{"status":"ok","message-type":"work","message-version":"1.0.0","message":{"indexed":{"date-parts":[[2026,2,27]],"date-time":"2026-02-27T15:46:19Z","timestamp":1772207179827,"version":"3.50.1"},"reference-count":44,"publisher":"MDPI AG","issue":"12","license":[{"start":{"date-parts":[[2021,6,20]],"date-time":"2021-06-20T00:00:00Z","timestamp":1624147200000},"content-version":"vor","delay-in-days":0,"URL":"https:\/\/creativecommons.org\/licenses\/by\/4.0\/"}],"funder":[{"DOI":"10.13039\/501100012166","name":"National Key Research and Development Program of China","doi-asserted-by":"publisher","award":["2016YFC1401004"],"award-info":[{"award-number":["2016YFC1401004"]}],"id":[{"id":"10.13039\/501100012166","id-type":"DOI","asserted-by":"publisher"}]},{"name":"Zhejiang Provincial Natural Science Foundation of China","award":["LGF21D060002"],"award-info":[{"award-number":["LGF21D060002"]}]},{"name":"Scientific Research Fund of the Second Institute of Oceanography, Ministry of Natural Resources of China","award":["JG1708"],"award-info":[{"award-number":["JG1708"]}]}],"content-domain":{"domain":[],"crossmark-restriction":false},"short-container-title":["Remote Sensing"],"abstract":"<jats:p>The interferometric imaging radar altimeter (InIRA) aboard the Chinese Tiangong-2 space laboratory is the first spaceborne imaging radar working at low incidence angles. This study focuses on the retrieval of significant wave heights (SWHs) from InIRA data. The retrieved SWHs can be used for correcting the sea state bias of InIRA-derived sea surface heights and can supplement SWH products from other spaceborne sensors. First, we analyzed tilt, range bunching and velocity bunching wave modulations at low incidence angles, and we found clear dependencies between the SWH and two defined factors, range and azimuth integration, for ocean waves in the range and azimuth directions, respectively. These dependencies were further confirmed using InIRA measurements and collocated WaveWatch III (WW3) data. Then, an empirical orthogonal SWH model using the range and azimuth integration factors as model inputs was proposed. The model was segmented by the incidence angle, and the model coefficients were estimated by fitting the collocation at each incidence angle bin. Finally, the SWHs were retrieved from InIRA data using the proposed model. The retrievals were validated using both WW3 and altimeter (JASON2, JASON3, SARAL, and HY2A) SWHs. The validation with WW3 data shows a root mean square error (RMSE) of 0.43 m, while the average RMSE with all traditional altimeter data is 0.48 m. This indicates that the InIRA can be used to measure SWHs.<\/jats:p>","DOI":"10.3390\/rs13122413","type":"journal-article","created":{"date-parts":[[2021,6,20]],"date-time":"2021-06-20T21:50:15Z","timestamp":1624225815000},"page":"2413","update-policy":"https:\/\/doi.org\/10.3390\/mdpi_crossmark_policy","source":"Crossref","is-referenced-by-count":13,"title":["Preliminary Significant Wave Height Retrieval from Interferometric Imaging Radar Altimeter Aboard the Chinese Tiangong-2 Space Laboratory"],"prefix":"10.3390","volume":"13","author":[{"ORCID":"https:\/\/orcid.org\/0000-0003-1121-8980","authenticated-orcid":false,"given":"Lin","family":"Ren","sequence":"first","affiliation":[{"name":"State Key Laboratory of Satellite Ocean Environment Dynamics, Second Institute of Oceanography, Ministry of Natural Resources, Hangzhou 310012, China"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"ORCID":"https:\/\/orcid.org\/0000-0002-7514-3212","authenticated-orcid":false,"given":"Jingsong","family":"Yang","sequence":"additional","affiliation":[{"name":"State