Research Of Ionospheric Faraday Rotation In Spaceborne Lower-band Polarimetric SAR

As an active sensor,spaceborne Synthetic Aperture Radar(SAR)has the characteristics of wide-swath high-resolution reconnaissance and high-precision mapping for surface targets at all-day and all-weather.Therefore,it is widely used in the military applications and the development of national economy,such as target detection,attack guidance,resource exploration and marine remote sensing.Compared with the conventional spaceborne SAR,spaceborne lower-band polarimetric SAR can detect hidden targets under vegetation and shallow surface with the strong penetrability;on the other hand,the system is more sensitive to biomass and can measure the changes in forest resources;in addition,it can also obtain the information of earth objects under different polarimetric combinations.Therefore,the spaceborne lower-band polarimetric SAR has more wide prospect in military and civilian fields.However,the design and signal processing of spaceborne lower-band polarimetric SAR systems will be severely affected by the Faraday Rotation(FR)in the ionosphere,so the FR is necessary to be analyzed and calibrated.In this paper,FR is studied from three difficult and hot topics: "analysis of ionospheric FR effect in spaceborne lower-band full-polarimetric SAR","correction of ionospheric FR effect in spaceborne lower-band full-polarimetric SAR" and "research of ionospheric FR effect in spaceborne lower-band compact polarimetric SAR".The main work and innovation of this paper are as follows:The effect of FR on the spaceborne lower-band full-polarimetric SAR is analyzed in chapter 2.Firstly,the medium characteristics of background ionosphere and ionospheric irregularities are introduced.Secondly,the propagation characteristic of electromagnetic wave in the background ionosphere is analyzed by the geometrical relations and signal characteristics.Finally,the effect of FR and Polarimetric Dispersion(PD)on spaceborne lower-band full-polarimetric SAR are detailly studied by using the existing models and the model proposed based on the Scattering Impulse Unit Matrix(SIUM)in this paper.Chapter 3 studies the correction methods of FR in spaceborne lower-band full-polarimetric SAR.Firstly,these existing Faraday Rotation Angle(FRA)estimation methods are derived.Secondly,two effective FRA estimation methods are proposed based on the Polarimetric Covariance Matrix(PCM)and the circular biasis respectively.Compared with the existing methods,both new methods achieve good estimation performance,especially the method proposed by PCM,shows better estimation performance and stronger robustness in the aspect of system additive noise,amplitude imbalance,phase imbalance and crosstalk.Finally,to solve the issue of ambiguity arisen in FRA estimated,a new ambiguity-resolution method combining pixel-level ambiguity and image-level ambiguity is proposed,and experimental results show that the method can improve the accuracy of FRA estimation effectively.In Chapter 4,the FR effect of spaceborne lower-band compact polarimetric SAR is studied.Firstly,the existing signal model of Circular-Transmit and Linear-Receive(CTLR)compact polarimetric SAR is introduced.Secondly,a group of commonly used radar calibrators are given and the corresponding scattering matrices are derived.Finally,the FRA estimation methods of CTLR compact polarimetric SAR are proposed,and the performance of estimation methods is analyzed.Chapter 5 is the conclusion and prospect.The main work in this paper is summarized,and the future research direction is described according to the developing trend of related subjects.