Direct And Inverse Remote Sensing Theories Of Polarimetric Synthetic Aperture Radar

SYNTHETIC APERTURE RADAR (SAR) has become one of the most important tools of earth observation and monitoring, attributable to its merits of day-and-night and all-whether operation and the capability of high-resolution imaging. In recent decades, SAR related techniques, e.g. interferometry, polarimetry, bistatic and multistatic SAR, polarimetric interferometry, and applications, e.g. topographic information retrieval, terrain classification, urban planning, military surveillance, forest management, have been extensively studied and developed.Research of SAR remote sensing can be categorized as electromagnetic scattering modeling, imaging and raw data compression, scattering signature interpretation and parameter inversion, image processing, etc. The purpose of SAR imaging is to acquire the information we need for applications in different areas. It has to resort to corresponding inverse theories and then extract the information delivered in SAR imagery. However, most inverse theories are based on direct analysis including theoretical modeling and simulation. It goes without saying that both direct studies and inverse approaches are very important and mutually beneficial.During the past several years of my doctoral research, I started from basic theories of electromagnetic scattering modeling for different types of terrain surfaces, rough surface to vegetation canopy, urban buildings to complex targets, and developed novel theories and approaches to both direct modeling and simulation and inverse retrieval and interpretation, for the applications on various circumstances, like vegetated areas, urban environments, ocean surfaces etc. The dissertation is organized as three main parts, the direct research, the inverse research and the extended research.The first topic of direct research is scattering modeling for vegetated terrain surfaces. A radiative transfer model for a layer of randomly oriented non-spherical particles with underlying rough surface was developed. Randomly oriented cylinders, disk-like and needle-like small particles are used for simulation of trunks, branches, deciduous and evergreen leaves, respectively, with underlying randomly rough surface for simulation of soil ground. It takes account of five scattering mechanisms of volume, surface scattering and their interactions. Numerical results match well with experimental data which shows the effectiveness of this model. It is useful to analyze and simulate scattering from vegetated areas and has been adopted in most of my research including scattering prediction, analysis and validation.The second topic of direct research is simulation of radar echoes from terrain surfaces. The abovementioned model is further extended to the time domain, by introducing the time variable into the radiative transfer equation and solving it in the frequency domain. Based on this, a more complicated model for a subsurface media layer with randomly distributed particles embedded in the middle and both rough upper and bottom interfaces is developed. It is used to simulate polarized echoes from lunar regolith for the lower frequency pulse radar detection. It is observed from the simulation that information of regolith structure and properties are hiding in the varying pattern of echo profiles. Therefore, a new mode for lunar exploration is proposed, using low orbit wideband pulse radar.A more advanced level of direct study should be the imaging simulation based on fundamental scattering models. Regarding the fact that natural scenes are more complicated including randomly distributed, penetrable or impenetrable objects, such as vegetation canopies, manmade structures and perturbed surface topography, a more adaptive simulation tool was developed. It takes account of scattering, attenuation, shadowing, foreshortening, overlay and multiple scattering of spatially distributed volumetric and surface scatterers, where a novel Mapping and Projection Algorithm (MPA) is devised to speed up the simulation of the entire process of scattering, extinction, mapping and projection in association with grid partitioning of the three-dimensional terrain scene. It also includes speckle simulation and raw data generation. It can simulate medium-resolution SAR images of comprehensive terrain scenes, which could have rivers, hills, urban and suburban buildings, timberlands, and farmland crops. It will be very useful in evaluating, interpreting and validating real SAR imagery.The inverse studies were conducted sequentially as the dimension of information increasing from oD scattering information to 1D echo signal, and to 2D SAR image. At the level of oD polarimetric scattering information within one single pixel, radar polarimetry and target decomposition theorems play important roles in the interpretation of polarimetric SAR images and its application on terrain surface classification. A novel de-orientation theory is proposed for the analysis of polarimetric scattering targets, plus a set of de-orientation parameters is defined with their respective physical meanings, namely the de-orientation angle, the projected dielectric polarity and the dielectric skippness. Together with the well-know entropy, a new unsupervised terrain surface classification scheme is proposed. Through theoretical simulation and real data experiments, it is demonstrated that the new parameters possess a higher