Simulation Study Of Sea Surface Doppler Characteristics Of Real Aperture Radar With Small Incidence Angle

Ocean currents are continuous,predictable,directional movements of seawater driven by gravity,wind(Coriolis effect)and water density,and play a key role in the Earth’s climate,influencing almost all processes occurring in the ocean.Remote sensing is the primary means of measuring surface currents in the ocean,and it has major advantages over buoy observations in terms of observational range,periodicity,and cost.Real aperture radar rotating antenna combined with Doppler principle measurement can achieve a large range of two-dimensional flow field observation,and it shows great advantages as a new observation system in two-dimensional current field measurement,but today there is no such on-board payload for sea surface current observation,and the research of related models and parameter(including hardware and sea state)sensitivity experiments are relatively few,and the analysis of the effect of sea surface waves on Doppler frequency shift is still in The analysis of the effect of sea surface waves on Doppler shift is still at an early stage.The Doppler shift of radar sea surface echo is the basis of sea surface current field inversion,which includes the Doppler shift caused by platform motion and waves,etc.in addition to the contribution of sea surface currents.Whether there is sufficient knowledge of the sea surface Doppler characteristics is the key to understand the sea surface motion,and whether the effects of platform motion and waves can be effectively removed is the key to accurately obtain the sea surface current field.In this paper,we analyze the influence of key radar and sea state parameters on the Doppler characteristics and pulse correlation by means of simulation for the small incidence rotary scanning observation system,and explore a new method for direct observation of sea surface motion by small incidence real aperture radar,which provides a reference for the hardware design of the new system satellite system and the application of sea current observation.The main findings of this paper are as follows:(1)When real aperture radar uses the phase difference between two pulses to measure the sea surface motion,it is necessary to ensure that the correlation requirements are met between the two pulses.By analyzing the effects of key parameters such as radar pulse repetition frequency,instrument noise,azimuth angle,incidence angle and wind speed on pulse correlation,it is verified that each parameter can meet the accuracy requirements of phase difference measurement well,and only the lower side wind speed conditions will cause the correlation coefficient to drop abruptly and affect the data quality.(2)The sea surface Doppler information acquired by radar contains both nongeophysical and geophysical components.The geophysical Doppler shift due to the relative motion of the satellite and the Earth is influenced by the satellite flight speed and the attitude angle of the satellite,however,the Doppler frequency is calculated from the phase difference of adjacent pulses,which has less influence on the solution of the phase difference after removing the non-geophysical Doppler shift due to the small error of the satellite attitude change rate.For the geophysical Doppler frequency shift due to sea surface motion,the Elfouhaily spectrum performed the most stable in the three typical wave spectrum comparison experiments,and the higher the radar frequency,the smaller the standard deviation of the simulation results,while the radar parameters such as pulse repetition frequency,noise equivalent sigma zero,and polarization mode did not have significant effects on the geophysical Doppler shift.(3)The wave-induced Doppler velocity is the key error affecting the inversion of the surface currents.The simulation results show that the azimuth direction and incidence angle of radar electromagnetic wave irradiation will have a large impact on the observation results,and the wind field,wind and wave development,swell scale and current field and other sea state conditions have different degrees of contribution to the wave-induced Doppler velocity.The simulation results are verified by using the classical analytical solution of the mean velocity of the mirror scattering point on the sea surface,and they are in good agreement under different sea state parameters.The above simulation analysis of dynamic sea surface Doppler characteristics of small incidence angle real aperture radar yields some reliable results,which provide references for the parameter design of new regime rotating beam radar and current measurement applications.