## 4 Discussion
### Optimal Relative Azimuth Angles in the Multi-Angle Remote-Sensing Platform  
In the simulation model, the sun azimuth angle and sensor azimuth angle only affect the relative azimuth angle (\(\Delta\phi\)) (Equation (5)). In future SSR estimation model applications, such as drone formation, the platform would allow for more flexibility in adjusting and controlling the sensor parameters compared with satellite platforms that have fixed orbital parameters. Aviation platforms can change their flight path and, thus, for a given sun azimuth angle, they can change the sensor azimuth angle to optimize the relative azimuth angle. To analyze the influence that the sensor azimuth has on the SSR estimation results, we chose different latitudes or simulation. The locations of \(5^{\circ},20^{\circ}\), and \(40^{\circ}\) N at 1030 local time on 21 August 2018 were selected as the background, and the imaging geometry and environment parameters were set as follows:(\(P=0^{\circ}\), \(W=5\) m/s), and the preset SVA combination is (\(\theta_{I}=10^{\circ}\), \(\theta_{II}=20^{\circ}\)). Within the sensor azimuth range of [\(0^{\circ}\), \(360^{\circ}\)), the distribution of SSR estimation error and normalized SG radiance are simulated, as shown in Figure 16.  
Figure 15: A comparison of the predicted and actual errors.  
According to the result, it is always possible to reduce \(\Delta\sigma_{0}^{2}\) to 10% or less by adjusting the sensor azimuth. And we use the \"X\" to mark the main optimal area. However, the optimal sensor azimuth combinations changes in different latitude locations, and there are abnormal bands, such as S1 and S2 in Figure 16a. The sensor azimuth combinations marked by \"X\" can be selected according to actual location and flight detection. When the latitude is higher, the combinations that be marked are less because of the weaker SG radiance, such as location of 40\({}^{\circ}\) N. The result shows that optimal sensor azimuth combinations can always obtain strong SG radiance. In the locations of 5\({}^{\circ}\), 20\({}^{\circ}\), and 40\({}^{\circ}\) N,  
Figure 16: **(a)** Distribution of SSR estimation error (\(\Delta\sigma_{0}^{2}\)) at the simulation location [5\({}^{\circ}\) N, 120\({}^{\circ}\) E]. **(b)** The normalized SG radiance of the profile a in Figure 16a. **(c)** Distribution of SSR estimation error at simulation location [20\({}^{\circ}\) N, 120\({}^{\circ}\) E]. **(d)** The normalized SG radiance of the profile a in Figure 16c. **(e)** Distribution of SSR estimation error at simulation location [40\({}^{\circ}\) N, 120\({}^{\circ}\) E]. **(f)** The normalized SG radiance of the profile a in Figure 16f.  
respectively, the sun azimuth angles are 71.2\({}^{\circ}\), 105.7\({}^{\circ}\) and 137.1\({}^{\circ}\). When SG radiance is strongest, the sensor azimuth is approximately 250\({}^{\circ}\), 285\({}^{\circ}\) and 315\({}^{\circ}\) respectively. Therefore, the optimal relative azimuth angle (\(\Delta\phi\)) is confirmed to be about 180\({}^{\circ}\). Conversely, when sensor azimuth angles are about 70\({}^{\circ}\), 100\({}^{\circ}\) and 130\({}^{\circ}\) respectively, \(\Delta\sigma_{0}^{2}\) will increase. Without being limited by the sun azimuth, the relative azimuth can be controlled by adjusting the flight direction. Therefore, the flexibility of an aviation platform as a multi-angle remote-sensing platform allows for more optimized designs for imaging angle. According to the normalized SG radiance of the selected profiles, the resulting set of SG radiance are the same under certain sensor azimuth combinations. We mark the corresponding combinations in Figure 16 respectively. The abnormal bands occur when both sensors have the same SG radiances; thus, combinations that cause the same set of SG radiance should be avoided.