A tutorial on tomographic synthetic aperture radar methods
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A tutorial on tomographic synthetic aperture radar methods Seyed Alireza Khoshnevis1 · Seyed Ghorshi2 Received: 18 February 2020 / Accepted: 5 August 2020 © Springer Nature Switzerland AG 2020
Abstract Synthetic Aperture Radar (SAR) is a type of radar that is mounted on an airborne platform and aims to increase the resolution of the acquisitions by traveling over the target area. The signals acquired by SAR are two dimensional, but it is possible to create three dimensional models using signal processing methods. Tomography is the method of creating three dimensional models from multiple two dimensional signals. This method can be applied to SAR acquisitions to create three dimensional models of the landscape. The goal of this study is to create a comprehensive tutorial on how to work with SAR data, what toolbox to use for analysis, how to create a tomographic SAR dataset, and how to use different methods of spectral estimation for SAR tomography. In this work, first, we focus on the main problem of SAR tomography followed by SAR preprocessing steps, a brief description of data levels, and finally, we discuss the different spectral estimation methods used in SAR tomography along with examples at every step. Keywords Synthetic aperture radar (SAR) · Comperssive sensing (CS) · Nonlinear least squares (NLS) · Tomographic SAR (Tomo-SAR) · Interferometric SAR (In-SAR) · Singular value decomposition (SVD)
1 Introduction In the late 1940s, after the second world war, the United States army was looking for an all-weather, 24-h remote surveillance device. The ability of the radar to penetrate cloud and fog and its independence from daylight made it the logical choice for the army. The only issue was that in order to achieve a high enough resolution the antenna would need to be the size of a football field, far too large for any aircraft to carry. Synthetic aperture radar (SAR) was the solution to this problem. It was invented by Carl A. Wiley, a mathematician at Goodyear Aircraft Company, in 1951. This technology was released to the civilian communities in the 1970s. This type of radar increases the resolution by using the movement of the platform to create a synthetic aperture. SAR stores the data of the scanned area in the form of a 2-dimensional signal (similar to an image); however, the actual landscape is 3-dimensional. Therefore, SAR in
fact maps the information of the 3-dimensional area into two dimensions. During this mapping process, not only we lose the data for the third dimension, but also since the model is summed in one direction, it becomes less accurate. The process of tomography aims to untangle this mapped version of the landscape and estimate the original 3-dimensional model by calculating the reflectivity and elevation of the scatterers using multiple SAR acquisitions. Any object or surface that comes into contact with the beam scatters the signal in all direction which is why we use the term “scatterer” to describe them. The goal of this study is to create a tutorial for the SAR tomography (tomoSA
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