# Dual-Polarization Observations of Slowly Varying Solar Emissions from a Mobile X-Band Radar
## 3 Deriving Solar Flux Values Using Observations of a Mobile Radar Working at the X-Band  
### The Swiss Confederation Dual-Polarization X-Band Weather Radar  
A few years ago, the Swiss Confederation acquired an innovative, state-of-the-art, mobile, dual-polarization, Doppler weather radar to assess the effect of precipitation on the performance of precision approaching radars (PAR). MeteoSwiss is currently operating the X-band radar in parallel with the PAR site acceptance tests at designated locations. The products generated by the weather radar are used to assess the state of the atmosphere within and around the PAR cells. Refer to Section 2.3 of [16] for further details. The system is provided with an accurate antenna control unit that allows tailored, powerful and precise 3D scanning operations to be conducted. Hence, it was possible to implement not only conventional, meteorological scan programs, but also innovative ones. The following implemented scan programs are worth mentioning: (A) 10-min co-located PAR cell monitoring; (B) 2-min adaptive track of thunderstorm cells; (C) 30-min adaptive and fully automatic Sun-tracking. As far as (A) and (B) are concerned, more information can be found in [18; 19], respectively. As for (C), it is worth noting that, because of our adaptive scan, the sun is automatically tracked every half hour, just after and before the ordinary meteorological scans, which is, Polar Plane Indicator (PPI) and Range Height Indicator (RHI). The system is also equipped with a powerful Radar Signal Control and Processing software (Ravis(r)) that allows, among many other features, an accurate manual and/or automatic Sun-tracking. However, even in automatic mode, the Ravis software needs an operator to launch the sun raster scan, while our adaptive scan is totally unmanned. Note that during the Sun-tracking, Ravis turns the transmitter off, while during our 30-min adaptive scan (interleaved between PPI and RHI) the transmitter is on. The list of benefits associated with this mobile system is long and the spectrum of possible atmospheric observations is wide, e.g., [18; 19; 20; 21]. Because of the low receiver losses, the presence of a Low Noise Amplifier (LNA) at the first stage of the receiver chain and a directive antenna (HPBW = 1.3\({}^{\circ}\)), the system is sensitive enough to detect the solar flux, although with a relatively poor Signal-to-Noise ratio. The ratio between a measurement with the antenna beam axis pointed towards the center of the sun (Signal + Noise) and a noise measurement acquired at high elevation is in fact around 4 dB. This implies that noise subtraction reduces the raw observations by approximately 1.8 dB (as can be seen in Appendix A, Table 1, which lists the results of the retrieval for both the original observations and after noise subtractions). From a practical viewpoint, in order to precisely quantify the amount of solar power that is reaching the LNA at the entrance of the radar receiver (Rx) during a clear sky day, it is necessary to know the equivalent area of the radar antenna and the Rx losses (including dry radome losses) along the path. These characteristics are listed in Table 1 for both polarization channels. It should be noted that the antenna performance is expressed in terms of maximum gain, as measured by the manufacturer at the test-range. In order to derive the equivalent area, Aeq, in m\({}^{2}\) from the (horizontal and vertical polarization) gain measured at the test range, Equation (5) is used. The X-Band Radar Calibration Concept and the Conversion of the Solar Signal from Log-Transformed Analogue-Digital-LUnits (dBadu) to Solar Flux Units (dBsfu)  
During the 30-min automatic, dedicated Sun-tracking, the maximum possible amount of incoming spectral irradiance from the sun hits the parabolic antenna reflector (the antenna beam axis is pointed towards the center of the sun).