Evaluation of Several Filtering and Unwrapping Methods for the Interferometric Imaging Radar Altimeter

Radar altimeters using interferometric synthetic aperture radar (InSAR) techniques at near-nadir angles have been proposed to measure elevations of the ocean and the inland water with wide swath and high spatial resolution. The Ku-band Interferometric Imaging Radar Altimeter (InIRA) on the Tiangong-2 space laboratory, which was launched on 15 September 2016, is the first spaceborne instrument of such kind. Another similar instrument is the forthcoming Ka-band Radar Interferometer (KaRIn) of the Surface Water and Ocean Topography (SWOT) mission. In the interferometric processings, which have to be done to derive the elevations with the interferometric altimeter, the phase filtering and the phase unwrapping are two key steps. There are many filtering and unwrapping methods for conventional InSAR published in literature, but very few for interferometric altimeters. In this paper, several common filtering and unwrapping methods were evaluated with real InIRA acquisitions over three different scenes, including an open ocean, a coastal area, and a land with a mountain. The phase filtering was evaluated by the pseudo-correlation coefficient, the residue number, and the Root Mean Square (RMS) of the difference between the filtered phase and the original phase. The phase unwrapping was evaluated by the statistics of the difference between the rewrapped phase and the input wrapped phase, the e value, and the discontinuity number. The evaluation results were shown and some recommendations were made on the method selection and the parameter setting for interferometric altimeters.