Haiyang-2A Research Articles

Impact of Pseudo-Stochastic Pulse and Phase Center Variation on Precision Orbit Determination of Haiyang-2A from Experimental HY2 Receiver GPS Data

Haiyang-2A (HY-2A) is the first marine dynamic environment satellite established by China, which has made significant contributions to the marine scientific research field. It carries the satellite-based GPS receiver named HY2, which was independently developed by China. It is an experimental satellite-borne GPS receiver for low earth orbit satellites, and during its operational period in orbit, the satellite-borne GPS data are not made accessible to the public. Therefore, this paper assesses the quality of HY-2A satellite-borne GPS data based on indicators such as satellite visibility, multipath effect, and ionospheric delay. The results indicate that the data acquired by the HY2 receiver are of high quality. The precise orbit determination (POD) uses the reduced-dynamic (RD) method. The adjustment effects of the pseudo-stochastic pulse time interval and a priori sigma on POD are analyzed, and the antenna phase center variation (PCV) is estimated using the direct method and residual method. Furthermore, this paper investigates the impact of PCV models with different resolutions (10° × 10° and 5° × 5°) on satellite orbit determination. To evaluate the orbit precision, three methods are used for validation, including carrier phase residual analysis, external precise science orbit (PSO) validation, and SLR three-dimensional (3D) validation, respectively. The results indicate that the highest orbit precision is achieved when the pseudo-stochastic pulse time interval is configured to 15 min, with the a priori sigma of 1 × 10−8 m/s2. The orbit carrier phase residuals reach the millimeter level. The 10° × 10°PCV model was estimated using the direct method and residual method, respectively; the root mean square of the external orbit validation for both methods show a millimeter-level improvement. The results obtained from the direct method and residual method are comparable. The resolution is increased from 10° to 5°, and the improvement in orbital precision is relatively small. The results obtained from the SLR 3D validation are consistent with those from the external PSO validation. The experimental results contribute valuable information for the POD of the HY2 series satellites.

An improved triple collocation-based integration of multiple gravity anomaly grids from satellite altimetry: Contribution of ICESat-2

Satellite altimetry has been widely used to determine the marine gravity fields. Different altimeter missions lead to different gravity grids with varying spatial resolutions and accuracies. Marine gravity accuracy in offshore waters is often low. Integrating marine gravity anomalies (MGA) derived from various altimeter missions are key to obtaining high-precision and high-resolution global marine gravity fields. Here an improved triple collocation (ITC) method is first proposed to establish a new 1´×1′ marine gravity anomaly in the South China Sea (SCS), named MGAITC, by merging the grids of MGA from the Haiyang-2A (HY-2A), CryoSat-2, and ICESat-2 altimeter measurements. A combined MGA (MGACOM) is also constructed by using the least squares collocation (LSC) method from three sets of altimeter-derived geoid gradients. The qualities of MGAITC, MGACOM and each altimeter-derived MGA grids are assessed using shipborne gravity and different global gravity models over waters of varying depths and distances from the coastline. The accuracy of the MGAITC gravity against the shipborne gravity is 5.52 mGal, which represents improvements of 7%, 9%, 13%, and 3% relative to the gravity grids derived only from ICESat-2, CryoSat-2, HY-2A and from MGACOM. In all assessments involving different sub-regions, seafloors, and distances from the coastline, MGAITC performs the best, and ICESat-2 yields the best single-mission MGA grid, especially in offshore waters. We find that ICESat-2 has a high potential to improve shallow-water gravity accuracy and the global marine gravity field can be improved by integrating gravity grids from different altimetry missions using ITC.

