Capabilities of Chinese Gaofen-3 Synthetic Aperture Radar in Selected Topics for Coastal and Ocean Observations

Abstract

Gaofen-3 (GF-3), the first Chinese spaceborne synthetic aperture radar (SAR) in C-band for civil applications, was launched on August 2016. Some studies have examined the use of GF-3 SAR data for ocean and coastal observations, but these studies generally focus on one particular application. As GF-3 has been in operation over two years, it is essential to evaluate its performance in ocean observation, a primary goal of the GF-3 launch. In this paper, we offer an overview demonstrating the capabilities of GF-3 SAR in ocean and coastal observations by presenting several representative cases, i.e., the monitoring of intertidal flats, offshore tidal turbulent wakes and oceanic internal waves, to highlight the GF-3's full polarimetry, high spatial resolution and wide-swath imaging advantages. Moreover, we also present a detailed analysis of the use of GF-3 quad-polarization data for sea surface wind retrievals and wave mode data for sea surface wave retrievals. The case studies and statistical analysis suggest that GF-3 has good ocean and coastal monitoring capabilities, though further improvements are possible, particularly in radiometric calibration and stable image quality.kilometers. Wide-swath or ScanSAR images generally have a spatial resolution of tens of meters and, more importantly, can map a large area of the open sea and coast, which makes them particularly suitable for studying meso-scale oceanic and atmospheric processes, e.g., by mapping the distribution of internal ocean waves [2], observing atmospheric solitons [3], estimating the wind speed of tropical cyclones [4], and measuring sea surface velocity [5].The year 2007 marks an important advance in the development of spaceborne SAR; two X-band spaceborne SAR, the TerraSAR-X (TSX) and Cosmo-SkyMed and the C-band SAR, Radarsat-2 (R2), were launched. Compared with previous spaceborne SAR missions, the new generation of SAR sensors has several advantages. One advantage is that the new generation can acquire images with a high spatial resolution of up to 1 m in spotlight mode [6][7][8]. This offers a unique opportunity to detect targets in the ocean and coast, e.g., for ship detection [9][10][11]. The other advantage is that these SAR sensors have polarimetric capabilities of acquiring data in different polarization combinations of VV, HH, VH and HV. These SAR polarimetric data are widely exploited for oil spill detection or classification [12][13][14], analysis of objects scattering or their classification in coastal intertidal flats [15][16][17], and sea ice detection and classification [18][19][20]. In addition to the general advantages of the aforementioned high spatial resolution and polarimetry, advanced SARs have constellation configuration design. The Cosmo-SkyMed, TSX/TanDEM-X (TDX) and Sentinel-1A/1B missions, as well as the forthcoming Radarsat Constellation Mission (RCM), all operate in constellations, which significantly reduces the temporal intervals of SAR data acquisition and therefore enhances the capture of dynamic sea surface information ...

Retrieval of Sea Surface Wind Speeds from Gaofen-3 Full Polarimetric Data

“…The accuracy of the VV polarization radiation calibration is the key factor in SSWS retrieval, as noted in Equation 1. Our previous study [42] examined the calibration inaccuracy of the GF-3 QPS mode data based on a simulation experiment. Here, we conducted similar experiments to comprehensively analyze the GF-3 radiometric calibration accuracy in terms of SSWS retrieval.…”

“…One of the primary tasks of GF-3 is ocean and coastal observation, as a spaceborne SAR instrument was previously absent among the Chinese ocean-observing satellites [41]. Therefore, in a previous study [42], we presented a few examples and statistical analysis to demonstrate the capabilities of GF-3 for ocean and coastal observations. In general, GF-3 meets some requirements for monitoring oceans and coasts, although there are a few aspects that should be considered and studied further.…”

“…Wang et al [43] Shao et al [44] Li et al [42] Number As listed in Table 1, most studies are based on GF-3 data acquired in the quad-polarization stripmap (QPS) and the standard stripmap (SS) mode data. Wang et al [43] conducted a preliminary analysis of GF-3 wind retrieval using several images.…”

“…A new set of calibration constants for GF-3 SAR data was released in May 2018. Li et al [42] compared the SSWS retrievals from GF-3 QPS data using the new and old calibration constants. Compared with the collocated WindSat SSWS measurements, the QPS VV polarization data achieve an RMSE of 1.72 m/s when the new calibration constants were used to radiometrically calibrate the data, which is by approximately 0.60 m/s better than the result when the GF-3 QPS data were calibrated using the old calibration constants.…”

A Global Sea State Dataset from Spaceborne Synthetic Aperture Radar Wave Mode Data

“…Although the WM covers a relatively area of the sea surface (approximately 6 km by 10 km), these data are automatically acquired by spaceborne SAR over the global oceans. From the European Remote Sensing Satellite missions (ERS-1, 1991and ERS-2, 1995 [12][13][14][15] to the Environment Satellite (ENVISAT) mission (2002 -2012) [16][17] and the current Sentinel-1A/1B (2014 -) 18 and Chinese Gaofen-3 (2016 -) missions 19 , WM data have been available for nearly 30 years and will continue to be acquired into the future, constituting a valuable dataset for global sea state measurements. On the basis of SAR WM data, some interesting investigations of global ocean waves, particularly with respect to the characteristics of ocean swells [20][21][22] , have been reported.…”