## 4 Discussion of the Results Including a Preliminary Attempt of Noise Subtraction  
As previously stated, our radar-retrieved estimates of solar flux can be improved in two directions, which are characterized by opposite signs: (1) on the one hand, the retrieved estimates are slightly \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Date**} & \multicolumn{2}{c}{**DRAO**} & \multirow{2}{*}{**I\({}_{\textbf{5.5}}\) Rx H**} & \multirow{2}{*}{**I\({}_{\textbf{5.5}}\) Rx V**} & **10 log** & **10 log** & **10 log** \\  & **Reference** & & & **(H/V)** & **(H/DRAO)** & **(V/DRAO)** \\ \hline
22 February 2016 & 21.30 \(\pm\) 0.024 & 21.50 & 21.55 & \(-\)0.05 dB & +0.20 dB & +0.25 dB \\
14 March 2016 & 21.27 \(\pm\) 0.024 & 21.50 & 21.65 & \(-\)0.15 dB & +0.23 dB & +0.38 dB \\
22 March 2016 & 21.14 \(\pm\) 0.023 & 21.05 & 21.50 & \(-\)0.45 dB & \(-\)0.09 dB & +0.36 dB \\ \hline Average & 21.24 & 21.35 & 21.57 & \(-\)0.22 & +0.11 & +0.33 \\ St. Deviation & \(\pm\)0.09 & \(\pm\)0.18 & \(\pm\)0.07 & \(\pm\)0.21 & \(\pm\)0.18 & \(\pm\)0.07 \\ \hline \hline \end{tabular}
\end{table} 
Table 6: Solar flux values from three Sun-tracking observations at the PPM site (next to Crans Montana) and corresponding nearest-in-time reference values by the DRAO in Canada. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Date**} & \multicolumn{2}{c}{**DRAO**} & \multirow{2}{*}{**I\({}_{\textbf{5.5}}\) Rx H**} & \multirow{2}{*}{**I\({}_{\textbf{5.5}}\) Rx V**} & **10 log** & **10 log** & **10 log** \\  & **Reference** & & & **(H/V)** & **(H/DRAO)** & **(V/DRAO)** \\ \hline
2 November 2013 & 22.34 \(\pm\) 0.079 & 21.27 & 21.67 & \(-\)0.40 dB & \(-\)1.07 dB & \(-\)0.67 dB \\
2 April 2014 & 22.51 \(\pm\) 0.070 & 21.57 & 21.92 & \(-\)0.35 dB & \(-\)0.94 dB & \(-\)0.59 dB \\
6 August 2014 & 22.19 \(\pm\) 0.031 & 20.97 & 21.37 & \(-\)0.40 dB & \(-\)1.22 dB & \(-\)0.82 dB \\
28 January 2015 & 22.56 \(\pm\) 0.039 & 21.47 & 21.87 & \(-\)0.40 dB & \(-\)1.09 dB & \(-\)0.69 dB \\
7 April 2015 & 21.76 \(\pm\) 0.125 & 20.47 & 20.87 & \(-\)0.40 dB & \(-\)1.29 dB & \(-\)0.89 dB \\
28 May 2015 & 21.28 \(\pm\) 0.021 & 20.07 & 20.47 & \(-\)0.40 dB & \(-\)1.21 dB & \(-\)0.81 dB \\
21 March 2016 & 21.16 \(\pm\) 0.017 & 20.07 & 20.37 & \(-\)0.30 dB & \(-\)1.09 dB & \(-\)0.79 dB \\ \hline Average & 21.97 & 20.84 & 21.22 & \(-\)0.38 & \(-\)1.13 & \(-\)0.75 \\ Standard dev. & \(\pm\)0.58 & \(\pm\)0.64 & \(\pm\)0.65 & \(\pm\)0.04 & \(\pm\)0.12 & \(\pm\)0.10 \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Solar flux values from seven Sun-tracking observations at the Albis site (near Zurich) and corresponding nearest-in-time reference values by the DRAO in Canada. \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Date**} & \multicolumn{2}{c}{**DRAO**} & \multirow{2}{*}{**I\({}_{\textbf{5.5}}\) Rx H**} & \multirow{2}{*}{**I\({}_{\textbf{5.5}}\) Rx V**} & **10 log** & **10 log** & **10 log** \\  & **Reference** & & & **(H/V)** & **(H/DRAO)** & **(V/DRAO)** \\ \hline
28 January 2015 & 22.56 \(\pm\) 0.039 & 22.24 & 22.04 & 0.20 dB & \(-\)0.32 dB & \(-\)0.52 dB \\
7 April 2015 & 21.76 \(\pm\) 0.125 & 21.59 & 21.54 & 0.05 dB & \(-\)0.17 dB & \(-\)0.22 dB \\ \hline Average & 22.10 & 21.91 & 21.79 & 0.13 & \(-\)0.19 & \(-\)0.31 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Solar flux values from two Sun-tracking observations at the _La Dole_ site (near Geneva) and corresponding nearest-in-time reference value by the DRAO in Canada. low because atmospheric attenuation is neglected; (2) on the other hand, the retrieved estimates are somewhat too high because of the contamination of receiver noise, which is not negligible compared to the solar signal. Total clear sky atmospheric attenuation at the zenith is 0.076 (0.060) dB at an altitude of 1000 (3000) m--this by assuming a one-way attenuation value of 0.01 dB/km at sea level, and an equivalent atmospheric height of 8.5 km.