TEC and Ionospheric Height Estimation by Means of Azimuth Subaperture Analysis in Quad-Polarimetric Spaceborne SAR Data

The total electron content (TEC) is probably the most important single parameter to characterize the ionospheric state and its impact on synthetic aperture radar (SAR) data. Its accurate estimation is therefore essential when it comes to the correction and calibration of SAR data distorted by the ionosphere. In recent years, the estimation of Faraday rotation (FR), i.e., the rotation of the polarimetric plane of the transmitted and scattered EM pulses as they propagate through the ionosphere, has become a key element of TEC estimation. FR is proportional to TEC and to the line-of-sight (LOS) component of the geomagnetic field. The availability of quad-polarimetric SAR data allows the precise estimation of FR, and thus TEC. However, an accurate TEC estimation requires knowledge of the geomagnetic field that varies with height. In this sense, the knowledge of the ionospheric height becomes an important issue for a correct FR to TEC conversion. While the estimation performance of FR is well understood, our understanding of the ionospheric height estimation by means of SAR is yet not established. In this article, a new estimator that allows the simultaneous estimation of TEC and ionospheric height from FR measurement is proposed. It exploits the variation of the parallel-to-LOS component of the geomagnetic field across azimuth sublooks. The performance of the proposed estimator is demonstrated using ALOS-2/PALSAR-2 data. The achieved estimation accuracy, precision, and the prerequisite for an accurate estimation are discussed.