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Study Of Ionospheric Effects And Correction Methods On Space-borne SAR Imaging

Posted on:2016-09-17Degree:DoctorType:Dissertation
Country:ChinaCandidate:C WangFull Text:PDF
GTID:1108330482453187Subject:Radio Physics
Abstract/Summary:PDF Full Text Request
Due to the dispersion of background ionosphere and ionospheric fluctuations, the low-band space-borne SAR signals therefore pass through and are affected by the ionosphere inevitably, which in turn degrades the SAR imaging. The performance of space-borne SAR imaging will be significantly degraded and can not image at all when working at lower bands, wider bandwidth and higher ionospheric activities. Thus, before the design of low-band space-borne SAR system, the effects of ionosphere should be considered. Aiming at this issue, this paper analyzes the ionospheric effects on space-borne SAR and corresponding methods to compensate the SAR imaging, which includes the background ionosphere and ionospheric irregularities. This work will be meaningful and helpful for the applications of low-band space-borne SAR system. The main work follows as below:1. Based on numerical simulations of point target and measurements, the effects of background ionosphere on SAR imaging is analyzed, which includes image shift and resolution degradation. For the range direction, besides linear and quadratic phase errors due to the background ionosphere, the cubic phase error is considered. The results show that the background ionosphere significantly reduces the quality of the point response with the increased bandwidth and TEC level. For the azimuth direction, the results show that the performance is not serious from the background, and can be ignored.2. Based on moment equation, the time delays due to the multiple scattering under the condition of oblique incident and anisotropic ionospheric irregularities are studied. For the case of strong fluctuation regimes, the path delay due to the multiple scattering is comparable to that of dispersion. Thus, the effects of multiple scattering on path delay should be considered. In addition, the decorrelation distance is also analyzed, it shows that the azimuthal resolution is particularly severe even under weak fluctuations, and will be sharply degraded as the electronic density fluctuation increases. For the numerical simulations, the multiple phase screen model is used to analyze the effects of irregularities on space-borne SAR, and the simulation results show that azimuthal resolution can be seriously degraded by ionospheric irregularities, which agree with theoretical results.3. Based on the mechanism of how ionosphere affects SAR imaging, a tri-band technique of TEC retrieval based on the SAR data is proposed. Due to the large errors for retrieving ionospheric information, the traditional dual-band technique can not compensate the SAR imaging well. Considering both background ionosphere and multiple scattering, the tri-band technique can retrieve ionospheric information, including TEC and irregularities information. Thus, tri-band technique can compensate degraded SAR imaging well. In addition, based on tri-band technique, we also do some researchs on computerized ionospheric tomography (CIT).4. Similar to the compensation of random phase fluctuations due to the atmosphere turbulence and change of attitude angle for onboard SAR, the phase gradient algorithm (PGA) is used to compensate the phase fluctuations due to the ionospheric irregularities for case of space-borne SAR. In order to accommodate with strip-map SAR, traditional PGA is modified under the conditions of single point and multipoint in azimuth direction, respectively.
Keywords/Search Tags:Space-borne SAR imaging, ionosphere, multiple phase screen method, tri-band technique, phase gradient algorithm
PDF Full Text Request
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