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With the continuous development of the China ocean dynamic environment satellite series (Haiyang-2, HY-2), it is urgent to explore the potential application of HY-2B in Arctic sea ice thickness retrievals. In this study, we first derive the Arctic radar freeboard and sea ice thickness during two cycles (from October 2019 to April 2020 and from October 2020 to April 2021) using the HY-2B radar altimeter and compare the results with the Alfred Wegener Institute (AWI) CryoSat-2 (CS-2) products. We evaluate our HY-2B sea ice freeboard and thickness products using Operation IceBridge (OIB) airborne data and Ice, Cloud, and land Elevation Satellite 2 (ICESat-2) products. Finally, we estimate the uncertainties in the HY-2B sea ice freeboard and sea ice thickness. Here, we derive the radar freeboard by calculating the difference between the relative elevation of the floe obtained by subtracting the mean sea surface (MSS) height and sea surface height anomaly (SSHA) determined by an average of the 15 lowest points method. The radar freeboard deviation between HY-2B and CS-2 is within 0.02 m, whereas the sea ice thickness deviation between HY-2B and CS-2 is within 0.2 m. The HY-2B radar freeboards are generally thicker than AWI CS-2, except in spring (March and April). A spring segment likely has more floe points than an early winter segment. We also find that the deviations in radar freeboard and sea ice thickness between HY-2B and CS-2 over multiyear ice (MYI) are larger than those over first-year ice (FYI). The correlation between HY-2B (CS-2) sea ice freeboard retrievals and OIB values is 0.77 (0.84), with a root mean square error (RMSE) of 0.13 (0.10) m and a mean absolute error (MAE) of 0.12 (0.081) m. The correlation between HY-2B (CS-2) sea ice thickness retrievals and OIB values is 0.65 (0.80), with an RMSE of 1.86 (1.00) m and an MAE of 1.72 (0.75) m. The HY-2B sea ice freeboard uncertainty values range from 0.021 to 0.027 m, while the uncertainties in the HY-2B sea ice thickness range from 0.61 to 0.74 m. The future work will include reprocessing the HY-2B L1 data with a dedicated sea ice retracker, and using the radar waveforms to directly identify leads to release products that are more reasonable and suitable for polar sea ice thickness retrieval.

In the context of global warming, sea ice changes have received increasing attention as "indicators" and "amplifiers" of climate change. With the development of satellite altimeters, satellite altimeter technologies have been increasingly used to retrieve Arctic sea ice thicknesses and have achieved rapid development and application. At present, the CryoSat-2 radar altimeter and Ice, Cloud and land Elevation Satellite-2 (ICESat-2) laser altimeter are the main data sources used in Arctic sea ice thickness retrievals. With the continuous development of the China Ocean Dynamic Environment Satellite Series (HY-2), it is of great significance to explore the potential application of this dataset in Arctic sea ice thickness retrievals. In this study, we first estimated the Arctic radar freeboard and sea ice thickness values during two sea ice growing cycles (from October 2019 to April 2020 and from October 2020 to April 2021) using the China HY-2B radar altimeter and then compared the results with the Alfred Wegener Institute (AWI) CryoSat-2 sea ice freeboard and sea ice thickness products recorded during the same period. The accuracies of the HY-2B radar freeboard and sea ice thickness were then verified with the Operation IceBridge (OIB) airborne data and ICESat-2 laser altimeter data, and the random uncertainties in the HY-2B sea ice freeboard and sea ice thickness results were finally estimated. Although the spatial distributions of the HY-2B radar freeboard and sea ice thickness results agreed well with those of AWI CryoSat-2, the deviation between the HY-2B radar freeboard and CryoSat-2 radar freeboard data was within 2 cm, while the deviation between the HY-2B sea ice thickness data and CryoSat-2 sea ice thickness data was within 0.2 m. In addition, the growth trends of the HY-2B radar freeboard and sea ice thickness were slower than those of AWI CryoSat-2. This finding was related to the applied sea surface height anomaly (SSHA) extraction method. Comparisons with the OIB sea ice freeboard and sea ice thickness values recorded in April 2019 showed that the correlation between the HY-2B sea ice freeboard retrievals and OIB sea ice freeboard data was 0.58, the root mean square error (RMSE) was 0.17 m, and the mean absolute error (MAE) was 0.14 m. The correlation between the HY-2B sea ice thickness retrieval and OIB sea ice thickness data was 0.41, the RMSE was 2.05 m, and the MAE was 1.91 m. Based on the Gaussian error propagation theory, we estimated the uncertainties of the HY-2B sea ice freeboard and sea ice thickness data: the uncertainty of the former ranged from 8.5 cm to 12.0 cm, while the uncertainty of the latter ranged from 26.8 cm to 37.7 cm. Due to the influence of the SSHA uncertainty (σSSA) and the number of observation points inside the grid, the uncertainties in the HY-2B sea ice freeboard and sea ice thickness data were higher at low latitudes than at high latitudes.

