flux units (SFUs), where 1 SFU is equal to \(10^{-22}\) W\(\cdot\)m\({}^{-2}\)\(\cdot\)Hz\({}^{-1}\). The S-band solar flux measurements distributed by the Dominion Radio Astrophysical Observatory (DRAO) of the National Research Council of Canada provide a useful reference for weather radar receivers. Such measurements, precisely acquired three times per day, are consistent and accurate; furthermore, the same source can be observed simultaneously over a large area. A problem encountered with C-band radar receivers is the frequency conversion from 10.7 to 5.5 cm. Section 5 of the paper by Tapping Tapping (2016) provides a simple conversion formula that allows one to overcome such a problem. In particular, we use the coefficients presented in Equation (1) of Tapping (2016), which are tailored to the Swiss frequency band (5430-5470 MHz). In this paper, we present results focusing on absolute calibration of vertical and horizontal polarization receivers. In other words, we also put emphasis on the absolute difference (bias) between the log-transformed radar-retrieved value and the reference value accurately measured by the DRAO.  
Such absolute calibration results are obtained thanks to a convenient and effective method recently presented in Tapping (2016). The method is complementary to the on-line technique that automatically detects and analyzes Sun signals stored in the polar volume radar reflectivity data acquired during the operational weather scan program ([PERSON] et al., 2015; Tapping (2016); Tapping (2016); Tapping (2016)). Such a technique, which allows relative calibration and mutual inter-comparison between vertical and horizontal channels, has the great advantage of requiring no interruption of the weather surveillance. On the contrary, the method here applied and presented in Tapping (2016) tackles the absolute calibration of the receiver by pointing the antenna beam axis towards the center of the Sun; hence, it requires that the weather radar be off-line for a few minutes during such a special purpose scan. This idea has been applied many times in radio astronomy. In radar meteorology, it was developed in the late 1970s by [PERSON] and [PERSON] at the National Center for Atmospheric Research, preliminarily presented in 1984 [PERSON] and [PERSON] (1984) and thoroughly described by [PERSON] and [PERSON] in 1989 [PERSON] and [PERSON] (1989). Subsequently, [PERSON], [PERSON] and [PERSON] have further extended it and eventually transferred to Nexrad (USA) where [PERSON] and colleagues developed it for use with the WSR-88D [PERSON] and [PERSON] (1989).  
The method has been applied to the five weather radar receivers of the recently renewed Swiss weather radar network. As described in Tapping (2016), radar observations of the solar signal are performed only in fair weather conditions. In this way, wet-radome attenuation and attenuation along the path are avoided. Section 2 briefly presents the operational Swiss weather radar receivers, the acquired and reference data, as well as the formula for the retrieval. Results are given in Section 3 for the ten dual-polarization receivers of the Swiss weather radar network: a set of 27 observations by the five radar receivers is presented. A detailed discussion and interpretation of the results is presented in Section 4. A summary, conclusions, and an outlook are provided in the last section.