## 5 Summary, Conclusions, and Outlook  
An effective method for the absolute calibration of vertical and horizontal polarization radar receivers has been applied to the five dual-polarization radar receivers of Switzerland. Five observations are available for two radar receivers (_La Dole_ and PPM); for the other three radar receivers, a total of 22 observations are available. Regarding the absolute error, which is related to the absolute radar calibration, WEI and _Monte Lema_ show the best results, _La Dole_ and PPM show intermediate results, while Albis shows an underestimation of approximately \(-1.62\) (\(-1.25\)) dB for the horizontal (vertical) channel (including noise subtraction but neglecting clear sky attenuation). The standard deviation of such an error in terms of stability is better than \(\pm 0.18\) dB for both channels of Albis, Lema, and WEI. For _La Dole_ and PPM, the sample is too small to calculate the standard deviation. It is interesting to compare these results obtained during solar relative minima and Sun-tracking with those obtained by the operational, automatic method for the continuous monitoring of weather radar receivers through the comparison of daily radar-derived solar flux values with the DRAO reference [2; 3; 4]. Using such a different technique for more than two hundred days in 2014, the standard deviation of the error of the horizontal channel of Albis (_Monte Lema_) was, as expected, much larger than with this Sun-tracking method: \(\pm 0.35\) (\(\pm 0.48\)) dB (see Table 2 in [4]). By restricting the analysis to 100 days (see Table 4 in [5]), the standard deviation of the error results is smaller: \(\pm 0.26\) (\(\pm 0.34\)) dB for Albis (_Monte Lema_), which is still considerably larger than with the Sun-tracking method. Similar results are obtained if we consider other C-band radar receivers in Europe: for the same 100-day period, the standard deviation of the error was \(\pm 0.26\) dB for the Dutch Den Helder radar and \(\pm 0.36\) dB for the Finnish Anjalankoski (ANJ) radar. When comparing the figures of the Swiss radar receivers with those from the Dutch and Finnish radar receivers, it is worth noting that the MeteoSwiss operational algorithm is based on the median of the strongest 21 daily hits, while the others are based on a full five-parameter linear model fit to both polarization channels separately. Not surprisingly, the 22 Swiss observations using Sun-tracking and avoiding solar relative maxima show a considerably smaller standard deviation of the error: better than \(\pm 0.16\) dB for the horizontal channels of Albis, Lema, and WEI. The correlation between radar estimates and the DRAO reference is also significant (see Table 9 in the previous section). For Albis and WEI, it increases after the noise subtraction. The method shows stability not only of the absolute error between the radar and the DRAO reference, but also in terms of the difference between the vertical and horizontal channels: for WEI, the average of 9 values of 10 log(H/V) is \(\sim\)0.2 dB with \(\pm 0.1\) dB standard deviation. For _Monte Lema_ (6 values), it is \(\sim\)0.1 dB with \(\pm 0.08\) dB standard deviation. For Albis (7 values), the standard deviation is as small as \(\pm 0.04\) dB. It is interesting to compare this very small dispersion of the difference between \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multicolumn{1}{c}{**Standard Deviation of the Error**} & \multicolumn{2}{c}{**Albis**} & \multicolumn{2}{c}{_Monte Lema_} & \multicolumn{2}{c}{_Weissflubgifel_} \\ \cline{2-7} \multicolumn{1}{c}{**between Radar and DRAO Reference**} & **H** & **V** & **H** & **V** & **H** & **V** \\ \hline \(\mathrm{S_{Sun}+N_{unknown}}\) (as per Section 3) & 0.12 dB & 0.10 dB & 0.12 dB & 0.13 dB & 0.13 dB & 0.10 dB \\ \(\mathrm{S_{Sun}+N_{unknown}-N_{EL40}}\) & 0.16 dB & 0.17 dB & 0.15 dB & 0.18 dB & 0.16 dB & 0.12 dB \\ \hline \hline \end{tabular}
\end{table} 
Table 8: Same as in Table 7, but in terms of dispersion of the error (Log-transformed values).