Study On Ocean Surface Current Inversion Algorithm With Distributed High-frequency Sky-surface Wave Radar

As one of the key dynamic parameters of marine environmental monitoring,ocean surface currents have important application and research value in fishery fishing,marine rescue,marine transportation,oil drainage,nearshore engineering,and military navigation.At present,the equipment used for ocean surface current observation mainly includes high-frequency(HF)radar,drifting buoy,X-band radar,spaceborne/airborne Synthetic Aperture Radar(SAR),etc.Among them,HF radar is widely used for its advantages such as high temporal and spatial resolution,low cost and long-distance detection ability.However,as the detection mode of the HF surface wave radar(SWR)is single and its detection coverage is limited,it is not conducive to the ocean current monitoring for long-distance and wide-range seas,which promotes the research of new radar systems with farther detection capabilities.HF sky-surface wave radar(SSWR)uses the ionospheric layer to reflect electromagnetic waves,so that electromagnetic waves are spread to the ocean surface and then received by shore-based radars or mobile offshore platforms.SSWR has the characteristics of long propagation distance and flexible layout of receiving stations,enabling the long-distance and large-scale observation of seas.Therefore,it is of great practical significance and application prospect to study the ocean surface current detection technologies of the HF SSWR.Because the electromagnetic wave propagation paths are complex and the echoes are usually seriously polluted by the ionosphere in sky-surface wave mode(SSWM),there are still many problems in the inversion of ocean current.The existing ocean current inversion methods are generally based on simplified planar ionospheric model and approximate sky-surface wave scattering mechanism,resulting in large errors of inversion results.To avoid the assumption of the simplified ionospheric model and improve the accuracy of ocean current detection with the SSWR,this paper explores the current inversion algorithm of SSWR,supported by the “12th Five-Year” National 863 Project “Distributed High-frequency Over-the-horizon Radar Detection and Networking Technology Study”.The ocean current detection basis of the SSWR,data quality evaluation,first-order Bragg frequency extraction model,and current inversion algorithm of the SSWR are studied.The main work accomplished is as follows:1.Basis research on ocean current detection of SSWR.Firstly,the influence of the tilt of the ionosphere on the distance detection is analyzed,which indicates that the detection error caused by the tilt of the ionosphere is not negligible.Secondly,aiming at the approximation problem in the existing bistatic model of the SSWR,the Bragg scattering mechanism and the spatial geometric relationship of the wave scattering model in SSWM are studied.And the wave direction and Doppler projection relationship of the wave phase velocity are determined,which provides an important theoretical basis for inverting the ocean current.The first-order ocean echo spectrum in the SSWM is simulated,and the influences of ionospheric motions on the echo spectrum are analyzed.Finally,the key techniques in the signal pre-processing of the current inversion of the SSWR are introduced.The above research results are the basis of ocean exploration of SSWR,which is of great significance for ocean current inversion.2.Echo characteristic analysis and data quality assessment of the SSWR.Aiming at the problem that the echo data of SSWR is seriously affected by the ionosphere,a comprehensive statistical analysis of the experimental data from nearly 60 days of the SSWR is carried out.The analysis objects include the reflex virtual height,spectral width,frequency shift,echo power,direction of arrival of the direct wave,and the spectral width and signal-to-noise ratio(SNR)of first-order ocean echoes.Based on the empirical parameters obtained from the analysis of a large number of echo data,a preliminary scheme for evaluating the data quality of the SSWR with four indicators is proposed.The four indicators are the fitting parameters of the frequency shift of the direct wave,standard deviation of virtual height of the direct wave,azimuth estimation error of the direct wave and the sum of the SNR of first-order ocean echoes,which respectively characterize the motion state of the ionospheric reflection point,the fluctuation of echo spectrum in the range dimension,the phase pollution degree and the reception of the ocean echo signal.For the first time,the data quality of the SSWR is quantitatively evaluated by this work,and a four-indicators evaluation model is proposed to provide a data qualityjudgment criteria for the ocean current inversion of the SSWR.3.First-order Bragg frequency extraction model.Aiming at the problem that the first-order Bragg frequency in the SSWR is difficult to calculate due to the unknown ionospheric state,a first-order Bragg frequency extraction model based on the Fourier series expansion is proposed,which is verified by the ocean current inversion of monostatic and bistatic SWRs.The model expands the first-order Bragg frequency and Doppler frequency shift caused by the radial current into a two-dimensional Fourier series expansion function about the range bin and the azimuth angle.Using the relationship among the positive and negative spectral points of the ocean echo,the Bragg frequency,and the Doppler deviations of radial current,the overdetermined linear equations are constructed.Then the least squares algorithm is applied to solve the unknown coefficient terms in the Fourier series expansion,so as to obtain the first order Bragg frequency values corresponding to each scattering element,and further calculate the radial current velocity.Simulations and error analysis of the algorithm are carried out,and the influences of the data coverage rate and the matching degree of positive and negative spectral points on the inversion result of ocean current are evaluated.The measured monostatic and bistatic radar data are used for verification.The inversion results are compared with the buoy data,and the Root Mean Square Error(RMSE)can reach 8.8 cm/s.The detection accuracy of the new algorithm is comparable to that of traditional method in the deep waters,and better than that of traditional method in the coastal waters,which prove the feasibility and effectiveness of the new algorithm.4.The ocean currents inversion algorithm of distributed HF SSWR.Aiming at the problem that the bistatic angle,glancing angle and position of the scattering element are difficult to determine under the unknown ionospheric state,the current inversion algorithm based on the Fourier series expansion and dual station joint ergodic search is studied.Firstly,the first-order Bragg frequency of the SSWR is extracted by using the Fourier series expansion model.Secondly,the common scattering patch of two receiving stations are determined by ergodic search method.The obtained firstorder Bragg frequencies corresponding to the co-scattering patch are combined with the theoretical computation formula to solve the bistatic angle and the grazing angle.Finally,based on the wave scattering model in the SSWM,the velocity and direction of the current are calculated.The inversion of the current is carried out using the measured data of the SSWR,and the result is compared with the current field observed by the traditional SWR.In the core detection area,the root mean square error(RMS)of the current velocity and direction of the two results can reach 9.5 cm/s and 9.5°,respectively.Finally,aiming at the problem that the group range bin resolution of the SSWR is low and the constraint condition of the ergodic is fuzzy,a spectral refinement method based on chirped Z-transformation(CZT)is proposed to realize the refinement of the distance spectrum.The inversion results are in good agreement with the current results of SWR.The RMSE of the current velocity and direction in the core detection area can reach 7.2 cm/s and 8.6 °,respectively.The detection range increases from the original 155 km to about 215 km after the improvement,which confirms the longdistance and wide-range ocean monitoring capability of the SSWR.