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Evaluation of the Significant Wave Height Data Quality for the Sentinel remote sensing Technical Note Evaluation of the Significant Wave Height Data Quality for the Sentinel-3 Synthetic Aperture Radar Altimeter Yong Wan 1,* , Rongjuan Zhang 2, Xiaodong Pan 3, Chenqing Fan 4 and Yongshou Dai 1 1 College of Oceanography and Space Informatics, China University of Petroleum, No. 66, Changjiangxi Road, Huangdao District, Qingdao 266580, China; [email protected] 2 College of Control Science and Engineering, China University of Petroleum, No. 66, Changjiangxi Road, Huangdao District, Qingdao 266580, China; [email protected] 3 Marine Environmental Monitoring Center of Wenzhou, the State Oceanic Administration, No. 2, Xinanjiang Road, Wenzhou Avenue, Wenzhou 325000, China; [email protected] 4 Remote Sensing Office of The First Institute of Oceanography, Ministry of Natural Resources, No. 6, Xianxia Road, Laoshan District, Qingdao 266061, China; fanchenqing@fio.org.cn * Correspondence: [email protected]; Tel.: +86-150-5325-1676 Received: 21 August 2020; Accepted: 19 September 2020; Published: 22 September 2020 Abstract: Synthetic aperture radar (SAR) altimeters represent a new method of microwave remote sensing for ocean wave observations. The adoption of SAR technology in the azimuthal direction has the advantage of a high resolution. The Sentinel-3 altimeter is the first radar altimeter to acquire global observations in SAR mode; hence, the data quality needs to be assessed before extensively applying these data. The European Space Agency (ESA) evaluates the Sentinel-3 accuracy on a global scale but has yet to perform a detailed analysis in terms of different offshore distances and different water depths. In this paper, Sentinel-3 and Jason-2 significant wave height (SWH) data are matched in both time and space with buoy data from the United States East and West Coasts and the Central Pacific Ocean. The Sentinel-3 SWH data quality is evaluated according to different offshore distances and water depths in comparison with Jason-2 SWH data. In areas more than 50 km offshore, the Sentinel-3 SWH accuracy is generally high and less affected by the water depth and sea conditions (root-mean-square error of 0.28 m and correlation coefficient of 0.98); in areas less than 50 km offshore, the SWH data accuracy is slightly affected by water depth and sea conditions (especially the former). Compared with Jason-2, the observation ability of the Sentinel-3 altimeter in nearshore areas with water depths of 0 m-500 m is greatly improved, but in some deep water areas with stable sea conditions, the Jason-2 SWH data accuracy is higher than that of Sentinel-3. This work provides a reference for the refined application of Sentinel-3 SWH data in offshore deep water areas and nearshore shallow water areas. Keywords: SAR altimeter; time-space matching; significant wave height; data accuracy evaluation 1. Introduction Waves represent an important dynamic marine process. At present, wave parameters are obtained mainly by the following means: wave buoy observations, wave numerical model predictions, and remote sensing observations, including altimeters, synthetic aperture radar (SAR), and video monitoring. Among them, wave buoy observations have been recognized as the relatively accurate method for obtaining wave parameters; nevertheless, information observed by buoy is limited [1]. Moreover, the accuracy of wave numerical model predictions is restricted by many factors, such as the accuracy of the driving wind field, the setting of initial conditions, and the assimilation of observation data; Remote Sens. 2020, 12, 3107; doi:10.3390/rs12183107 www.mdpi.com/journal/remotesensing Remote Sens. 2020, 12, 3107 2 of 14 the accuracy is especially affected by the influence of the water depth, and this influence is more obvious in coastal areas [2,3]. In contrast, the development of microwave remote sensing technology provides a new approach for observing waves. SAR and altimetry can achieve long-term observations as a result of field observations and thus are extensively used in wave research, and video monitoring can be used to measure breaking wave height. Although SAR has the advantage of a high resolution, the inversion of wave parameters is complex, as the inversion requires the first-guess spectrum to be input. Satellite radar altimeters are used mainly to observe changes in the sea surface height (SSH), which is of great significance in oceanography, meteorology, and geophysics [4]. At present, with respect to wave observations, altimeters are employed for two primary reasons: verifying the accuracy of wave models [5–7] and assimilation into wave models to improve the prediction accuracy [8,9]. The inversion of the SSH and significant wave height (SWH) from altimeter data is usually achieved by tracking echo waveforms. Traditional radar altimetry adopts the real aperture working mode of the pulse-limited system, which has low resolution and is easily affected by land near the shore; furthermore, sheltered