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 ε 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.
KeywordsTiangong-2 Altimetry Interferometry Phase filtering Phase unwrapping Synthetic aperture radar
Thanks to China Manned Space Engineering for providing space science and application data products. Thanks to Xiao Dong and Yunhua Zhang in the Key Laboratory of Microwave Remote Sensing, Chinese Academy of Sciences, for their discussions on the instrument design and for their help in generating the complex images.
- 2.Esteban-Fernandez, D., Fu, L.L., Rodriguez, E., Brown, S., Hodges, R.: Ka-band SAR interferometry studies for the SWOT mission. In: Proceedings of the IEEE IGARSS, pp. 4401–4402. IEEE, Honolulu (2010)Google Scholar
- 3.Moller, D., Farquharson, G., Esteban-Fernandez, D.: Assessment of near-nadir correlation characteristics over water bodies using interferometric SAR: Implications for the swot mission. In: Proceedings of the IEEE IGARSS, pp. 3219–3222. IEEE, Beijing (2016)Google Scholar
- 5.Ren, L., Yang, J., Zhang, Y., Zhai, W., Zheng, G., Wang, J.: Exploring for the wind speed retrieval from Interferometric Imaging Radar Altimeter. In: Proceedings of the IEEE IGARSS, pp. 4637–4640. IEEE, Beijing (2016)Google Scholar
- 6.Fu L., Alsdorf D., Morrow R., Rodríguez E. and Mognard N.: SWOT: The Surface Water and Ocean Topography mission: Wide-swath altimetric measurement of water elevation on Earth. Jet Propulsion Lab., NASA Technical Reports Server, Pasadena (2012)Google Scholar
- 8.Bao, Q., Lin, M., Zhang, Y., Jia, Y., Lang, S.: Wind speed inversion for imaging microwave altimeter. J. Remote Sens. 21, 835–841 (2017)Google Scholar
- 9.Fu, L.-L., Rodriguez, E.: High-resolution measurement of ocean surface topography by Radar Interferometry for Oceanographic and Geophysical Applications. In: The State of the Planet: Frontiers and Challenges in Geophysics, 1st edn. American Geophysical Union, Washington, D. C. (2004)Google Scholar
- 10.Durand, M., Fu, L.L., Lettenmaier, D.P., Alsdorf, D.E., Rodriguez, E., Esteban-Fernandez, D.: The surface water and ocean topography mission: observing terrestrial surface water and oceanic submesoscale eddies. In: Proceedings of the IEEE IGARSS, pp. 766–779. IEEE, Honolulu (2010)CrossRefGoogle Scholar
- 13.Jin, G., Xu, Q., Zhang, H.: Synthetic Aperture Radar Interferometry, 1st edn. National Defense Industry Press, Beijing, China (2014)Google Scholar