Key Laboratory of Satellite Ocean Environment Dynamics, Second Institute of Oceanography, Ministry of Natural Resources, Hangzhou 310012, China"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"given":"Xiao","family":"Dong","sequence":"additional","affiliation":[{"name":"National Space Science Center, Chinese Academy of Sciences, Beijing 100190, China"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"given":"Yongjun","family":"Jia","sequence":"additional","affiliation":[{"name":"National Satellite Ocean Application Service, Ministry of Natural Resources, Beijing 100081, China"}],"role":[{"role":"author","vocabulary":"crossref"}]},{"ORCID":"https:\/\/orcid.org\/0000-0001-8125-9425","authenticated-orcid":false,"given":"Yunhua","family":"Zhang","sequence":"additional","affiliation":[{"name":"National Space Science Center, Chinese Academy of Sciences, Beijing 100190, China"}],"role":[{"role":"author","vocabulary":"crossref"}]}],"member":"1968","published-online":{"date-parts":[[2021,6,20]]},"reference":[{"key":"ref_1","doi-asserted-by":"crossref","first-page":"321","DOI":"10.1111\/j.1365-246X.1995.tb05714.x","article-title":"The accuracy and applications of satellite altimetry","volume":"121","author":"Shum","year":"1995","journal-title":"Geophys. J. Int."},{"key":"ref_2","doi-asserted-by":"crossref","first-page":"67","DOI":"10.1109\/JOE.1977.1145328","article-title":"The average impulse response of a rough surface and its applications","volume":"2","author":"Brown","year":"1977","journal-title":"IEEE J. Ocean. Eng."},{"key":"ref_3","doi-asserted-by":"crossref","first-page":"687","DOI":"10.1109\/TAP.1980.1142398","article-title":"Radar altimeter mean return waveforms from near nominal-incidence ocean surface scattering","volume":"28","author":"Hayne","year":"1980","journal-title":"IEEE Trans. Antennas Propag."},{"key":"ref_4","doi-asserted-by":"crossref","unstructured":"Hauser, D., Tourain, C., and Hermozo, L. (2020). New observations from the SWIM radar on-board CFOSAT: Instrument validation and ocean wave measurement assessment. IEEE Trans. Geosci. Remote Sens., 1\u201322.","DOI":"10.1109\/TGRS.2020.2994372"},{"key":"ref_5","doi-asserted-by":"crossref","first-page":"987","DOI":"10.1029\/JC090iC01p00987","article-title":"Aircraft and satellite measurement of ocean wave directional spectra using scanning-beam microwave radars","volume":"90","author":"Jackson","year":"1985","journal-title":"J. Geophys. Res. Ocean."},{"key":"ref_6","doi-asserted-by":"crossref","first-page":"981","DOI":"10.1109\/36.175333","article-title":"RESSAC: A new airborne FM\/CW radar ocean wave spectrometer","volume":"30","author":"Hauser","year":"1992","journal-title":"IEEE Trans. Geosci. Remote Sens."},{"key":"ref_7","doi-asserted-by":"crossref","first-page":"421","DOI":"10.1175\/1520-0426(2001)018<0421:SARART>2.0.CO;2","article-title":"SWIMSAT: A real aperture radar to measure directional spectra of ocean waves from space\u2014Main characteristics and performance simulation","volume":"18","author":"Hauser","year":"2001","journal-title":"J. Atmos. Ocean. Technol."},{"key":"ref_8","first-page":"1","article-title":"Satellite oceanography from the ERS synthetic aperture radar and radar altimeter: A brief review","volume":"1326","author":"Johannessen","year":"2013","journal-title":"Eur. Space Agency"},{"key":"ref_9","doi-asserted-by":"crossref","first-page":"10713","DOI":"10.1029\/91JC00302","article-title":"On the nonlinear mapping of an ocean wave spectrum into a synthetic aperture radar image spectrum and its inversion","volume":"96","author":"Hasselmann","year":"1991","journal-title":"J. Geophys. Res. Ocean."