Sea surface current is a research hotspot in oceanography. Space-borne along-track interferometric synthetic aperture radar (along-track InSAR, ATI) is a promising sensor for measuring high-resolution sea surface current field, and there is no operational system in orbit yet. To support future space-borne ATI systems, based on the ATI experimental mode of the Gaofen-3 (GF-3) satellite, the first sea surface current observing experiment was conducted in the Jiaozhou Gulf in China, in 2019. Meanwhile, SAR observations and in situ instrument measurements of the current are obtained in the experiments. The data is first preprocessed by a processor specially developed for the GF-3 ATI data. Then, the current is extracted based on the M4S mode. The retrieved current of the Jiaozhou Gulf is compared with ground-based high frequency surface wave radar data. The results show that the root mean square error of the surface current observed by the GF-3 satellite is less than 0.2 m/s.

A microwave range transponder has been operating at the CDN1 Cal/Val site on the mountains of Crete for about 6 years to calibrate international satellite radar altimeters at the Ku-band. This CDN1 transponder is part of the European Space Agency Permanent Facility for Altimetry Calibration (PFAC), and has been producing continuously as of 2015 a time series of range biases for Sentinel-3A, Sentinel-3B, Sentinel-6A, Jason-2, Jason-3 and CryoSat-2. A second range and sigma-0 transponder at a crossover location of Sentinel-6 “Michael Freilich” on the island of Gavdos is also operational as of 11-Oct-2021. This work presents a thorough examination of the transponder Cal/Val responses in an effort to understand and determine which factors are responsible for the observed changes in the Cal/Val results. Fourteen uncontrolled and three controlled sources of variations have been investigated. The latest calibration results for the Jason-3, the Copernicus Sentinel-3A and -3B, Sentinel-6A and the HY-2B radar altimeters will be described based on four sea-surface and two transponder Cal/Val sites of the PFAC in west Crete, Greece. Absolute biases for Jason-3, Sentinel-6A, Sentinel-3A, Sentinel-3B and HY-2B will be given using various techniques, infrastructure and settings.

The HaiYang-2B (HY-2B) altimeter satellite has been launched by the Chinese National Satellite Ocean Application Service (NSOAS) on 24 October 2018, to monitor the marine dynamic environment. In this work, HY-2B observations have been assessed by the Permanent Facility for Altimetry Calibration in Crete, Greece and in the context of the Dragon research initiative of ESA. This action supports Earth observation research activities between European and Chinese institutes. At first, a preliminary analysis of the HY-2B Geophysical Data Records revealed minor inconsistencies between the 1-Hz and 20-Hz products, as in the Net Instrument Correction and the Ku-band range. These have been reported to and immediately corrected by NSOAS, resulting in a consistent release of the HY-2B data. Two years of altimetric observations of HY-2B have been calibrated using two sea-surface Cal/Val sites in Crete. Results will be given for two Cal/Val sites: the CRS1 site in the south west tip of Crete, where the descending Pass No. 66 is calibrated, and the RDK1 site in the central south Crete used for the calibration of its ascending pass No. 161. In addition, relative calibration of HY-2B has also been carried out around Crete with reference to Jason-3, Sentinel-3A and Sentinel-3B. Finally, an assessment of the HY-2B’s microwave radiometer has been performed by comparing radiometric tropospheric delays with tropospheric delays derived by the ECMWF model and the permanent GNSS stations in Crete.