| By using vertical polarization electromagnetic wave that travels along the curvature of the ocean surface with low loss, high frequency ground wave radar (HFGWR) can be used for real-time ocean remote sensing over the horizon range, including not only the measurement of the ocean surface dynamic parameters such as wind, wave and current, but also the detection and tracking of targets that move on the ocean surface or fly over low altitude. There are two main kinds of HFGWRs, phased-array radar and portable radar. The former is characterized by perfect angular resolution based on the large aperture of the antennas, while the latter is typically small and easy for installation and maintenance. The representative product of portable radar is the so-called SeaSonde developed by CODAR Ltd., which employs crossed-loop antenna to estimate the direction of the arrival and also achieve good angular resolution via super-resolution algorithm. Both the two kinds of systems have been widely applied in the field of ocean state monitoring, however, there still exists some problems as follows1) The accuracy and range of waveheight measurement are limited by the conventional method. In high sea state, the second-order part of echo spectra get saturated and mixed with the bragg part which makes it hard to separate them. In low sea state, the second-order spectra may be too weak to be observed from noise floor. The waveheight cannot be measured accurately in both cases.2) When the target echo is located inside bragg spectra or second-order spectra of ocean echo, the detection of target and the extraction of ocean parameters will both be affected negatively.3) The above problem also exists in condition of serious external noise in high frequency.4) The topography and nearby interferent such as metal rod, buildings, trees and stones about the experimental environment are all contributed to the distortion of antenna pattern, which leads to the increase of angular estimation error.To solve these problems, this paper designed a fully-digital High-Frequency(HF) radar system based on SeaSonde, and proposed a new method of waveheight inver-sion with dual-frequency, also a DOA(Direction of Arrival) estimation method with only cross-loop was presented based on the influence of the environment on the cross- loop/monopole antennas. The research work of this paper is listed as follow1) This paper proposed the design and framework of the fully-digital dual-frequency HF radar including dual-frequency transmitting antenna, receiving antenna and receiver. Compared with the single-frequency antenna, dual-frequency antenna was achieved by adjusting hardware structure, which kept the same performance as the former. The system employs FMICW(Frequency Modulated Interrupted Continuous Wave) waveform and operates at frequency of 4-6MHz and 12-14MHz.2) This paper developed a fully-digital high-frequency dual-frequency radar receiver, including the design and implementation of the hardware module circuit, signal processing flow, system control software, and the birth of an engineered prototype. The conventional IF (intermediate frequency)receiver contains the analog mixing circuit, IF filter and LO (local oscillator) signal generating circuit. It is hard to overcome the disadvantages,such as IF image frequency interference,limitation of mixer dynamic range, phase error between local oscillator and transmitted signal. In contrast the fully-digital receiver utilizes direct radio sampling, digital converter and digital pulse compression technology. It reduces restrictions arisen from the analog devices on sensitivity,dynamic range and phase coherence between different frames.3) The performance of dual-frequency waveform based on time-division and frequency-division is compared. When the ratio of the maximum detection distances at two frequencies is greater than 3, time-division is better than frequency-division. In this paper, time-division waveform is employed and waveform design method followed by design examples is given. The design of data transfer and system control are detailed based on the synchronous timing control design. The design and function of the system control software is briefly introduced. Besides, the hardware platform is extended to be capability of monitoring external ambient noise in real-time and automatically selecting the appropriate radar frequency.4) By testing the environmental impacts on cross-loop/monopole antenna, analyzing the causes of antenna pattern distortion, we arrive at the following conclusion: the impacts on DOA estimation that caused by distortion can be eliminated via normal-izing the voltage of two loops according to their root mean square(RMS). On this basis, we proposed a new method to estimate DOA by using only two loops. Com-pared with the traditional cross-loop/monopole processing, this method doesn’t rely on antenna’s erection environment nor the actual antenna pattern, and economizes the receiving channels as well.5) Based on the first and second-order scattering mechanism of electromagnetic wave, the classical wave height inversion by Barrick has been derived, and the limitations of this method has been analyzed. For breaking through the restrictions above, dual-frequency fusion was proposed to achieve the ability of detecting large-scale waves. According to the characteristics of sea echo at two frequencies, different weights were obtained to weight the waveheight inversed on double frequency. The coverage and range of waveheight measurement, and anti-interference capability have been improved.Upon completion of the design and manufacture of the radar system, the closed-loop tests and field tests are carried out. Test results indicate that sensitivity of the system is -135dBm, dynamic range is greater than 100dB, channel isolation is greater than 60dB; within 4.6 minutes of continuous coherent integration time, magnitude vari-ation within different sweep frame is less than 0.005dB, and phase variation is less than 0.01°; receiver work normally in constant temperature environment of 45℃ over more than 40 hours of test time, while the maximum temperature difference between internal circuit and external environment is 13℃. The system satisfies the requirement com-pletely. The echo spectrum, radial current and result of waveheight from field test show the correctness of system design. Data obtained in field test proves the correctness and effectiveness of azimuth estimation of crossed-loop and wave-height inversion through dual-frequency. This thesis provides a good solution to improve the waves measurement accuracy and overcome the environment impact on antenna direction characteristics.
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