degree of separation of terrain attributes than conventional methods.In order to utilize the lD information of the varying pattern of echo profiles, a novel multi-parametric inversion approach both for vegetation canopy and underlying ground is developed. As echo profiles provide rich temporal information showing the process of wave propagation and scattering through the complex media, those might identify contributions from different scattering layers or different volumetric and surface scattering. By treating the scattering process as some sort of signal excited system, the scattering can be described by a system response function which is explicitly expressed using a set of system parameters. These system parameters are estimated by fitting the echo profiles and they are further used to inverse multi-parameters of both vegetation canopy and the underlying ground based on their theoretical relationships.In the stage of high-resolution SAR applications, what we are concerned about is not an indicator parameter of the observed area, but rather the detailed information of the target, either in geometric or physical aspects. HR SAR images actually provide us with the possibility for extracting the specific parameters of an imaged target from the HR scattering object-image, for example, to invert the 3D sizes and position of a simple target, i.e. 3D reconstruction. Multi-aspect observations are especially important for 3D reconstruction due to the ambiguity of 3D objects in SAR images at one single aspect.An automatic method for detection and reconstruction of 3D objects from multi-aspect SAR images is proposed and realized with four steps. First, as a prior knowledge, the imaging features of the object are first generalized, such as geometric profile and spatial distribution. Then, to identify and extract the scattering image of the object from the SAR image, and the delivered information is inverted. Thereafter, the statistical description of the extracted object-image and its coherency to those of other aspects are provided. Finally, an automatic algorithm is designed to match object-images of different aspects in order to reconstruct the object. The potential application of this work is to effectively obtain the detail information of built-up areas from multi-aspect space-borne or airborne HR SAR data. It will be especially useful for urban planning and urban related research studies.Since the SAR techniques are developing rapidly, my research is also transfer to newly rising topics after I finished the direct and inverse studies. The first part of extended research is bistatic SAR. At first, bistatic image formation is studied. Under the translational invariant configuration of stripmap imaging BISAR, the frequency domain expression of signal model is derived. Two forms, respectively with an approximation of extracting the range dependence and an accurate iterative solution, are presented as well as their respective focusing methods. Meanwhile, the range-Doppler method is extended to the bistatic case. Imaging simulation of point targets shows the feasibility of these methods as well as their merits and drawbacks.Thereafter, by employing three-dimensional mapping and projection algorithm, imaging simulation of BISAR observation over complex scenario is realized. Some cases of simulated BISAR image are studied. Polarimetric characteristics of BISAR image are then discussed. It is found that some typical polarimetric parameters might become unable to describe scattering mechanism under bistatic observation. To circumvent this problem, a transform of unified bistatic polar bases for BISAR image is proposed. The conventional polarimetric parameters are redefined to retain the property of orientation independent in bistatic circumstance. Analysis of simulated images shows that the redefined parameters after the unified bistatic polar bases transform well describe different scattering mechanisms in BISAR imaging. It provides a primary tool for BISAR image interpretation and terrain classification.Scattering prediction of complex targets embedded in natural environments is the last topic of the extended research. Here, a new bidirectional and analytical ray tracing technique is proposed. Among the targets modeled by patches and edges, ray tracing is carried out both along the incident and inverse-scattered directions. By recording illumination and shadows information of rays at different orders, scattering of different orders can be calculated by using high frequency methods, such as physical optics and physical theory of diffraction. Besides, the concept of rough patch with coherent and diffused scattering parts is introduced to model rough surface, so as to deal with the co-presenting problems of targets and environments. This technique possesses the advantage of an electrical-size independent complexity of computation. Its precision for scattering predication is proved by comparing to conventional methods of computational electromagnetics.The entire set of remote sensing theories of polarimetric synthetic aperture radar consisted by the direct studies, the inverse studies and the extended studies is a summary of my doctoral research. It is expected to provide primary solutions to part of remote sensing applications of synthetic aperture radar. However, many aspects and issues are still remained for further study, not mentioning the rapidly developing techniques of synthetic aperture radar.