Research On Echo Simulation And Ocean Current Parameters Estimation Of Along-Track Interferometry

Marine resources are a new field developed by many countries.Marine remote sensing and ocean parameter inversion are the hot fields of radar research nowadays.Synthetic Aperture Radar has great advantages in ocean observation because of its all-day,all-weather and wide-area coverage capability.Two or more antennas are placed along the flight direction of Along-Track Interferometric Synthetic Aperture Radar on the basis of SAR.Two observations are made in the same area in a very short time,and the phase difference information of observation images is used to obtain the information of moving targets(large-area moving scenes such as rivers and ocean currents).The information is used to retrieve the parameter information of the moving target by using the interferometric phase information.Based on the analysis of the fluctuation model and scattering model of sea surface SAR,the simulation of ocean SAR echo is got in this paper,and ATI echo is also got on the base of this simulation.Combining the research of airborne along-track interferometric data and satellite formation data,the ocean current parameters are inverted.The specific work is as follows:(1)According to the ocean SAR imaging model,the difference of phase history between ocean imaging and ground fixed scene imaging is analyzed,and the expression of interference phase of ATI-SAR is further analyzed based on the ocean SAR imaging model.(2)Starting from the simulation of sea surface elevation model,this paper simulates the sea surface by two methods(double superposition method and Monte Carlo method)and compares the advantages and disadvantages of the two methods.The modulation mechanism of sea surface SAR is deeply analyzed.The main reason of SAR sea surface imaging is that the modulation of long wave and short wave makes sea surface wave and electromagnetic wave satisfied the Bragg scattering so that SAR can receive real sea surface echoes.At the same time,the theoretical models of electromagnetic scattering of different scales are further studied: mirror scattering model,perturbation scattering model and two-scale scattering model.But these models are based on backscattering coefficients of large-scale sea surface,which can not meet the requirements of sea surface SAR echoes for electromagnetic scattering model.In this paper,the backscattering coefficients of sea surface SAR are calculated based on the small-panel electromagnetic scattering model under the Kirchhoff approximation theory.Then,the electromagnetic scattering model is modulated by long-wave tilt modulation,hydrodynamic modulation and velocity bunching modulation caused by long-wave orbital motion,which more vividly reflects the statistical characteristics of the backscattering coefficients of sea surface SAR echoes.On the basis of sea modeling,amplitude modulation modeling and scattering modeling,the echo of sea surface SAR is simulated and imaged.The statistical characteristics of the imaged SAR conform to the theoretical model,which verifies the validity of the simulation method.The echo simulation of sea surface ATI-SAR is further realized,and the ocean current parameters of the simulated data are inverted.The inversion results validate the simulation of sea surface ATI-SAR and the inversion effectiveness of the method.(3)Ocean current parameter inversion of ATI-SAR is realized by using measured data.Firstly,the airborne measurements along the track are imaged and ocean current parameters are estimated,and the Tan DEM-X satellite measurements under the mixed baseline are used to estimate the velocity of the Weihe River in Xi'an.Because of the short coherence time of sea surface imaging,this paper uses different Doppler bandwidths to analyze the coherence time of sea surface imaging and proposes an image entropy method to evaluate the imaging quality of airborne data with different Doppler bandwidths.After obtaining the imaging data,the main and auxiliary images along the track are interfered(registration,phase filtering,phase unwrapping).Then the Doppler velocity in line-of-sight direction of radar is directly retrieved from the interferometric phase,and the ocean current velocity is estimated from the Doppler velocity.In the interferometric processing of Tan DEM-X data with mixed baseline,because there are both along-track baselines and vertical-track baselines,the ocean current interferometric phase model with mixed baseline configuration is established in this paper.The terrain phase and ground phase are removed by assistant DEM combined with double-base positioning method,and then the river velocity parameters are estimated by interferometric processing.The validity and correctness of the proposed method are verified by the comparison of parameters inversion results with hydrological observation results.