Global statistical assessment of Haiyang-2B scanning microwave radiometer precipitable water vapor

Introduction: As a successor to Haiyang-2A (HY-2A), HY-2B is China’s second marine dynamic satellite. Equipped with a scanning microwave radiometer (SMR), it can measure the precipitable water vapor (PWV) over the oceans, providing valuable climate and weather insights. This study aims to evaluate the accuracy of HY-2B SMR PWV data from January 2019 to December 2021 using various validation methods.Methods: to validate HY-2B SMR PWV, fifth-generation European Centre for Medium-Range Weather Forecasts (ECMWF) reanalysis (ERA5) data, radiosonde data, and global navigation satellite system (GNSS) observations are used.Results: the validation shows that the HY-2B SMR PWV data agree well with the ERA5 PWV data, with a root mean square error (RMSE) of 1.61 mm and a mean value of 0.71 mm. However, RMSEs of approximately 3.5 mm are observed when comparing the HY-2B SMR PWV data to GNSS and radiosonde data, likely due to temporal and spatial gaps. Geographically, there are greater differences between the HY-2B SMR and ERA5 in lower-latitude areas. The reason could be attributed to a higher water vapor content and more frequent cloudy weather in the areas.Discussion: The results demonstrate that the HY-2B SMR PWV meets requirement (RMSE≤3.5 mm). The HY-2B SMR PWV has a high quality, with a slight observational drift of only 0.1 mm/year, but regular validation and calibration are still necessary.

Estimation of Geopotential Value W0 for the Geoid and Local Vertical Datum Parameters

Unification of the global vertical datum has been a key problem to be solved for geodesy over a long period, and the main challenge for unifying the global vertical datum system is to determine the geopotential value W0 of the geoid and to calculate the vertical offset between the local vertical datum and the global vertical datum W0. The geopotential value W0 can be calculated using the grid mean sea surface (GMSS) data and the global geopotential model (GGM). In this study, this GMSS data was measured with adjustment methods and 24 years of merged multi-satellite altimetry data. The data of HaiYang-2A (HY-2A) and Jason-3 were first used to calculate W0. The geopotential value W0 was determined to be 62,636,856.82 m2s−2 by combining the EIGEN-6C4 (European Improved Gravity Model of the Earth by New Techniques) and the GMSS data. Then, the geopotential difference approach and geodetic boundary value problem (GBVP) approach were used to determine the vertical datum parameters in this study. To compensate for the omission error of the GGM, this study utilized the remove–compute–restore (RCR) technique and the residual terrain model (RTM)-recovered high-frequency gravity signals. Finally, as a result of the GBVP solution, the geopotential value of the Australian Height Datum (AHD) was 62,636,851.935 m2s−2, and the vertical offset of the AHD relative to the global vertical datum W0 was 0.4885 m. As a result of the geopotential difference approach, the geopotential value of the Chinese Height datum was 62636861.412 m2s−2, and the vertical offset of the Chinese Height datum was −0.4592 m.

Performance of Haiyang-2 Derived Gravity Field Products in Bathymetry Inversion

Haiyang-2A (HY-2A), China’s first altimetry satellite mission, was launched more than ten years ago, and its follow-up satellites, HY-2B, HY-2C, and HY-2D, have also been launched. More attention has been paid to the evaluation of these satellite observations in marine gravity field inversion. However, this is not the case for bathymetry inversion. This study is aimed at evaluating the performance of HY-2 gravity field products in bathymetry recovery. Not only gravity anomaly, but also deflection of the vertical from the HY-2 series’ observations is also used. The results show that the bathymetry derived from the deflection of the vertical from HY-2A has a precision of around 128~130 m, and the north-south component performs better than the east-west component. Three versions of the gravity anomaly are used in bathymetry inversion, i.e., HY2ONLY_GRA, WHU16_GRA, and NSOASS22_GRA, and three bathymetry models are derived correspondingly, named as HY2ONLY_BAT, NSOASS22_BAT, and WHU16_BAT, respectively. The results show that HY2ONLY_BAT has a precision of 82.93 m, which is a little poorer than WHU16_BAT; NSOAS22_BAT has the best performance in bathymetry inversion among the three versions of the gravity anomaly. It indicates that HY-2 observations can also contribute to bathymetry inversion compared to current altimetry datasets, since the main difference between WHU16_GRA and NSOASS22_GRA is the use of HY-2 observations. According to spatial analysis results, considerable improvements appear in the west of the Pacific and Indian oceans, and most of the improvements are within 20 m. Meanwhile, the improvements are stronger in the regions with depths ranging between 2600~5500 m. Correlation analysis demonstrates that NSOASS22_BAT is very close to SIO V19.1 and DTU21BAT, which once again indicates the excellent performance of NSOASS22_BAT.

