Ocean Remote Sensing Technologies: High frequency, marine and GNSS-based radar
A vast array of ocean instrumentation has been developed for research purposes since the middle of the twentieth century, among which remote sensing technologies have become increasingly important. Within this class of instruments, high frequency (HF) surface and skywave radar, microwave marine radar and global navigation satellite systems (GNSS)-based radar have been successfully implemented in gathering information on large tracts of the ocean surface. This book provides a systematic introduction to the principles, state-of-the-art methods and applications of HF surface and sky wave radar, microwave marine radar and GNSS-based radar, as well as an exploration of ongoing challenges in the field. Ocean Remote Sensing Technologies: High frequency, marine and GNSS-based radar includes 23 chapters that are organized into three parts, mainly according to sensor types. The first part covers work related to HF radar, the second focusses on microwave marine radar, and the third concentrates on GNSS-based radar. Each part consists of an introductory chapter that provides an overview of the corresponding sensor, followed by chapters focussing on fundamental theory, specific applications, or advanced algorithm development. Each of the chapters is self-contained and readers should be aware that there may be across-chapter differences in symbols used for various parameters. The book is intended for a variety of readers in the radar and remotes sensing communities, and content has been selected with a range of interests and backgrounds in mind.
Inspec keywords: remote sensing by radar; oceanographic regions; marine radar; ocean waves
Other keywords: remote sensing by radar; radar imaging; marine radar; ocean waves; oceanographic regions; oceanographic techniques; radar signal processing; satellite navigation; height measurement; remote sensing
Subjects: Radar equipment, systems and applications; Education and training; Textbooks; Surface waves, tides, and sea level; General electrical engineering topics
- Book DOI: 10.1049/SBRA537E
- Chapter DOI: 10.1049/SBRA537E
- ISBN: 9781839531613
- e-ISBN: 9781839531620
- Page count: 655
- Format: PDF
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Front Matter
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1 HF radar in a maritime environment
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In this chapter, an overview is given of the science, technology and techniques associated with the development of HF radar as an important instrument in the arsenal of tools available for remotely sensing the ocean surface and objects on or near it. This particular aspect of ocean sensing has matured to being generally referred to as radiowave oceanography, or, simply, radio oceanography.
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2 Oceanographic applications of high-frequency (HF) radar backscatter
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The collection of high-frequency (HF) radar backscatter observations has many potential uses for management and scientific studies in coastal waters. This is because HF backscatter observations contain information related to the interaction of the electromagnetic waves and the ocean waves. Radiowaves in the HF portion of the electromagnetic (EM) spectrum have physical wavelengths that match those of wind-driven gravity waves on the ocean surface. Because of these facts, there is the potential to extract environmental information about the ocean, and about the atmosphere that is forcing it, by examining details of the HF backscatter. Indeed, this has been shown to be a viable remote sensing method to infer ocean surface currents and, to a lesser extent, ocean surface wave conditions and overwater wind conditions. The nature of HF backscatter and the theory for how to use it to extract environmental parameters are discussed in detail throughout this book. In this chapter, the goal is to review potential applications of these environmental parameters with a particular focus on the relevant space and timescales. When does it make sense to use observations from one or more coastal HF radar stations and what are the pitfalls and limitations of these data?
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3 Symbiosis of remote sensing and ocean surveillance missions of HF skywave radar
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HF over-the-horizon radar (OTHR) systems are widely used for ocean surveillance, that is to say, for detecting and tracking ships and aircraft over wide areas extending far beyond the range of shore-based microwave radars. For ranges less than about 400 km, the surface wave mode of propagation can be exploited, while, for ranges beyond 1000 km, skywave illumination is generally available. The signal transfer characteristics of the skywave channel are highly variable, necessitating sophisticated frequency management, flexible choice of waveform, and adaptive signal processing, but the potential coverage is vast - the one-hop zone for a single skywave radar may exceed 5×106 km2 in area. Moreover, the earth-ionosphere waveguide supports multi-hop propagation, and it is by this mechanism that large ships have been detected at ranges in excess of 6000 km.
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4 Sea surface current mapping with HF radar – a primer
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Shore-based oceanographic high frequency (HF) radars are frequently used to remotely sense and map coastal sea surface currents. This chapter begins with a review of the development and utilization of HF radar sea-echo interactions and their relationship in the determination of the radial component of the sea surface current and vector coverage map, followed by a brief discussion of recent ongoing HF radar observations on the West Florida Shelf (WFS). Reported are HF radar performance and its complicated relationships with environmental factors.
