The Chinese Ocean Salinity Mission (COSM) is dedicated to the global sea surface salinity (SSS) using the interferometric aperture synthesis radiometry. The airborne COSM was conducted during July to August 2023 at Qixia of Shandong Province. The introduction of airborne COSM experiment and the pre-simulation results based on the design of airborne instruments, which are the scaled-down model of spaceborne payloads, are presented. The preliminary sea surface salinity (SSS) retrieval results show that, the best accuracy of SSS is achieved when considering all the available measurements from two payloads together in a combined retrieval. The results from the airborne COSM experiment verify the interferometric synthetic aperture payloads performance and will also help to improve the data processing technique and the key error correction methods in the future in-orbit operational phase.

After over 12 years of development, the HY-2 series microwave scatterometers have been providing stable and reliable Ku-band observation data for operational forecasting departments and marine scientific research departments. The observation data of the satellite constellation comprising the HY-2B/C/D series satellites have played an important role in fields such as numerical forecasting models, typhoon monitoring, and polar sea ice edge identification. In 2019, the HY-2A scatterometer was retired after eight years of operation, having accumulated valuable experience for subsequent payloads and improvements in the processing algorithms of ground systems. Beginning from the HY-2B satellite, the performance of the scatterometer payload has been greatly improved, and the ground data processing system has also been improved accordingly. In addition, compared to the ECMWF Reanalysis v5 (ERA5) wind field, the wind speed accuracy has been increased from 1.35 to 0.9 m/s, and the wind direction accuracy has been increased from 27° to 12°, which is closely related to the improvement of the signal-to-noise ratio (SNR) of the observed data. In this article, the optimization process from experimental satellite payload to operational payload is introduced, then the calculation method of the normalized radar cross section (NRCS) is described in detail, and the consistency of observation data of four payloads is analyzed. The results show that the change in signal bandwidth, increase in signal slice, and improved slice finding and accumulation method have improved the data’s SNR reduced the measurement error, thus fundamentally ensuring the improvement of wind field inversion accuracy.

Microwave radiometers are widely used in Earth observation and ocean monitoring for their strong penetration ability. Nevertheless, their utilization is somewhat constrained by their intrinsic low-resolution capability, particularly in complex applications such as coastal zone monitoring and analysis of typhoons. To overcome this limitation, resolution enhancements are necessary. Here, we present a novel resolution enhancement technique, the alternating proximal gradient (APG) method, applied to Haiyang-2B (HY-2B) Scanning Microwave Radiometer (SMR) data. This method is based on the decorrelation approach and incorporates total variation constraint. The APG method, along with the Backus-Gilbert (BG) method, are both applied to the 6.925GHz data of HY-2B SMR. Both simulated and real HY-2B SMR data from two representative regions, including land-sea transition zones and cyclonic storms, are analyzed. Results show that both the BG method and the APG method can significantly enhance details obscured in the original SMR data. Furthermore, the enhanced brightness temperatures are compared to the AMSR-2 25 km product, revealing that the APG method provides more consistent results and contains more reliable information than the BG method in terms of resolution enhancement.

Satellite Calibration Spectrometer (SCS) onboard HY-1C/HY-1D could support a direct calibration using the Simultaneous Nadir Overpass (SNO) approach for imaging spectroradiometers with various band configurations on the same or different satellite platforms by providing precise hyperspectral radiance after onboard calibration employing a solar diffuser (SD). However, a unique phenomenon was discovered in the analysis of annual variations of onboard calibration coefficients of the SCS. Although the solar calibration scheme employing an SD was used, a season-dependent oscillation still existed, and the oscillation trend of calibration coefficients is highly correlated with the variation trend of solar beta angle. In this article, we analyzed the entire solar calibration results of SCS from HY-1C using more than three years of data and discovered that season-dependent oscillation is primarily due to the variations in the transmittance of the solar attenuation screen affected by satellite platform attitude. Furthermore, an accurate variation model of the incident angle on the solar attenuation screen was developed, and the findings demonstrate that the season-dependent oscillation of onboard calibration coefficients of SCS could be effectively removed. Finally, the model established in this article is validated by optical simulation, which shows its consistency and reliability.

This study develops a new sea surface temperature (SST) retrieval algorithm based on statistical regression that utilizes WindSat satellite data and in situ SST quality monitor (iQuam) buoy data. The proposed algorithm introduces new rules to remove abnormal brightness temperatures (TB) due to land, sea ice, radio frequency interference (RFI), and sun glint. Besides, SST data from 2019 to 2022 are generated from the HY-2B satellite’s scanning microwave radiometer (SMR) using a two-step retrieval method. Our study reveals that wind speed (WS), cloud liquid water (LW), and wind direction have significant crosstalk effects on SST retrieval, which increases exponentially when the cloud LW exceeds 0.1 mm. The crosstalk effect can be significantly reduced by incorporating WS fitting in the SST retrieval algorithm. By fitting iQuam SST data, the multiple localizing sea surface WS and latitude algorithms are developed, significantly reducing the impact of crosstalk parameters on SST retrieval and improving the SST retrieval accuracy. Also, the algorithm performed well in correcting the nonlinear response of TB and SST. The results reveal that the retrieval algorithm eliminates bias across different WSs, water vapor (WV), and cloud LW ranges and performs consistently on ascending and descending orbits. The overall bias of the SMR four-year SST product is$-0.004\,\,^{\circ }\text{C}$, with a standard deviation (STD) of 0.588 °C, posing a substantial improvement over the existing product with a bias of$-0.022\,\,^{\circ }\text{C}$and an STD of 0.803 °C. Moreover, the four-year fluctuation of STD is very small, indicating excellent stability of the retrieval algorithm.

Model-based Incoherent Polarimetric Decomposition (MIPD) is a frequently used technique to analyze the scattering characteristics of multilook Polarimetric Synthetic Aperture Radar (PolSAR) data. A reflection symmetry decomposition (RSD) algorithm was proposed to overcome two limitations of Freeman–Durden decomposition. In this letter, a modified RSD (MRSD) algorithm with an improved approach to determining the surface scattering power and the double-bounce scattering power of RSD is proposed. MRSD causes no loss of polarimetric information, and its outputs consist of nine independent real-valued parameters. Based on these two properties, a new polarimetric product is created for Chinese GF-3 satellite. The product is very convenient for PolSAR data application users because it can be directly displayed by geographic information system (GIS) software, and its ground pixel sizes are very close to be square. The case study with six real GF-3 images demonstrates the effectiveness of both MRSD and the new product.

