Networking Observation and Applications of Chinese Ocean Satellites

This paper presents the networking observation capabilities of Chinese ocean satellites and their diverse applications in ocean disaster prevention,ecological monitoring,and resource development.Since the inaugural launch in 2002,China has achieved substantial advancements in ocean satellite technology,forming an observation system composed of the HY-1,HY-2,and HY-3 series satellites.These satellites are integral to global ocean environmental monitoring due to their high resolution,extensive coverage,and frequent observations.Looking forward,China aims to further enhance and expand its ocean satellite capabilities through ongoing projects to support global environmental protection and sustainable development.

Current Status and Main Application Achievements of Ocean Satellites

Ocean satellites have realized multi-satellite networked operation.The HY-1D satellite launched in June 2020 realized networked with HY-1C satellite,and completed the construction of ocean color satellite constellation.The HY-2D satellite launched in May 2021 is networked with the on orbit HY-2B and HY-2C satellites to complete the construction of marine dynamic environment satellite constellation.The 1 mC-SAR satellite 01 launched in November 2021 is networked with GF-3,which initially forms the marine monitoring satellite constellation.This year,the networking of 1 mC-SAR satellite 02 with satellite 01 and GF-3 is realized,and the construction of marine monitoring satellite constellation is completed.At present,the ocean satellites have the operational application capabilities of remote sensing investigation,monitoring,evaluation and supervision of marine ecology,marine disaster prevention and reduction,global oceans and Polar Regions,Sea Islands,rights and interests maintenance.

Applications of China's Ocean satellites in 2009

Considerable progress was achieved in China's ocean satellite missions in 2009. The ground application system for ocean satellites operates well,and the applications of ocean satellites have been further extended. The HY-1B satellite operates stably in orbit.We received 2018 orbits of measurements from the HY-1B satellite during 2009 and the accumulated raw data reached 8472GB;2778 orbits of data from the EOS/MODIS satellite

The Development Status and Main Application Progress of China’s Ocean Satellites

China’s ocean satellites are divided into three series based on ocean color satellites(HY-1),ocean dynamic environment satellites(HY-2)and ocean monitoring satellites(HY-3).The three series of ocean satellites operate today in a multi-satellite network.The HY-1 D satellite launched in June 2020 and the HY-1 C satellite,already in orbit,realized a network observation capability and completed the formation of the ocean color satellite constellation.The HY-2 D satellite launched in May 2021 joined the HY-2 B and HY-2 C satellites,which have been on orbit already and completed a network observation capability,thus establishing the ocean dynamic environment satellite constellation.The GF-302 satellite(1 m C-SAR 01)launched in November 2021 has networked with GF-3,initially establishing an ocean monitoring satellite constellation,which has finally completed its construction with the launch of the GF-303 satellite(1 m C-SAR 02)in April 2022.The GF-3 three-satellite network effectively boasts a wide capability in applications of satellite data products and services in many fields,such as ocean environmental monitoring,ocean disaster prevention and mitigation,marine scientific research and polar research.

FY-3A/MERSI, ocean color algorithm, products and demonstrative applications

A medium resolution spectral imager （MERSI） on-board the first spacecraft of the second generation of Chinas polar-orbit meteorological satellites FY-3A, is a MODIS-like sensor with 20 bands covering visible to thermal infrared spectral region. FY-3A/MERSI is capable of making continuous global observations, and ocean color application is one of its main targets. The objective is to provide information about the ocean color products of FY-3A/MERSI, including sensor calibration, ocean color algorithms, ocean color prod- ucts validation and applications. Although there is a visible on-board calibration device, it cannot realize the on-board absolute radiometric calibration in the reflective solar bands. A multisite vicarious calibration method is developed, and used for monitoring the in-flight response change and providing post-launch cal- ibration coefficients updating. FY-3A/MERSI ocean color products consist of the water-leaving reflectance retrieved from an atmospheric correction algorithm, a chlorophyll a concentration （CHL1） and a pigment concentration （PIG1） from global empirical models, the chlorophyll a concentration （CHL2）, a total sus- pended mater concentration （TSM） and the absorption coefficient of CDOM and NAP （YS443） from Chi- na＇s regional empirical models. The atmospheric correction algorithm based on lookup tables and ocean color components concentration estimation models are described. By comparison with in situ data, the FY-3A/MERSI ocean color products have been validated and preliminary results are presented. Some suc- cessful ocean color applications such as algae bloom monitoring and coastal suspended sediment variation have demonstrated the usefulness of FY-3A/MERSI ocean color products.

