## 2 Instrument for Observation  
### The System of GMR  
The GMR (model MWP967 KV) used for this experiment, shown in Figure 1, was developed and manufactured by our research team, and the system performance parameters are given in Table 1. The GMR comprises an antenna system, two sensitive heterodyne receivers, a detector unit, a data retrieval system and additional sensors. The GMR also contains a high-precision elevation and azimuth stepping scanning system to scan the sky, and the angular resolution is 0.1\({}^{\circ}\) for both the elevation and azimuth. The schematic internal structure is shown in Figure 2. To ensure high observation accuracy of atmospheric radiation, it is calibrated by liquid nitrogen (LN2), hot load, noise diode and the Tipping curve method. These calibrations can provide an absolute accuracy of at least 0.5 K [15].  
Figure 1: The ground-based multichannel microwave radiometer (GMR) used at this experiment.  
\begin{table}
\begin{tabular}{c c} \hline \hline
**Parameter** & **Specification** \\ \hline  & K-band (22–30 GHz): 22.235, 22.5, 23.035, 23.835, \\ Channel frequency & 25.0, 26.235, 28.0, 30.0 \\  & V-band (51–59 GHz): 51.25, 51.76, 52.28, 52.8, 53.34, \\  & 53.85, 54.4, 54.94, 55.5, 56.02, 56.66, 57.29, 57.96, 58.8 \\ Parabolic reflector & Size: 320.5 \(\times\) 186.3 mm; Focal length: 180 mm \\ K-feedhorn & D = 80 mm; L = 115 mm; flare angle: 15\({}^{\circ}\) \\ V-feedhorn & D = 38.2 mm; L = 56 mm; flare angle: 15\({}^{\circ}\) \\ Beamwidth & K-band: \(\leq\)5\({}^{\circ}\); V-band: \(\leq\)3\({}^{\circ}\) \\ Gain & \(\geq\)25 dB \\ Sidelobe level & \(\leq\)\(-\)25 dB \\ TB accuracy & 0.5 K \\ TB sensitivity & K-band: \(\leq\)0.2 K (RMS); \\ V-band: \(\leq\)0.3 K (RMS) \\ Integration time & Typically 1 s \\ Antenna scanning capability & +/\(-\)180\({}^{\circ}\) stepping scanning \\ 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: Schematic internal structure of MWP967 KV radiometer.  
The GMR receiver system consists of two heterodyne receivers covering K- and V-bands, it is nominally a 22-channel instrument, the centre frequencies of the channels are given in Table 1, each band is received and detected, and all channels use a common Local Oscillator signal. The antenna system contains a parabolic reflector, beam splitter and the compactness of a corrugated feedhorn, and it is shown in Figure 2. The parabolic reflector can focus the beam and be used to scan the beams in elevation. The corrugated feedhorn offers a wide bandwidth, low cross-polarization level, low sidelobe level, and a rotationally symmetric beam. In order to scan the sun, we tried to improve the antenna servo control system of the GMR so that it could point and track the sun. Hence, it was possible to implement not only meteorological observation mode, but also innovative ones. The upgraded GMR was set up at the Xi'an field experiment site, as shown in Figure 1. We used the GMR to observe the solar microwave radiation.  
The period considered in this study extends from December 2019 to January 2021. During this period, the GMR undergoes regular maintenance, including antenna radome cleaning, sanity checks and absolute LN2 calibration. The LN2 calibrations were performed twice a year.