Global marine gravity anomalies from multi-satellite altimeter data

In this study, China’s first altimeter satellite Haiyang-2A (HY-2A) data combined observations from CryoSat-2, SARAL/AltiKa, and Jason-1&2 are used to calculate the global (60°S–60°N) marine deflections of the vertical and gravity anomalies named Global Marine Gravity Anomaly Version 1(GMGA1), with grid resolution of 1′ × 1′. The deflections of the vertical from each satellite observations are first derived from the gradients of the geoid height through the least squares method. The deflections of the vertical are then merged by assigning different weights to each satellite product based on their accuracy. Finally, gravity anomalies are obtained by the remove-restore method. The results reveal that the fused deflections of the vertical have an accuracy of 0.4 arcsec in the north component and 0.8 arcsec in the east component. HY-2A’s contribution to the north component of the integrated deflections of the vertical is second only to Cryosat-2. Jason-1/2 accounts for a large proportion of the integrated east components. Compared to worldwide products such as DTU17, Sandwell & Smith V31.1, as well as values from EGM2008, EIGEN-6C4 and XGM2019e_2159, GMGA1 has an accuracy of around 3.3 mGal. By not using HY-2A data, the precision of GMGA1 is reduced by about 0.1 mGal. To further improve the accuracy, seafloor topography information is used to provide short wavelength gravity anomaly. It is verified in the South China Sea (112°E–119E°, 12°N–20°N) using the Parker formula. By combining shipborne depth generated data and GMGA1 through a filtering technique, a new version of gravity anomaly grid with an accuracy improvement of 0.4 mGal in the South China Sea is obtained.Graphical

Accuracy comparison of marine gravity derived from HY-2A/GM and CryoSat-2 altimetry data: a case study in the Gulf of Mexico

SUMMARYThe HaiYang-2A (HY-2A) satellite was launched in August 2011, filling a gap in the field of Chinese altimetry. The application of HY-2A-measured altimeter data in gravity research needs to be studied. CryoSat-2 altimetry data play an important role in constructing global marine gravity models. Based on HY-2A and CryoSat-2 altimetry data of Level-2 plus products from March 2016 to June 2020, marine gravity anomaly models on 1′ × 1′ grids were derived by the inverse Vening–Meinesz (IVM) method over the Gulf of Mexico (80–100°W, 17–32°N). The accuracy of the altimeter-derived gravity was assessed by ship-borne gravity anomalies and the SIO V30.1 global marine gravity model. Then, integrated gravity models on 1′ × 1′ grids were constructed from the altimeter-derived gravity anomaly models by the weighted modification method and the combined altimetry data by IVM method. The results show that the accuracy of altimeter-derived marine gravity anomalies is affected by the coastline and complicated submarine topography. The accuracy of gravity anomalies derived from HY-2A is slightly lower than that derived from CryoSat-2 in the offshore region and regions containing complicated submarine topography, and is consistent with that derived from CryoSat-2 in other regions. The HY-2A-measured altimetry data play a significant role in constructing integrated gravity models. To conclude, the gravity anomalies derived from HY-2A have similar accuracy to those derived from CryoSat-2 during the same period, and HY-2A altimetry data can be used to construct high-precision global/local marine gravity models.

