Faraday rotation effects on SAR images

Kurzfassung

Linearly polarized radio waves that traverse the ionosophere are affected by ionospheric refraction. Due to the anisotropy which is caused by the geomagnetic field, the polarization plane rotates as a function of the plasma density and the angle between the ray path and the geomagnetic field (Faraday rotation). Ionosphere induced Faraday rotation causes distortion of Synthetic Aperture Radar (SAR) data especially under high solar activity conditions when the ionization level is high. In this study, the Faraday rotation effect has been investigated at the center frequencies of the Advanced Land Observing Satellite (ALOS) and Canadian RADARSAT-1 satellite 1270 and 5300 MHz, respectively. The rotation angle has been computed using a two dimensional ray tracing program. The Earth's magnetic field is modelled by the International Geomagnetic Reference Field (IGRF). We have found that at a certain incident angle, the maximum Faraday rotation is observed in southward direction(azimuth α ≈ 180°) and the minimum in northward direction (azimuth α ≈ 0°) at a target position (48° N, 15° E) in Europe. Considering typical variations of the Faraday rotation with the azimuth angle, we have developed a correction formula to calculate the Faraday rotation angles at L- and C-band frequencies. To check the validity of the correction formula over Europe, ray tracing simulations are done at different geographic locations over Europe with 1-degree latitude-longitude resolution. The root mean squared (RMS) and maximum remaining errors (Faraday rotation by ray tracing – Faraday rotation by correction formula) are computed over Europe.