China-France Oceanography Satellite (CFOSAT) simultaneously observes the typhoon-induced wind and wave fields

The China-France oceanography satellite(CFOSAT)developed by the China National Space Administration(CNSA)and Centre National D’Etudes Spatiales(CNES)was successfully launched into its orbit on October 29,2018.The Chinese wind scatterometer(SCAT)with swath width of about 1000 km and French wave spectrometer(Surface Wave Investigation and Monitoring,SWIM)with swath width of about 180 km onboard the CFOSAT are in line with all requirements and performing operationally.Thus,it is the first time that CFOSAT provides simultaneous and co-located observations of wind and wave fields with high spatial resolutions of 12.5 km×12.5 km for the winds and 70 km×90 km for the wave directional spectrum.The real-time and large-scale monitoring of wind and wave fields are of great significance for navigation and human activities on the sea(Xu et al.,2010;Tan et al.,2018;Sun et al.,2019),especially during severe typhoon processes when violent winds and hazardous waves occur(Walsh et al.,2002;Zhou et al.,2008).

Ocean Observation from Haiyang Satellites: 2012–2014

During 2012 and 2014, China has two Haiyang(which means ocean in Chinese, referred to as HY) satellites operating normally in space which are HY-1B and HY-2A. HY-1B is an ocean color environment satellite which was launched in April 2007 to observe global ocean color and sea surface temperature, and HY-2A is an ocean dynamic environment satellite which was launched in August 2011 to obtain global marine dynamic environment parameters including sea surface height,significant wave height, ocean wind vectors, etc. Ocean observation data provided by HY-1B and HY-2A have been widely used by both domestic and international users in extensive areas such as ocean environment protection, ocean disaster prevention and reduction, marine environment forecast,ocean resource development and management, ocean investigations and scientific researches, etc.

Results About Ocean Satellite Observation

Ocean observing satellites have become an important part of the China's three-dimensional marine observation system.They have played more and more important roles in marine pollution monitoring,marine environment and marine disaster monitor and forecasting,marine resource investigation and marine scientific research.In this paper,the author will give a brief review of China's operational and scientific activities in satellites ocean observation during 2008 to 2010.These activities include the application of the HY-1B for red tide and green tide detecting and monitoring,sea ice monitoring,fishery resources assessment at coastal zone and ocean,marine water quality assessment,sea surface temperature monitoring and forecasting,ocean primary productivity and carbon cycle research,and the development in the ocean dynamic environment parameter application technology before launch of HY-2.The achievements we have made in the ocean observation and monitoring during the past 3 years indicate that,the development of the ocean satellite series and the application of the ocean satellite can greatly promote the development of China's three-dimensional marine observation system and earth-observation system from space,accordingly we can improve the ability of acquiring the marine environmental parameter to sustain the marine economy development and the sea defense construction.

