## 3 Description of Acquired Data and Instruments  
MeteoSwiss has recently renewed its weather radar network: antenna-mounted, fully-digital receivers have been introduced into the current network; each system is equipped with two orthogonal receiving channels, which are able to measure vertical and horizontal linear polarizations. Three radar systems have already passed the extensive acceptance tests (test readiness review, factory acceptance test, integration and site adaptation test, system equipment acceptance test; see, for instance, [5, 6]) and have been operating for more than two years. These three radar sites are: Lema (162 m altitude, near Lugano; Lat 46.04\({}^{\circ}\); Lon 8.83\({}^{\circ}\)), Albis (935 m, near Zurich; Lat 46.29\({}^{\circ}\); Lon 8.51\({}^{\circ}\)) and Dole (1681 m, near Geneva; Lat 46.43\({}^{\circ}\); Lon 6.10\({}^{\circ}\)). From an operational Sun check method viewpoint, probably one of the most relevant aspects to emerge from such acceptance tests was the necessity of a modification of the original calibration unit (CU); updated, new generation calibration units [7] were installed on both the Albis and Lema radars at the end of 2013. Hence, the data shown in this note (from the beginning of 2014 up to mid-August 2014) refer to the new concept CU in two out of three radars.  
The solar flux at a 10.7-cm wavelength (S-band) is continuously monitored at the Dominion Radio Astrophysical Observatory (DRAO). This observatory is located near Penticton, British Columbia, Canada, and is characterized by low interference levels at the decimeter and centimeter wavelengths; the quality of the environment is maintained through strong local, provincial and federal protection [8]. Observations of the daily solar flux were started in 1946 and have continued to the present day. The current solar flux at 10.7 cm, \(F_{10.7}\), can be obtained from the DRAO observatory website ([[ftp://ftp.geolab.nrcan.gc.ca/data/solar_flux/daily_flux_values/fluxtable.txt](ftp://ftp.geolab.nrcan.gc.ca/data/solar_flux/daily_flux_values/fluxtable.txt)]([ftp://ftp.geolab.nrcan.gc.ca/data/solar_flux/daily_flux_values/fluxtable.txt](ftp://ftp.geolab.nrcan.gc.ca/data/solar_flux/daily_flux_values/fluxtable.txt))). Such values are corrected for atmospheric attenuation. In particular, they refer to precise measurements acquired three times a day. In summer, this takes place at 17, 20 (local noon) and 23 UTC. The hilly horizon and 50\({}^{\circ}\)N latitude cause the Sun to be too low for the first and last timings during the \"winter season\"; as a consequence, the flux determinations are made at 18, 20 and 22 UTC during \"darker months\".  
The 10.7 cm solar flux measurements can be converted to other wavelengths with some uncertainty (approximately 1 dB according to [8]). This is possible thanks to the remarkable stability of the blackbody spectrum of the slowly varying component of solar activity. However, it should be recalled that the spectral component can be distorted by superimposed contributions from flares, especially for \"short\" sampling periods, like radar \"hits during operational weather radar scanning\". In short, on the basis of Equation (13) in Section 5 of [8], the estimated solar flux in solar flux units (sfu) used in this note for the Swiss wavelength (5.5 cm) is:  
\[F_{5.5}=0.7155\times(F_{10.7}-64)+113. \tag{1}\]  
If the receiver bandwidth (in Hz), the effective antenna area (in m\({}^{2}\)) and the receiver and transmitter losses (dimensionless) are known, then the radar received power can be converted into sfu (see Section 2 in [2] for details); this is also the approach that has been followed in this paper (see the next section). Because of the multiplicative nature of the errors that affect radar measurements, radar-related results are often presented using decibels (dB). In the present case, \(s=10\) log(\(S\)/S\({}_{0}\)) is used, where S\({}_{0}=1\) sfu = 10\({}^{-19}\) mW\(\cdot\)m\({}^{-2}\cdot\)Hz\({}^{-1}\) and \([s]=\) dBsfu.