Seven satellite scatterometers, namely, the C-band MetOp-A/Advanced Scatterometer (ASCAT), MetOp-B/ASCAT, MetOp-C/ASCAT, Ku-band HY-2A/SCAT, HY-2B/SCAT, Chinese-French Oceanography Satellite (CFOSAT)/SCAT, and SCATSAT-1/OceanSat Satellite Scatterometer (OSCAT2), are operating in orbits. These satellites make a high-frequency observation of sea surface winds possible, particularly at high latitudes. This work analyzes the passing time, passing frequency, coverage of these seven scatterometers, and the quality of high-latitude surface wind products. All the scatterometer wind products were reproduced with the same processing procedures, in terms of backscatter calibration, wind retrieval, numerical weather prediction (NWP) wind, and quality control. The target region (74–78 N, 167°–171 W) was chosen to analyze the passing frequency. A spatial grid of$0.25^{\circ } \times 0$.25° in the range of 60° N–88° N was chosen to analyze the spatial coverage of the scatterometers. The results show that more than 40 observation times can be provided daily by all the seven scatterometers; however, there is a 9-h gap between UTC 9:00 and UTC 18:00, suggesting that international cooperation is needed for optimizing the equatorial crossing time in future scatterometer missions, such that an optimal virtual scatterometer constellation can be achieved. The scatterometer winds are compared with the European Centre for Medium-Range Weather Forecasts (ECMWF) Reanalysis 5th Generation (ERA5) winds, and with each other. The comparison results confirmed noticeable wind speed biases due to sea surface temperature (SST) in all Ku-band scatterometer winds with respect to the C-band scatterometer and ERA5 winds.

The Chinese Ocean Salinity Satellite is dedicated to global sea surface salinity (SSS) mapping with two payloads onboard, which are the interferometric microwave radiometer (IMR) and the microwave imager combined active and passive (MICAP), both using the interferometric aperture synthesis radiometry. One of the payloads is an L-band interferometric radiometer system with a Y-shaped antenna array. The other one, MICAP, is a 1-D passive/active combined system, where the L-, C-, and K-band interferometric radiometers’ antenna arrays are arranged in a line, and the active device is an L-band scatterometer. Based on the payload configurations, a series of simulations is applied to analyze the payloads performance, including the brightness temperature (TB) characteristic, the SSS accuracy, and the effects of Sun and land contamination. The TB simulation results show that the IMR possesses a finer spatial resolution compared to the MICAP L-band radiometer, whereas the latter achieves a better TB radiometric resolution. With the assistance of the C- and K-band radiometers and the L-band scatterometer, the combined retrieval accuracy improves compared with the SSS retrieved using only L-band TB and auxiliary parameters. The best accuracy of SSS is achieved when considering all the available measurements from two payloads together in a combined retrieval. Finally, the results related to Sun and land contamination that endanger the interferometric radiometer measurements are presented and discussed. The performance simulation in this article has been used as a reference for the design phase of two payloads and will be updated with the development of payload manufacture.

The HY-2 series scatterometers (HSCAT) are now providing global Ku-band radar observations. In order to build two-decade normalized radar cross section ($\sigma ^{\circ }$) datasets with high quality and high stability, increased attention should first be paid to the instrument variations caused by space environments. The HY-2B scatterometer (HSCAT-B) has operated to yield a two-year dataset in orbit. The monitoring of long-term ocean calibration results and telemetric temperatures indicate that beam radiometric imbalances and long-term instability exist in$\sigma ^{\circ }$due to the gain variations related to temperature changes. Such variations will result in estimated$\sigma ^{\circ }$biases with a peak-to-peak of approximately 0.5 dB. Gain-related calibration models depicting the functions of temperatures were developed using ocean calibration results. The beam radiometric imbalances were corrected using linear gain compensation models of antenna temperatures. The long-term instability was corrected using a linear dependence on microwave front-end and antenna temperatures. Following gain-related calibration, the beam imbalances and long-term instability of the HSCAT-B$\sigma ^{\circ }$were eliminated. The seasonal responses over the Amazon rainforest were also found to correspond to the QuikSCAT$\sigma ^{\circ }$measurements, with amplitudes of approximately 0.15 dB for the morning passes and approximately 0.1 dB for the evening passes. The corrected$\sigma ^{\circ }$will be more suitable for the generation of climate data records. In addition, with the exception of the prelaunch thermal vacuum test, the external calibrations using natural targets are considered to be alternative approaches to gain-related calibrations due to the temperature variations of radar electronics.

GaoFen-3 (GF-3) is a C-band multipolarization synthetic aperture radar (SAR) satellite with 12 imaging modes. However, its initial positioning accuracy remains unsatisfactory, thereby hindering its use for large-area surveying and mapping. This study proposes a block orthorectification method without ground control points (GCPs) using the GF-3 Fine strip II (FSII) mode. To address the challenges with the accuracy and efficiency of this method, an integrated block orthorectification method was developed to conduct integrated processing of large-scale GF-3 satellite images without GCPs. Geometric calibration was used to improve the absolute positioning accuracy of each SAR image. Then, several tie points (TPs) were extracted using the SAR scale-invariant feature transform (SIFT) operator. A parallel matching strategy was used in the block images registration. The block adjustment model was constructed to solve the orientation parameter of all SAR images. The experimental results of 1,468 GF-3 images of China’s entire land area show a TPs root-mean-square error of 0.724 pixel and 8.014 m for the independent checkpoint, suggesting that the proposed method can effectively improve the geometric accuracy of GF-3 satellite images and demonstrate the feasibility of large-scale SAR mapping without GCPs.