Ocean Observation from Haiyang Satellites

In 2018,China successfully launched three new Haiyang(which means ocean in Chinese,referred to as HY)satellites which are an ocean color observation satellite HY-1C(operational),an ocean dynamics environment satellite HY-2B(operational)and the China-France ocean satellite CFOSAT(experimental).In 2019,all the three satellites had finished their commissioning phases and were declared operational.HY-2A satellite continues to operate in-orbit,and its operational status is basically normal.So in 2020,China has 4 Haiyang satellites in-orbit,China’s ocean satellites enter into a new operational application phase.The operation of the ground application system of Chinese ocean satellites is stable.In 2019,Beijing,Hainan,Mudanjiang,and Hangzhou ocean satellite ground stations had received the data of HY-1C,HY-2A,HY-2B,and CFOSAT 5012 orbits and 26.46 TB data had been distributed to both domestic and international users.Chinese ocean satellite data has played an important role in marine disaster prevention and mitigation,development and management of marine resources,maintenance of marine rights and interests,marine environment protection,scientific researches,and blue economy development.

Assimilation of Ocean Surface Wind Data by the HY-2B Satellite in GRAPES: Impacts on Analyses and Forecasts

The ocean surface wind(OSW)data retrieved from microwave scatterometers have high spatial accuracy and represent the only wind data assimilated by global numerical models on the ocean surface,thus playing an important role in improving the forecast skills of global medium-range weather prediction models.To improve the forecast skills of the Global/Regional Assimilation and Prediction System Global Forecast System(GRAPES_GFS),the HY-2B OSW data is assimilated into the GRAPES_GFS four-dimensional variational assimilation(4DVAR)system.Then,the impacts of the HY-2B OSW data assimilation on the analyses and forecasts of GRAPES_GFS are analyzed based on one-month assimilation cycle experiments.The results show that after assimilating the HY-2B OSW data,the analysis errors of the wind fields in the lower-middle troposphere(1000-600 hPa)of the tropics and the southern hemisphere(SH)are significantly reduced by an average rate of about 5%.The impacts of the HY-2B OSW data assimilation on the analysis fields of wind,geopotential height,and temperature are not solely limited to the boundary layer but also extend throughout the entire troposphere after about two days of cycling assimilation.Furthermore,assimilating the HY-2B OSW data can significantly improve the forecast skill of wind,geopotential height,and temperature in the troposphere of the tropics and SH.

An assessment of arctic sea ice concentration retrieval based on “HY-2” scanning radiometer data using field observations during CHINARE-2012 and other satellite instruments

A retrieval algorithm of arctic sea ice concentration （SIC） based on the brightness temperature data of “HY-2” scanning microwave radiometer has been constructed. The tie points of the brightness temperature were selected based on the statistical analysis of a polarization gradient ratio and a spectral gradient ratio over open water （OW）, first-year ice （FYI）, and multiyear ice （MYI） in arctic. The thresholds from two weather filters were used to reduce atmospheric effects over the open ocean. SIC retrievals from the “HY-2” radiom-eter data for idealized OW, FYI, and MYI agreed well with theoretical values. The 2012 annual SIC was calcu-lated and compared with two reference operational products from the National Snow and Ice Data Center （NSIDC） and the University of Bremen. The total ice-covered area yielded by the “HY-2” SIC was consistent with the results from the reference products. The assessment of SIC with the aerial photography from the fifth Chinese national arctic research expedition （CHINARE） and six synthetic aperture radar （SAR） images from the National Ice Service was carried out. The “HY-2” SIC product was 16% higher than the values de-rived from the aerial photography in the central arctic. The root-mean-square （RMS） values of SIC between “HY-2” and SAR were comparable with those between the reference products and SAR, varying from 8.57% to 12.34%. The “HY-2” SIC is a promising product that can be used for operational services.

