Correction of Ionospheric Effects on SAR Interferometry using an Combined TEC Estimator

Abstract

Owing to the deep penetration into vegetation and low temporal decorrelation, the low-frequency spaceborne SAR observations are suited for global forest mapping and InSAR applications over natural terrains. For these reason, missions including JAXA’s ALOS-II, NASA’s DESDynI, DLR’s Tandem-L (heretofore L-band) and ESA’s BIOMASS (P-band) have been scheduled to be launched in the next years or proposed. These low-frequency missions products, however, requires compensation of the disturbing effects induced by the ionosphere. In the ionosphere the group velocity of the transmitting/receiving pulses delays, their phase(s) advances, and polarisation state rotates. On the processed SAR images, shifts and blurring of focusing are introduced due to the inhomogeneiety of the ionosphere. They distort intensity, as well as polarimetric, interferometric and polarimetric interferometric observation spaces. In SAR applications, the ionosphere can be characterized using the total electron content (TEC), which is defined as the integrated electron number density per unit volume along the direction of propagation. Most of the free electrons are distributed within a relatively narrow altitude range allowing the application of thin layer model at a fixed altitude. Then the ionosphere is a 2-D scalar field of TEC [1], [2]. We propose a concept integrating the detections of the ionospheric disturbing effects. In accordance, a novel generic correction schema enabling the use of the combined (and improved) estimates of the 2-D TEC field (or the associated differential TEC field in the interferometric case) is also proposed and assesses by means of the interferometric coherence [3]. As a special case, a combined 2-D differential TEC field estimator based on the differential Faraday rotation measurements and the relative azimuth shifts is presented. Both observations (Faraday rotation and azimuth shift) are independent. A linear inverse problem for the (differential) TEC estimation can be established. Geophysical knowledge, e.g. the anisotropic nature of the TEC distribution, has been incorporated as a priori information in the “combined” (differential) TEC estimator. In addition the method to estimate the ionospheric altitude from SAR data is also presented. The performance of the proposed approach is tested using ALOS quad-pol interferometric data sets over Alaska. The achieved estimates are characterized by an improved performance: While the Faraday rotation-based estimator suffers from the random deviation pattern of TEC after conversion, the proposed combined estimator effectively is free of such artifacts. References [1] F. Meyer and J. Nicoll, “Prediction, detection, and correction of Faraday rotation in full-polarimetric L-band SAR data”, IEEE Trans. Geoscience and Remote Sensing, vol. 46, no. 10, October 2008. [2] X. Pi, A. Freeman, B. Champman, P. Rosen, and Z. Li, “Imaging ionospheric inhomogeneities using spaceborne synthetic aperture radar”, Journal of Geophysical Research, vol. 116, A04303, 2001. [3] J. S. Kim, K. Papathanassiou, S. Quegan, and N. Rogers, “Estimation and correction of scintillation effects on spaceborne P-band SAR images”, Proc. of IGARSS 2012, pp. 5101-3104, 23-37. July 2012.