The Chinese new-generation ocean color observation satellite (HY-3A), launched on November 16, 2023, carries the 2nd Chinese Ocean Color and Temperature Scanner (COCTS2) as its primary sensor. COCTS2 offers a swath width of over 3000 km, a spatial resolution of 500 m, and 15 reflective solar bands (RSBs) spanning 360 to 1640 nm. This study reviews COCTS2 intrinsic characterization and radiometric performance during its first year on-orbit and assesses the accuracy of global ocean color products. It demonstrates a high signal-to-noise ratio (SNR) under typical ocean radiance, outperforming specification values by a significant margin of 50%. Meanwhile, the image non-uniformity is better than 1%, and the linear response remains close to 1 within the radiance dynamic range under oceanic ocean color conditions. Notably, COCTS2 is the first Chinese sensor equipped with an onboard calibration system, achieving solar calibration accuracy within 2%. Through solar calibration, we established a radiometric degradation model that maintains stable performance throughout nine months, with radiometric degradation across RSBs below 1%. Subsequently, we derived absolute calibration gain factors using cross calibration approaches, achieving consistency with the solar calibration gain factors. On this basis, we established a relative calibration gain factors by vicarious calibration, and after application, the radiometric accuracy was better than 95%. Ultimately, in validating global ocean color products, we compare COCTS2-derived remote sensing reflectance (Rrs) with Sentinel-3A OLCI and AERONET-OC data, confirming the high quality of the ocean color products. And compared with PACE OCI, although there is a certain deviation in ultraviolet band, it still has a high correlation.

China’s marine dynamic satellite constellation was formed in 2021 with the launch of HY-2D, with existing HY-2C and HY-2B. The SSH (Sea Surace Height) calibration by tide-gauges, buoys as well as range calibration utilizing transponders have been performed extensively. In order to correlate the SSH and range calibration in one experiment, search the association between the two biases, furtherly, to compile the range bias obtained by transponders into the altimeters data products, a novel calibration method was developed utilizing a coastal transponder. In the new method, the altimeter works at normal SSH tracking mode, and the transponder captures the pulses instantaneously, transmits them back entering the altimeters receiving windows. From the altimeter spectra, the correlation between the SSH and the transponder echoes are achieved, and the two biases could be validated. In this paper, all the China’s coastal tracks of HY-2B/C/D were examined, and four sites were selected. In March and August in 2023, two calibration missions were carried out. Besides the range bias, the SSH validation was analyzed for the first time. The new method shows potentials in combing the transponder and the sea surface methods. At present, some new calibration techniques based on the signal-rebuilt transponder are in research, involving the fully-focused SAR processing and the absolute measurements of ranges and the instrument resolutions. These new methods are expected to serve for the next advanced altimeters which would be launched in a few years.

The construction of a radiometric degradation model is vital for elucidating the performance alterations of ocean color satellite sensors following in-orbit operations. The Chinese ocean color satellites HaiYang-1C (HY1C) and HaiYang-1D (HY1D), operating as afternoon and morning satellites, respectively, conduct networked observations of the global ocean. However, there has been limited analysis of the radiometric performance of each ocean color sensor on HY1C/D. Therefore, to understand the performance change sensor, this study relies on the Satellite Calibration Spectrometer (SCS), a hyperspectral sensor mounted on HY1C/D. The radiometric degradation model of each sensor is constructed, to realize the tracking of the long-term radiometric performance trend of the sensors. First, the SCS ensures the accuracy of its observations through a high-precision solar calibration method. Using the abundant simultaneous earth observation data gathered by the SCS, this study realizes the radiometric calibration of each sensor, and the radiometric degradation model is further constructed to adjust the radiometric level and track and correct the sensor performance trend. The Chinese Ocean Color and Temperature Scanner (COCTS) is taken as an example, and the results show that the annual average radiometric degradation rate of COCTS-HY1C after launch ranges from 0.8% yr−1to 2.4% yr−1, while that of COCTS-HY1D ranges from 0.5% yr−1to 3.9% yr−1. These degradations were achived by the SCS that allowed the sensors onboard HY1C/D to perform in-orbit radiometric calibration independently of external data sources and continuously generate high-quality Climate Data Records (CDRs).

