Ionospheric effects on synthetic aperture radar imaging

This thesis presents analytical and numerical studies on ionospheric effects on synthetic aperture radar (SAR) imaging at UHF-VHF band.; A homogeneous ionosphere is assumed in the analytical study. The irregularity of the ionosphere is given by the two-parameter spectrum. The SAR image resolution is obtained by computing the 3 dB width of the second order of the ambiguity function. Results indicate a substantial change in azimuthal resolution caused by the ionosphere for a UHF-VHF SAR system. The change of range resolution is negligible. The analysis also indicates that Faraday rotation is significant for a UHF-VHF SAR system.; A numerical model is developed to further evaluate effects of the ionosphere on SAR imaging. The horizontal structure of the ionosphere is described by the 1-D two-parameter spectrum. The Chapman formula is used to generate the vertical profile of the electron density of the ionosphere. Simulation results show good agreement with analytical studies. In addition, simulation results indicate: (1) ray bending causes further image degradation and image shift. The average change of path length is 5.18 wavelength at f = 500 MHz frequency and image shift is 6 m for ionosphere with average Total Electron Content (TEC) and 10% TEC variation. (2) TEC gradient within the SAR aperture leads to substantial image shift in azimuth direction. (3) The SAR image degradation caused by the ionosphere shows strong frequency dependency. 600 MHz to 500 MHz is a transition zone where image resolution degrades dramatically with the decreasing of the SAR operating frequency. The difference is in the order of 102 m. The impact of the ionospheric variations on SAR image resolution decreases to a negligible level for frequencies above 600 MHz. (4) Ionospheric effects are negligible for ionosphere with average TEC and 5% or less variation. (5) The signal dispersion caused by the ionosphere is negligible for SAR operating at 500 MHz with 100 MHz bandwidth.