Calibration and validation (Cal/Val) of the sea surface height as measured by satellite radar altimeters is essential to understand altimeter biases, observation trends, and instrument aging. It also supports the long-term stability of the produced climate change records of sea level as determined by altimetry. In this article, we report the calibration of HY-2B and Jason-2/3 using the established research infrastructure and data sharing initiative introduced by the Altimetry Calibration Cooperation Plan (ACCP) of China. Currently, three ACCP calibration sites encompass the Wanshan Islands, and two national oceanic sites are located along the China coastline. For each Cal/Val site, the components of the facilities—the geodetic, sea level, and Global Navigation Satellite System (GNSS) infrastructure—and the followed monitoring procedures and calibration methods are described. The HY-2B performance was primarily evaluated using about two years data, which indicated a mean bias of −0.2 ± 4.2 cm. Confidence in the results is strong, because the HY-2B biases were cross compared and confirmed by all the three independent sites and the three satellite ground tracks. Compared with its predecessor HY-2A, HY-2B shows very stable observations with no linear drift at present. In addition, Jason-2 and Jason-3 were mainly assessed using Qianliyan site. Our results indicate that the Jason-3 sea-surface height bias is approximately 2–3 cm smaller than that of Jason-2 and that the long-term stability of Jason-2/3 shows no significant trend, which in good agreement with the international dedicated sites. The instrument noises of Jason-2/3 and HY-2B were estimated based on the ACCP sites. The results show that the instrument noise in the previous literature is underestimated. This was also consolidated by the result from wavenumber spectrum and global crossover point analysis. The code and Wanshan data used in these Cal/Val experiments are publicly available to facilitate further work in this domain (https://github.com/GenericAltimetryTools/CalAlti).

The Chinese Ocean Salinity Satellite is part of the Chinese ocean dynamic satellite series, and is dedicated to global SSS observation with two payloads onboard. These two payloads, named the Interferometric Microwave Radiometer (IMR) and the Microwave Imager Combined Active and Passive (MICAP), both adopt interferometric aperture synthesis technology. IMR is a two-dimensional L-band interferometric radiometer system with a Y -shaped antenna array. MICAP is a one-dimensional passive/active combined system, where the L-, C-, and K-band interferometric radiometers' antenna arrays are arranged in a line, and the active device is an L-band scatterometer. Based on the payload configurations, an end-to-end simulation shows that, with the assistance of the C- and K-band radiometers and the L- band scatterometer, the combined retrieval accuracy improves compared with the SSS retrieved using only L-band TB and auxiliary parameters. The best accuracy of SSS is achieved when considering all the available measurements from two payloads together in a combined retrieval.

China has launched five satellites scatterometers, namely, the Haiyang-2A scatterometer (HSCAT-A), Haiyang-2B scatterometer (HSCAT-B), China-French Satellite scatterometer (CSCAT), Haiyang-2C scatterometer (HSCAT-C) and Haiyang-2D scatterometer (HSCAT-D). This article aims to introduce calibration and validation methods for products of these Chinese scatterometers. Active transponders are used for post-launch absolute calibration, while the Numerical Weather Prediction Ocean Calibration (NOC) method is applied for post-launch relative calibration for the sigma0 observed in these Chinese scatterometers. As a complement to NOC, a natural terrestrial target of the Amazon rainforest is chosen as a calibration site. A long time-series analysis of sigma0 over the Amazon rainforest is also used to assess the temporal stability. To validate the winds from these Chinese scatterometers, the HSCAT-A/B/C and CSCAT winds are compared to in-situ buoy, ECMWF and ASCAT winds, respectively. The comparison results show that all these four Chinese scatterometers winds are comparable and meet the mission requirements, i.e., the error is less than 2 m/s or 10% in terms of the speed and 20° in terms of the direction. HSCAT-B and HSCAT-C perform better than HSCAT-A and show similar qualities to that of ASCAT-A.

Model-based incoherent polarimetric decomposition is a frequently used technique to analyze multilook data of polarimetric synthetic aperture radars (POLSARs). The purpose of this study is to analyze and compare different model-based incoherent polarimetric decomposition algorithms from the polarimetric information change aspect. For the input of a model-based incoherent polarimetric decomposition algorithm, polarimetric entropy was used to represent the polarimetric information of a coherency matrix. For the output of a model-based incoherent polarimetric decomposition algorithm, there are usually several decomposed components. To quantitatively represent their total polarimetric information, a new concept, generalized polarimetric entropy, was proposed which generalized the concept of polarimetric entropy based on the information entropy additivity of information theory. Generalized polarimetric entropy consists of two parts named as polarimetric power entropy and polarimetric residual entropy, respectively. Polarimetric power entropy describes the distribution status of the Span values of all decomposed components. Polarimetric residual entropy represents the residual randomness of all decomposed components. With the three new concepts, eight model-based incoherent polarimetric decomposition algorithms were compared and analyzed. Two real POLSAR images, respectively, derived by the E-SAR airborne system of Germany and the GF-3 satellite of China were used for the experiments. Experimental results had illustrated several useful conclusions.

Ocean color satellite sensors have become an indispensable component in the Earth Observing System, in which the use of multiple ocean color satellite sensors not only improves the spatiotemporal coverage of the global oceans but also maintains the continuity of the data products for long-term monitoring. In this research, the performance of a new ocean color satellite sensor Chinese Ocean Color and Temperature Scanner (COCTS) from HY1C launched in September 2018 is thoroughly evaluated with two important aspects: the signal-to-noise ratio (SNR) at the top of the atmosphere and the uncertainty in the remote-sensing reflectance Rrs) products. The results showed that the SNR of the COCTS can satisfy the requirements of the ocean color applications, and the uncertainty in the Rrs at the blue bands in the ocean waters meets the demand of less than 5%. A further comparison with other well-known ocean color sensors indicates that not only the COCTS can provide reliable ocean color data but also the processing system is robust and reliable. These results provide a solid base for merging the COCTS products with other ocean color sensors for the studies of ocean biogeochemistry.

The Chinese ocean color and temperature scanner (COCTS) onboard the HY-1C satellite was launched on September 7, 2018, and has been providing global multispectral Earth observation data as a new spaceborne sensor for ocean color detection since September 10, 2018. In this study, an atmospheric correction algorithm using a composite of the algorithms developed by Wang and Gordon (1994) and Heet al.(2004) and a chlorophyll-a concentration retrieval method that is a composite of the OC4 and color index (CI) algorithms are used to derive the global chlorophyll-a concentration from COCTS. The retrieval products are validated againstin situdata measured in the East China Sea and the South China Sea during the in-orbit testing activity for the HY-1C satellite with an unbiased percentage difference (UPD) of 39%, and the data are also validated against the Aerosol Robotic Network-Ocean Color (AERONET-OC) data with a UPD of 39%. The daily global chlorophyll-a concentration from COCTS with a gridded resolution of 9.2 km and a time period from September 10, 2018 to February 29, 2020 is compared with the same products from MODIS and VIIRS. The UPDs of COCTS against MODIS onboard Terra are approximately 20%, and the mean biases (in logarithm form) are approximately zero. Examples of chlorophyll-a concentration retrieval results in specific cases along the eastern coast of China and the Kuroshio Current are also presented to show the performance of the algorithms used in this study. The retrieval and evaluation results show that COCTS onboard HY-1C demonstrates satisfactory performance for global chlorophyll-a concentration observations.

