## 4 Result and Discussion
### Long-Term Observation and the System Stability Analysis  
During the experiment, the TB increment because of the solar radiation was observed by the GMR and recorded at four frequencies. In the operational use of the method, the solar radiation collected over a certain period (typically solar elevation >25\({}^{\circ}\)) was analyzed. We observed and recorded data every 10 min, the daily mean and the standard deviation were calculated and analyzed. Figures 9 and 10 show the annual variation of daily mean beamwidth, gain and aperture efficiency from December 2019 to January 2021, the sun tracking and monitoring was typically performed once or twice a week on sunny days and the solar observations were taken on a total of 53 days. The observation elevation was more than 25\({}^{\circ}\) and the maximum elevation angle was 79\({}^{\circ}\) on 18 June 2020. As the atmospheric refraction and attenuation both depend on elevation pointing, the sun observation at low elevations should be avoided. Scans under different solar elevations and seasons have been completed in order to study the effect of the solar elevation and season variation to the measurement of antenna pattern. It can be seen that these parameters do not have obvious seasonal variations. The statistics of the antenna parameters are shown in Table 2. One can see that the gain is greater than 30 dB at each frequency, the beamwidth is less than 5\({}^{\circ}\), and the GMR aperture efficiency is around 45%-55%. The results of measurement indicate that the parameters of the antenna pattern comply with the design specification. Comparison of the beamwidth between H- and E-planes shows that the antenna beam is circular in  
Figure 8: The antenna patterns measured in the anechoic chamber as compared with the results from the solar scanning at 30 GHz. (**a**) H-plane and (**b**) E-plane. Figure 7: The antenna power pattern on the H- and E-planes. The markers are for observed data and the lines are for fitting with the Gaussian function by least-square method, on March 14, 2020, 13:00 (local time). (**a**) H-plane and (**b**) E-plane. general. These results show that we can monitor the antenna and receiving system of the GMR based on the sun monitoring. Figure 10a presents the daily analysis of the system gain. The standard deviation of the daily average gain varies within roughly 0.15 dB at K-band and 0.28 dB at V-band during this period. The low standard deviation found for the GMR demonstrates the stability of the sun-based monitoring and the GMR receiving system. The standard deviation of the estimated gain serves as a sensitive quality measure of the daily analyses. In addition, the Tipping curve calibration has been widely applied to absolute calibration in order to improve the long-term stability and reduce the retrieval error. However, the antenna beamwidth and pointing errors are very important influential factors for Tipping curve calibration uncertainties [15; 18]. When Tipping curve calibration is enabled, the radiometer performs a scan from zenith to 30\({}^{\circ}\) elevation, the calibration uncertainties increase by increasing beamwidth [27]. In this case, the TB calibration needs the antenna \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Frequency** & **Gain** & \(\eta\) & \multicolumn{2}{c}{**Beamwidth (\({}^{\circ}\))**} \\ \cline{2-5}
**(GHz)** & **(dB)** & **(\%)** & **H-Plane** & **E-Plane** \\ \hline
22.235 & 32.69 \(\pm\) 0.02 & 46.25 \(\pm\) 0.31 & 4.62 \(\pm\) 0.13 & 4.56 \(\pm\) 0.11 \\
26.235 & 34.59 \(\pm\) 0.02 & 51.42 \(\pm\) 0.25 & 3.70 \(\pm\) 0.05 & 3.69 \(\pm\) 0.03 \\
30.000 & 35.42 \(\pm\) 0.04 & 47.54 \(\pm\) 0.39 & 3.31 \(\pm\) 0.04 & 3.40 \(\pm\) 0.04 \\
51.250 & 40.33 \(\pm\) 0.03 & 50.65 \(\pm\) 0.33 & 1.90 \(\pm\) 0.05 & 1.92 \(\pm\) 0.03 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: The statistical results of the GMR antenna parameters. Figure 10: The annual variation of daily mean gain and aperture efficiency. The markers are observed daily mean and minus are standard deviation.