Research On The GNSS-based Spatial States Determination Method Of Distributed SAR Satellite System

The distributed SAR satellite system is realized by fixing the synthetic aperture radars on formation flying small satellites. Through the collaboration of the formation flying satellites and the spaceborne SAR, the system is capable of performing various surveying and mapping tasks which cannot be imagined under the conventional single satellite SAR mode. It is a kind of new conceptual radar systems with enormous potential. There are many challenges in both the basic theories and the technical level before it comes into reality. One of the most important issues is the determination of the spatial states of the satellite system. The determination method depends on several factors, such as the states measurement schemes and the small spacecraft design. It also serves the payload missions, the formation cooperative control, and so on. The acquisition of high-precision satellite spatial states is the important guarantee for the success of distributed SAR missions. Based on the idea of utilizing the future space resources properly and effectively, this dissertation tries to probe into the spatial states determination method of distributed SAR satellite system based on the multi-frequency and multi-constellation GNSS observations.The researches and contributions are mainly embodied in the following areas.Firstly, the precision requirement analysis and internal relationships of parameters are presented based on the concepts and models arrangements of the distributed SAR satellite system spatial states. The significances of the spatial states determination, especially the relative position determination are illustrated through requirement analysis of the entire distributed SAR satellite system. The levels of the spatial states and the InSAR interferometric baseline are defined in the system. The characteristics of the spatial states are analyzed to study the clues of accuracy improvement. The interactions between the absolute and relative states in the classifications are associated with modeling and error analysis. The work can offer references for the top-level design of the distributed SAR satellite system.Secondly, the whole relationship is established between the measurement baseline and the interferometric baseline of the distributed InSAR satellite system. The conversion and precision analysis are also carried out. The entire process from the measurement baseline to the interferometric baseline is decomposed in detail. The parameters among the spatial states, except the relative position, function as boundary conditions of the conversion. The parameters information flow and precision analysis are put forward. The conversion models in the space domain are established from the measurement baseline to the interferometric baseline. The non-autonomous measurement mode with the autonomous one is taken into account in the modeling, and comprehensive studies are performed on error propagation relationships. Due to the reality of the low measurement sampling rate, the conversion models in the time domain are established from the low rated spatial states to the high one. The precision-keeping and high-rate interpolation method for the baseline is raised and the combined influences of approximation errors and random errors are analyzed from the perspective of function approximation. The influences of coregistration offset are also taken into consideration, which makes the conversion from the measurement baseline to the interferometric baseline complete.Thirdly, the parameter-saving spline representation model for the inter-satellite relative positioning based on the multi-frequency GNSS is established. On the one hand, the multi-frequency GNSS ambiguity resolution methods are raised tailored for the formation flying environment. The geometric relationship between the observation data and the estimated parameters is utilized, combining with the constraint of the continuous variation of the estimated parameters with epochs. Based on the function approximation and parameter-saving modeling theory, the relative positioning spline representation model is brought forward based on multi-frequency GNSS. The theoretical basis and simulation results of the relative positioning precision improvement are presented. The advantages of multi-frequency data over dual-frequency data and the ones of spline representation model over the traditional pointwise model are shown. On the other hand, the key issue of ambiguity resolution in the GNSS relative positioning is investigated. The performance diversity introduced by different conditions is illustrated, such as the geometry-free or geometry-based model, the original or combinational observations, and searching or sequential rounding strategies. A cascade tri-frequency ambiguity fixing method is presented, based on geometry-based model and rounding following integer decorrelation transformation. It is capable of instantaneous ambiguity fixing and robust to observation errors in the formation environment.Finally, the evaluation for the satellite spatial states determination performances based on multi-GNSS is given, and the navigation satellites selection criteria are raised oriented the distributed InSAR performance. The advantages of integrated GNSS are presented qualitatively. The quantitative performance evaluation indices are given for the satellite spatial states determination, such as the internal and external reliability, the success rate of carrier phase integer ambiguity resolution, and the observation geometry in particular. The comprehensive simulation examples and rigid theoretical foundations for the performance enhancement are presented. The redundancy and contributions of the observation data are considered synthetically. By combing the GNSS measurement sub-system and the InSAR sub-system, a novel selection criterion for the navigation satellites is raised from the aspect of elevation accuracy inversion, and higher elevation reconstruction accuracy is achieved under the same observation conditions.