The HY-2B scatterometer (HSCAT-B) has now completed a one-year orbit. The instrument was declared operational after six months of in-orbit testing. The ocean wind field products are routinely being made available to the global science community. The HY-2B will work with the HY-2A to provide the Ku-band sea surface observations on a long-term scale, and this collaboration is expected to be important for disaster prevention and mitigation, as well as globally focused climate change research. However, to obtain reasonable evaluations of the system’s functional, performance, and instrument stability results, the temperature, internal calibration parameters, attitude angles, and normalized radar cross section (NRCS) of the instrument were rigorously analyzed during the in-orbit tests. This study provided an overview of the six-month in-orbit testing process results. The temperature levels of some important parts of the scatterometer were found to meet the design requirements. The internal calibration and noise power had shown similar trends and were both within the normal range. The attitude angles, which mainly included the roll and pitch, had displayed variations within a very small scope. The evaluation results of the Amazon rainforest observations indicated that the scatterometer instrument had met all the design requirements and had established a good database for geophysical processing.

China's ocean dynamic monitoring satellite Haiyang-2B (HY-2B) has been in orbit for more than one year. Payload named scanning microwave radiometer (SMR) with five frequency channels onboard the HY-2B is an instrument mainly used to monitor and study ocean environment and to obtain ocean dynamic parameters like sea surface wind, temperature, etc. One issue is that high wind speeds in typhoons cannot be monitored by brightness temperature (TB) data at high frequency channels due to heavy rain, while the low frequency channel TB are capable of high wind speed retrieval but are limited to the low spatial resolution. This paper intends to enhance SMR's spatial resolution in low frequency channels by applying the interpolation method developed by Backus and Gilbert. Preliminary results indicate that TB data after spatial resolution enhancement show more details of typhoon structures and will provide more information for the high wind speed retrieval.

Chinese ocean salinity satellite is a mission designed for global ocean salinity monitoring. Two main payloads equipped onboard the satellite are the Interferometric Microwave Radiometer (IMR) and the Microwave Imager Combined Active and Passive (MICAP) which both have L-band radiometer and adopt aperture synthesis technology. A simulation is designed according to the payloads' configurations and consists of three parts: the brightness temperature generator, the radiometer system and the parameters retrieval. Preliminary results, which contain IMR and MICAP's brightness temperature resolution, spatial resolution, and parameter retrieval accuracy, are presented.

The daily-composited 9 km sea surface temperature (SST) data from the 10-band Chinese Ocean Color and Temperature Scanner (COCTS) onboard the Haiyang-1C (hy -1C) satellite are evaluated for global area against daily 9 km SST product from the Moderate Resolution Imaging Spectroradiometer (MODIS) onboard the Terra satellite and in situ data. Around four months of the data are used for the validation from Jun. 18, 2019 to Jan. 9, 2020. The in situ SST data are averaged at a spatial resolution of 9 km which is similar to that of the COCTS and MODIS SST data. For daytime, the bias, standard deviation (STD) and robust standard deviation (RSD) of the SST difference between COCTS and buoy data are -0.31 °C, 1.09°C and 0.66°C. And the bias, standard deviation and RSD of the SST difference between COCTS and MODIS data are -0.14°C, 0.95°C and 0.60°. The error sources are discussed. The results indicate that studies should focus on error sources to improve the HY-1C/COCTS SST products accuracy.

This letter preliminarily assesses the accuracy of ocean wind products from the Ku-band scatterometer (HSCAT) onboard the recently launched Chinese satellite HY-2B. The wind vectors derived from HSCAT during the period from November 15, 2018 to April 30, 2019 are evaluated. The reference wind data include in situ measurements from the offshore meteorological buoys of the National Data Buoy Center, USA, and westerlies mooring of the National Ocean Technology Center, China, and scatterometer winds from the European Advanced Scatterometer (ASCAT) and Indian difference is limited to 25/V2 km, while the HSCAT winds are SCATSAT-1 Oceansat Scatterometer (OSCAT). The spatial temporally collocated with buoys and other scatterometers by less than 0.5 and 1.5 h, respectively. The comparison results show that the HSCAT data have a root-mean-square error (RMSE) of 0.95-1.20 m/s (14.7°-25.7°) regarding the wind speed and direction, respectively, indicating consistency between the HSCAT winds and the reference. Furthermore, better agreement is found for the HY-2B HSCAT winds processed using the Pencil-beam Wind Processor (PWP) algorithm, regarding wind speed (RMSE of 0.95-1.07 m/s) and particularly with respective to the wind direction (RMSE of 14.7°-19.6°), both satisfying the mission specification (<; 2 m/s and <; 20 for wind speed and direction, respectively). The encouraging validation results over the first 5 months demonstrate that the HY-2B HSCAT wind products will be useful for the scientific community.

Polarimetric symmetry, including reflection symmetry, rotation symmetry, and azimuthal symmetry, are useful concepts for describing the scattering characteristics of polarimetric synthetic aperture radar (SAR) data. In this study, for the coherency matrices of multilook polarimetric SAR data, three new scattering models were first proposed based on the polarimetric symmetry. Then, a four-component incoherent decomposition algorithm was introduced, which was based on the three aforementioned new models, along with a classic volume scattering model. The proposed algorithm was considered to be a complete incoherent decomposition algorithm. No polarimetric information was lost during the decomposition. Also, there were no negative powers in its output. Then, by utilizing three polarimetric SAR images which had been derived by ESAR, RADARSAT-2, and GF-3, respectively, the experimental results had revealed that the proposed decomposition algorithm was effective in the analysis of the scattering mechanisms of terrain targets. It was observed that more than 90% of the coherency matrices of the real polarimetric SAR data had been completely decomposed by the proposed algorithm into four components. These components were found to be exactly consistent with the scattering models for surface scattering, double-bounce scattering, volume scattering, and helix scattering. The proposed decomposition algorithm had provided insight into the limitations of incoherent decomposition.

