| Tethered InSAR system is a special distributed InSAR system,which consists of two or more SAR satellites connected by tethers.The system could accomplish on-orbit deployment rapidly and supply long-term stable baseline,which is effective for interferometry at arbitrary latitude.These features make the system capable of acquiring information quickly and dealing with emergency which needs topographic mapping and moving-target detection.However,the successful and rapid deployment,steady configuration maintenance and high-precision beam synchronization under effect of tether tension are the prerequisites for tethered InSAR system to work properly.Hence,the configuration stability,rapid deployment and maintenance of the system,as well as beam synchronization,are investigated in this dissertation.The main contents of the dissertation are as follow:The dynamic model of tethered InSAR system which uses lumped mass representation of the tether is established under the influences of atmospheric drag and oblateness of the earth and the simplified model suitable for calculation and analysis is derived.Moreover,the attitude dynamics model of satellites is described with quaternion method.Then the effects of tether discrete number,atmospheric drag and J2 perturbation on the spatial distribution of tether are analyzed.The static equilibrium configurations of a chain-type multi-tethered satellites only exist when the system moves on a circular orbit and are solved and analyzed to study their worth in earth observation missions.Further,the system is linearized around its equilibria and the stability of the equilibrium configurations are analyzed.The rapid deployment strategies and stable control methods of two-satellite tethered InSAR system are determined considering mission constraints and stability of system.For the system used in DEM missions,the optimal trajectory of deployment is designed based on Gauss Pseudospectral Method with least time and minimum final in-plane angle.Then an adaptive sliding mode controller is put forward to track the optimal trajectory under the effects of external disturbances and model errors.As for system used in GMTI mission,the free-deployment method is chosen since the short tether.Subsequently,the speed and direction of the initial separation are optimized by means of Particle Swarm Optimization and jet-control method is selected to keep the system stay around horizontal direction.The deployment of chain-type multi-tethered satellite system is studied.The workable separation modes are determined and their influence factors are analyzed.Then the parametric analysis method is used to study the effects of control parameters and separation speed on the deployment of system under different separation modes in a qualitative way.Moreover,based on the analysis results,the deployment methods of system applied in DEM missions or GMTI missions are determined.Lastly,the beam synchronization of tethered InSAR system is investigated.Given the residual Doppler central frequency,the maximum overlap ratio synchronization and maximum coherent synchronization are designed to obtain the maximum SNR and minimum Doppler centroid of system.Then an adaptive sliding mode controller is designed to achieve the beam synchronization in presence of external disturbations and system uncertainties. |