Surface waves investigation and monitoring (SWIM) can provide global wave spectra, but under small sea conditions, the presence of parasitic peaks at low wavenumbers, surfboard effects, and residual speckle noise lead to performance degradation of SWIM wave height spectrum products. To reduce the impacts of the above factors on the SWIM wave height spectrum, in this article, a convolution neural network (CNN) method based on BU-Net is proposed for calibrating SWIM omnidirectional wave height spectra with buoy measurements under sea states (wind wave mainly/swell mainly) and sea surface conditions (wind speed from 9 to 19 m/s, and significant wave height (SWH) from 0.8 to 3.4–4.2 m). The calibration results show that the impact of the above factors on the SWIM omnidirectional wave height spectrum can be corrected. The correlation coefficients between the corrected SWIM beams 6°, 8°, and 10° and the buoy mean omnidirectional wave height spectrum are all greater than 0.90, and the relative error of the peak wavenumber is within 10%. The relative error of the integrated energy is mostly less than 20%. In addition, the performance of spectral integration parameters (effective wave height$H_{s}$, and energy wave period$T_{m-10}$) of each spectral beam of SWIM has been verified using Meteo-France WAve Model (MFWAM) reanalysis data. The validation results show that RMSE of$H_{s}$and$T_{m-10}$, for the corrected SWIM beam 6° (8°, 10°) under wind wave sea conditions are 0.31 m (0.32, 0.25 m) and 0.50 s (0.51, 0.49 s), respectively; those for swell cases are 0.16 m (0.16, 0.13 m) and 0.87 s (0.77, 0.72 s), respectively.

Scatterometer wind data are critical for meteorological and oceanographic applications. The differences between scatterometer winds and buoy reference winds largely depend on the type of reference wind used in the fitting of the scatterometer’s geophysical model function (GMF). This study evaluates scatterometer winds using advanced buoy reference winds, specifically stress-equivalent winds (U10S), and equivalent-neutral winds (U10N). We utilized HSCAT-B scatterometer wind products retrieved using both NSCAT-4 and NSCAT-5 GMFs). NSCAT-4 winds were corrected to stress-equivalent winds, and these scatterometer winds were compared with various buoy reference winds, including true buoy winds, stress-equivalent winds, and equivalent-neutral winds. The results show that in extratropical regions, stress-equivalent winds provided a closer match to scatterometer winds, while in tropical regions, equivalent-neutral winds exhibited smaller errors. Scatterometer winds derived from the NSCAT-5 GMF demonstrated superior consistency with buoy reference winds, further reducing wind speed biases compared to NSCAT-4. An extended triple collocation (ETC) analysis was conducted to address the uncertainties arising from differences in spatial resolution between scatterometer and buoy measurements. The findings emphasize the importance of using appropriate wind correction methods for scatterometer wind validation, particularly in regions with significant atmospheric variability.

The HY-2A satellite, launched on October 16, 2011, is the first dynamic environments satellite in China, carrying the radar altimeter, the radar scatterometer and microwave radiometer. The orbit error is the most error sours of the measurement of the sea surface height for altimeter observations. Therefore the HY-2A satellite is equipped with GPS, Doppler Orbitography by Radiopositioning Integrated by Satellite (DORIS), and Satellite Laser Ranging (SLR) tracking system in order to realize the 10cm accuracy goal for precise orbit determination (POD). In this paper the method and strategies of DORIS, GPS and DORIS/GPS solution are introduced for HY-2A satellite POD system, which is in dynamic, induced-dynamic and induced-dynamic methods respectively. The two orbit performance assessment tools of SLR validation and independent orbit comparison are to be applied to verify the Medium accuracy Orbit Ephemeris(MOE) radial accuracy calculated by DORIS, GPS and DORIS/GPS and DORIS CNES) solutions for HY-2A satellite system. Finally the verification results are presented in this paper.

Since August 2011, the HY-2 satellite has been launched successfully, and it has provided almost continuous wave height measurements for more than 4 years. Before using these data, the measurements need to be validated. Based on the in-situ buoys from the National Data Buoy Center (NDBC) and Jason-2 altimeter, the HY-2 Ku-band significant wave height (SWH) measurements were validated and have been corrected using a linear regression with in-situ measurements. Compared with NDBC SWH, the validation results of HY-2 SWH show a RMS (Root Mean Square) of 0.33 m, which is similar to Jason-1 and Jason-2 data with the RMS of 0.35 m and 0.37 m respectively; the RMS of corrected HY-2 SWH is 0.30 m, similar to 0.27 m and 0.23 m of corrected Jason-1 and Jason-2 data. Therefore the accuracy of HY-2 SWH products is close to that of Jason-1/2 SWH data, and the linear regression function derived can improve the accuracy of HY-2 SWH products.

The HY-2A launched in August 2011 is the first satellite for ocean dynamic environment measurement. The calibration of HY-2A altimeter is performed using cross-calibration methods with Jason-2 mission. Range absolute calibration activities is ongoing based on transponders. The main parameters in IGDR products are validated by in-suit NDBC buoys and Jason-2 measurements. The calibration of HY-2A scatterometer is implemented using open sea measurements, Amazon rainforest measurements and transponders. Wind vectors products are validated by NDBC buoy observations. The key results are as follows: The accuracy of the sea surface height is about 7.0cm by crossover analysis of HY-2A alone. Dual-crossover analysis with Jason-2 altimeter shows HY-2A altimeter total performances is close to the Jason-2. The calibration coefficients of sigma0 for scatterometer are 1.7dB for VV and HH polarization using ocean calibration technique, the monitoring result of the scatterometer measurement stability by Amazon rainforest shows that the instrument system is stable, the RMS of wind speed and wind direction retrieved by scatterometer are about 1.19m/s and 18.74°.