The new Ku-band scatterometer (HSCAT-B) onboard the HY-2B satellite was launched on October 25, 2018, and soon after the C-band scatterometer (Advanced Scatterometer (ASCAT)-C) onboard the MetOp-C satellite was launched on November 6, 2018. This article aims to validate the new sea surface wind products from them, and also to summarize the common issues in current scatterometer wind products. Thus, other scatterometer data are also used for comparisons, including the C-band MetOp-B/ASCAT, and Ku-band SCATSAT-1/OSCAT2 and HY-2A/SCAT winds. In this study, the C-band and Ku-band scatterometer wind products were each reproduced using the same procedures, in terms of backscatter calibration, wind retrieval, and quality control. The scatterometer winds are compared to the European Centre for Medium-Range Weather Forecasts (ECMWF) ERA5 winds or buoy winds, and the results show that the quality of ASCAT-C winds is almost the same as the well-known ASCAT-B; the HSCAT-B winds show quite good quality and similar validating statistics as ASCAT winds. Noticeable wind-speed-dependent biases are found in all Ku-band scatterometer winds, which suggests that refinements are needed for the NSCAT-4 geophysical model function, especially in terms of wind speed dependence for all incidence angles.

Freeman-Durden decomposition is a frequently used technique to analyze the scattering characteristics of multilook Polarimetric Synthetic Aperture Radar (POLSAR) data. When it is applied to real POLSAR data, two problems emerge, which are the volume scattering overestimation and negative powers. Many researchers think these two problems are caused by the insufficient decomposition algorithm, and several improvements are proposed. However, the improved decomposition algorithms become more and more complicated, and some new problems such as the decomposed component is not model-based also emerge. In this article, we try to solve the two problems through another way. We think they are caused by the dogmatic input rather than the insufficient decomposition algorithm. Freeman-Durden decomposition explicitly assumes reflection symmetry. Its input is a direct truncation of the measured coherency matrix. The truncation can be regarded as a Reflection Symmetry Approximation (RSA) of the measured coherency matrix. We firstly show some reasons why we think the truncation is not a good RSA. Then a new RSA is proposed based on the sum of three reflection symmetry components derived from the measured coherency matrix. Experimental results with several real POLSAR images show that, if the new RSA is used as the input of Freeman-Durden decomposition, the abovementioned two problems no longer exist.

HY-2B microwave scatterometer(HY-2B/SACT) is a Ku band rotating pencil-beam scatterometer that carried on-board the HY-2B satelllite. This paper constructed a retrieval algorithm of operational wind fields for HY-2B/SACT. Microwave scatterometer wind field inversion uses its L1B product. To prepare the sigma0 measurements for conducting wind vectors retrieval, the time ordered L1B product was regrouped to the sub-track aligned wind vector cells (WVCs). The maximum-likelihood estimator (MLE) was applied to retrieve wind vector for each WVC with the NSCAT-4 as geophysical model function (GMF). Furthermore, circular medium filter (CMF) is used to remove the ambiguities solutions. The sea surface wind field retrieved by HY-2B/SACT were evaluated by comparing with ECMWF wind field, in-situ measurements and NDBC buoy observations. The wind speed and wind direction retrieval accuracy is better than 1.5m/s and 19° respectively. This result showed that the wind products provided by HY-2B/SCAT are able to meet the requirements of science and operational users.

The HY-2B satellite is the second dynamic environment satellite in China. It was successfully launched on October 24th 2018 with a sun-synchronous orbit at an altitude of~970km. Repeat cycles of 14 days are planned for the first two years with oceanographic purpose and 168 days geodetic cycles will follow for the third year of the mission. The satellite is equipped with a Ku/C bands altimeter and the orbit is determined thanks to SLR, GPS and DORIS systems.

To analyze the wind power and develop tools for the offshore wind farm siting in the coastal region of Guangdong with water depth 30~50m, wind data from satellites, the CCMP analysis, Lidars, wind towers, buoys, a high-resolution numerical model are used to calculate wind resources and wind energy reserves. Overall, the northeastern wind with speed above 8m/s is prevailing in Autumn and Winter and the southern wind with low speed is prevailing in Spring and Summer. The eastern part of the off-shore region of Guangdong province is better for wind farm siting than the western region.

To analyze the wind power and develop tools for the off-shore wind farm siting in the coastal region of Guangdong with water depth 30~50m, wind data from satellites, the CCMP analysis, Lidars, wind towers, buoys, a high-resolution numerical model are used to calculate wind resources and wind energy reserves. Overall, the northeastern wind with speed above 8m/s is prevailing in Autumn and Winter and the southern wind with low speed is prevailing in Spring and Summer. The eastern part of the off-shore region of Guangdong province is better for wind farm siting than the western region.

The algorithm of inversing the SST from the SMR onboard on HY-2B satellite is an empirical regression method based on the physical model to calculate the empirical relationship between the brightness temperature and the different physical parameters of the ocean and atmosphere. In order to test the preliminary result of SST retrieved from HY-2B satellite SMR, the SST is validated according to the IQUAM in-situ SST and the space-borne WindSat SST. The performance of the instrument is still being estimated and SST retrieval algorithm is still improving.

HY-2B satellite is launched on 25thOct, 2018 at Taiyuan satellite launch center. It carried four main payloads: radar altimeter, microwave scatterometer, scanning microwave radiometer, and corrected microwave radiometer. HY-2B microwave scatterometer (HY-2B/SCAT) is a pencil-beam rotating scatterometer, which is simillar to HY-2A microwave scatterometer (HY-2A/SCAT). HY-2B satellite is in the same orbit of HY-2A, and about 30 min ahead of HY-2A. The HY-2B microwave scatterometer started to work on 30thOctober, after days of testing. The performance of HY-2B/SCAT become stable since 14th, Nov. This paper provied some preliminary results of HY-2B/SCAT data, such as pulse power DN value, sigma0, retrieved sea surface wind field.

