# Ocean Wave Inversion Based on a Ku/Ka Dual-Band Airborne Interferometric Imaging Radar Altimeter
## 2 Method
### Ocean Wave Inversion  
According to the data processing procedure shown in Figure 4, the echo data of the dual-band and the 1D 5 Hz SSH in situ data were processed, and the OWPs could be further extracted. Upon obtaining the OWPs from different data sources, the OWPs (in red) using the in situ data were exploited to validate the OWPs (in blue) based on the interferometric data. The SSH was established based on the World Geodetic System 1984 (WGS-84) ellipsoid surface. The signal intensity of the single look complex (SLC) images after the SAR focusing is illustrated in Figure 5. The x-axis, here, refers to the ground-range direction, and the y-axis shows the plane flight direction. As displayed in Figure 6, the intensity images were averaged along the azimuth direction, and the result revealed that the mean signal intensity curves of the Ku and Ka bands decreased along the ground-range direction, and their intensity trends were consistent due to the adjusted incidence angles. The major reason for this trend was that the backscatter coefficient diminished and the ground range resolution declined along with the incidence angle. \begin{table}
\begin{tabular}{c c c} \hline \hline \multirow{2}{*}{**Parameters**} & \multicolumn{2}{c}{**Frequency Band**} \\ \cline{2-3}  & **Ku-Band** & **Ka-Band** \\ \hline Center frequency & 15.8 GHz & 35.8 GHz \\ Baseline length & 0.6 m & 0.34 m \\ Incidence angle & 6–18\({}^{\circ}\) & 4–17\({}^{\circ}\) \\ Bandwidth & & 900 MHz \\ Sample frequency & & 1.2 GHz \\ Antenna operation mode & Single transmission and dual reception \\ Pulse repetition frequency & & 2500 Hz \\ Flight altitude & & 3.38 km \\ Flight velocity & & 68 m/s \\ Baseline roll angle & & 10\({}^{\circ}\) \\ \hline \hline \end{tabular}
\end{table} 
Table 1: The main parameters of the DInIRA in this campaign. Figure 3: (a) Mobile weather station; (b) GNSS buoy. In recent years, registration methods have been developed significantly [30; 31; 32; 33]. For regions with consistent scattering characteristics and high coherence, an early study indicated that the registration accuracy of the complex-number correlation coefficient registration method was significantly better than that of the real-number registration function method [34]. In this paper, due to the consistent scattering characteristics of targets and  
Figure 4: Flowchart of ocean waves inversion and verification. Figure 5: Signal intensity of the master image in: (**a**) The Ku band; (**b**) the Ka band. Figure 6: Mean signal intensity curves with the ground range direction for DInIRA. the high coherence (Figure 7a,b) between master and slave images, the high-quality interferometric phase diagrams of the Ku and Ka bands were further calculated based on the image-based complex-number correlation algorithm, whose results are shown in Figure 7c,d. According to Figure 7a,b, the coherences of the master and slave SLC for the Ku and Ka bands were good after the registration. The statistical results of the coherence coefficient (not displayed here) also demonstrated that \(\sim\)90% of the areas could reach above 0.9 in the Ka band, indicating that the master and slave images were highly correlated in phase. In addition, the coherence coefficients of the Ku-band data were more uniformly distributed as compared with those of the Ka band, and the primary reason for this issue was that the Ku-band electromagnetic waves were less affected by the volumetric decorrelation than those of the Ka band. Figure 7c,d shows the interferometric phase diagrams of the Ku and Ka bands, respectively. Even if the interferometric phase was not filtered, the interference fringes of the dual-band data were still relatively bright, suggesting that the measured data were of high quality and the SAR-focus processing was sufficiently reliable. The interference phases included the flat-earth phase, elevation phase, and random phase errors.