Oceanic eddy, which is an important subject in oceanic scientific research, plays an important role in the ocean thermal cycle. Because oceanic eddy is a kind of rotating three-dimensional water body, the sea surface height anomaly caused by it is very significant for eddy detection. At present, the satellite altimeter is the main sensor to detect the height anomaly of eddy. However, the satellite altimeter can only obtain the one-dimensional sea surface height information of eddy along the satellite track, and the resolution of the data fusion product is also rough. It is difficult for an altimeter to map mesoscale or sub-mesoscale (15—300 km) oceanic processes, since a 200—300 km gap usually exists between two successive tracks. Therefore, an altimeter is normally used to study large-scale (>300 km) oceanic processes, the sub-mesoscale or small-scale eddies cannot be detected using conventional radar altimeters. Interferometric Synthetic Aperture Radar (InSAR) not only has very high spatial resolution, but also can measure height based on interferometric phase, so it has great potential in eddy height anomaly detection. However, the height measurement accuracy of the existing InSAR system is generally in the meter level, which cannot meet the centimeter precision needed for eddy detection. The imaging altimeter based on SAR interferometry is expected to be the next generation of satellite altimeter. The Interferometric Imaging Radar Altimeter (InIRA) on Tiangong-2 is the first spaceborne Ku-band interferometric Synthetic Aperture Radar (SAR) that is specially designed for ocean surface topography altimetry. InIRA provides a large number of images for the observation and investigation of sub-mesoscale or small-scale eddies. This altimeter not only acquires the SAR image of oceanic eddies, but also obtains the interferometric phase of the complex images of two antennas and identifies the ocean surface height anomaly of eddies, thereby providing a new possibility for eddy detection. In order to prove the ability of InIRA in ocean eddy detection, this paper carries out the research of oceanic eddy detection based on InIRA interferometric data. An eddy detection method based on InIRA complex image is proposed by calculating the relative height anomaly of the ocean surface. The interferometric data of InIRA are processed to detect eddies after a series of procedures, including image coregistration, flat-earth phase removal, system parameter calibration, and phase noise suppression. Results show that the interferometric phase change corresponds to the ocean surface height anomaly induced by the eddy, and the relative height anomaly of the eddy is 23 cm. The moderate resolution imaging spectroradiometer Sea Surface Temperature (SST) and chlorophyll-a (CHL) data are used to verify the eddy identified from the InIRA images. The core of the eddy and the spiral arms emanated from the core are about 2.5°—3.0° cooler than the surrounding water. The CHL concentration in the core of the eddy and the spiral arms is about 1.0—1.5 mg·m^-3 higher than the adjacent water. And the center of the eddy with lowest SST corresponds to the highest CHL concentration. The findings indicate that the proposed method can realize the detection of oceanic eddy and preliminarily prove the ability of Tiangong-2 InIRA in detecting eddy, which reflects the great potential and application value of Tiangong-2 InIRA data in the investigation of sub-mesoscale oceanic dynamic environment.

oceanic eddies;Tiangong-2;near-nadir cross-track interferometric SAR;eddy detection