## 4 Result and Discussion
### Scanning the Sun and Measuring the Antenna Pattern  
GMR antenna pattern measurement is important for reliable and accurate antenna temperature measurement. We used the GMR to track and scan the sun at some frequencies. Figure 5 shows an example of scanning the sun on 14 March 2020, and the maximum solar elevation is about 40\({}^{\circ}\). Measured TB increment distributions for a 29 by 29 matrix of measurement scatterplots and corrected the angle distortion are shown in Figure 5. There are more raster scanning points to collect a sufficient number of points for fitting near the sun, and this scanning method will help to get the complete 3D antenna pattern. The solar power data exhibit a radial pattern with a maximum in the center; this maximum value and the size of the halo are different for each frequency, and they are related to the antenna beamwidth and solar radiation.  
Figure 6 shows the results from fitting the Gaussian model, and the 3D pattern of the antenna can be obtained by least-square fitting of the antenna raster scanning data. From the observation results, the scanning data are relatively symmetrical, the maximum value of TB increment is different for each frequency and they are related to the antenna beamwidth, and TB increment is inversely proportional to the beamwidth. The azimuth and elevation cut position for the sun is chosen to be the expected main beam direction (Figure 7).  
Figure 4: Scatterplots of the pointing bias at each frequency. The observations were taken on a Table for 53 days from December 2019 to January 2021. (**a**) 22.235 GHz, (**b**) 26.235 GHz, (**c**) 30.0 GHz, (**d**) 30.0 GHz. The vertical axis gives the difference between the observed antenna elevation and the calculated elevation of the sun, and the horizontal axis gives the same for the azimuth.  
Figure 5: _Cont._Generally, the antenna power pattern is measured in at least two principal planes (H- and E-plane). Therefore, we only observed the solar azimuth and elevation scanning data to determine the beamwidth of the antenna through the center of the sun as it can save a lot of scanning time. Besides the sun measurement, a point source measurement was performed for comparison in the microwave anechoic chamber. We measured the antenna pattern at 30 GHz, the pattern derived from the sun and a point source were compared, and the maximum error was less than 0.1\({}^{\circ}\) at 30 GHz (Figure 8). It is shown that the main-lobe matches well to the pattern based on the point source measurements. The results show that when the antenna gain decreases more than 25 dB, the GMR cannot  
Figure 5: Two-dimensional distribution of the TB increment observed from the sun raster scanning on March 14, 2020, 13:00 (local time), at four specific frequencies. The vertical axis is the elevation difference between the antenna and the sun, and the horizontal axis is their azimuth difference. (**a**) 22.235, (**b**) 26.235, (**c**) 30.000, and (**d**) 51.250 GHz.  
Figure 6: The 3D antenna power patterns were fitted by the least-square method on March 14, 2020, 13:00 (local time), at four specific frequencies. \(A_{d}\) is the maximum TB increment, \(\theta_{H}\) and \(\theta_{E}\) are the half-power beamwidth of the H- and E-planes (**a**) 22.235 GHz, (**b**) 26.235 GHz, (**c**) 30.000 GHz, and (**d**) 51.250 GHz.  
measure the TB increment from the sun. The results show that the sun method is simple to employ, especially when the antenna cannot be moved from its staged installation site.