## 2 Description of Instruments, Acquired Data, and Retrieval Formula  
In recent years, MeteoSwiss has renewed its weather radar network with an innovative, state-of-the-art solution. From 2011 to 2015, five new polarimetric systems have been installed. Fully digital, antenna-mounted receivers, which are capable of simultaneously measuring vertical and horizontal linear polarization components, have been introduced. With respect to the conventional solution, where the receivers are installed next to the transmitter inside the technical room, this system architecture does not require the installation of a dual channel rotary joint, which may introduce differential errors in amplitude and phase; it also provides a significant reduction of Rx-losses. Another key aspect of the Swiss weather radar receivers is the use of a stable, white signal generated by a noise source (NS) as an absolute reference for the calibration of the radar Rx. Every 5 min, which is the time required to accomplish a full-volume in the 20-elevation Swiss scan program ([PERSON] et al., 2015), the NS reference signal (NS\({}_{\text{ref}}\), around \(-90\) dBm) is injected into the Rx front-end (input of the LNA) and the corresponding log-transformed value in analog-to-digital-Units (ADUs) at the output of the A/D converter is read, NS\({}_{\text{out}}\). In this way, the factor for the transformation from dBADU to dBm of any received signal is known and updated every 5 min ([PERSON] et al., 2015). The Dominion Radio Astrophysical Observatory (DRAO) is located at a site near Penticton (British Columbia, Canada), which is characterized by low interference levels at decimeter and centimeter wavelengths. Accurate observations of the daily solar flux have been performed since 1946; they are acquired three times per day at 10.7 cm, which is the wavelength where the slowly varying solar component is more significant compared to the quiet radio flux [13]. The 10.7-cm solar flux measurements can be converted into other radio frequencies with some bias uncertainty. This is possible thanks to the remarkable stability of the spectrum of the slowly varying component of solar activity. As stated in the introduction, for the conversion from 10.7 cm to 5.5 cm, we used the formula listed in [6] with the coefficients presented in Equation (1) of [4]. For the set of measurements presented in this paper (a total of 17 days from 2 November 2013 until 22 March 2016), we have, on the one hand, checked that the spectral component was not distorted by contributions from flares or other events having spectra and variability with time that are a function of frequency, but, on the other hand, pointed the radar antennas toward the Sun in days close to the Sun minima. During active periods (_i.e._, the last few years), in fact, the number of active regions (Sun spots) that enhance the radio emission with respect to the quiet component, as seen from the Earth, varies because of the Sun's rotation around its axis. This result is an oscillation with an amplitude of a couple of decibels and a period of \(\sim\)27 days. For instance, using the above cited Equation (1), in 2014, the min/max values (excluding solar flares) at 5.5 cm were around 21.3 and 23.5 dBsfu; in 2015, they were around 21.0 and 22.7 dBsfu. Note that, in Equation (1), the quiet Sun emission is assumed to be 113 sfu (_i.e._, 20.53 dBsfu). Note also that, in this paper, we will use log-transformed values of solar flux units (spectral irradiances) coming from the Sun: \(S=10\) log(\(s/S_{0}\)), where S\({}_{0}=10^{-19}\) mW\(\cdot\)m\({}^{-2}\cdot\)Hz\({}^{-1}\) and [\(S\)] = dBsfu. Finally, we need a way to transform the observed Sun signal at the output of the receiver A/D converter, S\({}_{\rm out}\) ([S\({}_{\rm out}\)] = dBADU), into equivalent spectral irradiance at 5.5 cm, incident on the radome and subsequently on the antenna, I\({}_{5.5}\), where [I\({}_{5.5}\)] = dBsfu. This transformation is conceptually done in three steps that are thoroughly described in Section 2 of [7].