Abstract

Ionospheric Faraday rotation distorts satellite radar observations of the Earth's surface. While its impact on radiometric observables is well understood, the errors in repeat-pass interferometric observables and hence in geodetic deformation analysis are largely unknown. Because the Faraday-induced errors cannot rigorously be compensated for in non-quadpol systems, it is imperative to determine their magnitude and nature. Focusing on distributed targets at L-band, we combine theoretical and empirical analyses for a range of land covers using airborne observations with simulated Faraday rotation. We find that the typical deformation error is up to 2 mm in the co-pol channels but may exceed 5 mm for intense solar maxima. The cross-pol channel is more susceptible to severe errors. We identify the leakage of polarimetric phase contributions into the interferometric phase as a dominant error source. The polarimetric scattering characteristics induce a systematic dependence of the Faraday-induced deformation errors on land cover and topography. Also their temporal characteristics, with pronounced seasonal and quasi-decadal variability, predispose these systematic errors to be misinterpreted as deformation. While the relatively small magnitude of 1--2 mm is of limited concern in many applications, the persistence on semi- to multi-annual time scales compels attention when long-term deformation is to be estimated with millimetric accuracy. Phase errors induced by uncompensated Faraday rotation constitute an important and hitherto neglected error in interferometric deformation measurements.