## 5 Conclusions  
In this paper, we presented a method for determining the GMR antenna pattern, calibrating antenna alignment and checking the system stability using the solar radiation signals. Experiment to track and scan the sun was carried out by using the GMR at the Xi'an experiment field. The use of the sun for the GMR antenna alignment and checking the effective antenna system gain and stability are suited for routine application to overcome the disadvantages of complexities and high costs in operational field applications after installation.  
As a successful application example of the sun-based monitoring for GMR, an upgraded GMR has been discussed. It has been shown with the TB increment because of the fact that solar radiation can be observed and the 3D antenna pattern can be measured by raster scanning of the sun. A comparison between the sun and a point source measurement showed an agreement with each other in terms of the beamwidth in both H- and E-planes.  
During the solar observation, operational the GMR can accurately observe the microwave signal of the sun to monitoring of the receiving system gain. We can use the system gain of the measurement as a check of the GMR system stability. During the observation, the standard deviation of the system gain is within 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. Results from a daily analysis of the sun signals in radiometer data can be used for monitoring the alignment of the GMR antenna and the stability of the receiving system on sunny days or high-cloud condition. This method allows for daily quality monitoring of operational GMRs, which is becoming more and more popular in observational networks.  
Figure 11: The refraction curve as a function of elevation is calculated by model. The interval between data points is fixed (1\({}^{\circ}\)) on the horizontal axis.  
Because accurate monitoring receiving system of radiometer is a prerequisite for a national network of operational radiometer, it is recommended that the sun-based monitoring method be used, which can be done independently and automatically in the field. This method presented in this paper has great potential for routine monitoring of the GMRs in national or international networks.  
Conceptualization, L.L. and Z.W; methodology, L.L., Z.W. and Y.M; software, L.L.; validation, L.L.; investigation, J.L., J.Q., L.Z. and R.C.; resources, J.Q., R.C. and L.Z.; data curation, L.L., L.Z. and R.C.; writing--original draft preparation, L.L., Z.W. and Y.M.; writing--review and editing, L.L., Z.W. and Y.M.; visualization, L.L.; project administration, Z.W. and J.L.; funding acquisition, Z.W. All authors have read and agreed to the published version of the manuscript.  
This work was supported by the National Natural Science Foundation of China (No. 41675028), and the Natural Science Foundation of Shanxi Province, China (No. 2020 JM-718) and the Xi'an Science and Technology Project of Shanxi Province, China (No. 20 SFSF0015), and A Project Funded by the Priority Academic Program Development of Jiangsu Higher Education Institutions [PAPD].  
Not applicable.  
Data Availability StatementData are contained within the article.  
We are also grateful to the Shanxi Provincial Atmospheric Sounding Technical Support Center and the Xi'an Meteorological Observation Center of Shaanxi Province, China, for the radiometer installation and for their support.  
The authors declare no conflict of interest.