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5 An initial evaluation of high-frequency radar radial currents in the Straits of Florida in comparison with altimetry and model products
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A long-range (4.9 MHz) CODAR SeaSonde was deployed at Marathon, Florida (along the Florida Keys chain), in December 2019 to observe surface currents in the Straits of Florida. An analysis of the initial seven months of High-Frequency Radar (HFR) data provides an opportunity to assess the CODAR performance in this area of complex ocean current dynamics and to compare these HFR radial data with the surface geostrophic currents derived from the along-track and gridded sea surface heights from satellite altimetry, and with surface currents from the data assimilative Gulf of Mexico HYbrid Coordinate Ocean Model (HYCOM). The HFR and the along-track altimetry-derived radial velocity components agree to within a root-mean- square difference (RMSD) of about 21 cm/s in a region of strong currents (the Florida Current, 100~200 cm/s). The agreement with the gridded altimetry product varies within the HFR footprint with an RMSD range of 16.2-61.2 cm/s and mean value of 34.1 cm/s when spatially averaged over the HFR domain. Lesser agreement is found with the HYCOM output, wherein the RMSD range is 15.2-82.3 cm/s and the mean value is 39.9 cm/s. The largest RMSD values are generally within the Florida Current frontal regions where frequent mesoscale eddy activity occurs. These findings have important implications for users of both altimetry data products and data assimilative numerical ocean circulation models.
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6 Ocean wave measurement
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The Doppler spectrum provides a measurement of the magnitude and frequency of the received radar signal scattered from ocean waves. We therefore need to understand something about ocean waves and how they interact with electromagnetic waves in order to understand how we can use the Doppler spectrum to extract quantitative oceanographic information. Therefore, we begin this chapter with an introduction to ocean waves.
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7 A non-linear method to estimate the wave directional spectrum by HF radar
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High-frequency (HF) ocean radar radiates HF radio waves and can measure surface currents and ocean wave spectra by analysing backscattered signals from the ocean. There are four types of wave estimation methods by HF ocean radars: Barrick's approximation method, parameter-fitting method, linear inversion method, and non-linear inversion method. This chapter introduces the non-linear inversion method to estimate wave spectrum from the HF radar. The non-linear inversion method is the one to obtain the wave spectra without approximating the integral equation of the relationship between the Doppler spectra and the wave spectra to a linear integral equation with respect to the wave spectra. As will be described later, Doppler frequencies are related to the frequencies and directions of the wave. This method uses not only integral equations but also other constraints on the wave spectrum. It is possible to estimate the spectral values in a wider wave number plane than other methods. Then, it was extended to a method applicable to both single and dual radars.
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8 HF radar observation of nearshore winds
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The objective of this chapter is to describe the principles and methods for extracting wind speed and direction from the signals received by oceanographic HF radars. We begin with a review of early efforts - including the use of over-the-horizon (OTH) radars and phased array ground wave HF radars- most of which utilized the second-order scatter, before reviewing the more recent use of first-order scatter to examine wind speed and direction. As an example of this methodology, the empirical model used to extract wind from the first-order scatter is detailed. Finally, we discuss theoretical factors that may have inhibited previous efforts and provide suggestions for future work.
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9 HF radar in tsunami detection
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HF radars can be optimised for the fine spatial resolution and a temporal resolution of a few minutes that are needed for tsunami detection. To reduce costs and provide flexibility, one of the challenges that has been faced by developers of HF radars is to be able to simultaneously meet the requirements of seemingly conflicting applications of different scales from one installation. For example, a method has been developed for phased-array radars to operate coherently for 1 hour with samples of time-series being taken out at short intervals of less than one minute for high temporal resolution applications.
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10 High-frequency surface wave radar for target detection
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This chapter presents a high-level view of land-based high-frequency surface wave radar (HFSWR) used to provide persistent, active surveillance of surface vessels throughout the EEZ.
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11 Introduction to ocean remote sensing with marine radars
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In this chapter, we first review background on marine radar hardware and software technology, and then provide a brief review of the technical challenges regarding the radar applications that are the subject of the subsequent chapters. Finally, each of the following chapters is briefly introduced.
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12 Observation of sea surface waves by noncoherent X-band marine radar
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In this chapter, two typical algorithms for retrieving wave information from X-band marine radar images are described, i.e., the traditional 3D FFT-based algorithm and the EOF-based algorithm, along with corresponding observation results.
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13 Wavelet-based methods to invert sea surfaces and bathymetries from X-band radar images
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This chapter aims to provide a clear entry point to modeling of radar intensity images from sea surface elevation stochastic realizations for the purpose of development and testing of the new analysis approaches. It also aims to clarify the advantages of wavelet transform-based methods for the ocean remote sensing applications. The chapter is structured as follows. Section 13.1 introduces sea surface and radar image realization simulation for shoaling wavefield. Section 13.2 describes the concept and application of the 2D direct and inverse CWT. In Section 13.3, 2D Wavelet-based Sea Surface Reconstruction (WSSR) method is introduced and applied to a realization of a shoaling wavefield. Section 13.4 describes and utilizes the new 2D Rapid Continuous Wavelet-based Bathymetry Inversion (RCWBI) method.
