# Dual-Polarization Observations of Slowly Varying Solar Emissions from a Mobile X-Band Radar
## 5 Discussion
The issue of well calibrated differential reflectivity is crucial for successful meteorological applications of dual-polarization radar [25]. The sun is a cost/benefit wise tool for calibrating the polarimetric receiving chain: offline sun measurements, in which operational scanning is stopped and the antenna beam axis is pointed towards the center of the sun (hence maximizing the sun-to-noise ratio) have been successfully used at S-band [25] and C-band [9]. National meteorological services prefer to use solar signals \"randomly\" detected during the operational scan program [12; 13; 14]: the signal-to-noise \begin{table}
\begin{tabular}{l c c c c} \hline \hline  & **Hor.** & **Ver.** & **Hor. N\({}_{\text{subtr.}}\)** & **Ver. N\({}_{\text{subtr}}\)** \\ \hline Explained variance in percentage & 44.4\% & 57.0\% & 41.1\% & 55.4\% \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Explained variance of the retrieved radar values when the DRAO reference measurements are used as the independent variable. ratio is less favorable, but the radar does not need to be offline. In this paper an automatic Sun-tracking scan program that combines the advantages of both approaches is presented. As it can be seen in Table 11, the reception differential bias (independently of DRAO reference values, just considering the ratio between H and V) is +0.06 dB for raw observations and +0.09 dB after noise subtraction. It should be 0.00 dB, because the quiet and slowly varying components of the (thermal) solar radiation have equal power at vertical and horizontal polarizations. Ideally, the variance of H and V should be equal and the (square of the) correlation coefficient between them should be 1. In our real case, the explained variance between H and V (57 samples) is 96.0% for raw data and 92.5% after noise subtraction (see Table 4 for a comparison). It is worth reminding that sun-based checks are useful to characterize only the receiving part of the radar, not the transmitting one. Similarly, the calibration with respect to the DRAO reference presented in this paper deals solely with the receiving chain of the radar. In order to characterize also the transmitting chain one could use a one-way passive calibrator, for instance the one presented in Section 2.1 in [26]. A calibration of the whole dual-polarization radar system (receiving and transmitting chains) would be the optimal solution, but this task is certainly more complex. The differential reflectivity bias can be (monitored and) assessed using vertical scan and echoes from light rain [27], while absolute calibration of the radar reflectivity is certainly more challenging. Attempts have been made using the idea of self-consistency between reflectivity, differential reflectivity and specific differential phase [25]. Very recently, a multisensor conceptual experiment based on collocated dischrometers, a 24 GHz FMCW Doppler radar and a dual-polarization weather radar under test was able to put in evidence a weather radar underestimation of reflectivity of the order of 1.8 dB [27]. The receivers were checked using the sun and the average bias was found small (-0.2 dB). Consequently, the bias derived using external co-located sensors is attributed to the transmitting chain.