Estimation of ionospheric Faraday rotation over ocean areas using L-band spaceborne PolSAR data

L-band spaceborne polarimetric synthetic aperture radar (PolSAR) data has been well used for the high-resolution retrieval of the ionospheric Faraday rotation (FR). A vast majority of these retrievals were, however, conducted on PolSAR data of lands, with only a very few over oceans. For the global imaging of the Earth's ionosphere, the retrieval of oceanic ionosphere is indispensable. This paper aims to furnish a deep insight into the FR retrieval on oceans using the widely used Bickel-Bates estimator based on the two cross-circular polarizations ${Z_{12}}$Z12 and ${Z_{21}}$Z21. The FR estimates (FRE) on oceans are evaluated in detail by comparing with those on lands on four ALOS PALSAR datasets. The achieved oceanic FRE are shown to possess comparable or even better quality than those on lands. It occurs in three of the four scenes that the selected ocean sub-area holds a smaller standard deviation (STD) of FRE (less than 0.7000 degrees) than all the selected land sub-areas. Especially, the quality of FRE is shown to be closely linked to the dispersed degree of the magnitude of the conjugate product of ${Z_{12}}$Z12 and ${Z_{21}}$Z21 via the analysis of coefficient of variation (CV), according to which the unusual occurrence of larger magnitude level corresponding to larger STD of FRE can be explained. Nevertheless, compared with CV, the magnitude of the polarimetric coherence between ${Z_{12}}$Z12 and ${Z_{21}}$Z21 (i.e. $\left| \gamma \right|$gamma) is identified as a better index to measure the quality of FRE. A smaller STD of FRE appears along with a larger $\left| \gamma \right|$gamma. Three of the four scenes show that the selected ocean sub-area bears a larger mean of $\left| \gamma \right|$gamma (greater than 0.9890) in comparison with all the selected land sub-areas. This work can promote the ionospheric research over oceans and to the correction of ionospheric effects in oceanic L-band spaceborne polarimetric radar data as well.