# HISEA-1: The First C-Band SAR Miniaturized Satellite for Ocean and Coastal Observation
## 2 Typical Data and Preliminary Results
### Ocean Wind, Wave and Rip Current  
The image of South America is shown in Figure 2a. Various landcover, including buildings, farmlands, highways, rivers, and lakes, could be identified. At the same time, striped information is observed on the sea, which contains wind and wave information. Using the LG-mod [15], the wind direction was calculated, which contained 180\({}^{\circ}\) blur. Figure 2c shows the SAR image spectra in the area marked by a red rectangle in Figure 2a. There is also a 180\({}^{\circ}\) blur, and the wind direction is similar to that in Figure 2d. Based on the SAR image spectra, wave length and wave direction of the ocean wave could be obtained, for which the estimation of ocean wave spectra is a necessary prerequisite due to the nonlinear SAR-wave relation. There are several methods for ocean wave spectra inversion, including the Max-Planck Institute (MPI) method [16; 17], the semi-parametric retrieval algorithm (SPRA) [18; 19], the parameterized first-guess spectrum method (PFSM) [20], and the partition rescaling and shift algorithm (PARSA) [21; 22]. In addition, the ocean wave parameters such as significant wave height, average wave period, and wave direction  
Figure 1: Sea ice image of the Bohai Sea in China obtained by HISEA-1 striping mode. (**a**) The complete obtained image whose gridding size is 31,764 \(\times\) 15,668 (horizontal \(\times\) vertical) and spatial resolution is 3 m; (**b**) Sea ice with a brighter view; (**c**) Sea ice with a darker view. Red circles show ships trapped in the sea ice.  
could also be estimated by the methods using the empirical relationship between wave parameters and SAR images, such as CWAVE [23; 24; 25], convolutional neural network (CNN) [26], and some empirical methods using azimuth cut-off, peak wavelength, and normalized variance to retrieve wave parameters [27]. These results suggest that HISEA-1 could observe winds and waves on sea surfaces. However, since the image was acquired under the commissioning stage, the image has not yet been calibrated. Therefore, the further works about ocean wave inversion would be done by the method mentioned above after testing step.  
Figure 2: SAR image of North America obtained by striping mode and some preliminary analysis results. (**a**) Original SAR image whose gridding size is 14,000 \(\times\) 7167 (horizontal \(\times\) vertical) and spatial resolution is 3 m. (**b**) SAR image spectra in the area enclosed by left red rectangle in Figure 2a. (**c**) Rip current in the right red rectangle in Figure 2a, which is shown in the red oval border. (**d**) Wind direction with 180\({}^{\circ}\) blur retrieved by LG-mod.  
There are some other interesting phenomena in the coastal area. Some signatures of rip currents were identified in the central enlarged area along the coast in the SAR image, as shown in Figure 2c. Bright elongated cells extending offshore from the coast are inferred to be the rip currents [28]. In addition, there are some irregular bright bands in the sand bar, which could be further analyzed by geologists.