The GaoFen-3 (GF-3) is the first Chinese spaceborne SAR in C-band for civil applications. In the paper, we provide an overview on demonstrating capabilities of the GF-3 SAR on ocean, coast and polar observations, by presenting some representative cases, such as polarimetric characteristics of typical intertidal flat in the Subei shoal, the German Bight, observation of offshore wind turbine wakes in the North Sea and East China Sea, observation of internal waves generating in the Luzon Strait and their propagation to DongSha Atoll, as well as discrimination of sea ice and open water in the Arctic. Derivation of marine-meteo parameters in high spatial resolution is one of the most attractive applications of spaceborne SAR for ocean observation. This is also a primary goal of launching the GF-3 SAR, we also presented the current problem of using GF-3 for retrieval of oceanic dynamic parameters and some possible solutions.

Radar altimeter is an important part of ocean phenomena monitoring. Profile radar altimeters (such as Topex/Poseidon, Jason-2 and Sentinel-3) have provided abundant data for the ocean phenomena research in the past decades. But they only measure one-dimensional profile height along the satellite track, a 200~300 km gap usually exists between two successive tracks and the spatial resolution of their data products is sparse. Wide-swath altimeter (such as SWOT) calculate the target point height using the interferometric phase measured by two antennas of the altimeter. It can greatly improve the spatial resolution of the data product and makes up for the lack of profile altimeter. However, in the process of generating a digital elevation model with a wide-swath altimeter, the baseline roll error will cause height error which increases in the cross-track direction. Taking the design parameters of SWOT as an example, a baseline error of only 0.36arcsec (1/10000°) would result in an average height error of roughly 6 cm at the swath of 10km to 60km. In the present case, it’s difficult to control the measurement accuracy of baseline roll within 0.1arcsec by measuring instruments such as a gyroscope, and it is impossible to meet the high-accuracy requirement of 1-2cm. Therefore, it is necessary to adopt other methods to calibrate the measurement value of the baseline roll.

Due to the special geometrical relationship between the nadir point and two antennas, using the nadir interference phase can obtain a more accurate baseline roll angle value, estimate the flight attitude more effectively and improve the height measurement accuracy. The nadir interference phase is related to baseline length and roll angle. The contribution of measurement error of baseline length to the measurement error of roll angle is much smaller than that contributed by the measurement error of interferometric phase. The accuracy of interferometric phase is decided by both the hardware guaranteed accuracy and the echo guaranteed accuracy. An accuracy of 0.05° for open ocean surface is not difficult to achieve. Under this assumption, the measurement accuracy of roll angle can be as high as 0.0246arcsec.

A two-dimensional ocean surface which is simulated by the Pierson-Moscowitz spectrum. The slant range and interferometric phase of the resolution unit are obtained based on geometrical relationship. It is assumed that the interferometer can measure the nadir interferometric phase with an accuracy of 0.05°, while a gyroscope can measure the baseline roll angle with an accuracy of 0.36arcsec simultaneously. Reconstruct the sea surface using parameters of the SWOT mission and combining with the baseline roll that calculated by the nadir interference phase or measured by the simulated gyroscope, and then calculate the elevation error within the swath. The numerical simulation results show that the accuracy of the baseline roll can reach 0.03arcsec with a nadir interferometric phase accuracy of 0.05°, and the average height error within the swath of 10km-60km is 0.48cm.

Using the relationship between the nadir interference phase and the baseline roll angle to measure the roll angle indirectly, can achieve a more accurate baseline roll angle measurement than the general hardware. The height error caused by the baseline roll error could be reduced and the accuracy of data products of wide-swath altimeter could be improved which make it an effective way to calibration height error of wide-swath altimeter.

Tracing the evolution of subaqueous topography in coastal water enables us to understand the effects of intensive coastal development on bays and estuaries. Analysis of a series of historical bathymetric acoustic surveys has revealed large changes in morphology from 1960s to 2010s in Longkou Bay, China. Water depths were extracted from digitized admiralty charts to explore the accretion-erosion characteristics in a Geographical Information System (GIS) environment, providing quantitative estimates of morphological changes. Multibeam echosounders (MBES) were used to map and analyze the geomorphologic features caused by the construction of artificial islands. Results illustrated that the shoreline and bathymetry of Longkou Bay changed dramatically in recent decades. The subaqueous area decreased by about 15%, while land area increased by more than 13 km2 in the study area during the last 50 years. From 1960s to 1990s, the evolution of Longkou Bay was mainly governed by natural processes with a patchy distribution of deposition and erosion, and there were few signs of being related to large-scale human activities. During the period of 1990s to 2010s, intensive coastal developments including large port engineering projects, channel dredging and artificial islands construction became the main processes affecting morphological changes in the Longkou Bay. The high-resolution bathymetric results near the artificial island showed that the seafloor was dredged at many sites, leaving large areas of borrow pits. The sudden change of the underwater topography will lead to the destruction of local benthic habitat and effective measures need to be taken to protect and remediate heavily disturbed subaqueous environment.

The refraction and reconnection of internal solitary waves (ISWs) around the Dongsha Atoll (DSA) in the northern South China Sea (SCS) are investigated based on spaceborne synthetic aperture radar (SAR) observations and numerical simulations. In general, a long ISW front propagating from the deep basin of the northern SCS splits into northern and southern branches when it passes the DSA. In this study, the statistics of Envisat Advanced SAR (ASAR) images show that the northern and southern wave branches can reconnect behind the DSA, but the reconnection location varies. A previously developed nonlinear refraction model (NRM) is set up to simulate the refraction and reconnection of the ISWs behind the DSA, and the model is used to evaluate the effects of ocean stratification, background currents, and incoming ISW characteristics at the DSA on the variation in reconnection locations. The results of the first realistic simulation agree with consecutive TerraSAR-X (TSX) images captured within 12 h of each other,which proves the validity of the NRM model around the DSA. Further sensitivity simulations show that ocean stratification, background currents, and initial wave amplitudes all affect the phase speeds of wave branches and therefore shift their reconnection locations while shapes and locations of incoming wave branches upstream of the DSA profoundly influence the subsequent propagation paths. This study clarifies the variation in reconnection locations of ISWs downstream of the DSA and reveals the important mechanisms governing the reconnection process, which can improve our understanding of the propagation of ISWs near the DSA.

Most researchers pay more their attentions on the characteristics of oil spills when they construct the feature set for establishing an automatic sea surface oil spill monitoring system by spaceborne SAR. In this paper, taking oceanic internal waves as an example of oil look-likes, a Fourier spectrum feature subset is proposed based on the analysis of the characteristics of look-alikes. The aim is to reduce the false alarms resulting from oceanic internal waves and then to decrease the false discovery rate (FDR) of the automatic monitoring system. The proposed feature subset consists of 10 Fourier spectrum features. 53 SAR images rich in internal waves acquired over South China Sea are collected for this experimental study. An adaptive threshold image segmentation algorithm and its post-processing are applied on the SAR images to automatically generate dark target set with reasonable proportion between oil spills and look-alikes. Training and testing of classifier are conducted with 77 original features and 87 features including the additional 10 Fourier spectrum features, respectively. The results show that the system's FDR decreases from 19.6% to 14% due to the introduction of the Fourier spectrum feature subset.