Radar Signal Decomposition and Quality Assessment for Wide Area Monitoring
Creators: Sun, Xinyao
Publisher: name: University of Alberta Library
Year: 2022

<p>The objective of signal decomposition is to extract and separate distinct signal components from a composite signal. Signal decomposition has been studied in many applications, such as image, video, audio, and speech signals. This thesis focuses on the category of signal decomposition on Interferometric Synthetic Aperture Radar (InSAR), a remote sensing technology that can monitor the earth from space. It provides measurements for thousands of square kilometres of ground, with a spatial resolution of around 10 m per pixel and a 1 mm precision on ground deformation estimation over time.</p>

<p>For wide-area monitoring, algorithms must handle tens of thousands of radar satellite images annually to measure ground stability over time. This thesis&#39; primary focus is to combine traditional signal/image processing techniques with recent deep learning approaches to improve the InSAR processing pipeline to deliver faster and better results. The task is very challenging because ground surface displacement or deformation signals are encoded in observed InSAR phase measurements with other contaminant signals (e.g., atmospheric distortion, orbital error, and digital elevation model error and noise). Each type of signal could be spatially correlated, temporally correlated, or both. It is also possible for the signals to be neither spatially nor temporally correlated. The phase values are wrapped by 2pi, which causes a non-continuous processing domain. Moreover, there is no real-world ground truth to reference in the training or validation stages. This thesis explores and addresses the deformation signal extraction problem using different strategies.</p>

<p>We start by focusing on the image filtering problem of removing spatially independent noise components. We demonstrate a novel deep learning model for Gaussian denoising in natural images and then adapt it to data from the InSAR modality. We designed a teacher-student paradigm for supervised training in the absence of real-world ground truth data. The framework uses a standard stack-based filtering method as the &quot;teacher&quot; (requiring more than 30 observations) and a deep differentiable model to learn the behaviour of the teacher method. After training, the student model can produce results comparable to, or even better than, those produced by its teacher method. Moreover, the student model relies on just a single pair of observations. Additionally, the proposed model is designed to provide a coherence map, which indicates the signal quality at the pixel level. Furthermore, we present an extension in the form of a novel self-supervised framework. This framework can be used to remove noise signals and estimate pixel-level quality using only noisy observations for training and inference.</p>

<p>In addition to the previous outcome, we investigate how to separate deformation and DEM error signals using a 2D optimization problem for each spatial location in a time series. In general, current approaches suffer from a non-continuous solution space. They are limited to small-scale displacement use cases, making them unsuitable for high-velocity scenarios such as mining, construction, and earthquakes. We propose a two-stage optimization strategy that effectively locates global optima by combining an iterative global coarse search with a stochastic derivative-free local fine search.</p>

<p>Almost all of the research on InSAR deforming signal estimation is based solely on temporal analysis and requires pre-removal of the atmospheric phase. We further investigate the spatial-temporal cross-domain optimization by developing an adaptive kernel that performs convolutional optimization on the entire 3D InSAR stack, resulting in accurate and robust deformation and DEM error signal extraction. The approach should be capable of processing wrapped phases directly and even working on phases that have not had their atmospheric component removed.</p>

<p>Despite these signal decomposition processes, accurately validating and optimizing the developed algorithms remains a challenge due to the lack of relevant ground truth data in a real-world environment. We developed a stochastic InSAR simulator to address this problem. The simulator provides a highly flexible modeling framework for generating various phase fringes and coherence distributions. This simulator is suitable for conducting thorough quantitative evaluations of various filtering and coherence estimation algorithms. The simulator features 2D and 3D modes that support stack and non-stack analysis. The 3D version is expected to simulate time-series deformation signals to evaluate signal separation methods. Additionally, to mimic realistic signals, we also study the intelligent generative InSAR simulator with adversarial training to learn the real-world deformation signal&#39;s distribution and its correlations to the DEM error.</p>