Analytical And Correction Research On Ionospheric Effects On Spaceborne Multimode Synthetic Aperture Radar Imaging

The space-borne synthetic aperture radar(SAR)is an important technology in remote-sensing area.Compared with the air-borne SAR,the space-borne SAR has better platform stability and larger swath width.There has been an upward trend of developing low-frequency multimode space-borne SAR system.The low-frequency SAR takes the advantages in strong penetrating capability and better time coherence that has extensive applications in agriculture,marine monitoring,military reconnaissance and biomass measurement.Furthermore,multimode SAR,such as sliding spotlight mode and terrain observation by progressive scans(TOPS)mode,has been widely developed to satisfy the different requirements in terms of resolution and swath width.However,the low-frequency SAR system is vulnerable to the ionospheric effect which fatally restricts its development.This paper focuses on the analysis and correction technology of ionospheric effect on the space-borne low-frequency multimode SAR system.Both of the background ionospheric effect and scintillation effect induced by ionosphere irregularities are studied systematically.The main work of this paper are listed as follow:In chapter 2,the performance analysis of ionospheric effect on space-borne multimode SAR system is studied.Firstly,the imaging geometries of sliding spotlight and TOPS mode are discussed,then the modified two-frequency and two-position coherence function(TFTPCF)is proposed to perform the theoretical analysis of scintillation effect for sliding spotlight and TOPS mode.Simultaneously,a multimode SAR ionospheric effect simulator is proposed based on the reverse back projection(Re BP)method.In chapter 3,a joint absolute ionospheric total electron content(TEC)estimation approach from the single-look complex SAR data is proposed.This approach combines two complementary STEC(Slant TEC,STEC)estimation methods including the global estimation with spilt-spectrum method and local estimation with range autofocusing.Combining the advantages of these two methods,the proposed method is capable of estimating absolute STEC with high accuracy and fine resolution which can be used in compensating the dispersive phase or interferometric phase compensation for TOPS mode.In chapter 4,the ionospheric scintillation mitigation methods based on the minimum-entropy autofocusing are studied.Comparing with the traditional minimum-entropy autofocusing methods,the proposed refined particle swarm optimization(Re-PSO)algorithm is a heuristic method which has extremely strong exploring abilities to the global optimum.Based on the simulation and experiment with real scintillation-contaminated SAR data,the effectiveness of Re-PSO is validated.