Synthetic aperture radar (SAR) is widely used to detect sea surface wind. It has been found that the signals of co-polarization SAR tend to become saturated under high wind-speed conditions. Also, low signal-to-noise ratios of cross-polarization SAR have been observed under low wind-speed conditions. In this letter, a typhoon wind-field retrieval method is developed by compositing the wind speeds retrieved from dual-polarized SAR images. This letter then applies the proposed method to eight typhoons (four from the Canadian RADARSAT-2 satellite and four from the Chinese Gaofen-3 satellite). The results show that the method is successful in regard to fine typhoon wind structure retrieval, with a root-mean-square error of wind direction of 12° and a wind speed of 1.7 m/s, against 12 oceanic buoy measurements in the South China Sea.

Freeman-Durden decomposition is a frequently used technique to analyze the scattering characteristics of multilook Polarimetric Synthetic Aperture Radar (POLSAR) data. When it is applied to the real POLSAR data, two problems emerge, namely, the volume scattering overestimation and negative powers. Many researchers think these two problems are caused by the insufficient decomposition algorithm, and several improvements are proposed. However, the improved decomposition algorithms become more and more complicated, and some new problems such as the decomposed component is not model based also emerge. In this paper, we try to solve the two problems through another way. We think they are caused not by the insufficient decomposition algorithm but by the dogmatic input. Freeman-Durden decomposition explicitly assumes reflection symmetry. Its input is a direct truncation of the measured coherency matrix. The truncation can be regarded as a reflection symmetry approximation (RSA) of the measured coherency matrix. We first show some reasons why we do not think the truncation is a good RSA. Then, a new RSA is proposed based on the sum of three reflection symmetry components derived from the measured coherency matrix. Experimental results with several real POLSAR images show that, if the new RSA is used as the input of Freeman-Durden decomposition, the above-mentioned two problems no longer exist.

Nowadays, several unitary transformations have been used in model-based incoherent decomposition of polarimetric synthetic aperture radar data. However, the physical meanings of these unitary transformations are still vague (except the orientation angle compensation). In this paper, we propose calling a unitary transformation as the Helix Angle Compensation (HAC), and detailed analyses on its physical meanings are presented. We show that HAC denotes the structure information of a target that it contains different double-bounce scattering components with both different orientation angles and different distances form radar along the line of radar sight. HAC is used to modify a three-component incoherent decomposition algorithm, and makes its third component exactly consistent with the surface scattering model or the double-bounce scattering model. Experiments with real polarimetric SAR date are presented to demonstrate the effectiveness of the modified decomposition and illustrate several properties of HAC.

Imaging ALTimeter (IALT) is a new type of radar altimeter system, which observes the earth from 2° to 7° incident angles. In comparison to the conventional altimeters such as HY-2A altimeter, Jason-1/2, TOPEX/Poseidon, which observe the ocean at nadir, the swath of IALT is much wider and its spatial resolution is much higher. The IALT on board Tiangong-2 space station is launched on 15th September, 2016 at Jiuquan Satellite Launch Center. The in-orbit assessment of IALT is done until 30th April, 2017. In this paper, the ocean surface wind speed inversion method based on IALT is established. The neural network algorithm is used for ocean surface wind speed retrieval, and the spatial resolution of retrieved wind speed is 25km. The wind speed inversion accuracy is evaluated by comparing with the ECMWF reanalysis wind speed, buoy wind speed, and boat measurement wind speed. The results show that the Root-Mean-Square (RMS) of retrieved wind speed is 1.85m/s, and the Bias of retrieved wind speed is about -0.21m/s. The wind speed inversion accuracy satisfies performance requirement.

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.

Ocean surface wind field is a very important marine dynamic parameter in marine environment forecasting and climatological studies. Spaceborne scatterometer is one of the most efficient remote sensors than can provide global ocean surface wind measurement. Polarimetric scatterometer (PolScat) simultaneously measures co-polarized and cross-polarized backscattering coefficient and the correlation coefficient of the co- and cross-polarized component of radar echoes which can significantly improve the performance of the sea surface wind field measurements. In this paper, we derive the error model of correlation scattering coefficient from radar echo signals. The effect of antenna polaxis deflection on correlation scattering coefficient error is analyzed. Moreover, an “end-to-end” system simulation model of PolScat is established. A modified Maximum Likelihood Estimation (MLE) is used for ocean surface wind field inversion. Both the co-polarized backscattering coefficient and correlation scattering coefficient are included in the objective function of MLE. The simulation results show that PolScat can effectively reduce the probability of ambiguous solutions. The wind speed and wind direction inversion accuracy of PolScat is better than 1m/s and 15° respectively.

The Chinese HY-2A satellite microwave scatterometer (HSCAT) is designed to measure the ocean surface wind vector with a large observation swath. HSCAT is a Ku-band scatterometer that has continuously observing the earth’s surface for more than 6 years. Approximately, 30 000 orbit revolutions of scientific data are archived in its ground application system. To further understand the characteristics of the instrument, deeper analyses of these scientific data must be carried out. Currently, the geolocation accuracy of the normalized radar cross section (NRCS) has yet to be documented. However, geolocation errors can cause significant measurement bias, and have large impacts on the calibration and validation results. In this paper, the coastline crossing detection method is used to evaluate the geolocation errors of the NRCS data. Some coastal areas and islands where the NRCS data have large gradients are selected for the geolocation error analysis. The coastline inflection points can be identified on the basis that the radar backscatter of land and water exhibits a significant difference when the ocean surface wind speed is relatively low. The longitudes and latitudes of these points from different time periods are then fit to the global self-consistent hierarchical high-resolution shorelines map. The comparison results show that the overall distance error of HSCAT is approximately 2.59 km. This geolocation accuracy meets the instrument design requirements. The sources of geolocation errors are also analyzed in this paper in order to improve the geolocation accuracy.

Based on multi-temporal observations of static targets on sea surface, a positioning shift that relates to geodetic terrain heights (GTH) is found. The positioning shift mainly exists in the range direction of a SAR image and has an almost linear relationship with the difference between the real GTH of a target and the GTH of a SAR product. An approach of compensating the positioning shift is proposed based on rigorous projection models of RADARSAT-2. Experimental results illustrate that the positioning error of a static target which is derived using SCW products with different GTHs is 446.63m. After applying the proposed positioning shift compensation approach, the positioning error of a static target becomes 121.09m, which demonstrates the effectiveness of the proposed positioning shift compensation approach.

