Analytical Research On Ionospheric Effects On Geosynchronous Synthetic Aperture Radar Imaging

Geosynchronous Synthetic Aperture Radar(GEO SAR)is a new type of satellite payload of the spaceborne SAR,which works at the geosynchronous orbit.Similar to the existing spaceborne SAR,GEO SAR obtains high-resolution two-dimensional images via signals' coherent integrations in time or frequency domain.The altitude of GEO SAR is 36,000 kilometers,and its revisit time can be even less than one day.Therefore,GEO SAR has higher time resolution and wider swath,compared to Low Earth Orbit SAR(LEO SAR).In order to obtain high signal-to-noise ratio(SNR),its carrier frequency is always within the low-frequency band,e.g.,the L-band or S-band,which means it is susceptible to ionospheric effects.This dissertation gives in-depth analyses of dispersion effects caused by the background ionosphere and scintillation effects caused by the ionospheric irregularities,along with their impacts on GEO SAR imaging.Primary works of this dissertation are shown as follows:Chapter 2 gives the Doppler performance analysis of the GEO SAR system in an ideal condition,which means that the Doppler performance is analyzed without ionospheric effects.By deriving formulas,we obtain the expression of the zero-Doppler centroid frequency,and give two solutions of the attitude steering and corresponding solutions of the antenna phase centroid scanning,which can realize the zero-Doppler centroid control.Then,line-of-sight trajactories of GEO SAR with different conditions are illustrated;total zero-Doppler centroid performance for a wide swath is also analyzed.Moreover,variations of the beam centroid direction during the integration time are considered;Doppler performance of the GEO SAR system is precisely formulated,and relevant simulative analyses are operated.Background ionospheric effects on GEO SAR imaging are analyzed in Chapter 3.Based on the expression of the dispersive phase advance error induced by the background ionosphere,we derive the signal model of GEO SAR under its influence,which can be divided into the large-scale constant of the total electron content(TEC)and temporal-variant slant TEC(STEC),and will have effects on GEO SAR range and azimuth imaging,respectively.When analyzing the effect of the large-scale constant on the range-imaging,Taylor expansion of the frequency-dependent phase error are operated;thereby,we obtain the formulas about the range shift and quadratic phase error.When we analyze the effect of temporal-variant slant total electron content on the azimuth-imaging,it can be attributed three factors: the temporal-variant,spatial-variant background ionosphere and varying propagation paths.The temporal-variant background ionosphere will introduce the linear,quadratic,and cubic phase error in the azimuth direction,which will cause the azimuth-imaging shift and defocusing.The spatial-variant background ionosphere will cause the extra azimuth-imaging shift.Varying propagation paths will introduce the extra QPE in the azimuth direction.At last,by using experiment-measured data of the temporal-varying background ionosphere and spatial TEC distributions provided by the International Reference Ionosphere model,we operate the point target simulation to validate the analytical model.Scintillation effects on GEO SAR imaging are analyzed in Chapter 4.Beginning with the existing generalized ambiguity function(GAF)of the LEO SAR,we obtain the GAF model applicable to analyzing the ionospheric scintillation effect on GEO SAR imaging resolution,which is based on the particular geometric model of GEO SAR.On account of the phase screen theory and Rino's spectral mechanism,two-frequency and two-position coherence function is analyzed in detail when the dispersive effect of the irregularities is considered;meanwhile,simplified two-position coherence function is also introduced when we neglect the freguency-dependent feature of the scintillation phase.Furthermore,via numerical simulation and Monte Carlo signal-level simulation,further analysis and verificaiton of the theoretical model are operated.Finally,compared with the ionospheric penetration point,convention velocity of ionospheric turbulence for the GEO SAR system is non-negligible.By using the two-dimensional space-time coherence function to illustrate the spatial correlation with different time separations,we discuss the effect of the relative velocity of the irregularities scanned by IPP on the azimuth resolution.