Calibrating the Haiyang-2A Calibration Microwave Radiometer When the 18.7-GHz Band Fails

The wet tropospheric correction (WTC) retrieved from the onboard calibration microwave radiometer (CMR) of Haiyang-2A (HY-2A) is critical in monitoring the global sea level. However, the CMR WTC became significantly biased from June 2017 due to the failure of the 18.7-GHz band, which caused massive errors in the sea surface height (SSH) measurements. We investigate the accuracy of the CMR WTC derived from the two remaining bands to address this problem. A comprehensive evaluation using multisource data demonstrates that the dual-band + backscattering coefficient (BC) algorithm achieves comparable accuracy to the three-band algorithm, and it does not suffer from any large errors when the equipment works well. Hence, we calibrated the HY-2A CMR data with the dual-band + BC algorithm when the 18.7-GHz band failed, and the accuracy of the CMR WTC is improved from 2.34 to 1.39 cm compared with European Center for Medium-Range Weather Forecasts (ECMWF) ERA5 data. In addition, the SSH measurements are improved significantly by a maximum of 2 cm in mean value using the dual-band + BC WTC during the failure period of HY-2A CMR. Compared with Jason-3 SSH measurements, the HY-2A with dual-band + BC shows a slightly larger difference than HY-2A with three-band by 0.1 cm in rms. This method prolongs the operational lifetime of the HY-2A CMR and could be used in the reprocessing of HY-2A observations.

On Deflections of Vertical Determined From HY-2A/GM Altimetry Data in the Bay of Bengal

Satellite altimetry is an important technique to derive oceanic deflections of vertical (DOV). HaiYang-2A (HY-2A) is the first Chinese altimetry satellite launched in August 2011 to observe the global marine dynamic environment. To assess the reliability of DOVs derived from HY-2A geodetic mission data, the Bay of Bengal and its adjoining area (0°–23°N, 80°–100°E) are selected as the research area, and the accuracy of HY-2A/GM-derived DOVs is compared with that of DOVs determined from CryoSat-2-measured sea surface heights (SSHs) and SARAL/AltiKa (SARAL)-measured SSHs which are in the same period as HY-2A-measured SSHs. Compared with the along-track DOVs calculated from Sentinel-3B-measured SSHs (from cycle 20 to cycle 33), the RMS of differences for HY-2A-determined DOVs is 0.1 <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">μ</i> rad smaller than that for SARAL-determined DOVs and CryoSat-2-determined DOVs. The differences of the along-track DOVs in the frequency domain are also analyzed by power spectral density. In addition, the XGM2019e model is adopted to evaluate the quality of the gridded DOVs model established by least-squares collocation method. Marine gravity anomaly models (SIO V30.1) and ship-borne gravity anomalies are also used to assess the accuracy of gravity anomalies derived from gridded DOVs. The whole results show that the accuracy of HY-2A-determined DOVs is coincident with that of CryoSat-2-determined DOVs and slightly lower than that of SARAL-determined DOVs. Therefore, HY-2A/GM altimetry data can be applied in establishing the global high-precision DOV model.

Assessment of HY-2A GM data by deriving the gravity field and bathymetry over the Gulf of Guinea

Nine cycles of Haiyang-2A (HY-2A) IGDR (Interim Geophysical Data Record) data are used to derive gravity products over the Gulf of Guinea (15° W–5° E, 4° S–4° N). Firstly, the sea surface heights (SSH) and vertical deflections are derived and their precisions are evaluated. The comparison results show that the east component of vertical deflections has a poorer precision than the north component by 4.15 times. A theoretical proof was given to explain this point according to the error propagation rule. Gravity anomalies are then derived from vertical deflections using the remove–restore method. The precision of the derived HY-2A gravity anomalies is evaluated with SIO, DTU13, EGM2008, EIGEN-6C4 products. The results showed that the differences between HY-2A-derived gravity anomalies and these models have mean values larger than 0.5 mGal and std values around 7.0–7.3 mGal. In order to improve the precision, an improved new version of gravity anomalies was derived by assigning a small weight to the east component of vertical deflections, since the precision of which is poorer than the north component. Comparison with the initial model showed that the precision of the new gravity anomalies is an improvement of the initial model by approximately two times. When compared with EGM2008, EIGEN-6C4, SIOv28 and DTU13, the mean values of the differences are close to zero and standard deviation of the differences are around 2.7–3.0 mGal. The improved gravity anomalies were used to invert the bathymetry of the region using the gravity-geologic method. The modeled bathymetry compared well with a previous bathymetric study by the authors that used DTU13 gravity anomalies. It also performed well against ETOPO1 and SRTM15+V2; with difference means, standard deviations and correlation coefficients of 26.67 m, 183.09 m, 0.9562; and 12.26 m, 174.55 m, 0.9590, respectively. This implies that SSH data from HY-2A are geophysically reliable; and hence, can be incorporated with SSH data from other satellite altimeters.

