## 4 Results
### Flight Experiment  
Experiments were carried out on an airborne demonstrator. The instrument is an 11-element fully polarimetric aperture synthesis radiometer operating at 1.415 GHz and arranged in a Y-shape array with three elements per arm. Before the flight experiment, antenna patterns for all elements' X and Y polarization were measured in an anechoic chamber, as illustrated in Figure 8a. These antenna patterns are utilized for image reconstruction and the correction of antenna pattern errors. The average antenna pattern is obtained by averaging the measured antenna pattern of all elements. The co-polar (CO) and cross-polar (CR) average antenna patterns at X polarization and Y polarization are illustrated in Figure 9. After calculation, the amplitude difference of the array is 6.8% and the phase difference is 4.4\({}^{\circ}\). Before the flight experiment, indoor experiments were carried out in an anechoic chamber. The results of the indoor experiments were used to conduct a quantitative assessment of the effectiveness of the correction method. The absorptive material background wall of the anechoic chamber is used as the observed scene for imaging. The absorbing material's physical temperature is measured using a temperature sensor, and it is approximately 293 K. The BT of the absorbing material is approximately equal to the physical temperature. Therefore, the original BT of the observed scene can be set as 293 K. By using Equation (27) and combining the original BT with the reconstructed BT, the BT error can  
Figure 8: (**a**) Picture of antenna array in anechoic chamber. (**b**) Picture of the instrument mounted on the aircraft. Figure 9: (**a**) Co-polar (CO) average antenna pattern at X polarization. (**b**) Cross-polar (CR) average pattern at X polarization. (**c**) Co-polar (CO) average antenna pattern at Y polarization. (**d**) Cross-polar (CR) average pattern at Y polarization. be calculated. The reconstructed images are shown in Figures 10 and 11. The absorptive material background wall has a uniform and stable temperature, so the reconstructed BT in the alias-free field of view is theoretically a constant value. The experimental results obtained by the two-step correction method are more consistent with expectations. Before the correction, the reconstructed BT in the alias-free field of view exhibited significant errors, with substantial variations in brightness temperature. After correction, the brightness temperature of the image became more uniform and consistent. From the results of BT errors before and after correction in Table 3, it can be seen that after the two-step correction method is applied, the error has been reduced from over 10 K to approximately 3 K.  
Following the initial validation through indoor experiments, the instrument was subsequently transported to the coastal site for the execution of flight experiments. During the flight experiment, the instrument is mounted on the aircraft, with the antenna array located in the underpart of the aircraft's fuselage, as shown in Figure 8b. The signal of the target scene enters the receiving channel through the antenna array. The visibility function \begin{table}
\begin{tabular}{c c c} \hline \hline \(\mathbf{\Delta T}\) & **X Polarization** & **Y Polarization** \\ \hline Before calibration (K) & 11.56 K & 10.27 K \\ After calibration with the two-step correction method (K) & 3.01 K & 2.64 K \\ \hline \hline \end{tabular}
\end{table} 
Table 3: The BT error \(\Delta T\) of the indoor experiment. Figure 11: The results of the indoor experiment after correction. (**a**) \(T^{XX}\), (**b**) \(T^{YY}\). Figure 10: The results of the indoor experiment before correction. (**a**) \(T^{XX}\), (**b**) \(T^{YY}\). is obtained after the complex correlation operation of signals. The reconstructed BT image can be obtained by the measured visibility function. The experiment selected a coastal-land junction area as the observation scene.