Validation of Chinese HY-2 satellite radar altimeter significant wave height

Chinese Haiyang-2（HY-2） satellite is the first Chinese marine dynamic environment satellite. The dual-frequency （Ku and C band） radar altimeter onboard HY-2 has been working effective to provide operational significant wave height （SWH） for more than three years （October 1, 2011 to present）.We validated along-track Ku-band SWH data of HY-2 satellite against National Data Buoy Center （NDBC） in-situ measurements over a time period of three years from October 1, 2011 to September 30, 2014, the root mean square error （RMSE） and mean bias of HY-2 SWH is 0.38 m and （-0.13±0.35） m, respectively. We also did cross validation against Jason-2 altimeter SWH data, the RMSE and the mean bias is 0.36m and （-0.22±0.28） m, respectively. In order to compare the statistical results between HY-2 and Jason-2 satellite SWH data, we validated the Jason-2 satellite radar altimeter along-track Ku-band SWH data against NDBC measurements using the same method. The results demonstrate the validation method in this study is scientific and the RMSE and mean bias of Jason-2 SWH data is 0.26 m and （0.00±0.26） m, respectively. We also validated both HY-2 and Jason-2 SWH data every month, the mean bias of Jason-2 SWH data almost equaled to zero all the time, while the mean bias of HY-2 SWH data was no less than -0.31m before April 2013 and dropped to zero after that time. These results indicate that the statistical results for HY-2 altimeter SWH are reliable and HY-2 altimeter along-track SWH data were steady and of high quality in the last three years. The results also indicate that HY-2 SWH data have greatly been improved and have the same accuracy with Jason-2 SWH data after April, 2013. SWH data provided by HY-2 satellite radar altimeter are useful and acceptable for ocean operational applications.

Ocean color products retrieval and validation around China coast with MODIS

Waters along China coast are very turbid with high concentrations of suspended sediment nearly all the time,especially at the Hangzhou Bay,the Changjiang （Yangtze） River Estuary and the shoal along Jiangsu Province.In these turbid and optically complex waters,the standard MODIS ocean color products tend to have invalid values.Because the water-leaving radiances in the near-infrared （NIR） are significant resulting from the strong scattering of suspended particles,the standard MODIS atmospheric correction algorithm often gets no results or produces significant errors.And because of the complex water optical properties,the OC3 model used in the standard MODIS data processing tends to get extremely high chlorophyll-a （Chl-a） concentrations.In this paper,we present an atmospheric correction approach using MODIS short wave infrared （SWIR） bands based on the fact that water-leaving radiances are negligible in the SWIR region because of the extreme strong absorption of water even in turbid waters.A regional Chl-a concentration estimation model is also constructed for MODIS from in situ data.These algorithms are applied to MODIS Aqua data processing in the China coastal regions.In situ data collected in the Yellow Sea and the East China Sea in spring and autumn,2003 are used to validate the performance.Reasonably good results have been obtained.It is noted that water-leaving reflectance in the NIR bands are significant in waters along the China coast with high sediment loadings.The satellite derived and in-situ reflectance spectra can match in the turbid waters along China coast,and there is relatively good linear relationship between satellite derived and in-situ reflectance.The RMSE value of Rrs（λ） is 0.0031 sr ？1 for all the nine ocean color bands （412 to 869 nm）.The satellite-derived Chl-a value is in the reasonable range and the root mean square percentage difference is 46.1%.

Assessment of the initial sea surface temperature product of the scanning microwave radiometer aboard on HY-2 satellite

HY-2 satellite is the first satellite for dynamic environmental parameters measurement of China,which was launched on 16th August 2011.A scanning microwave radiometer（RM） is carried for sea surface temperature（SST）,sea surface wind speed,columnar water vapor and columnar cloud liquid water detection.In this paper,the initial SST product of RM was validated with in-situ data of National Data of Buoy Center（NDBC） mooring and Argo buoy.The validation results indicate the accuracy of RM SST is better than 1.7 C.The comparison of RM SST and WindSat SST shows the former is warmer than the latter at high sea surface wind speed and the difference between these SSTs is depend on the sea surface wind speed.Then,the relationship between the errors of RM SST and sea surface wind speed was analyzed using NDBC mooring measurements.Based on the results of assessment and errors analysis,the suggestions of taking account of the affection of sea surface wind speed and using sea surface wind speed and direction derived from the microwave scatteromter aboard on HY-2 for SST product calibration were given for retrieval algorithm improvement.