},{"key":"ref_10","doi-asserted-by":"crossref","first-page":"1047","DOI":"10.1109\/36.406690","article-title":"SAR-ocean wave inversion using image cross spectra","volume":"33","author":"Engen","year":"1995","journal-title":"IEEE Trans. Geosci. Remote Sens."},{"key":"ref_11","doi-asserted-by":"crossref","first-page":"682","DOI":"10.1007\/BF02995562","article-title":"Wave and wind retrieval from SAR images of the ocean","volume":"56","author":"Chapron","year":"2001","journal-title":"Ann. Des T\u00e9l\u00e9commun."},{"key":"ref_12","first-page":"10182","article-title":"An empirical approach for the retrieval of integral ocean wave parameters from synthetic aperture radar data","volume":"42","author":"Konig","year":"2007","journal-title":"J. Geophys. Res."},{"key":"ref_13","doi-asserted-by":"crossref","first-page":"155","DOI":"10.1109\/TGRS.2010.2052364","article-title":"Ocean wave integral parameter measurements using Envisat ASAR wave mode data","volume":"49","author":"Li","year":"2011","journal-title":"IEEE Trans. Geosci. Remote Sens."},{"key":"ref_14","doi-asserted-by":"crossref","first-page":"1827","DOI":"10.1002\/2016JC012364","article-title":"Significant wave heights from Sentinel-1 SAR: Validation and applications","volume":"122","author":"Stopa","year":"2017","journal-title":"J. Geophys. Res. Ocean."},{"key":"ref_15","doi-asserted-by":"crossref","unstructured":"Wang, H., Wang, J., Yang, J., Ren, L., Zhu, J., and Yuan, X. (2018). Empirical algorithm for significant wave height retrieval from wave mode data provided by the Chinese satellite Gaofen-3. Remote Sens., 10.","DOI":"10.3390\/rs10030363"},{"key":"ref_16","doi-asserted-by":"crossref","first-page":"122","DOI":"10.1007\/s13131-017-1010-6","article-title":"The first quantitative ocean remote sensing by using Chinese interferometric imaging radar altimeter onboard TG-2","volume":"36","author":"Yang","year":"2017","journal-title":"Acta Oceanol. Sin."},{"key":"ref_17","doi-asserted-by":"crossref","first-page":"4993","DOI":"10.1029\/JB091iB05p04993","article-title":"Topographic mapping from interferometric SAR observations","volume":"91","author":"Zebker","year":"1986","journal-title":"J. Geophys. Res. Atmos."},{"key":"ref_18","doi-asserted-by":"crossref","first-page":"1803","DOI":"10.1080\/01431169608948741","article-title":"SAR interferometry-issues, techniques, and applications","volume":"17","author":"Gens","year":"1996","journal-title":"Int. J. Remote Sens."},{"key":"ref_19","first-page":"2349","article-title":"Spaceborne imaging altimeter for topographic mapping","volume":"5","author":"Zhang","year":"2000","journal-title":"IEEE Geosci. Remote Sens. Symp."},{"key":"ref_20","first-page":"190","article-title":"Design and preliminary experiment of China imaging altimeter","volume":"4894","author":"Zhang","year":"2003","journal-title":"SPIE Microw. Remote Sens. Atmos. Environ. III"},{"key":"ref_21","unstructured":"Zhang, Y., Zhang, X., Xin, M., Luo, W., and Jiang, J. (2007). An interferometric imaging altimeter applied for both ocean and land observation. IEEE Geosci. Remote Sens. Symp., 3821\u20133824."},{"key":"ref_22","doi-asserted-by":"crossref","first-page":"4718","DOI":"10.1109\/JSTARS.2018.2879753","article-title":"Sea surface wind speed retrieval and validation of the interferometric imaging radar altimeter aboard the Chinese Tiangong-2 space laboratory","volume":"11","author":"Ren","year":"2018","journal-title":"IEEE J. Sel. Top. Appl. Earth Obs. Remote Sens."