Study On Inversion Method Of Surface Seawater Conductivty Distribution Using Over-the-horizon Radar

First-order and second-order scattering wave appears when high frequency radio wave propagating upon random rough surface. By receiving the scattering wave, HFSWR (High Frequency Surface Wave Radar) can obtain current, wind, wave and other marine information, achieving remote detection of ocean dynamics parameters. The domestic study of HFSWR for environment monitoring began in the 1980s. With the supports of the National High Technology Research and Development Program of China (863 Program) from Ninth Five-Year Plan to Eleventh Five-Year Plan, the Radio Oceanography Laboratory of Wuhan University has developed a series of matured products called OSMAR (Ocean State Monitoring and Analyzing Radar). The research of this paper is supported by the Twelfth Five-Year Plan of '863 Program' under contract No.2012AA091701. Radar data measured in the open sea area to the southwestern area of Taiwan Straits are used to carry out the research.At present stage, the emphasis of HFSWR remote sensing is on the inversion mehord of wind, wave, and flow. Little work has been done in the field of remote sensing for sea surface conductivity. Sea surface conductivity is one of the most important factors affecting radio wave propagation upon seawater, especially for surface wave model. For smaller conductivity, radio wave attenuation is greater, and detection range is shorter. Variation of conductivity leads directly to the change of radio wave attenuation character.This paper studies the inversion method of sea surface conductivity based on these attenuation characters. Details of the research are presented below:1) Studied the propagation model and scattering model of high-frequency electromagnet wave acts with random rough surfaces. Radar Equation is deduced and analyzed to classify influence factors:transmitted power, transmitting antenna gain, received power, received antenna gain, antenna array, radio frequency, waveform, range, attenuation factor, scattering cross-section. As for HFSWR, transmitted power, transmitting antenna gain, received power, received antenna gain, antenna array, radio frequency and waveform are invariable in a frequency sweep cycle. Range information can be obtained from first fast-Fourier-transform (FFT). Attenuation factor describes wave energy loss propagating upon hydro-pneumatic interfaces. Scattering cross-section describes scattering-energy from random rough surfaces.2) Scattering cross-section has been deduced based on the previous results. Considered non-perfect conducting seawater, and find that scattering cross-section is close to zero with grazing incidence. This conclusion does not conform to Bragg scattering echo from measured data. This paper believes that contradiction is due to the mismatching of perturbation method and sea-wave model, which requires longer radio wavelength than sea-wave. But in fact, this hypothesis is imprecision because of the difference of dominate wave and resonance wave. Two-scale model is used to solve this problem.3) Attenuation factor is analyzed under two different conditions:over homogeneous medium and inhomogeneous one. The former one supposes propagation medium is with same conduction properties; the later one supposes the conduction properties are various over the propagation path. Differences of two attenuation factors are compared by simulation. Influences of initial value and variation of conductivity to attenuation factors are analyzed. These help us understand the propagation process. Conductivity inversion method will differ with different attenuation factors.4) Influences that factors have on radar equation are discussed, such as conductivity, wind and wave. These factors effect radar equation indirectly through scattering cross-section and attenuation factor in different ways. Conductivity exists in both scattering cross-section and attenuation factor. The one in former is independent of distance. The one in latter has an accumulative effect with distance. Wind and wave exist in scattering cross-section which is constant in simplified model.5) Five inversion methods for sea surface conductivity are proposed. The first method supposes that sea water is homogeneous; conductivity is constant in propagation path, which can be inversed from attenuation curve. The second method supposes that sea water is homogeneous in each range cell; conductivity changes from cell to cell, which can be inversed from attenuation curve in each range cell. The third method supposes that sea water is inhomogeneous; conductivity changes in every distance, which can be inversed from near to far using iteration method. The fourth method supposes that sea water is inhomogeneous, conductivity can be obtained by solving nonlinear equations. The fifth method find a experience curve of attenuation factor from empirical data, conductivity is estimated base on the different of experience curve and measure curve.6) The methods are investigated using three days HFSWR data. Statistics characters of radar data are analyzed. Fourier transform for radar data in fast-time-field and slow-time-field helps separating the wave sign from original data. Quality control and curve fit are used to reduce interference and noise in echo sign. Preliminary result of conductivity is list.