IEEE TRANSACTIONS ON GEOSCIENCE AND REMOTE SENSING, VOL. 53, NO. 12, DECEMBER 2015 6319 Correcting Distortion of Polarimetric SAR Data Induced by Ionospheric Scintillation Jun Su Kim, Konstantinos P. Papathanassiou, Fellow, IEEE, Rolf Scheiber, and Shaun Quegan, Member, IEEE Abstract—A correction methodology for distortions induced by ionospheric scintillation on fully polarimetric synthetic aperture radar (SAR) data is proposed. The correction is based on deriving the phase distortion induced by the ionosphere from Faraday rotation estimates. The estimated phase distortion is then used for correction. In order to compensate the phase and time–Doppler history distortions, the correction has to be performed at the slant range of the ionospheric layer, i.e., on partially focused single-look complex data. Accordingly, the performance of the proposed correction methodology depends, among other factors, on knowledge of the altitude of the effective ionospheric layer (assuming the thin ionospheric layer model). Its estimation from the SAR data itself is therefore also addressed. The methodology was applied and validated on simulated P-band data for various ionospheric conditions and on real L-band data acquired by the Advanced Land Observation Satellite Phased Array L-band SAR (PALSAR). Index Terms—Faraday effect, ionosphere, polarimetry, space- borne radar, synthetic aperture radar (SAR). I. I NTRODUCTION T HE ionosphere is the upper part of the Earth’s atmosphere where solar radiation ionizes gas molecules and atoms sufficiently to affect the propagation of radio waves [1]. Elec- tron density significantly varies with altitude as a result of the competition between photochemical, collisional, and diffusion processes. A large fraction of the ionization is concentrated in a relatively narrow layer (known as the F2-layer [2, p. 1]) whose peak value is usually located at an altitude between 250 and 400 km. For the purpose of propagation calculations, the iono- sphere is often approximated by a thin layer whose spatial variation is characterized by the integrated value of the electron density along a given direction, known as the total electron con- tent (TEC). TEC varies on spatial scales extending from a few meters to thousands of kilometers [3]. Small-scale ionospheric irregularities caused by particle precipitation and plasma in- stabilities induce scintillations [4] and are common features of the polar and post-sunset equatorial ionosphere [4]. Larger Manuscript received August 1, 2014; revised December 23, 2014 and April 1, 2015; accepted April 22, 2015. This work was supported in part by the European Space Agency BIOMASS mission project. J. S. Kim, K. P. Papathanassiou, and R. Scheiber are with the Mi- crowaves and Radar Institute, German Aerospace Center (DLR-HR), Wessling 82234, Germany (e-mail: junsu.kim@dlr.de; Kostas.papathanassiou@dlr.de; rolf.scheiber@dlr.de). S. Quegan is with the Centre for Terrestrial Carbon Dynamics, The Univer- sity of Sheffield, Sheffield S3 7RH, U.K. (e-mail: S.Quegan@sheffield.ac.kr). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TGRS.2015.2431856 scale ionospheric features, which we will regard as constituting the background ionosphere, extend to global scale and are regulated by solar radiation and auroral precipitation, together with photochemical and transport processes [2]. Note that, in the context of synthetic aperture radar (SAR) data correction, the relevant scales of spatial variation in the ionosphere are with respect to the length of the synthetic aperture at the height of the ionospheric layer, which is usually on the order of several kilometers depending on radar frequency and imaging mode. Due to the presence of free electrons in the ionosphere, pulses transmitted by the SAR and backscattered by a scat- terer experience phase advance and group delay [5]. This can cause several types of distortion in the SAR image, including defocusing and/or spatial shifts in both range and azimuth [6], [7]. In addition, in the presence of the Earth’s magnetic field, the ionosphere is anisotropic, and only circularly polarized waves propagate through it without changing their polarization (although at different velocities for the right and left circular waves). This effect is known as Faraday rotation (FR) [2], [8]–[10]. In the case of repeat-pass interferometric SAR, the individual images are distorted by the different ionospheric conditions at each acquisition time; if not accounted for, this difference induces a loss of interferometric coherence and/or a distortion of the interferometric phase [9], [11]–[13]. Ionospheric distortions become stronger as frequency de- creases, so are more critical for low-frequency spaceborne SAR [5], [11] implementations. Nonetheless, a number of low-frequency spaceborne SAR missions operating at L- and P-bands are in space or planned to be launched in the next decade. These include the ESA BIOMASS mission for global forest biomass mapping, which is the first-ever spaceborne mission operating at P-band (435-MHz center frequency) in a fully polarimetric mode [14]. At L-band, Japan Aerospace Exploration Agency’s (JAXA) Advanced Land Observation Satellite-2 (ALOS-2) mission [15] launched in 2014 and the first of the two SAOCOM (CONAE/ASI) SARs is expected to be placed in orbit in 2016 [16]. Other planned/proposed L-band SAR missions include the NISAR (NASA/ISRO) [17] and TanDEM-L of the German Aerospace Center (DLR) [18]. All of them will be, to some degree, affected by distortions induced by ionosphere irregularities; hence, appropriate mit- igation approaches need to be developed. These can include selection of a dawn/dusk orbit that minimizes the effects [19], but correction may be still needed along some parts of the orbit, such as in the auroral zones. In this paper, a methodology to correct amplitude and phase distortions of focused SAR images induced by scintillations in the azimuth direction is proposed. While gradients in the 0196-2892 © 2015 EU