Ocean currents and winds are crucial parameters to understand ocean-atmosphere interactions, while the simultaneous retrievals using Doppler scatterometry can provide direct observational support. To obtain high-quality airborne Doppler scatterometer wind and current products, accurate estimation of the observed backscatter coefficient and reliable wind field inversion are essential. Based on the Ocean Surface Current Observation Mission (OSCOM) airborne experiment data collected using a Ka-band rotating pencil-beam Doppler scatterometer, we propose two different calibration methods for the backscatter coefficient to account for the larger-than-expected azimuthal modulation of the backscatter signal, as predicted by consolidated Geophysical Model Functions (GMFs) used in Ka-band scatterometry. Both methods are based on the numerical ocean calibration (NOC) approach, which is in turn based on the estimation of the mean backscatter differences between real measurements and simulated ones with the use of the GMF and reference winds. The first method employs an azimuth-dependent calibration, which can be implemented using either an overall ratio or a ratio per flight leg. The second method involves modifying the GMF to match the observed azimuthal modulation, with options for one or two GMF coefficient adjustments. The retrieved wind speeds range from 4 to 7 m/s, with wind directions around 155°. In comparison with collocated ECMWF winds, the wind speed biases of different methods are all lower than 1.2 m/s, and the wind direction standard deviations (SD) are lower than 9.3 °. The azimuth-dependent calibration method yields smaller wind speed biases but larger wind direction SDs compared to the modified GMF method. The azimuth-dependent calibration using leg-dependent ratios leads to the closest retrieved wind speeds and directions to ECMWF. The calibration methods proposed in this study provide data support for future simultaneous retrieval studies of ocean winds and currents. Additionally, these methods can be applied to other airborne Doppler scatterometer experiments.

Satellite-derived, coincident ocean surface winds and currents are of great importance to enhance our understanding of air-sea interactions. The data from a flight campaign, carrying a Ka-band rotating pencil-beam Doppler scatterometer (i.e., the so-called OSCOM prototype), are exploited in this work. In particular, data calibration and wind/current retrievals are performed. The Normalized Radar Cross-Sections (NRCS, σ0) or backscatter measurements are calibrated using two different methods to account for the larger than expected (by consolidated Geophysical Model Functions or GMFs used in Ka-band scatterometry) azimuthal modulation of the backscatter signal. Both methods are based on the so-called target or numerical ocean calibration (NOC). The first method consists of applying an azimuth-dependent calibration, while the second is based on a modification of the GMF to match the observed modulation after calibration. The retrieved wind speeds range from 3 to 6 m/s, and the wind directions are around 155°. The standard deviations for wind speed and direction against ECMWF winds are lower than 0.9 m/s and 9°. The winds derived using azimuth-dependent calibration present lower wind speed bias and larger wind direction standard deviation with respect to the modified GMF calibration.

An airborne validation experiment using a pencil-beam Doppler scatterometer was carried out in south of Yangjiang River, Guangdong Province, China. Its purpose is to validate the feasibility of spaceborne Doppler scatterometers and simultaneous observations of ocean wind and currents. This study presents the pencil-beam airborne Doppler scatterometer system, and its data processing algorithm. To investigate the accuracy of measured ocean wind and currents in the airborne campaign, we made a comparison with other datasets. The results show that 1) The speed error of wind is 1.2m/s, and wind direction error is around 13.19°. 2) The current speed error is better than 0.15 m/s, the current speed is better than 15 degree.

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.

Space-borne Doppler scatterometer is a new and promising sensor for wide-swath, high-accuracy, and simultaneous measurements of ocean surface current and wind vector. Doppler shift of ocean surface backscattering is determined by the relative motion between the sensor and ocean surface, it includes contributions from ocean surface current, wind-wave, and satellite motion. Accurate Doppler shift due to ocean surface current is estimated by removing the wind-wave and satellite motion effects. Satellite motion contribution can be accurately estimated from the satellite attitude motion of spacecraft. The main challenge for ocean surface current retrieval is uncertainty/error in the estimation of wind-wave induced Doppler shift. In this study, effects of wind estimation errors on surface current retrieval are evaluated through the Doppler scatterometer system simulations. Simulation results show that (1) surface current retrieval is affected more by estimation error in wind direction than in wind speed; and (2) wind estimation errors have larger effect on retrieval accuracy of ocean surface current direction than that of surface current speed.

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 simultaneous measurements of ocean surface current and wind have attracted considerable interest. Since they are closely coupled at sea surface, wind errors (speed and direction) have a transfer effect on current retrieval. In this paper we investigate the effects of wind transfer error on current retrieval for a Doppler scattterometer. We analyze current retrieved errors in different cross-track distances, and find the best current retrieval position is at sweet spot (i.e., mid-swath). For a single measurement in a current resolution cell of 10km, if no wind errors in 7m/s wind speed, at sweet spot current retrieved errors are around 0.25m/s and 30°, respectively. However, with wind errors existing, current retrieved errors are 0.33m/s and 41°. For comparisons, the results show that wind errors degrade current retrieval performances. Additionally, we also analyze current retrieved errors in different wind and current conditions, the results show that, wind direction error has a larger effect on current retrieval compared with wind speed error, and current direction error is more susceptible to wind errors. Our results indicate that, a simultaneous and accurate wind measurement is critically needed to accurately mitigate its effect on current retrieval.

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.

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.

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.

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 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.

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.

Ocean surface current is a very important parameter of ocean dynamic environment, which has been connected to global climate change, marine environment forecasting, marine navigation, engineering security and so on. Doppler scatterometer is a new type of scatterometer that can be used to measure ocean surface current as well as the ocean wind vector in space. The Doppler Scatterometer is based on a real aperture radar, which can achieve a very wide swath. It can provide the ocean surface current and wind vector information in a certain resolution and achieve global coverage quickly, which is very important for the marine environment forecasting and climate changes research.

In order to satisfy a high resolution for the measurement of snow water equivalent, we use the system of using the dual frequency(X-band 9.6GHz and Ku-band 17GHz ) and full polarization. This paper discusses the various system options(scanning pencil-beam, high PRF, high SNR) and tradeoffs considered for improving the azimuth resolution of scatterometer are required.

In order to measure the ocean surface current vector in a direct way, an interferometric method using pencil-beam scatterometer is proposed and the parameters of the traditional scatterometer are modified. What's more, the measurement principle and correlation model are introduced in this paper. The simulation results show that the Std of ocean surface current speed in both along-track and cross-track direction can be smaller than 10cm/s when the wind speed is larger than 4m/s. And the effective swath is greater than 445km when the wind speed is larger than 7m/s. The Kpcof the modified scatterometer is better than that of the tradition scatterometer.

For the operational spaceborne scatterometers, there is no effective way to eliminate the influence of the rain. Echoes contaminated by the rain are discarded directly. In this paper, a special designed spaceborne scatterometer and its decoupling methods of echoes from sea surface and rain are introduced. The main steps of decoupling methods are as follows. First, the scatterometer system parameters are slightly adjusted to acquire both the rain and sea surface echoes simultaneously. Next, the data of the scatterometer are copied and processed separately, one copy for rain retrieving and the other copy for sea surface backscatter retrieving. Finally, the rain attenuations are compensate to modify the bottom rain rate and the sea surface backscatter energy.