Abstract
The radio noise that comes from the Sun has been reported in literature as a reference signal to check the quality of dual-polarization weather radar receivers for the S-band and C-band. In most cases, the focus was on relative calibration: horizontal and vertical polarizations were evaluated versus the reference signal mainly in terms of standard deviation of the difference. This means that the investigated radar receivers were able to reproduce the slowly varying component of the microwave signal emitted by the Sun. A novel method, aimed at the absolute calibration of dual-polarization receivers, has recently been presented and applied for the C-band. This method requires the antenna beam axis to be pointed towards the center of the Sun for less than a minute. Standard deviations of the difference as low as 0.1 dB have been found for the Swiss radars. As far as the absolute calibration is concerned, the average differences were of the order of −0.6 dB (after noise subtraction). The method has been implemented on a mobile, X-band radar, and this paper presents the successful results that were obtained during the 2016 field campaign in Payerne (Switzerland). Despite a relatively poor Sun-to-Noise ratio, the “small” (~0.4 dB) amplitude of the slowly varying emission was captured and reproduced; the standard deviation of the difference between the radar and the reference was ~0.2 dB. The absolute calibration of the vertical and horizontal receivers was satisfactory. After the noise subtraction and atmospheric correction a, the mean difference was close to 0 dB.
Highlights
The use of extraterrestrial radio noise sources as calibration aids has been reported in the literature since the 1950s, with the aim, in particular, of evaluating the performance of radio telescopes [1,2,3].As far as the calibration of weather radar receivers is concerned, Whiton et al [4] first introduced the idea of using the Sun in 1976, and after further discussions on the topic, Frush presented a practical methodology in Zurich in 1984 [5]
Such a two-digit average of Dominion Radio Astrophysical Observatory (DRAO) flux values is expressed in dBsfu: the values are listed in the 4th column in Table A1 (Appendix A), and are followed by a three-digit standard deviation, calculated using the 5 values on the logarithmic decibel scale
It is possible to state that the dual-pol receivers of the X-band radar were well calibrated during the 241-day period (2016) in Payerne, in both absolute and relative terms
Summary
The use of extraterrestrial radio noise sources as calibration aids has been reported in the literature since the 1950s, with the aim, in particular, of evaluating the performance of radio telescopes [1,2,3].As far as the calibration of weather radar receivers is concerned, Whiton et al [4] first introduced the idea of using the Sun in 1976, and after further discussions on the topic, Frush presented a practical methodology in Zurich in 1984 [5]. In 1989, Pratte and Ferraro [6] presented the first quantitative comparison of S-band horizontal polarization radar-derived solar flux values at the National Center for Atmospheric Research (NCAR) and accurate reference measurements acquired by several observatories (e.g., the Dominion Radio Astrophysical Observatory (DRAO) of the Canadian National Research. According to the observations of the NCAR CP-2 radar, where DRAO measurements were used as a reference, the standard deviation of the multiplicative error that affected 14 retrieved values was 0.14 dB. The 14 semi-automated radar observations that were considered had been acquired between November 1987 and August 1988, which was an active solar period. Using their semi-automated sun-tracking technique, a radar operator was able to perform the data acquisition
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