Validating HY-2A CMR precipitable water vapor using ground-based and shipborne GNSS observations

Abstract. The calibration microwave radiometer (CMR) on board the Haiyang-2A (HY-2A) satellite provides wet tropospheric delay correction for altimetry data, which can also contribute to the understanding of climate system and weather processes. The ground-based global navigation satellite system (GNSS) provides precise precipitable water vapor (PWV) with high temporal resolution and could be used for calibration and monitoring of the CMR data, and shipborne GNSS provides accurate PWV over open oceans, which can be directly compared with uncontaminated CMR data. In this study, the HY-2A CMR water vapor product is validated using ground-based GNSS observations of 100 International GNSS Service (IGS) stations along the global coastline and 56 d shipborne GNSS observations over the Indian Ocean. The processing strategy for GNSS data and CMR data is discussed in detail. Special efforts were made in the quality control and reconstruction of contaminated CMR data. The validation result shows that HY-2A CMR PWV agrees well with ground-based GNSS PWV with 2.67 mm as the root mean square (rms) within 100 km. Geographically, the rms is 1.12 mm in the polar region and 2.78 mm elsewhere. The PWV agreement between HY-2A and shipborne GNSS shows a significant correlation with the distance between the ship and the satellite footprint, with an rms of 1.57 mm for the distance threshold of 100 km. Ground-based GNSS and shipborne GNSS agree with HY-2A CMR well.

Vertical Deflections and Gravity Disturbances Derived from HY-2A Data

The first Chinese altimetry satellite, Haiyang-2A (HY-2A), which was launched in 2011, has provided a large amount of sea surface heights which can be used to derive marine gravity field. This paper derived the vertical deflections and gravity disturbances using HY-2A observations for the major area of the whole Earth’s ocean from 60°S and 60°N. The results showed that the standard deviations (STD) of vertical deflections differences were 1.1 s and 3.5 s for the north component and the east component between HY-2A’s observations and those from EGM2008 and EIGEN-6C4, respectively. This indicates the accuracy of the east component was poorer than that of the north component. In order to clearly demonstrate contribution of HY-2A’s observations to gravity disturbances, reference models and the commonly used remove-restore method were not adopted in this study. Therefore, the results can be seen as ‘pure’ signals from HY-2A. Assuming the values from EGM2008 were the true values, the accuracy of the gravity disturbances was about −1.1 mGal in terms of mean value of the errors and 8.0 mGal in terms of the STD. This shows systematic errors if only HY-2A observations were used. An index of STD showed that the accuracy of HY-2A was close to the theoretical accuracy according to the vertical deflection products. To verify whether the systematic errors of gravity field were from the long wavelengths, the long-wavelength parts of HY-2A’s gravity disturbance with wavelengths larger than 500 km were replaced by those from EGM2008. By comparing with ‘pure’ HY-2A version of gravity disturbance, the accuracy of the new version products was improved largely. The systematic errors no longer existed and the error STD was reduced to 6.1 mGal.