Satellite SAR observation of the sea surface wind field caused by rain cells

Rain cells or convective rain,the dominant form of rain in the tropics and subtropics,can be easy detected by satellite Synthetic Aperture Radar（SAR） images with high horizontal resolution.The footprints of rain cells on SAR images are caused by the scattering and attenuation of the rain drops,as well as the downward airflow.In this study,we extract sea surface wind field and its structure caused by rain cells by using a RADARSAT-2 SAR image with a spatial resolution of 100 m for case study.We extract the sea surface wind speeds from SAR image by using CMOD4 geophysical model function with outside wind directions of NCEP final operational global analysis data,Advance Scatterometer（ASCAT） onboard European Met Op-A satellite and microwave scatterometer onboard Chinese HY-2 satellite,respectively.The root-mean-square errors（RMSE） of these SAR wind speeds,validated against NCEP,ASCAT and HY-2,are 1.48 m/s,1.64 m/s and 2.14 m/s,respectively.Circular signature patterns with brighter on one side and darker on the opposite side on SAR image are interpreted as the sea surface wind speed（or sea surface roughness） variety caused by downdraft associated with rain cells.The wind speeds taken from the transect profile which superposes to the wind ambient vectors and goes through the center of the circular footprint of rain cell can be fitted as a cosine or sine curve in high linear correlation with the values of no less than 0.80.The background wind speed,the wind speed caused by rain cell and the diameter of footprint of the rain cell with kilometers or tens of kilometers can be acquired by fitting curve.Eight cases interpreted and analyzed in this study all show the same conclusion.

Measurement analyses and evaluations of sea-level heights using the HY-2A satellite’s radar altimeter

The HY-2A satellite is China’s first independent oceanic dynamic environmental satellite,and has been operating continuously for more than six years.The satellite’s radar altimeter,which is one of the main loads on the satellite,has the ability to realize all-weather and all-day observations of global sea-surface heights,as well as significant wave heights and sea-surface wind speeds.These observed data have been widely used in marine disaster prevention and reduction,along with resource development,maritime security and other fields.In order to achieve a comprehensive understanding of the multi-year overall observational performances of the HY-2A satellite’s radar altimeter,all of the observational data of the IGDR product from October 26,2012 to August 27,2017 were selected in this study for a comprehensive evaluation.The height measurement capability of the HY-2A satellite’s radar altimeter was evaluated using self-crossover and Jason-2 crossover methods.The height discrepancies at the self-crossover point of the HY-2A satellite’s ascending and descending orbits were also calculated.It was found that for the HY-2A satellite’s radar altimeter in global waters under the restriction conditions of ascending and descending orbits,the height anomaly differences were within a range of less than 30 cm.The absolute mean error was determined to be 5.81 cm,and the height anomaly standard deviation was 7.76 cm.Under the conditions of the observational areas being limited within a scope of 60°from the Equator,it was determined that the sea-level height anomaly differences were less than 10 cm at the junction of the ascending and descending orbits,the absolute mean error was 3.95 cm.In addition,the sea-level height anomaly standard deviation was observed to be 4.76cm.Using a mutual cross method with the Jason-2 satellite,it was found that under the conditions of the observational area being within the scope of 66°from the equator,the height anomaly differences at the junction were less than 30cm,and the absolute mean error of HY-2A and Jason-2 sea level height anomaly was 5.86 cm,with a standard deviation of 7.52 cm.It was observed that,if within the sea area the sea level height anomaly difference was limited to within 10cm,then the absolute mean error and standard deviation could reach 4.19cm and 4.98cm,respectively.It was confirmed that the HY-2A satellite’s radar altimeter had successfully reached the height measurement level of similar international altimeters.Therefore,it had the ability to meet the needs of marine scientific research and ocean circulation inversions.

