System Design And Simulation Of Doppler Scatterometer

Ocean surface current is a very important parameter of ocean dynamic environment. It has been connected to global climate change, marine environment forecasting, marine navigation, engineering security and so on. The observation and prediction of ocean current has become more and more concerned. Doppler Scatterometer is a new type of radar for ocean remote sensing. The ocean surface current filed(speed and direction) can be retrieved from the Doppler frequency shift of radar echoes caused by the motion of the sea surface, while ocean wind vector can be measured simultaneously. The Doppler Scatterometer is based on a real aperture radar, which can achieve a very wide swath. It can provide the ocean surface current and wind vector information in a certain resolution and achieve global coverage quickly, which is very important for the marine environment forecasting and climate changes research.Firstly, the measuring principles of the Doppler Scatterometer are introduced. Using the interferometric phase of two successive echoes, the radial speed of the ocean surface current can be calculated. Taking advantage of the antenna rotation and platform movement, Doppler Scatterometer can observe the ocean surface at different azimuth, from which the speed vector of ocean surface current can be obtained through models.Secondly, the ocean surface Doppler spectrum model and correlation coefficient model are established. Ocean surface Doppler spectrum model is a bridge between the oceanographic parameters and radar measurements. It is the basis of radar system design and inversion method research. Correlation coefficient model is the foundation of measurement accuracy analysis and radar system parameters’ optimization. The correlation coefficient of Doppler Scatterometer is affected by four factors: thermal noise decorrelation, mismatch decorrelation, spatial decorrelation, and temporal decorrelation. They are related to SNR, relative resolution shift caused by satellite movement and antenna rotation, Doppler bandwidth and PRF, ocean surface time response determined by Doppler spectrum.Thirdly, an “end-to-end” simulation method is adopted to establish the simulation model of Doppler Scatterometer. By system analyzing, it was confirmed that Doppler Scatterometer should use the rotating pencil-beam radar system. Based on the simulation model, the feasibility of ocean surface current vector measurement by Doppler Scatterometer are analyzed, the system parameters are optimized, and the signal processing diagrams are designed. The simulation results show that when the wind speed is 7 m/s, the effective swath that satisfy the ocean current speed accuracy(0.1 m/s) is about 604 km, which is about 60% of scatterometer’s swath(1000 km). Furthermore, the performance analyses of wind vector measurement show that the communication error(Kpc) of Doppler Scatterometer is better than that of fan-beam rotating scatterometer. When the wind speed is larger than 4 m/s, the Kpc of Doppler Scatterometer is smaller than that of pencil-beam rotating scatterometer.Finally, effects of satellite attitude and satellite speed measurement errors on ocean surface current speed accuracy are analyzed. The results show that the ocean surface current speed error is smaller than 5 cm/s when the temporal resolution is 10 days, the satellite attitude measurement errors are smaller than 0.001° and satellite speed measurement error is smaller than 1 cm/s, which is feasible for current satellite technology. Moreover, the sensitivity of the parameters to ocean surface Doppler spectrum was investigated. The influence of different ocean wave spectra models, different directional distribution functions, wind speed measurement errors, as well as wind direction measurement errors on ocean surface current speed inversion are analyzed. Based on the results, further study of ocean surface current speed inversion can be carried out.