| High frequency surface-wave radar(HFSWR) has been employed for early warning and ocean remote sensing for decades. HFSWR uses a vertically polarized electromagnetic wave(3–30 MHz) that follows the curvature of the Earth along the air–water interface and has a very low propagation loss along the ocean surface. Compared with conventional instruments such as buoys, anemometers, and microwave radars, HFSWR can provide continuous, all-weather, and real-time surveillance far beyond the visible horizon.According to the platform where it is mounted, HFSWR could be categorized as the land-based HFSWR and shipborne HFSWR. Due to land-based HFSWR is usually located on the shoreline, it is limited to the coastal ocean applications. In addition to the properties of land-based HFSWR, shipborne HFSWR can provide the agility and maneuverability to enlarge the applicable regions.Understanding the characteristics of echoes backscattered from the ocean surface is essential to further theoretical and experimental investigations in target detections and ocean remote sensing. However, the studies on first- and second-order ocean surface cross sections in existing literatures are almost conducted for land-based HFSWR. Fewer studies have been conducted to investigate the characteristics of ocean surface cross sections in shipborne HFSWR. The main purpose of this dissertation is to study the characteristics of ocean surface cross sections in shipborne HFSWR. In addition, on the basis of the space-time distribution and the spreading mechanism of the first-order sea echo, the potential of wind direction extraction with shipborne HFSWR is presented.The dissertation firstly analyzes the space-time distribution of the first-order echoes backscattered from the ocean surface in land-based bistatic HFSWR and shipborne HFSWR. To study the properties of the spread spectrum in land-based bistatic HFSWR, the space-time model and the spreading mechanism of the first-order sea echoe is proposed. The simulation and experimental results verify the spreading model. Meanwhile, based on the receiving beamwidth in shipborne HFSWR, we analyze the space-time model and the spreading mechanism of the first-order sea echoe. These analytical results provide theoretical basis for the subsequent derivations and analysis of the first- and second- ocean surface cross sections in shipborne HFSWR.On the basis of the scattering geometry, autocorrelation of the first-order field equation in the time domain and the Fourier transform, we derived the first-order ocean surface cross section for shipborne HFSWR with uniform linear motion, which could be used to mathematically verify the space-time model and the spreading mechanism of the first-order echoes backscattered from the ocean surface provided by Xie et al. In addition, the derived cross section can be reasonably regarded as that Barrick or Walsh’s result in land-based HFSWR is spread due to the uniform linear motion.The second-order ocean surface cross section in shipborne HFSWR is derived by combining the hydrodynamic and electromagnetic contributions for patch scatter. On the basis of the simulation results, the effects of radar system parameters and sea state on the second-order sea echo spectrum are discussed. With the previously proposed first-order ocean surface cross section, both cross sections could be reduced to Barrick’s or Walsh’s results in land-based HFSWR.The interaction between the platform and the sea environment, such as sea wave, wind and current, may result in more complicated characteristics of the echoes backscattered from the ocean surface. For interpretting the essential characteristics of sea echo, the first- and second-order ocean surface cross sections are mathematically derived for shipborne HFSWR, where the uniform linear motion and sway motion are involved simultaneously.The feasibility of extracting wind direction is studied based on the first-order ocean surface cross section in shipborne HFSWR. Meanwhile, a method of extracting wind direction without ambiguity is proposed. Based on the space-time model and the spreading mechanism of the first-order sea echoe in shipborne HFSWR, wind direction of the large sea area covered by radar can be obtained by use of a single receiving sensor rather than the receiving array in land-based HFSWR. Compared with the methods in land-based HFSWR, it can provide higher transverse resolution and be more easily realized with less system cost.
|
PDF Full Text Request