performed via Hamming window and via SVA are compared. High resolution interferometric products have been obtained with small variation of mean coherence when using ASLR products with respect to hamming windowed and no windowed one.
This paper focuses on the generation of Digital Elevation Models (DEMs) with COSMO SkyMed Spotlight data in providing DEMs. In particular, the peculiarity of Spotlight data (affected from Doppler centroid drift) is investigated, and the use of the processing chain included in the Delft Object-oriented Radar Interferometric Software (DORIS [1]). The effects of not correctly handled Doppler drift is shown. The standard interferometric processing, without Doppler drift handling, has been applied to Spotlight image pairs, resulting in interferometric coherence loss in interferograms as we move away from scene center. So, the standard processing chain has been modified to take in account the Doppler centroid drift affecting Spotlight data and very high resolution and accuracy DEMs have been obtained. Some Spotlight image pairs have been processed and the obtained DEMs have been shown and analyzed proving the high details and product accuracy.
Accurate geolocation of SAR data is nowadays strongly required because of the increasing number of high resolution SAR sensors available as for instance from TerraSAR-X / TanDEM-X and COSMO-SkyMed space-borne missions. Both stripmap and spotlight acquisition modes provide from metric to sub metric spatial resolution which demands the ability to ensure a geolocation accuracy of the same order of magnitude. Geocoding quality depends on several factors and in particular on the knowledge of the actual values of the satellite position along the orbit, and the delay introduced by the additional path induced by changes in the refractivity index due to the presence of the atmosphere (the so called Atmospheric Path Delay or APD). No definitive results are reported yet in the scientific literature, concerning the best performances achievable by the COSMO-SkyMed constellation in terms of geolocation accuracy. Preliminary studies have shown that sub-pixel geolocation accuracies are hardly achievable with COSMO-SkyMed data. The present work aims at inspecting the origin of the geolocation error sources in COSMO-SkyMed Single-look Complex Slant (SCS) products, and to investigate possible strategies for their compensation or mitigation. Five different test sites have been selected in Italy and Argentina, where up to 30 corner reflectors are installed, pointing towards ascending or descending passes. Experimental results are presented and discussed.
COSMO-SkyMed (CSK) satellites are providing images with a resolution in the meter regime using the sliding spotlight mode (SL). This is an imaging mode which can obtain better azimuth resolution at the expense of azimuth imaged area than stripmap mode .Spotlight SAR data processing is already an established topic; efficient and accurate solutions in frequency domain have been proposed over the last years. However, the assumptions of these algorithms start to be invalid when applied to high-resolution spotlight SAR data acquired in spaceborne low Earth orbit (LEO) configurations. The assumption of a hyperbolic range history is no longer accurate for sub-metric spatial resolutions due to the satellite curved orbit. Since velocity of a space-borne platform is quite uniform, a simple focusing scheme had been designed in order to handle no straight line trajectory, using both approximated and accurate ω-k focusing kernel. Moreover, when getting close to decimeter resolution (at X-band) other several effects appear; in particular the motion of the satellite during the transmission and reception of the chirp signal deteriorate the impulse response function (IRF), if not properly considered (so called stop-and-go approximation). This paper shows that also CSK SL SAR data, with a resolution close to 1 meter, are not immune to disturbance effects when the stop-and-go approximation is assumed. The ω-k algorithm with satellite curved orbit handling is used to focus CSK spotlight data, and the stop-and-go approximation correction is included in the data processing chain. Experimental results with CSK spotlight data are provided to show quality enhancement on SAR standard focused products.
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