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14 Wave field reconstruction using orthogonal decomposition of Doppler velocities
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In this paper, The wave field statistics from either radar backscatter or Doppler velocities of the sea surface have developed extensively over the last several decades. As previously described, these methods typically involve fast Fourier transform (FFT)-based techniques that filter 2D or 3D wave spectra using the linear dispersion relationship for ocean surface waves in order to isolate the wave field from other contributions to the backscatter or Doppler/surface velocities. The use of orthogonal decomposition is examined to isolate the wave field and generate 2D phase-resolved time series of orbital velocity, which could be converted to sea surface elevation using the transformations. This orthogonal decomposition will be referred to as proper orthogonal decomposition, or POD, but orthogonal decompositions go by many names across a variety of fields or sub disciplines, such as empirical orthogonal functions (EOF), principal component analysis (PCA), Karhunen-Loeve transform (KLT), or singular value decomposition (SVD). The POD method can be used to reconstruct phase-resolved ocean wave fields from Doppler radar measurements using the leading mode functions as a filter to separate wave contributions to the radar signal from non-wave contributions, assuming some of the basis functions can be associated with the physics of the ocean surface waves while others can be associated with unwanted artifacts of the radar measurement. Most wave number-frequency (k-ω) spectra of radar images of the sea surface exhibit a "group line" feature: a low-frequency feature below the first-order dispersion relationship that passes through the origin.
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15 Current mapping from the wave spectrum
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In this chapter, we review methods by which near-surface ocean currents can be measured remotely using images of the water surface, as obtained by X-band radar in particular.
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16 Bathymetry (and current) retrieval: phase-based method
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A fundamental goal of bathymetry retrieval is the reproduction of bedforms that affect navigation and recreational safety, and that have dominant roles in wave and circulation dynamics. Some bedforms span only a few ocean wavelengths, such as sand bars, rip channels, and complex tidal shoal/channel networks. The constituent local depths may be estimated by exploiting the dispersive relationship between water column thickness and the celerity (i.e., phase speed) of surface gravity waves, provided that the corresponding spatial variations in celerity can be spatially resolved. Direct inspection of variations in wave phase can achieve spatial estimate resolution on the order of one ocean wavelength and bathymetric structures with spatial scales of about two wavelengths. We describe a method of exploiting the spatial structure of surface gravity wave phase in X-band radar imagery for the primary goal of estimating bathymetry.
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17 Wind parameter measurement using X-band marine radar images
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This chapter reviews the wind parameter estimation using X-band marine radar images.
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18 Introduction to remote sensing using GNSS signals of opportunity
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This chapter provides an introduction to remote sensing by means of Reflectometry using Global Navigation Satellite Systems (GNSS) signals of opportunity, in short GNSS-R. This technique was originally proposed in the late 1980s and proven in the mid 1990s. However, it took two more decades until satellite data were widely available, and more affordable and reliable ground-based and airborne instruments were designed, before the number of researchers and applications began to significantly increase.
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19 Modeling and simulation of GNSS-R delay-Doppler maps over the ocean
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This chapter develops the main concepts concerned with modeling and computer simulation of the Global Navigation Satellite System (GNSS) bistatic ocean-scattered signal and illustrates how this signal can be processed to obtain an observable that is generally referred to as delay-Doppler map (DDM). The variability of the sea surface geometry in space and time makes this modeling task especially difficult, also in consideration of the fact that a full understanding of how physical processes translate into scattering mechanisms has not been acquired at this time.
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20 Wind estimation
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In this chapter, we focus on ocean wind estimation using space-borne platforms such as the retired TechDemoSat-1 (TDS-1) satellite and the currently operational Cyclone GNSS (CyGNSS) constellation.
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21 GNSS-R ocean altimetry
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Mesoscale ocean altimetry remains a challenging area for satellite observations and yet of great interest for oceanographers trying to validate and derive their ocean circulation models with real measurements. Global navigation satellite systems (GNSS) Earth-reflected signals can be used as sources of opportunity for mesoscale ocean altimetry with improved spatio-temporal resolution as compared to traditional monostatic radar altimetry.
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22 Sea ice sensing using the GNSS-R technique
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In this chapter, an overview of the state-of-the-art methods for sea ice remote sensing with the GNSS-R technique is presented. More specifically, recent progress in sea ice sensing including sea ice detection, sea ice concentration (SIC) estimation, sea ice type classification, sea ice thickness (SIT) retrieval, and sea ice altimetry from GNSS-R data is summarized. The fundamentals of these applications and corresponding performance are described.
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23 Triton – GNSS reflectometry mission in Taiwan
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The aim of this chapter is to introduce the recent development of a Taiwanese GNSS-R system, named Triton. The development of the GPSR and GNSS-R mission payloads is described. Subsequently, the validation of the GNSS-R payload is discussed. The development of the wind speed retrieval algorithm is then considered. Finally, the Triton programme is summarised.
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Appendix: List of Reviewers
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Back Matter
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