bays may exhibit an almost mirror-like surface and generate specular returns in waveforms under very calm conditions [10–12]. Therefore, traditional radar altimeters are unable to observe waves with high accuracy. With the introduction of SAR technology in the azimuth direction, radar altimeters have achieved a high resolution and have great potential in the observation of waves and water bodies in areas characterized by complex conditions. Cryosat-2 is the first instrument to operate in SAR mode [13]. Although the resulting model was used only for specific periods and limited regions over the oceans, several studies demonstrated good model performance over the open ocean and coastal areas [14,15]. Video monitoring systems are used to estimate the breaking wave height, which occurs when the wave height is approximately equal to the water depth [16]. This method has been proved to be an effective way to measure nearshore wave height and breaking wave height [17,18]. The European Space Agency (ESA) launched the Sentinel-3A and Sentinel-3B satellites in 2016 and 2018, respectively, with an orbit height of 814.5 km and an orbit inclination of 98.645589◦; both satellites are equipped with the Synthetic Aperture Radar Altimeter (SRAL), Microwave Radiometer (MWR), Sea and Land Surface Temperature Radiometer (SLSTR), and Ocean and Land Color Instrument (OLCI). Sentinel 3A/B has a cycle of 27 days, and each cycle contains 385 tracks, with a minimum track interval of 52 km. The Sentinel-3 topography mission is predominantly applied to study the ocean topography, including the mean sea level, wave height, sea surface wind speed, sea ice, ocean currents, Kelvin and Rossby waves, eddies, and tides. The SRAL instrument works in two modes: SAR mode and pulse-limited mode (PLRM). SRAL can operate in the Ku and C bands, and its azimuthal resolution is 250–300 m. The geophysical parameters in the data products include the SSH, significant wave height and sea surface wind speed, etc., all of which are of great significance for monitoring the SSH and for studying the ocean surface morphology characteristics, such as wave heights, currents and tides [19]. Considerable research has been carried out on the Sentinel-3 altimeter and its data. Birgiel et al. [20] compared Sentinel-3 SSH data with the data from three observation stations near Estonia and pointed out that the accuracy is reduced in cold months and that the impact of the atmospheric correction should be considered. Delikaraoglou and Flokos [21] summarized the technical characteristics and performance parameters of the Sentinel-3 SAR altimeter and discussed its advantages through a comparison with the observation capabilities of traditional altimeters in coastal areas. Gao et al. [22] retracked the Sentinel-3 altimeter waveform data in the Ebro River Basin, compared the results with in situ measurement data, and confirmed that SAR altimetry can measure the water level of small inland water bodies with high accuracy. Jiang et al. [23] assessed the observation ability of the Sentinel-3 altimeter on rivers in mountainous areas of China and analyzed the influences of different topographical features and river width on the measurement accuracy. Nencioli and Quartly [10] compared the data acquired in two kinds of measurement modes by the Sentinel-3A altimeter with buoy data from the coastal area of southwestern England; the analysis showed that within 15 km of the coast, the accuracy of SAR mode data is higher than that of PLM data, verifying the advantages of Remote Sens. 2020, 12, 3107 3 of 14 the former. Yang and Zhang [24] compared the SWH data of the Sentinel-3 altimeter with the data of National Data Buoy Center (NDBC) buoys in open sea areas and the Jason-3 altimeter in global sea areas; the results showed that the root-mean-square error (RMSE) is 0.2–0.3 m, and the accuracy slightly decreases with increasing SWH data, confirming that Sentinel-3 SWH data have high accuracy. Several studies have evaluated the accuracy of Sentinel-3 altimeter data in different regions and under different conditions and verified its high accuracy, although the accuracy is slightly reduced under complex geographical conditions, sea conditions and atmospheric conditions. The ESA evaluates the Level-2 products of the Sentinel-3 altimeter in each cycle and generates a quality report that is published on the Sentinel Online website (https://sentinel.esa.int/web/sentinel/ technical-guides/sentinel-3-altimetry/data-quality-reports). Taking Cycle 050 of Sentinel-3A and Cycle 031 of Sentinel-3B (10 October 2019 to 5 November 2019) as examples, the data quality verification employed by the ESA is briefly summarized. First, a comparison with European Centre for Medium-Range Weather Forecasts (ECMWF) data is carried out over a long period over the global oceans. The SWH data quality is evaluated in different regions in space, that is, in the Southern and Northern Hemispheres and in tropical regions, and under different sea conditions.
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