Preliminary marine gravity field from HY-2A/GM altimeter data

HY-2A (Haiyang-2A), launched in 2011, is the first ocean dynamic environment satellite of China and is equipped with a radar altimeter as one of the primary payloads. HY-2A shifted the drift orbit in March 2016 and has been accumulating geodetic mission (GM) data for more than three years with 168-day cycle. In this paper, we present the preliminary gravity field inverted by the HY-2A/GM data from March 2016 to December 2017 near Taiwan (21°–26°N, 119°–123°E). The gravity anomaly is computed by Inverse Vening Meinesz (IVM) formula with a one-dimensional FFT method during remove-restore procedure with the EGM2008 gravity model as the reference field. For comparison, CryoSat-2 altimeter data are used to inverse the gravity field near Taiwan Island by the same method. Comparing with the gravity field derived from CryoSat-2, a good agreement between the two data sets is found. The global ocean gravity models and National Geophysical Data Center (NGDC) shipboard gravity data also are used to assess the performance of HY-2A/GM data. The evaluations show that HY-2A and CryoSat-2 are at the same level in terms of gravity field recovery and the HY-2A/GM altimeter-derived gravity field has an accuracy of 2.922 mGal. Therefore, we can believe that HY-2A will be a new reliable data source for marine gravity field inversion and has the potentiality to improve the accuracy and resolution of the global marine gravity field.

Retrieval of Sea Surface Temperature From HY-2A Scanning Microwave Radiometer

The scanning microwave radiometer (RM) onboard the Haiyang-2A (HY-2A) satellite has low-frequency channels with the capability of observing sea surface temperature (SST) from space. To improve the accuracy of the HY-2A RM SST data, the intercalibration of RM brightness temperature (BT) and SST retrieval were carried out. Based on the simulated BTs using the microwave radiative transfer model (RTM), the double-difference approach was applied to perform the intercalibration of RM BTs for the 10.7, 18.7, 23.8, and 37.0 GHz channels, with the Global Precipitation Measurement (GPM) Microwave Imager (GMI) as the reference. The RM 6.6-GHz BTs were corrected with the RTM modeled BT on account of a lack of the similar channel in GMI. The comparison of RM original BTs with GMI and modeled BTs showed large biases and the strong dependence on latitude. We obtained latitude-dependent coefficients using robust linear regression for RM BT correction and applied recalibrated RM BTs for the SST retrieval. The validation of the RM retrieved SST showed the bias of −0.12 °C and the robust standard deviation (RSD) of 1.10 °C compared with buoy SST in the region between 70°S and 70°N. In the tropical and subtropical regions, the bias was −0.12 °C and the RSD was 0.93 °C. In addition, the relationships between the SST difference and the sea surface and atmospheric parameters were investigated. Both statistics of validation results and error analysis indicated significant improvement of RM SST accuracy.

Wind-Wave Relationship Model and Analysis of Typhoon Wave Fields in the South China Sea From HY-2A Satellite Observations

Significant wave height is an important parameter for characterizing ocean surface waves. With the development of remote sensing technology, satellite radar altimetry has become an essential tool for obtaining significant wave height estimations. However, its measurement only covers the satellite track on the ground and cannot be applied to large regions or areas. In this study, we obtained significant wave height from the Hai Yang-2A (HY-2A) radar altimeter and sea surface wind speed at a 10-m height from the HY-2A microwave scatterometer for October 2013, and then proposed a wind-wave relationship model for the South China Sea using linear/nonlinear regression analysis at high/low wind speeds (0–40 m s–1). By comparison with two other wind-wave models and validation with HY-2A observations in November 2013, our results show that the proposed wind-wave relationship model is credible, and at low wind speed exhibited good consistency with the wind-wave model from in situ observations. According to the proposed model, significant wave height from the HY-2A microwave scatterometer-retrieved wind speed and ocean wind wave analysis during the “1329” Typhoon Krosa were successfully obtained and determined. Data coverage of the computed significant wave height was far wider than that of the satellite radar altimeter observations and demonstrated development of typhoon wave fields over a large region. Overall, this study and proposed model provide useful information for the analysis and forecast of typhoon waves and potential storm surge disasters.