An improved method of absolute calibration to satellite altimeter: A case study in the Yellow Sea, China

An improved absolute calibration technology based on indirect measurements was developed through two probative experiments, the performance of which was evaluated by applying the approach to in situ sea surface height （SSH） at the Tianheng Island （tidal gauge） and the satellite nadir （GPS buoy）. Using Geoid/MSS （mean sea surface） data, which accounted for a constant offset between nadir and onshore tidal gauge water levels, and TMD （tidal model driver）, which canceled out the time-varying offsets, nadir SSH （sea surface height） could be indirectly acquired at an onshore tidal gauge instead of from direct offshore observation. The approach extrapolated the onshore SSH out to the offshore nadir with an accuracy of （1.88±0.20） cm and a standard deviation of 3,3 cm, which suggested that the approach presented was feasible in absolute altimeter calibration/validation （Cal/Val）, and the approach enormously facilitated the obtaining SSH from the offshore nadir.

Analysis of Wave Distributions Using the WAVEWATCH-III Model in the Arctic Ocean

In this work,we examined long-term wave distributions using a third-generation numerical wave model called WAVE-WATCH-III(WW3)(version 6.07).We also evaluated the influence of sea ice on wave simulation by using eight parametric switches.To select a suitable ice-wave parameterization,we validated the simulations from the WW3 model in March,May,September,and December 2017 against the measurements from the Jason-2 altimeter at latitudes of up to 60°N.Generally,all parameterizations ex-hibited slight differences,i.e.,about 0.6 m root mean square error(RMSE)of significant wave height(SWH)in May and September and about 0.9 m RMSE for the freezing months of March and December.The comparison of the results with the SWH from the European Centre for Medium-Range Weather Forecasts for December 2017 indicated that switch IC4_M1 performed most effec-tively(0.68 m RMSE)at high latitudes(60°-80°N).Given this finding,we analyzed the long-term wave distributions in 1999-2018 on the basis of switch IC4_M1.Although the seasonal variability of the simulated SWH was of two types,i.e.,‘U’and‘sin’modes,our results proved that fetch expansion prompted the wave growth.Moreover,the interannual variability of the specific regions in the‘U’mode was found to be correlated with the decade variability of wind in the Arctic Ocean.

Improvement and application of a saturation based wave dissipation function in SWAN model

Wave dissipation characteristics in SWAN （Simulating Waves Nearshore） model are investigated through numerical experiments. It is found that neither the fully developed integral parameters of wind waves （significant wave height and peak frequency） nor the high frequency spectral tail can be well reproduced by the default wave dissipation source terms. A new spectral dissipation source term is proposed, which comprises saturation based dissipation above two times of peak frequency and improved whitecapping dissipation at lower frequency spectrum. The reciprocal wave age （u./ep） is involved into the whitecapping model to adjust dissipation rate at different wind speed. The Phillips higher frequency saturation parameter in the saturation-based dissipation is no longer taken as a constant, but varies with wave age. Numerical validations demonstrate that both the wind wave generation process and higher frequency spectrum of wind waves can be well simulated by the new wave dissipation term.

Application of MODIS data in monitoring suspended sediment of Taihu Lake,China

Application of MODIS in ocean color is mainly based on bands 8-16 with the spatial resolution of 1 000 m.This spatial resolution,however,can not meet the application demand of inland waters where the areas are relatively small.With the assumption of the black water at shortwave infrared(SWIR) wavelengths(>1 000 nm),we first propose an atmospheric correction method for bands 1 and 2 with their spatial resolution of 250 m,and we then establish a quantitative retrieval model for suspended sediment concentration retrieval using the in-situ data collected in Taihu Lake.We also use MODIS data to retrieve the suspended sediment concentration of Taihu Lake with the retrieval model.The comparison between the retrieved and measured suspended sediment concentrations confirms that our algorithm can provide reliable data for monitoring the suspended sediment in Taihu Lake.