| Geosynchronous orbit spaceborne/airborne bistatic synthetic aperture radar(GEO SA-Bi SAR)is a nonstationary bistatic SAR system which employs GEO satellite as transmitter and high-altitude reconnaissance aircraft,air early waring or unmanned aerial vehicle as receiver.Compared with conventional optical radar,GEO SA-Bi SAR can detect and image the interest target in all-time and all-weather.Compared with spaceborne SAR,the influence caused by ionosphere and stratosphere is small and the requirement for the transmitted power is reduced.Compared with airborne SAR,the ‘far-transmit near-receive' strategy which locates the transmitter on geosynchronous orbit provides GEO SA-Bi SAR an additional layer of security for target detecting in dangerous battlefield.Hence,GEO SA-Bi SAR can be widely applied in civil and military field such as disaster warning,marine monitoring and intelligence investigation.During the coherent accumulated time(CAT)of GEO SA-Bi SAR,the motion of ship target contains not only the horizontal component which caused by propulsion,but also the heave and three-dimensional swing which caused by wave.When the transmitter and receiver platforms fly stably,the phase history of ship target's echo has periodic terms in azimuth.Hence,the SAR image of ship target which achieved by the conventional RD algorithm will be blurred.How to estimate the Doppler parameters which influenced by ocean waves and solve the fuzzy problem caused by the periodic terms are keys to improvement of the performance in ship target imaging by GEO SA-Bi SAR.On this basis,estimating the Doppler parameters based on echo data,overcoming the shortage of conventional moving target detection and imaging algorithm,and eliminating the influence of ocean wave in motion compensation are three research topics.In this thesis,the topic of ship target imaging based on GEO SA-Bi SAR as discussed above is analysed and researched in detail.The main achievements of this paper can be summarized as the following four parts:1.The motion trajectory of GEO satellite,the velocity of GEO satellite related to earth and the six-dimensional motion of ship induced by waves are described mathematically,and the motion characteristics of GEO satellite and ship target are analyzed.The geometry of GEO SA-Bi SAR is established,the work model of the system is analyzed combining with the motion characteristics of GEO satellite,and the signal function of the echo is derived.As the Doppler spectrum of ship target is influenced by wave,the image of ship target will be fuzzy.The possibility of improving the performance in GEO SA-Bi SAR ship target imaging is discussed.Based on the gradient method,the accurate 2-D resolution of a GEO SA-Bi SAR system and the size of resolution cell are analytically calculated.We obtain the conclusion that the performance of GEO SA-Bi SAR depends on the designs of aircraft attitude and sliding factor.Through simulations,the influence induced by the initial attitude of receive platform on 2-D resolution is discussed.2.The ship position estimation and clutter suppression based on GEO SA-Bi SAR are studied,the estimation method of the direction-of-arrival(DOA)which based on maximum likelihood estimation(MLE)and the approach of clutter suppression based on adaptive beam forming(ABF)are proposed.In this thesis,the geometry of GEO SA-Bi SAR with multi-receive antenna is established,and the signal model of a moving ship target which has no rotational movement in this geometry is derived.The kernel of ship position estimation in this system is to calculate the bearing angle of ship target respect to the receiver,which makes the energy of echo most in this projection direction.In order to estimate the DOA angle,the phase function is designed to compensate the difference in the Doppler shift that exists between the channels and to beat out the nominal azimuth chirp corresponding to a stationary target,then the MLE algorithm is applied to search for the best projection angle.As the asymmetry of GEO SA-Bi SAR,the conventional DPCA method can not be applied in clutter suppression anymore,meanwhile the method based on ABF can be used to signal enhancement.In simulations,the clutter suppression performance of the proposed method and DPCA method are compared.The simulation results validate the effectiveness of the proposed method in this paper.3.The 3-D rotational ship target imaging algorithm of GEO SA-Bi SAR which is based on the adaptive Chirplet decomposition is proposed in this thesis.The kernel of the proposed method is Chirplet decomposition,time-frequency characteristics analysis and Doppler parameters estimation in every range cell.According to the estimated Doppler parameters,the azimuth compression function can be designed to solve the defocus problem caused by random rotation of target.As the proposed method is based on the 3-D rotational model which is close to the practical situation,it has good performance in GEO SA-Bi SAR ship target imaging when sea condition is bad.The simulations demonstrate that compared with the conventional WVD and PWVD,the proposed method has not only high time-frequency resolution,but also less cross terms in the Doppler spectrum.The problems caused by the nonlinear character of the Doppler spectrum in Doppler parameter estimation can be solved and the performance of 3-D rotation ship target imaging is improved.4.The influence caused by the heave motion of ship target in imaging is researched.By using the echo data to reconstruct ocean waves,a ship target imaging strategy which based on the compensation of wave motion is proposed.This paper provides a signal model of the interaction of ocean waves with a moving ship target in GEO SA-Bi SAR system,and the characteristics of the Doppler history which influenced by the heave motion due to waves and the horizontal motion due to propulsion is analyzed.The compensation method proposed in this thesis contains three well established algorithms.They are: adaptive notch filtering(ANF),used to analyse time-frequency characteristics of the signal and extract wave-induced and cruising motion of the ship,Root-MUSIC combined with linear least squares algorithm(LLS)for parameters estimation in waveinduced motion retrieval.In this thesis,a signal model of ship target which influenced by the heave motion of ocean waves is derived.Then,the effect of the heave and horizontal motion induced by waves and propulsion in Doppler spectrum is analyzed.ANF is applied to the range compressed signal which captured along the range cell migration(RCM)line for instantaneous Doppler history extracting.After achieving the accurate Doppler spectrum,the slope of the instantaneous Doppler frequency can be estimated and removed.To the residual Doppler frequency which is the period terms induced by ocean waves,Root-MUSIC and LLS algorithm can be used to estimate the frequencies and amplitudes for wave retrieval.Finally,the motion caused by propulsion and waves can be compensated by applying the designed functions based on the estimated parameters.The simulation results demonstrate the correctness and effectiveness of the proposed method in this thesis. |
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