| UHF radar operating at decimeter band has high range resolution,small antenna aperture and inherent portable structure characteristics,which can effectively detect river dynamic parameters,such as river flow and discharge.At the same time,UHF radar can theoretically bridge the detection blind area of HF and X-band radar,with high potential in the offshore dynamic parameters monitoring.In addition,UHF radar can indirectly analyze the Bragg and non-Bragg scattering mechanism by combining with wave tank reduced-scale experiment.Therefore,UHF radar can provide a reliable non-contact measurement method for river and ocean hydrodynamic parameters.Based on the classic water wave Bragg backscattering enhancement theory,the river detection mechanism with UHF radar has been relatively mature.However,when the existing radar system booming in engineering,there are still some defects,such as complex hardware structure,high requirements for station building environment,and difficulty in the implementation of array calibration,which can not meet the needs of all-weather online measurement of modern hydrology.For the detection of offshore hydrodynamic parameters,because UHF band is out of the effective range of the classical electromagnetic scattering theory,the connection between the echo spectrum and the sea state cannot be established strictly,and there is still a lack of reliable inversion mechanism.In the existing study of Bragg and non-Bragg scattering mechanism,it is only limited to the preliminary explanation of wave tank experiment phenomenon,and there is no in-depth theoretical research on the internal relationship of radar cross section(RCS),scattering angle,and Doppler frequency shift with periodic water wave height,wave period,and incident wavelength.In order to deal with the above problems,this paper redesigns a compact digital multi-channel UHF radar system,and proposes a linear array amplitude and phase error self calibration algorithm based on the river echo characteristics,which will further promote the engineering application process of UHF radar river detection.Then,combining with the theory deduction,numerical analysis and the wave tank experimental results,the Bragg and non-Bragg scattering mechanism and Doppler shift characteristics are studied in depth.Finally,from numerical simulation and field experiment,the response of UHF radar ocean echo spectrum to sea state is analyzed,which lays the foundation for the offshore dynamic parameters monitoring via UHF radar.The specific work and related conclusions include the following aspects:1.A full digital multi-channel UHF radar system named RISMAR-U is developed.The detailed design and implementation of the radar hardware system,logic circuit and software system are given.With the adoption of technologies such as direct RF sampling,full digital pulse compression,distributed software structure,4G network data transmission,solar and AC complementary power supply,and remote status monitoring and power management,it greatly simplifies the RISMAR-U system structure and requirements for the station environment,and can realize the integrated unattended station building mode.The test results of various indexes of the system in the laboratory environment show RISMAR-U with a dynamic range of higher than 100 d B,a sensitivity of lower than 155 d Bm/Hz,a channel isolation of higher than 55 d B,and an amplitude and phase stability of lower than 0.006 d B and 0.04 degrees,respectively.2.A complete technical scheme for the inversion of radial flow field,vector flow field,cross-section velocity and discharge of RISMAR-U is proposed.According to the river flow geometric distribution characteristics,a linear array self calibration algorithm is proposed,which can calibrate the array amplitude and phase errors using the river echo in real time,and further reduce the complexity of RISMAR-U engineering implementation.In order to verify RISMAR-U's comprehensive performance,detection accuracy,and the robustness of array self calibration algorithm,this paper has carried out several field experiments comparing with the traditional contact river flow and discharge measurement methods.In straight,curve and more complex river channels,RISMAR-U all has achieved high long-term detection accuracy.3.The Bragg and non-Bragg scattering mechanism and Doppler frequency shift characteristics from periodic water wave are explored from three aspects of numerical simulation,theoretical analysis,and field experiment.The RCS and Doppler spectrum are calculated by the method of moment(Mo M)numerically.The general form of the nth order perturbation solution to scattering waves is derived by the small perturbation method(SPM).The radar wave tank reduced-scale experiment is conducted to use RISMAR-U to measure the Doppler spectrum of water wave.The wave tank observation results show that when the water wavelength is an integral multiple of the Bragg wavelength,backscattering enhancement will occur,and multiple harmonics with equal frequency interval will appear in the Doppler spectrum.However,the numerical simulation shows that except for the Doppler peak related to the phase velocity of water wave,the other harmonics are caused by the wave tank boundary effect.Theoretical analysis,numerical simulation and wave tank experiment all show that the scattering field of periodic water wave is composed of a series of plane waves propagating to the directions determined by the ratio between the water wavelength and the incident wavelength,and there is a clear exponential relationship between the RCS and the water wave height.4.The Doppler spectrum gradual characteristics of one-dimensional sea surface in HF,VHF and UHF bands are compared and analyzed numerically.In order to deal with the problem of full grazing incidence,the rough surface is transformed into a local perturbation plane,the surface current exciting the incoherent scattering wave is regarded as the surface unknowns,and the improved surface integral equation is solved by the Mo M.The validity of the improved scattering model is verified by comparing with the SPM in HF band,and the applicable frequency range of the SPM is evaluated quantitatively in the Doppler spectrum domain.Then,the Doppler spectrum characteristics in different frequency bands and different sea conditions are analyzed in detail,and the influence of different sea wave nonlinear models on the Doppler spectrum is also compared.5.The characteristics of two-dimensional sea surface backscattering RCS and Doppler spectrum of shore-based UHF radar are studied experimentally and numerically.The small slope approximation and choppy wave model are used to calculate the nonlinear sea surface backscattering RCS and Doppler spectrum numerically,and RISMAR-U is employed to measure the offshore sea surface echo information under different sea states.After compensating for the influence of wind direction,the responses of the radar echo power and the numerical predictive RCS to the wind speed are basically consistent,which are both more sensitive in low sea state.During the whole experiment,the correlation coefficient of Doppler spectrum between radar measurement and numerical simulation is more than 0.96.With the rise of sea state,the high-order peaks intensity increases rapidly,while the Bragg peak intensity decreases slightly,and the overall shape of Doppler spectrum is wider and flatter.The influence of ocean surface current on the echo spectrum is consistent with that of HF radar,and the radial surface current in the radar coverage will produce an overall frequency shift in Doppler spectrum. |
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