Μελέτη, ανάλυση και βελτίωση της λειτουργίας αερομεταφερόμενου ραντάρ συνθετικής απεικόνισης (SAR) χαμηλών συχνοτήτων

Abstract

Because of its high resolution, frequency scattering properties and indifference to day/night or cloud cover, Synthetic Aperture Radar (SAR) has become into vogue in the last years. The field of SAR remote sensing has changed dramatically with the operational introduction of new high performance signal processing techniques and new operational modes, like the polarimetry in 1980’s and the interferometry in 1990’s. Additionally, technological advances in antenna design, low noise amplifiers, band-pass filters, digital receiver technology and high frequency digital sampling devises, increase the availability and the performance of airborne as well as spaceborne SAR sensors. All these technological advances result to real time SAR system operation and in most of the frequency bands of the electromagnetic spectrum. These advanced hardware components combined with the new radar techniques result to large variety of operational and research applications. In several of the new coming applications there is the need for a SAR system to penetrate vegetation and foliage. As a result of this, a new class of SAR systems, using low frequencies, has emerged. The combination of low frequency with high bandwidth allows a variety of new military as well as civilian applications. In the frame of this thesis, several hardware and software modifications made in the E-SAR P-Band system operated by DLR aiming the improvement of the collected and processed data quality is described. The basic P-Band inherent problems like the low Signal-To-Noise-Ratio (SNR), the presence of Radio Frequency Interferences (RFI) as well as the high dynamic range of the backscattered signal are addressed. A new mode of operation called “Listen Only” (LO) channel mode gave us the unique opportunity to study and analyze the special characteristics of the interfering signals and the nature of the low frequency backscattered signal. Based on this analysis new RFI suppression algorithms have been developed and the system operation parameters have been set to the correct value resulting to high quality collected data. The effect of RFI signals in fully polarimetric SAR data processing and applications are analyzed in detail. One of the principal items of this thesis is the development of a new robust sub-aperture algorithm for improved Motion Compensation (MoCo) in wide azimuth beam SAR data processing. The new algorithm is incorporated to the Extended Chirp Scaling SAR data processing algorithm. The improved MoCo algorithm results to focused images with high SNR, contrast, higher resolution and better geometric correctness. The performance and the correction accuracy of the proposed algorithms are analyzed by using mainly real data collected by the E-SAR system of DLR.