Radar Imaging for Maritime Observation

Radar Imaging for Maritime Observation

The term maritime observation has been deliberately chosen to encapsulate both the concept of remote sensing of the sea surface and maritime surveillance. The two scenarios are significantly different in terms of applications related to them although they share a fundamental common ingredient, which is the sea surface. The presence of the sea surface characterizes uniquely the radar echoes and the way they must be processed either when some sea surface parameters must be estimated or when targets must be detected or identified. The use of radar for maritime observation finds its roots in the early days of radar when radars were installed on ships as support for the navigation and for maritime surveillance purposes. In fact, the ability of radar to operate in all weather/all day conditions made it appealing to a number of applications, from civil to military scenarios.

The interaction of radar microwave signals with the sea surface is very complex and it requires a good understanding of basic physics of backscattering phenomena to maximize the information that can be extracted. Theoretical and empirical studies have been conducted for decades with the aim to understand such physical phenomena and consequently improve detection, estimation and classification algorithms. With the advent of high spatial resolution radar and subsequently synthetic aperture radar (SAR), radar images became available that could be used also in the area of maritime observation. Nowadays, two-dimensional (2D) radar imaging is recognized as one of the most important tools for monitoring the sea surface. As a matter of fact, a large number of airborne and spaceborne platforms for Earth observation are equipped with radar imaging systems. Moreover, some coastal and ship-borne radars have radar imaging capability to improve the maritime situational awareness.

Motivated by these facts and based on the experience of the Radar Laboratory of the University of Pisa and of the Radar and Surveillance System (RaSS) national laboratory of the National Interuniversity Consortium of Telecommunication (CNIT), we decided to propose this book with the aim to present the most recent results in radar imaging for maritime observation. This book is intended both for beginners and for experts in this field as it treats the theoretical aspects as well as the applications.

The book is organized into two sections. The first section (SAR and ISAR image processing) contains the conceptual and theoretical aspects of both SAR and ISAR. More specifically, standard SAR/ISAR and novel imaging techniques, including bistatic ISAR, passive ISAR (P-ISAR) and the 3D Interferometric ISAR (3D InISAR) are detailed. The second section (Applications) focuses on the use of such techniques for maritime observation. Results based on the use of real data and related to scenarios of interest are presented throughout this section.

Section I is composed of six chapters. Chapter 1 introduces the principle of radar imaging and specifically the concepts of high range and cross-range resolutions, the relationship between SAR and ISAR and a generic received signal model that will be used throughout the remainder of the book. Chapter 2 is focused on SAR. The SAR signal model is first defined and the main SAR image formation techniques known in the literature are described. Chapter 3 refers to ISAR. The main steps of the ISAR image processing chain, namely image formation, time windowing, motion compensation and image scaling are detailed. A mention of ISAR parameter setting for ISAR system design is also made. Chapter 4 introduces the new concept of bistatic ISAR (B-ISAR). The bistatic geometry and modeling is introduced first, then bistatic image formation is introduced by making use of the equivalent monostatic configuration. The distortions caused by the bistatic geometry are also analyzed directly using a mathematically derived ISAR point spread function. The behavior of B-ISAR in the presence of synchronization errors is also dealt with. Chapter 5 concerns the use of passive radar for target imaging. In particular, the signal pre-processing chain for the formation of range Doppler maps is described by making use of the batch algorithm. Passive ISAR (P-ISAR) theory and signal modeling is then introduced. Reconstruction of P-ISAR images is illustrated together with a performance analysis. Chapter 6 regards 3D interferometric ISAR (3D-InISAR), where the height of each single target scatterer point is reconstructed in order to have a 3D plot of objects. A multi-channel ISAR signal model is defined to derive the main InISAR signal processing. The use of multiple interferometric radars also allows for the estimation of the image plane orientation and the effective rotation vector for cross-range scaling. Analysis of the performance is provided through the use of suitable parameters.

Section II is organized in five chapters, each containing applications of maritime observation by means of radar images. Chapter 7 is devoted to the detection of ships from SAR images. Image cleaning, several detectors and post-processing techniques are included. Three different case studies are shown to demonstrate the validity of the proposed techniques. Chapter 8 refers to the detection of oil spills with SAR images. Fractal modeling and analysis of SAR images is used for oil spill detection and discrimination to other lookalike dark areas. Results based on the use of real spaceborne SAR images are presented to show the effectiveness of the approach. Chapter 9 concerns the application of ISAR processing to refocus moving targets in SAR images. Real cases related to maritime scenarios are shown to demonstrate the capability of the proposed techniques. Chapter 10 is an interesting application of passive ISAR for harbor surveillance and protection. Two case studies are proposed to see how this processing can be applied to real scenarios. Chapter 11 deals with the application of 3D InISAR from a radar network to reconstruct the 3D shape and to extract features such as the size of a ship for traffic monitoring and safe navigation in internal port waters or along the coastline.

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