Congratulations to our ESR 7, Muhammad Amjad Iqbal, who successfully defended his PhD thesis on the topic of “Sparse Reconstruction for high-resolution inverse SAR imaging” on November 10, 2023 at the University POLITEHNICA of Bucharest under the supervision of Prof. Dr. ing. Andrei ANGHEL.
The radar acts as a primary and active sensor; it works regardless of weather conditions. Synthetic aperture radar (SAR) and inverse SAR are vital for earth observation (EO) and target detection in remote sensing. The Doppler effect, owing to the movement of the radar or target, is key to imaging. This thesis focuses on the signal processing and spectral analysis of SAR data. Doppler centroid (fDC) is significant for ocean scientific pursuits. Three fDC methods were implemented to address their capabilities in the context of SAR scene analogy. The de-ramping of SAR data is performed in the SNAP toolbox, which is a necessary step before the fDC estimation. In addition, the competency of the complex-valued (CV) convolution autoencoder is assessed to preserve the original complex SAR properties, including fDC for ocean surface current (OSC) analysis.
In the coastal zone, the static coastline paves zero Doppler compared to dynamic sea waves. A novel coastline extraction method is proposed using fDC images based on the constant false alarm rate (CFAR) approach. The impact of natural hazards on coastline degradation was investigated by using Doppler parameters. The proposed technique was compared with polarimetric correlations and in-situ data. By observing theoretical distributions from empirical data, CFAR effectively discriminates between targets and non-targets via thresholding. Subsequently, subaperture (SA) decomposition was deployed for ship detection using CFAR. SAs were used to calculate the ship velocity.
Expanding on CFAR, the ice cover was delineated using a dual-pol descriptor, and the ice cover retreat velocity was calculated using Euclidean displacement. The experimental outcomes demonstrate the practicality of CFAR methods for real-time target detection.
Nevertheless, ISAR provides images of rotating objects. Experiments evaluated the performance of the imaging methods, and the results indicated that the mm-wave radar achieved an ample range and azimuth resolution. The point spread function (PSF), which is an impulse response, demonstrates the performance of each method. Following ISAR imaging, the dual-pol decomposition technique accurately extracts the scattering properties from ISAR images, enabling a wide array of applications. Therefore, a novel case study involves the two-dimensional (2D) ISAR imaging of partial polarimetric data from natural targets. Eigen-descriptors were utilized to identify the scattering mechanisms through the Lee and Pottier Hโฮฑ plane. This study represents the first instance of ISAR images being used for Radar Vegetation Index (RVI) analysis.
Finally, coarse-to-fine (CTF) estimation, a 2D compressive sensing (CS) algorithm, was developed to discard a certain number of columns from the sensing matrix (SM) as the reconstruction progressed from coarser to finer scales, thereby achieving high-resolution imaging. This brings us closer to real-time applications for target imaging without any pre-processing or pipeline motion compensation.
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