Ocean surface current is a very important parameter of ocean dynamic environment. The observation and prediction of ocean surface current has attracted more and more concern. Doppler Scatterometer (DopScat) is a new type of radar for ocean surface wind and current field remote sensing. The ocean surface current inversion method of DopScat impacts the measurement accuracy directly. In this paper, we establishes the Maximum Likelihood Estimation(MLE) method to retrieve the ocean surface current and wind simultaneously. The retrieval accuracy for different position in cross-track, wind speed, and current speed are analyzed. The retrieval results show that the RMS of inversion current speed and direction can be smaller than 0.18m/s and 25°respectively, for medium wind speed condition. This is submitted for the special session of “New Developments of Chinese Oceanographic and Meteorological Satellites”.

Microwave remote sensing is one of the most useful methods for observing the ocean parameters. The Doppler frequency of the radar echoes can be used for ocean surface current speed retrieval. While the effect of the ocean currents and waves are interactional. In this paper the suitable ocean wave elevation spectrum and directional distribution function are selected by comparing the ocean Doppler spectrum in C band with the empirical geophysical model function (CDOP). The simulation results show that the ocean surface current speed error is sensitive to the wind speed and wind direction error. With VV polarization, the ocean surface current speed error is about 0.15 m/s when the wind speed error is 2 m/s, and the ocean surface current speed error is smaller than 0.3 m/s when the wind direction error is within 20° in the cross wind direction. This is submitted for the special session of “New Developments of Chinese Oceanographic and Meteorological Satellites”.

The Ocean Salinity (OS) Satellite mission of the State Oceanic Administration of China dedicates to an “all-weather” estimate of high-quality global SSS and to reduce geophysical errors due to surface roughness and sea surface temperature from space. The payload of this mission has the capability of L/C/K multi-frequency passive and L-band active measurement, and can implement the simultaneously remote sensing of SSS, SST, WS and atmospheric parameters. The OS Satellite data processing prototype software has been developed to simulate the instrument and to generate Level 0 to Level 2 data. Based on the prototype software, the end to end study of the OS Satellite mission is performed and the errors of simultaneous retrieval of multiparameters are analyzed and noises level and stability requirement of instruments are estimated.

The ocean surface current is a very important parameter of ocean dynamic environment. It is connected to global climate change, marine environment forecasting, marine navigation, engineering security, and so on. The observation and prediction of ocean surface current have attracted more and more concern. Doppler Scatterometer (DopScat) is a new type of radar for ocean surface wind and current field remote sensing. The ocean surface current inversion method of DopScat impacts the measurement accuracy directly. In this paper, we establish the simulation model of a DopScat and provide the radial velocity error model. The numerical ocean surface Doppler spectrum model is also introduced and validated with the empirical geophysical model function in C-band (CDOP). The suitable ocean wave elevation spectrum and directional distribution function are selected. What is more, this paper establishes the maximum likelihood estimation (MLE) method to retrieve the ocean surface current and wind simultaneously. The retrieval accuracy for different positions in cross track, different wind speeds, and different current speeds are analyzed. At last, the global ocean current field is observed by DopScat and the ocean current is retrieved. In our simulation, the orbit parameters and observation geometry of DopScat are the same as that of HY-2A scatterometer. The retrieval results show that global current speed standard deviation can be smaller than 0.18 m/s for five days and$0.5 {^{\circ }} \times 0.5 {^{\circ }}$grid average.

A three-component decomposition algorithm is proposed for polarimetric SAR data. After extracting the volume scattering component, both the orientation angle compensation and a unitary transformation are applied to the remaining matrix to derive the second and third components which are exactly consistent with either the surface scattering model or the double-bounce scattering model, respectively. We suggest calling the unitary transformation as the helix angle compensation and show that the remaining matrix is decomposed into two coherency matrices with an orientation angle difference of 45° and opposite helix angles. Experiments with real polarimetric SAR date are presented to demonstrate the effectiveness of the proposed decomposition algorithm and illustrate its several properties.

As the launch of radars, such as the Precipitation Radar (PR) on Tropical Rainfall Measuring Mission (TRMM)[1] satellite and the Surface Wave Investigation and Monitoring (SWIM) on China France Oceanography SATellite (CFOSAT)[2], that operate in low incident angles, more and more NRCS data at low incident angle will be obtained. In order to retrieve the sea surface wind speed using low incident angle NRCS, the empirical GMF of NRCS with wind speed is established. The empirical nadir reflection coefficient |R(0)2| are calculated and the empirical relationship between mean square slop s(u) and wind speed is established. The mean square slop s(u) can be retrieved by fitting the NRCS at certain incident angles with the theoretical Gaussian GMF model. Then the wind speeds are calculated using the empirical corresponding relation between mean square slop and wind speed. The retrieved wind speeds are compared with Tao and NDBC buoy. The results show that the standard deviation (STD) and bias of retrieved wind speeds are smaller than 1.7m/s and 0.1m/s respectively.

This paper reviews several wind quality-sensitive parameters derived from HY-2A scatterometer data, such as the wind-inversion residual (or Maximum Likelihood Estimator, MLE) and its spatially averaged value, and the singularity exponent (SE) derived from an image processing technique, called singularity analysis. Their sensitivity to data quality is evaluated using the collocated European Centre for Medium-range Weather Forecasting (ECMWF) model output and satellite radiometer rain data. It shows that SE is the best quality indicator, followed by the spatially averaged MLE and the conventional MLE. A set of MLE and SE thresholds are derived from the sensitivity analysis in order to optimize the quality control (QC) for the HY-2A scatterometer.

GF-1 is the first satellite of this Chinese civilian remote sensing satellites series. This paper presents some typical applications on marine environmental monitoring with high resolution and wide swath satellite data, especial on the marine disaster monitoring such as oil spill, sea ice, and red tide. With these data, we can get the detailed information about different disaster.

The absolute SSH (sea surface height) biases of three satellite altimeters Jason-2, Saral/AltiKa and HY-2 were determined using our GPS buoy at the Qian Li Yan Island of China, which are 9.3cm, 1.3cm and 66.8cm, respectively. In addition, the altimetry SWH (significant wave height) were assessed using GPS retrieved SWH, which shows good agreement between the GPS buoy and satellite altimeters. The detailed method and result are described in the paper.

HY-2 satellite was successfully launched on 16 August, 2011. It carried three main microwave instruments into space for operationally observing dynamic ocean environment parameters on a global scale. HY-2 satellite altimeter provides sea surface height, significant wave height, sea surface wind speed. Current status of HY-2 satellite altimeter is put forward in this study. By comparison with the other satellite data, NDBC data and other data, the accuracy of the HY-2's data products is evaluated in this work.