Mean sea surface model over China seas and its adjacent ocean established with the 19-year moving average method from multi-satellite altimeter data

A new mean sea surface (MSS) model (named Shandong University of Science and Technology (2018) (SUST2018) MSS model) with a grid of 1′ × 1′ over China seas and its adjacent ocean (0°N~41°N, 100°E~140°E) is established and validated. The SUST2018 model established with the 19-year moving average method is based on 26-year multi-satellite altimeter data covering the time period 1992–2018. The new altimetry data of HaiYang-2A (HY-2A), Jason-3 and Sentinel-3A have been used in establishing the SUST2018 model. The process of the establishment of the MSS model also have been presented, including collinear adjustment of exact repeat mission (ERM) data, removal of the temporal oceanic variability of geodetic mission (GM) data, crossover adjustment and gridding. The reliability and accuracy of SUST2018 are validated by comparing with the CLS15 and DTU18 MSS models as well as satellite altimeter data. By comparing SUST2018 and SUST(all)2018 (established with the traditional average method), it can be found that the accuracy of SUST2018 is higher than that of SUST(all)2018, which indicates that the 19-year moving average method can effectively improve the accuracy of an MSS model.

Validation of 7 Years in-Flight HY-2A Calibration Microwave Radiometer Products Using Numerical Weather Model and Radiosondes

Haiyang-2A (HY-2A) has been working in-flight for over seven years, and the accuracy of HY-2A calibration microwave radiometer (CMR) data is extremely important for the wet troposphere delay correction (WTC) in sea surface height (SSH) determination. We present a comprehensive evaluation of the HY-2A CMR observation using the numerical weather model (NWM) for all the data available period from October 2011 to February 2018, including the WTC and the precipitable water vapor (PWV). The ERA(ECMWF Re-Analysis)-Interim products from European Centre for Medium-Range Weather Forecasts (ECMWF) are used for the validation of HY-2A WTC and PWV products. In general, a global agreement of root-mean-square (RMS) of 2.3 cm in WTC and 3.6 mm in PWV are demonstrated between HY-2A observation and ERA-Interim products. Systematic biases are revealed where before 2014 there was a positive WTC/PWV bias and after that, a negative one. Spatially, HY-2A CMR products show a larger bias in polar regions compared with mid-latitude regions and tropical regions and agree better in the Antarctic than in the Arctic with NWM. Moreover, HY-2A CMR products have larger biases in the coastal area, which are all caused by the brightness temperature (TB) contamination from land or sea ice. Temporally, the WTC/PWV biases increase from October 2011 to March 2014 with a systematic bias over 1 cm in WTC and 2 mm in PWV, and the maximum RMS values of 4.62 cm in WTC and 7.61 mm in PWV occur in August 2013, which is because of the unsuitable retrieval coefficients and systematic TB measurements biases from 37 GHz band. After April 2014, the TB bias is corrected, HY-2A CMR products agree very well with NWM from April 2014 to May 2017 with the average RMS of 1.68 cm in WTC and 2.65 mm in PWV. However, since June 2017, TB measurements from the 18.7 GHz band become unstable, which led to the huge differences between HY-2A CMR products and the NWM with an average RMS of 2.62 cm in WTC and 4.33 mm in PWV. HY-2A CMR shows high accuracy when three bands work normally and further calibration for HY-2A CMR is in urgent need. Furtherly, 137 global coastal radiosonde stations were used to validate HY-2A CMR. The validation based on radiosonde data shows the same variation trend in time of HY-2A CMR compared to the results from ECMWF, which verifies the results from ECMWF.

A Study of the Technology Used to Distinguish Sea Ice and Seawater on the Haiyang-2A/B (HY-2A/B) Altimeter Data

