## 6 Conclusions  
This paper proposed an empirical model to retrieve SWHs from the InIRA aboard the Chinese Tangong-2 space laboratory. The data used include the InIRA and collocated WW3 and traditional altimeters (JASON2, JASON3, SARAL, and HY2A).  
First, the tilt, range bunching and velocity bunching modulations at low incidence angles were analyzed. From the analysis, the clear dependences between the SWH and two factors, range and azimuth integration, were found for waves in the range and azimuth directions, respectively. This type of dependence was further confirmed by the InIRA data. Finally, an orthogonal model using the two integration factors as model inputs was proposed. The model was segmented by the incidence angle and the model coefficients were estimated by fitting the collocations.  
SWHs were retrieved from the InIRA data using the proposed model. These retrievals were validated using the collocated WW3 and traditional altimeter SWHs. Through the validation, the InIRA SWH was found to have an RMSE of 0.43 m compared with the WW3 SWH, and an average RMSE of 0.48 m compared with all traditional altimeter data. These comparisons revealed the potential of InIRA for measuring the SWH.  
The main objective of the InIRA in oceanic applications is to measure the wide-swath SSH. In future work, the SWH retrievals from the proposed method will be used to correct the sea state bias of InIRA-derived SSH measurements.  
Conceptualization, L.R., J.Y. and Y.Z.; methodology, L.R., X.D. and Y.J.; validation, L.R., X.D. and Y.J.; writing--original draft, L.R.; writing--review and editing, L.R., J.Y., X.D., Y.Z. and Y.J. All authors have read and agreed to the published version of the manuscript.  
This research was funded in part by the National Key Research and Development Program of China (grant no. 2016 YFC1401004), the Zhejiang Provincial Natural Science Foundation of China (grant no. LGF21D060002), the Scientific Research Fund of the Second Institute of Oceanography, Ministry of Natural Resources of China (grant no. JG1708) and Southern Marine Science and Engineering Guangdong Laboratory (Zhuhai).  
Not applicable.  
The InIRA data and HY2A traditional altimeter data are from National Satellite Ocean Application Service. The former is copied by disk, while the latter is downloaded by ftp ([[ftp://osdds-ftp.nsoas.org.cn](ftp://osdds-ftp.nsoas.org.cn)]([ftp://osdds-ftp.nsoas.org.cn](ftp://osdds-ftp.nsoas.org.cn)), accessed on 15 March 2018, and the account registration instructions can be found on this link: [[https://osdds.nsoas.org.cn](https://osdds.nsoas.org.cn)]([https://osdds.nsoas.org.cn](https://osdds.nsoas.org.cn)), accessed on 15 March 2018). The WaveWatch III (WW3) global wave model data are from the Pacific Islands Ocean Observing System ([[https://pae-phaa.pacios.hawaii.edu/erddap/gridap/www3_global.html](https://pae-phaa.pacios.hawaii.edu/erddap/gridap/www3_global.html)]([https://pae-phaa.pacios.hawaii.edu/erddap/gridap/www3_global.html](https://pae-phaa.pacios.hawaii.edu/erddap/gridap/www3_global.html)), accessed on 15 March 2018). The JASON2, JASON3, and SARAL traditional altimeter data are from AVISO ([[ftp://avisoftp.cnes.fr/AVISO/pub/](ftp://avisoftp.cnes.fr/AVISO/pub/)]([ftp://avisoftp.cnes.fr/AVISO/pub/](ftp://avisoftp.cnes.fr/AVISO/pub/)), accessed on 15 March 2018). The ETOPO1 digital elevation model data are from NOAA's National Centers for Environmental Information ([[https://ngdc.noaa.gov/mgg/global/global.html](https://ngdc.noaa.gov/mgg/global/global.html)]([https://ngdc.noaa.gov/mgg/global/global.html](https://ngdc.noaa.gov/mgg/global/global.html)), accessed on 15 March 2018).  
The authors would like to thank the anonymous reviewers for their valuable comments which helped us improve the manuscript, and thank SOED/SIO/MNR satellite ground station and satellite data-processing center for their help with data processing.  
The authors declare no conflict of interest.