On August 16, 2011, the HY-2 satellite was successfully launched by a CZ-4B rocket from the Taiyuan site in Shanxi, China. HY-2 satellite, which is an ocean dynamic environmental satellite on-orbit operation in the world, has four payloads of microwave sensing. It can synchronous measure the ocean dynamic environmental information, such as wind field, significant wave height, sea surface height and sea surface temperature.

This paper presents a result of crossover analysis between HY-2 and Jason-2 mission over ocean. The major objectives of this paper are to assess HY-2 IGDR (Interim Geophysical Data Record) derived SSHs by comparing HY-2 measurements with the reference mission Jason-2, and further to illustrate the potential of HY-2 data in monitoring global sea level variability. The instrument-independent models and data are applied for both HY-2 and Jason-2 to correct the errors including range delays and geophysical effects, thus provide an objective assessment. All the results indicate a good performance of HY-2 measurements. A standard deviation of 6.6 cm and a bias of 0.26 cm between HY-2 and Jason-2 SSHs (Sea Surface Height) is acquired from 50°S to 50°N which is close to the Jason-1 and Jason-2 performance. The results suggest that a promising situation in terms of HY-2 observations.

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°.

The Bohai sea is a semi-enclosed sea located in the northeast of China. Safety of ship navigation and exploration platforms are important issues. Reliable near real time ice information is necessary and the ice thickness distribution is still a challenge. Here we present an ice thickness estimation method combining numerical sea ice model and SAR data. A high resolution thermodynamic snow and ice model (HIGHTSI) is applied to calculate the thermodynamic ice growth and used as the ice thickness background. SAR images are used to express the local ice statistics and to redistribute the modeled ice thickness. The results are evaluated by comparing with in-situ observations from oil platforms and ice forecast results from National Marine Environmental Forecasting Center (NMEFC).

The research constructed a retrieval algorithm of routine wind fields for microwave scatterometer onboard HY-2A (HY-2A/SACT). To prepare the sigma0 measurements for conducting wind vectors retrieval, the time ordered L1B product was regrouped to the sub-track aligned wind vector cells (WVCs). Besides, the authors utilized an extension land and ice mask to simplify the surface flag process. The maximum-likelihood estimator (MLE) was applied to retrieve wind vector for each WVC with the NSCAT-2 as geophysical model function (GMF). Furthermore, to greatly improve the ambiguity removal skill, the research used circular median filter to remove the ambiguities, and the NCEP wind field as a background to re-initialized wind field comprising of ambiguous wind vectors. Wind vectors observed by HY-2A/SACT were evaluated by comparing with NDBC buoy observations. The RMS difference for wind speed was less than 2 m/s or 10%; while the RMS difference for wind direction was less than 20°comparing with wind direction. This result showed that the wind products provided by HY-2A/SCAT are able to meet the requirements of science and routine meteo users.

To preserve the power information in the output of polarimetric whitening filter (PWF), a modified PWF is proposed for polarimetric synthetic aperture radar (PolSAR) imagery. Besides minimizing the standard deviation to mean ratio, the modified PWF also requires its output closest to the original polarimetric vector in a minimum mean-square error sense. This principle is consistent with that of the minimum mean-square error covariance shaping. A real PolSAR image derived by SIR-C is applied for the illustration. The experimental results demonstrate that the speckle reduction performance of PWF is enhanced by the proposed modification and the power information is well preserved in the output of the modified PWF.

According to the lower efficiency and larger wind direction errors in the nadir region of the swath, a new combined wind retrieval algorithm is proposed for conically scanning scatterometer in this paper. The presented algorithm has the dual advantages of both higher efficiency and higher wind direction retrieval accuracy by combining the wind speed standard deviation algorithm and the wind direction interval retrieval(DIR) algorithm. It adopts wind speed standard deviation as criterion for searching possible wind vector solutions and retrieves potential wind direction interval for the first and second ambiguities based on the change rate of the wind speed standard deviation. Some SeaWinds L2A data and collocated buoy data were used to validate the algorithm. Retrieval experiments indicated that the algorithm can significantly reduce the wind direction retrieval errors in the nadir region.

Experiments and theoretical analysis demonstrate that the polarimetric scatterometer has the potential in enhancing the accuracy of the wind vector retrieval. However, currently, there is no special wind vector retrieval algorithm for the polarimetric scatterometer. Based on the distribution characteristic of the objective function, a modified wind vector retrieval algorithm was designed for the conically scanning polarimetric scatterometer in this paper. Simulation experiments indicated that this algorithm could further improve the retrieval precision of the polarimetric scatterometer in comparison with the traditional algorithm, especially in the nadir- and outer-swath. By extending the wind direction range for the first and second ambiguity in the nadir- and outer-swath, the algorithm can effectively reduce the uncertainty of the wind direction solutions with error magnitude of 0° to 15°. Up to 2° improvement in wind direction retrieval can be achieved in the nadir track.

In order to make improved use of the altimetry waveform data near the China coastal area, we present a new retracker (OceanCS) consists of waveform Classification and Sub-waveform extraction based on coastal altimetry waveform analysis and existing retracking algorithm comparison. The OceanCS retracker combines the advantages of Ocean retracker and OCOG retracker. The new retracker is further compared to other retrakers (Ocean, Ice-2, OCOG, Threshold, Beta5, and GDR product) using one year (Mar. 2006 to Feb. 2007) Jason1 measurement and in-situ tide gauge station Sea Surface Height and buoy Significant Wave Height measurements. Comparison shows that the OceanCS retracker performs better than other trackers. Furthermore, Calculating the SWH by combining two retrackers is a way to compare different retracking algorithms without taking into account the error of the atomosphere and sea state corrections.

An automated typhoon identification algorithm using QuikSCAT wind data is developed. The histogram features of the wind speed and wind direction of typhoon is used for a coarse identification, and then a more precise identification which makes use of the circulation property of typhoon is applied to the wind data which can pass the coarse identification, so that the typhoon observed by QuikSCAT can be identified automatically. As an example, the identification method is applied to identify typhoon Morakat 2009 from a sequence of QuikSCAT wind data distributed by PO.DACC, the results show that the typhoon can be correctly identified with our method, which demonstrates the feasibility and usefulness of our method.