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

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.

Decades of ocean surface vector wind (OSVW) data acquired from space-based radar scatterometry have been providing short and long-term researches and applications information about ocean surfaces. The main objective of the project, stands and metrics of ocean surface vector wind by space-borne microwave remote sensing, of W orking Group on Calibration and Validation of the Committee on Earth Observation Satellites (CEOS WGCV), is to develop the standard and guideline for the requirement, procedure, processing and assessment for the space borne radar scatterometer measurement calibration, wind retrieval approaches, wind data validation and assessment for OSVW, which will be used to assure the consistency of the data quality of these satellites and instruments are the prerequisite for related scientific researches and applications. This synthesizes calibration, standardized practices of retrieval approaches for ocean surface winds, development of guidelines/standards of validation of ocean surface winds, and identifying and organizing collocation related data. This presentation will provide an overview of the proj ect and the recent progresses.

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.

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.

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.

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.

Taking Yongle Atoll of Xisha Islands as the research area, the information extraction of reef landforms is studied by using the characteristics of HY-1C satellite wide-band, wide-spectrum and short revisit period. The calculation of CZI band histogram and signal-to-noise ratio shows that the images signal to noise ratio in the four bands of CZI of HY-1C satellite is 0.84 and the coverage value of the blue band is 28.53, which is the most obvious in the island landform, while the information of red band is not so obvious about the island reef. The payload CZI can express the information of coral reefs, and unsupervised classification method can be used to obtain the information of reefs, but the information of underwater reef flats is easy to be misjudged. In order to make full use of the band information of the HY-1C satellite, the weights of each band to the reef information expression are established by using the analytic hierarchy process (AHP). Based on the weight composite image, the information of reefs is extracted by supervised classification method, which will lead to effective extraction of information of natural islands, sandy beaches, underwater reef flats, hidden sand, gullies and so on. By consistency test, the Kappa coefficient can reach 89.97%, which can meet the requirements of large-scale and high-frequency island reef survey on the basis of medium-resolution remote sensing images.

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.

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.

Rotating fan-beam scatterometer (RFSCAT) is a new type of satellite scatterometer that was proposed about one decade ago.However, just as other rotating scatterometers, relatively larger wind retrieval errors occur in the nadir and outer regions than in the middle regions of the swath. In order to address this problem, a modified wind vector retrieval algorithm for RFSCAT is presented in this paper. The new algorithm is featured with adaptively extending the range of wind direction for each wind vector cell position across the whole swath according to the distribution histogram of the retrieved wind direction bias. Simulation experiments demonstrated that the new established algorithm can effectively improve the wind direction retrieval accuracy in the nadir and outer regions of the RFSCAT swath.

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.

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.