## 3 Instrument and Experiments  
### The Radiometer and Observation Mode  
The GMR (model MWP967 KV) used for this experiment, shown in Figure 2, was developed and manufactured by our research team. It is a passive remote sensing instrument  
Figure 1: The relationship between the sun filled-beam efficiency and beamwidth. that measures the atmospheric radiation to retrieve temperature, humidity and liquid water profiles as well as integrated water vapor and liquid water paths in real time. The GMR usually measures T\({}_{\text{B}}\) in the K-band (22-30 GHz) and V-band (51-59 GHz). It is comprised of an antenna system and two sensitive heterodyne receivers as well as a detector unit and data retrieval system. It contains a high-precision elevation and azimuth-stepping scanning system to scan the sky, and the angle resolution is 0.1\({}^{\circ}\). The receiver system consists of two super-heterodyne receivers covering K- and V-bands, each band is independently received and detected, and all channels use a common local oscillator signal. The antenna system contains a parabolic reflector, beam splitter and compactness a corrugated feed-horn. In addition, it is calibrated by using liquid nitrogen (LN2), hot load, a noise diode and the tipping curve method. During observations, the GMR underwent regular maintenance and the LN2 calibrations were performed twice a year. The system performance parameters are given in Table 1.  
In order to track and observe the sun, the antenna servo control system was improved so that the GMR could be adjusted to control its antenna beam to scan the sun. When the GMR tracked and observed the sun, we were easily able to obtain the T\({}_{\text{B}}\) variation from the sun arriving at the antenna and thus study the solar activity. The upgraded GMR was set up at the Xi'an field experiment site (34.091\({}^{\circ}\)N, 108.89\({}^{\circ}\)E). The GMR works on two modes, the meteorological observation mode and the solar observation mode. \begin{table}
\begin{tabular}{c c} \hline \hline
**Parameter** & **Specification** \\ \hline Frequency & K-band (22–30 GHz); V-band (51–59 GHz) \\ Beam width & K-band: \(\leq\)5\({}^{\circ}\); V-band: \(\leq\)3\({}^{\circ}\) \\ Gain & \(\geq\)25 dB \\ Sidelobe level & \(\leq\)\(-\)25 dB \\ Brightness temperature accuracy & 0.5 K \\ Brightness temperature sensitivity & K-band: \(\leq\)0.25 K (RMS); V-band: \(\leq\)0.3 K (RMS); \\ Integration time & Typically, 1 s \\ Antenna azimuth and elevation scanning & \(+/-180^{\circ}\) stepping scanning \\ Antenna angular resolution & 0.1\({}^{\circ}\) \\ Calibration method & Hot load; Noise diode; Tipping method; LN2 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: The system performance of the GMR used in this study. Figure 2: The multichannel microwave radiometer used for this experiment. 1. Meteorological observation mode. Generally, the GMR adopts meteorological observation mode to obtain atmospheric T\({}_{\rm BS}\) which are used to retrieve atmospheric temperature and humidity profiles. 2. The solar observation mode. This observation mode was developed and was used to track and remotely observe solar radiation. Firstly, the antenna system was adjusted to point towards the center of the sun and then the antenna beam was tuned to scan the sun using a polar plane indicator (PPI) by adding a step value on the antenna azimuth. Secondly, the antenna was adjusted so that the sun was completely out of the beam in order to scan the sky using a range height indicator (RHI) by adding a step value to the antenna elevation, then the antenna was moved from the sun in order to obtain the sky background radiation as a reference and to calculate the atmospheric opacity with the tipping calibration method. Finally, we obtained the T\({}_{\rm B}\) increment of the sun without the atmospheric attenuation by utilizing Equations (1) to (5). This scanning can last up to 3 min for each observation, depending on the scanning step angle count.