When the Haiyang-2B (HY-2B) was launched into space to form a star network with the Haiyang-2A (HY-2A), it provided new data sources for the sea ice research of the Earth’s polar regions. The ability of altimeter echoes to distinguish sea ice and sea water is usable in operational ice charting. In this research study, the level 1B (L1B) data of HY-2A/B altimeter from November 2018 was used to analyze the altimeter waveforms from the polar regions. The Suboptimal Maximum Likelihood Estimation (SMLE) and Offset Center of Gravity (OCOG) tracking packages could maintain the waveform characteristics of diffused and quasi-specular surfaces by comparison. Also, they could be utilized to distinguish sea ice from seawater in the polar regions. It was determined that the types of echoes obtained from the seawater were diffuse. Also, some “ocean-like” waveform data had existed for the old ice formations in the Arctic regions during the study period. The types of echoes obtained from Arctic sea ice were found to be mainly quasi-specular. In the present study, three methods (Threshold segmentation, K-nearest-neighbor (KNN), and Lib-Support Vector machine (LIBSVM)) with four waveform parameters (Automatic Gain Control (AGC) and Pulse Peaking (PP) values of the Ku and C Bands) were adopted to distinguish between the sea ice and seawater areas. The accuracy rate of the separation results for the LIBSVM except band Ku from HY-2B ALT was found to be less than 40% in Antarctic. Meanwhile, the other two methods were observed to have maintained the waveforms correctly at accuracy rates of approximately 80% in Antarctic and the Arctic. In addition, the observed distinguishing errors were located in the regions of the old ice of the Arctic region. In addition, due to the summer melting processes, the large number of ice floes and the snow cover had made it difficult to distinguish the seawater and sea ice in the Antarctic regions.

Evaluation of HY-2A satellite-borne water vapor radiometer with shipborne GPS and GLONASS observations over the Indian Ocean

The Chinese Haiyang-2A (HY-2A) altimetry satellite is equipped with a calibration microwave radiometer (CMR) for correcting atmospheric water vapor path delay in radar altimeter observations. To evaluate the satellite-borne CMR, we retrieved 1 Hz high-frequency precipitable water vapor (PWV) using shipborne GPS and GLONASS from a two-month cruise in the Indian Ocean. This open-sea evaluation of the satellite-borne radiometer is free of the contamination of the satellite footprints induced by coastal lands, which occurs inevitably in ground-based or coastal stations. The estimates and errors of the retrieved PWV from shipborne GNSS kinematic precise point positioning were analyzed and then compared to the CMR-retrieved PWV. The results show that the shipborne GNSS kinematic precise point positioning can obtain marine PWV with an uncertainty of about 2.8 mm. When the HY-2A sub-satellite point and the ship cross, the HY-2A CMR-retrieved PWV is in good agreement with the GNSS PWV with the difference of about 0.8 mm, which demonstrates that the HY-2A satellite can contribute high-precision precipitable water vapor measurements to weather and atmospheric studies.

Performance Evaluation of Three Atmospheric Density Models on HY-2A Precise Orbit Determination Using DORIS Range-Rate Data

Abstract Doppler Orbitography and Radiopositioning Integrated by Satellite (DORIS) has become a matured and accurate space geodetic technique after more than two decades of development. DORIS’s main objective is precise orbit determination (POD) for the low earth orbit (LEO) satellite. For the POD of Haiyang-2A (HY-2A) satellite, a DORIS receiver was equipped on this mission. There are several nonconservative forces acting on a Haiyang-2A (HY-2A) satellite with an altitude of about 970 km. Out of the factors, the atmospheric drag is the most dominant and uncertain one during the process of POD with the dynamic method. The comparison between the computed orbits of HY-2A using DORIS range-rate data and the precise orbits computed by the Centre National d’Etudes Spatiales (CNES) has been performed, and the DORIS range-rate residuals have been analyzed in detail in normal period and frequent solar activity period. Tests demonstrated that the relative optimal atmospheric density model was the empirical MSIS-86 model for the HY-2A satellite. The root mean squares (RMSs) of orbit difference between the derived orbits using the MSIS-86 model and the CNES orbits were 0.0173 m in the radial direction and 0.0619 m in three dimensions. The RMS of the range-rate residual improved 0.024 mm/s during the frequent solar activity period with the atmospheric drag coefficient solved every three hours and then every six hours using the MSIS-86 model. This study will provide valuable references for POD and orbit prediction for the LEO satellites, especially for the other HY-2 series satellites of China.