## 2 Operational Observations of Solar Spectral Power at the Antenna Feed  
A complete description of the automated detection of Sun signatures in polar volume data of weather radar can be found in [1]. A similar approach is currently used at MeteoSwiss: a continuous, non-coherent, noise-like power signal from a non-polarized radio-frequency source is sought at long ranges in consecutive radar bins (75 m long and 1\({}^{\circ}\) wide); such a signal is expected to be between 5 and 12 dB stronger than the background noise. Eight-bit, \"uncorrected\" reflectivity data are used in this study. The pulse repetition frequency and rotation speed are optimized in the operational MeteoSwiss radar scan program as a function of the angle of elevation. Therefore, the number of pulses involved in each 1\({}^{\circ}\) half power beam width (HPBW) bin ranges from 28 to 42. In the MeteoSwiss approach, only the last 5 km (60 gates) of each radar bin are averaged for Sun power retrieval. Hence, each reflectivity value corresponding to a Sun hit is the linear average of a few thousand samples. Several tens of hits are found per day, depending on the scan program, latitude and season.  
In order to retrieve the solar spectral power, daily radar reflectivity values were analyzed by [PERSON] _et al._, in single [2] and dual-polarization [3] receiving chains using the five-parameter linear model fitting procedure described in detail in [1, 4]. It is worth noting that a correction factor has been introduced in the recent paper by [PERSON] _et al._[4] (Equation (3), page 1705) to characterize the residual bias between the true Sun azimuth from ephemeris and the value \"read\" using the antenna encoder: such a term, which is basically the sine of the Sun zenith angle, is more important in mid-latitude and equatorial regions, where the Sun reaches lower zenith angles. Once the five parameters are derived, for instance by means of the least squares method, the peak solar power that the radar would have received if the beam had hit the Sun's center can be estimated. At MeteoSwiss,the focus of the operational Sun check method is on a relative calibration between horizontal and vertical channels, as well as on an inter-comparison of different radars. Since the focus is not on absolute calibration, the median of the strongest 21 hits in a given day is taken in order to obtain a robust estimate of the Sun's apparent reflectivity. One important implication of this choice is that an accurate assessment of the absolute calibration of the receivers cannot be pursued: such a daily median value is of the order of 2 dB (or more) smaller than the true peak solar power value. However, the method allows an accurate monitoring to be made of the differential bias between the horizontal and vertical channels. Furthermore, assuming that the convolution between the antenna radiation pattern and the Sun's disk will on average lead to approximately the same amount of \"underestimation\", the stability of radar receiving chains can also be monitored. Section 4 will show that not only is this the case, but also that the monthly variability of the Sun's microwave signals is captured well by the Swiss radars.