A MODERN MICROWAVE RADIOMETER FOR SOLAR FLUX MONITORING

P. Schmid¹, M. Sartori², M. Gabella², A. Murk¹

¹University of Bern: Institute of Applied Physics (IAP), Microwave Physics Group, ²MeteoSwiss: Radar, Satellite and Nowcasting Division (MDR)

We present the latest results of a project collaboration between the Radar, Satellite and Nowcasting Division (MDR) of MeteoSwiss and the Institute of Applied Physics (IAP) of the University of Bern. The project focuses on the development of a radiometer system for measuring the solar flux density at centimetric wavelengths. Such measurements are useful for solar physics and space weather, and find application as calibration datum for other antenna systems such as the MeteoSwiss C-band and X-band weather radars [1; 2]. Continuous monitoring and regular calibration of the individual radar receivers are the basis for the quality of the measurements and thus essential for reliable and robust radar products. Comparing the radar system noise power in the direction of the sun against a well-calibrated reference telescope, allows to supervise – in a uniform, cost-efficient and computer automated manner – the stability of the receiver chains of radar networks on a routine basis.

Long-term observations since the mid-1940s at other observatories show frequency-dependent variations of several decibels on time-scales from milliseconds to years linked to a time-varying mix of emission mechanisms, see e.g. [3, 4]. Generally, accurate cross-calibration thus requires reference measurements at the same frequency and longitude. Using modern hardware and calibration techniques, the telescope under development will have orthogonal polarization channels both for C- and X-band. The channels are coherently sampled with a software-defined radio device (Ettus Research USRP X310) to measure all four Stokes parameters, computing broadband spectra and signal statistics in nearly real-time.

Summarizing requirements and challenges, we present the hardware design optimized for calibration accuracy and radiometric sensitivity. We show first measurements, and precis statistical radio-frequency interference mitigation strategies. Participating at the conference, we hope to discuss the concepts with technical radiometry experts. Furthermore, it will be an opportunity to review the design of the solar flux monitor according to the needs of the broader radiometry community.

1. I. Holleman, A. Huuskonen, M. Kurri, and H. Beekhuis, “Operational monitoring of weather radar receiving chain using the sun,” Journal of Atmospheric and Oceanic Technology, vol. 27, no. 1, pp. 159–166, 2010.

2. M. Gabella, “Checking absolute calibration of vertical and horizontal polarization weather radar receivers using the solar flux,” J. Electr. Eng, vol. 3, pp. 163–169, 2015.

3. K. F. Tapping, “The 10.7 cm solar radio flux (f10. 7),” Space weather, vol. 11, no. 7, pp. 394–06, 2013.

4. M. Shimojo and K. Iwai, “Over seven decades of solar microwave data obtained with toyokawa and Nobeyama radio polarimeters,” Geoscience Data Journal, vol. 10, no. 1, pp. 114–129, 2023.