Abstract
It is well known that the Faraday rotation (FR) is obviously embedded in spaceborne polarimetric synthetic aperture radar (PolSAR) data at L-band and lower frequencies. By model inversion, some widely used FR angle estimators have been proposed for compensation and provide a new field in high-resolution ionospheric soundings. However, as an integrated product of electron density and the parallel component of the magnetic field, FR angle measurements/observations demonstrate the ability to characterize horizontal ionosphere. In order to make a general study of ionospheric structure, this paper reconstructs the electron density distribution based on a modified two-dimensional computerized ionospheric tomography (CIT) technique, where the FR angles, rather than the total electron content (TEC), are regarded as the input. By using the full-pol (full polarimetric) data of Phase Array L-band Synthetic Aperture Radar (PALSAR) on board Advanced Land Observing Satellite (ALOS), International Reference Ionosphere (IRI) and International Geomagnetic Reference Field (IGRF) models, numerical simulations corresponding to different FR estimators and SAR scenes are made to validate the proposed technique. In simulations, the imaging of kilometer-scale ionospheric disturbances, a spatial scale that is rarely detectable by CIT using GPS, is presented. In addition, the ionospheric reconstruction using SAR polarimetric information does not require strong point targets within a SAR scene, which is necessary for CIT using SAR imaging information. Finally, the effects of system errors including noise, channel imbalance and crosstalk on the reconstruction results are also analyzed to show the applicability of CIT based on spaceborne full-pol SAR data.
Highlights
Due to the dispersive nature of ionosphere and the existence of Earth’s magnetic field, the polarization rotation of a linearly polarized wave will occur after traveling through the ionosphere
Error Analysis of the Proposed computerized ionospheric tomography (CIT) Technique In order to analyze the effects of system errors on the proposed CIT individually, the semi-physical simulations using synthetic data of calibrated Phase Array L-band Synthetic Aperture Radar (PALSAR) full-pol data, International Reference Ionosphere (IRI) and International Geomagnetic Reference Field (IGRF) models are required
We can see that when the all system errors are considered, the true ionospehric distribution can still be accurately reconstructed based on the proposed CIT technique, and the two small-scale disturbances are clearly visible
Summary
Due to the dispersive nature of ionosphere and the existence of Earth’s magnetic field, the polarization rotation of a linearly polarized wave will occur after traveling through the ionosphere. An autofocus algorithm is applied here to iteratively search the change of FM rate, which can further be used to derive the TEC value [22] It can provide a high resolution reconstruction, the autofocus algorithm is insensitivity to TEC because of the limitation of small bandwidth for current low-frequency spaceborne SAR systems, e.g., the ALOS Phase Array L-band Synthetic Aperture Radar (PALSAR) [23,24]. It requires strong point targets with high signal-to-clutter (SCR) ratio in a SAR scene [22,25].
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