},{"key":"ref_23","doi-asserted-by":"crossref","first-page":"016012","DOI":"10.1117\/1.JRS.10.016012","article-title":"Wind speed retrieval from Ku-band tropical rainfall mapping mission precipitation radar data at low incidence angles","volume":"10","author":"Ren","year":"2016","journal-title":"J. Appl. Remote Sens."},{"key":"ref_24","doi-asserted-by":"crossref","unstructured":"Dong, X., Zhang, Y., and Zhai, W. (2017). Design and algorithms of the Tiangong-2 interferometric imaging radar altimeter processor. Prog. Electromagn. Res. Symp., 3802\u20133803.","DOI":"10.1109\/PIERS.2017.8262420"},{"key":"ref_25","doi-asserted-by":"crossref","unstructured":"Ren, L., Yang, J., Dong, X., Zhang, Y., and Jia, Y. (2020). Preliminary Evaluation and Correction of Sea Surface Height from Chinese Tiangong-2 Interferometric Imaging Radar Altimeter. Remote Sens., 12.","DOI":"10.3390\/rs12152496"},{"key":"ref_26","doi-asserted-by":"crossref","unstructured":"Zhang, Y., Dong, X., Shi, X., Zhai, W., Yang, Q., Li, D., Kang, X., and Jiang, J. (2018). Demonstration of ocean target detection by Tiangong-2 interferometric imaging radar altimeter. Int. Microw. Radar Conf., 261\u2013264.","DOI":"10.23919\/MIKON.2018.8405194"},{"key":"ref_27","doi-asserted-by":"crossref","first-page":"829","DOI":"10.1029\/90JC02319","article-title":"The effect of the degree of wave development on the sea state bias in radar altimetry measurement","volume":"96","author":"Fu","year":"1991","journal-title":"J. Geophys. Res. Ocean."},{"key":"ref_28","doi-asserted-by":"crossref","first-page":"24981","DOI":"10.1029\/94JC01430","article-title":"Estimating the sea state bias of the TOPEX and POSEIDON altimeters from crossover differences","volume":"99","author":"Gaspar","year":"1994","journal-title":"J. Geophys. Res. Ocean."},{"key":"ref_29","doi-asserted-by":"crossref","first-page":"24995","DOI":"10.1029\/94JC02113","article-title":"The sea state bias in altimeter estimates of sea level from collinear analysis of TOPEX data","volume":"99","author":"Chelton","year":"1994","journal-title":"J. Geophys. Res. Ocean."},{"key":"ref_30","doi-asserted-by":"crossref","first-page":"108","DOI":"10.1007\/s13131-017-1116-x","article-title":"Nonparametric estimations of the sea state bias for a radar altimeter","volume":"36","author":"Miao","year":"2017","journal-title":"Acta Oceanol. Sin."},{"key":"ref_31","doi-asserted-by":"crossref","unstructured":"Reale, F., Dentale, F., Carratelli, E., and Fenoglio-Marc, L. (2018). Influence of sea state on sea surface height oscillation from doppler altimeter measurements in the north sea. Remote Sens., 10.","DOI":"10.3390\/rs10071100"},{"key":"ref_32","doi-asserted-by":"crossref","unstructured":"Reale, F., Carratelli, E., Leo, A., and Dentale, F. (2020). Wave orbital velocity effects on radar doppler altimeter for sea monitoring. J. Mar. Sci. Eng., 8.","DOI":"10.3390\/jmse8060447"},{"key":"ref_33","unstructured":"NOAA National Centers for Environmental Prediction and National Weather Service (2018, March 15). WAVEWATCH III Wave Model 2009, Pacific Islands Ocean Observing System. Available online: https:\/\/pae-paha.pacioos.hawaii.edu\/erddap\/griddap\/ww3_global.html."},{"key":"ref_34","unstructured":"NOAA National Geophysical Data Center (2018, March 15). ETOPO1 1 Arc-Minute Global Relief Model 2009. NOAA National Centers for Environmental Information, Available online: https:\/\/ngdc.noaa.gov\/mgg\/global\/global.html."