# Ocean Wave Inversion Based on a Ku/Ka Dual-Band Airborne Interferometric Imaging Radar Altimeter
## 1 Introduction  
The study of ocean waves is a hot topic in oceanographic research. In this respect, currently, satellite altimetry has been introduced as an effective technique for detecting ocean waves. The traditional satellite-based radar altimetry technology, developed in the 1960s, is a practical approach for this purpose since it can measure sea surface height (SSH) [1]. However, traditional altimeters can simply estimate the significant wave height (SWH) of a target sea area, but fail to retrieve other ocean wave parameters, such as the wave propagation direction (WPD) and the main wave wavelength (MWW) [2, 3]. As well, synthetic aperture radar (SAR) is widely used to detect ocean waves, due to its unique two-dimensional (2-D) imaging mechanism [4]. Two-dimensional ocean wavespectra (OWS) can be retrieved based on SAR images, and the WPD, SWH, MWW, as well as other related parameters can be further extracted using the OWS [5; 6; 7; 8]. Inversion methods based on OWS according to SAR images have been correspondingly developed to a great extent, mostly depending on modulation functions. However, modulus and phase calculations in these functions are relatively rough [9], limiting the inversion accuracy of ocean waves. The interferometric synthetic aperture radar (InSAR) is also an important tool for detecting ocean waves. [PERSON] et al., (2001) [10] simulated a distorted digital elevation model (DEM) based on OWS and a cross-track InSAR (XTI-SAR) imaging model, and found that the SWH error was within 10% for the low-amplitude swells, and the error was related to the WPD. In the same year, [PERSON] et al. [11] calculated SSH using airborne X-band XTI-SAR data, and demonstrated that SWH estimated in accordance with the DEM was similar to the results inversed by the buoy for the weak nonlinear imaging. However, the microwave scattering signals of the targets in the ocean were closely related to the wind field at moderate incidence angles, and the anisotropy and asymmetry [12] on the upwind/crosswind could reduce the inversion accuracy of the ocean waves. In addition, [PERSON] et al., (1985) [13] proposed a theory to measure ocean waves at small incidence angles. Accordingly, observations at small incidence angles could moderate the requirements of the radar transmit power and the gain of the antenna, while ensuring target homogeneity, and the ocean targets in the signal overlay area were primarily of specular scattering. [PERSON] et al., (1999) [14] correspondingly proposed a new sensor, named the imaging radar altimeter (InIRA), which could achieve high inversion accuracy of ocean waves due to its small incidence angles. [PERSON] et al., (2014) [15] further conducted the first Ka-band InIRA experiment, which proved the typical characteristics of Ka-band InIRA images, and then assessed the superiority of satellite-based Ka band for detecting ocean waves. In 2016, successful operation of the Tiangong-II InIRA sensor was the first to assess the effectiveness for detecting ocean waves using the Ku band at small incidence angles [16]. In addition, the forthcoming launch of the Surface Water and Ocean Topography (SWOT) satellite was equipped with a Ka-band InIRA to obtain high inversion accuracy of ocean waves. AirSWOT is an airborne instrument for supporting the SWOT mission, and it plays a key role in SWOT mission development. In recent years, some experiments have been conducted on water surface elevation reconstruction and algorithm development based on the actual data of AirSWOT, which have indicated that the AirSWOT has high measurement accuracy [17; 18; 19; 20]. However, there are many challenges [21; 22] facing observations using Ka band. For instance, Ka-band electromagnetic waves are easily affected by the atmospheric environment, in addition, the posture stability of the satellite platform and the accuracy of the antenna orientation both have a significant impact on the observations based on the high-frequency bands.