## 1 Introduction  
The sun is a natural external source with high radiation intensity, it radiates in a wide frequency band [1], and the solar emission is an approximate blackbody of 6000 to 20,000 K at millimeter and submillimeter wavelengths [2]. Observations of the thermal radio emission of the sun at millimeter wavelengths are important in the evaluation of theoretical atmospheric and surface models [3]. Generally, spectrographs, polarimeters and telescopes are used to observe the solar microwave radiation and activity [4, 5, 6, 7, 8, 3, 4], but these devices are generally large, expensive and complicated to operate. The GMR is a typical atmospheric passive remote sensing device, has high sensitivity and it can be also used to observe the solar radiation. [PERSON] et al. (2016) attempted to measure the T\({}_{\rm B}\) of solar radiation and propose the parametric retrieval of the atmospheric extinction at 23.8, 31.4, 72.5 and 82.5 GHz [1], but an operational GMR has rarely been used to track and observe solar radiation or a solar eclipse. At present, as GMRs can continuously observe the atmospheric microwave radiation T\({}_{\rm B}\) at K- and V-bands and provide valuable information on the temperature, humidity and liquid water structure of the troposphere [9; 10], they have been widely used in meteorology and climate monitoring in last few decades [11; 12; 13]. Although there have been some new applications of the technology for radiometers, their applications are limited to atmospheric observation and they are only used to observe and retrieve the temperature and humidity profiles, their value has not been fully utilized in other fields. Nowadays, new applications have been found for radiometers. For example, based on the theory of atmospheric radiation and the remote sensing potential of GMR, [PERSON] et al. (2014) proposed a method to measure the microwave radiation of a hot air cylinder caused by lightning [14], and [PERSON] et al. (2018, 2020) observed microwave heating and the duration of artificial rocket-triggered lightning using a GMR [15; 16]. In addition, because the sun has strong microwave radiation, theoretical analysis and experimentation show that GMRs are able to observe and monitor solar radiation, and radiometers can be used to observe solar radiation and measure the brightness temperature of solar radiation; the solar monitoring method can be used to measure the antenna pattern, monitor the antenna alignment and evaluate the receiver stability of a GMR in operational field applications [2; 17; 18; 19]. Therefore, we propose the improvement of the antenna servo control system of a GMR so that it can track and observe the sun automatically, and then attempt to observe the variations of solar radiation. Thus, an operational GMR can be used to observe the T\({}_{\rm B}\) variations which reach the antenna due to solar radiation and to study the solar activity and variations of solar radiation [17; 18]. A solar eclipse is one of nature's most impressive celestial performances and it happens rarely. So, we wanted to know whether a radiometer could respond to the variation of solar radiation during the solar eclipse. Fortunately, two solar eclipse events happened on 26 December 2019 and 21 June 2020 and were able to be observed in Xi'an, China. The solar eclipses provided a good opportunity for observing the variation of solar radiation. Based on the observation experiment of the sun with a radiometer, we attempted to observe the variation radiation of the sun during the solar eclipse. We monitored the complete duration of solar eclipse with the upgraded GMR, and the results showed that the GMR could respond to the variation of solar radiation and the effective radiation area of the sun during the solar eclipse. More importantly, we successfully observed the variation of the intensity of the solar radiation arising from the Earth-Sun distance and we used these data to evaluate the eccentricity of earth's orbit.