},{"key":"ref_35","doi-asserted-by":"crossref","first-page":"838","DOI":"10.1364\/JOSA.44.000838","article-title":"Measurement of the roughness of the sea surface from photographs of the suns glitter","volume":"44","author":"Cox","year":"1954","journal-title":"J. Opt. Soc. Am."},{"key":"ref_36","doi-asserted-by":"crossref","first-page":"61","DOI":"10.1007\/BF00913863","article-title":"Theories for the interaction of electromagnetic and oceanic waves\u2014A review","volume":"13","author":"Valenzuela","year":"1978","journal-title":"Bound. Layer Meteorol."},{"key":"ref_37","doi-asserted-by":"crossref","first-page":"9803","DOI":"10.1029\/93JC03373","article-title":"Estimation of the ocean wave-radar modulation transfer function from synthetic aperture radar imagery","volume":"99","author":"Schmidt","year":"1994","journal-title":"J. Geophys. Res. Atmos."},{"key":"ref_38","doi-asserted-by":"crossref","first-page":"6481","DOI":"10.1029\/JC086iC07p06481","article-title":"On the detectability of ocean surface waves by real and synthetic aperture radar","volume":"86","author":"Alpers","year":"1981","journal-title":"J. Geophys. Res. Ocean."},{"key":"ref_39","doi-asserted-by":"crossref","first-page":"1369","DOI":"10.1002\/2016JC012425","article-title":"Sun glitter imagery of ocean surface waves. part 1: Directional spectrum retrieval and validation","volume":"122","author":"Kudryavtsev","year":"2017","journal-title":"J. Geophys. Res. Ocean."},{"key":"ref_40","doi-asserted-by":"crossref","first-page":"7833","DOI":"10.1029\/97JC01579","article-title":"Analysis of ERS-1\/2 Synthetic Aperture Radar wave mode imagettes","volume":"103","author":"Kerbaol","year":"1998","journal-title":"J. Geophys. Res."},{"key":"ref_41","doi-asserted-by":"crossref","first-page":"C08013","DOI":"10.1029\/2005JC003048","article-title":"Dual-polarized c- and ku-band ocean backscatter response to hurricane-force winds","volume":"111","author":"Fernandez","year":"2006","journal-title":"J. Geophys. Res. Atmos."},{"key":"ref_42","doi-asserted-by":"crossref","first-page":"272","DOI":"10.1175\/JTECH-D-13-00006.1","article-title":"High-resolution hurricane vector winds from C-band dual-polarization SAR observations","volume":"31","author":"Zhang","year":"2014","journal-title":"J. Atmos. Ocean. Technol."},{"key":"ref_43","doi-asserted-by":"crossref","first-page":"4501","DOI":"10.1109\/TGRS.2016.2543502","article-title":"Application of AMSR-E and AMSR2 low-frequency channel 2 brightness temperature data for hurricane wind retrievals","volume":"54","author":"Mai","year":"2016","journal-title":"IEEE Trans. Geosci. Remote Sens."},{"key":"ref_44","doi-asserted-by":"crossref","unstructured":"Li, X., Zhang, B., Alexis, M., He, Y., and William, P. (2017). Ku-band sea surface radar backscatter at low incidence angles under extreme wind conditions. Remote Sens., 9.","DOI":"10.3390\/rs9050474"}],"container-title":["Remote Sensing"],"original-title":[],"language":"en","link":[{"URL":"https:\/\/www.mdpi.com\/2072-4292\/13\/12\/2413\/pdf","content-type":"unspecified","content-version":"vor","intended-application":"similarity-checking"}],"deposited":{"date-parts":[[2025,10,11]],"date-time":"2025-10-11T06:19:39Z","timestamp":1760163579000},"score":1,"resource":{"primary":{"URL":"https:\/\/www.mdpi.com\/2072-4292\/13\/12\/2413"}},"subtitle":[],"short-title":[],"issued":{"date-parts":[[2021,6,20]]},"references-count":44,"journal-issue":{"issue":"12","published-online":{"date-parts":[[2021,6]]}},"alternative-id":["rs13122413"],"URL":"https:\/\/doi.org\/10.3390\/rs13122413","relation":{},"ISSN":["2072-4292"],"issn-type":[{"value":"2072-4292","type":"electronic"}],"subject":[],"published":{"date-parts":[[2021,6,20]]}}}