## 4 Results and Discussion  
### Choice of the Reference Solar Signal and Scores for the Assessment of the Agreement  
As stated in the Introduction, the focus of the present analysis is on the relative agreement and not on the absolute calibration of each receiver. In other words, emphasis is on the variation of the error around the mean; it is not on the mean error itself. The error is defined in Sections 4.2 and 4.3 as the ratio between the value measured by the radar receiver and the reference (DRAO measurements); in Section 4.4, the error is the ratio between the horizontal and vertical polarization values. As a consequence, the selected scores emphasize the agreement between oscillations around the mean value. The first selected score is the explained variance, which is the square of the correlation coefficient (multiplied by a hundred). The second score is named \"scatter\" [9] and is measured in decibels (dB). It has the advantage of being conceived of to be robust, informative and independent of any multiplicative factor that could affect the measurements. The error distribution is expressed as the cumulative weighted contribution to the normalized total amount, which is the reference (\(y\)-axis) as a function of the log-transformed estimate/reference ratio (\(x\)-axis). Alternatively, it is possible to simply use the standard deviation of the log-transformed radar-to-DRAO solar flux value ratio, as shown, for instance, by [PERSON] _et al._[2].  
Figure 1 shows the daily variability of the three series of daily precise measurements acquired in 2014 by DRAO at 10.7 cm and extrapolated to the Swiss radar wavelength using Equation (1): the blue curve is for 20 UTC measured data; the green one is for the 18 (17) UTC data; while the red one is for the 22 (23) UTC data. Because of the Sun's rotation on its axis (variable, as a function of latitude), the number of active regions that enhances the radio emission with respect to the quiet (background) component, as seen from the Earth, varies. The result is an oscillation with an amplitude of approximately 1 dB and a period of \(\sim\)27 days. In January, there were a few episodes when radiation was outshone; for instance, up to 26.9 dBsfu on 7 January between 17 and 18 UTC. Apart from these few days in January, the three curves are almost indistinguishable. It should be noted that the flux values were log-transformed before being displayed and are shown on the \(y\)-axis in dBsfu. Let us now evaluate the agreement between the three series using the above-presented scores: the blue _vs._ green explained variance is 87%; the blue _vs._ red one is 98%; and the red _vs._ green one is 82%.  
Because of the rapidly varying solar component that consists of bursts (second and minute duration) produced by flares and other transient activity, which caused two extreme variability cases on 4 and 7 January, the explained variance, based on linear values of solar fluxes, is significantly smaller: 66%, 97% and 58%, respectively. As far as the scatter is concerned, the results are listed in Table 1 (the reference measurement is shown along the columns). According to the scatter values, the dataset acquired at 20 UTC again shows better agreement than the other two sets. For this reason, the radar derived data in Section 4.2 will be evaluated against a reference using the 20 UTC dataset.