PRELIMINARY TESTS AND MEASUREMENTS OF A PROTOTYPE TWO-ANTENNA RADIOMETER FOR SUN-TRACKING ATMOSPHERIC CHARACTERIZATION AT 89 GHZ

S. Barbieri¹, F. Consalvi², L. Amaduzzi², F. Nunziata¹, M. Cavagnaro¹, M. Biscarini¹

¹DIET, Sapienza University of Rome, ²Fondazione Ugo Bordoni

The increasing demand for high-capacity satellite communication systems is driving operations toward millimeter-wave frequencies, where accurate propagation models are still limited. Ground-based radiometry represents a valuable tool for atmospheric characterization at such frequencies, with the limitation of being reliable only in rain-free conditions. Recent advances have shown that Sun-tracking (ST) radiometry can overcome this constraint by exploiting the Sun as a stable reference source, thus enabling reliable measurements also in all-weather conditions [1].

This work presents MASTRad89, a novel dual-antenna ST radiometer designed and built to operate at 89 GHz. Unlike conventional single-antenna ST systems, which require continuous azimuthal switching between on-Sun and off-Sun directions [1], MASTRad89 employs two fixed conical antennas (beamwidth ≈ 1.5°, offset ≈ 10°), one pointing at the Sun and the other off the Sun. This configuration eliminates mechanical complexity and data-processing issues related to continuous scanning [2], while enabling simultaneous on/off Sun measurements. From their difference, the system retrieves attenuation and polarimetric brightness temperature of the atmosphere.

The instrument integrates complementary measurement techniques, such as mono-frequency tipping-curve calibration [3]. Local meteorological data are also ingested to improve calibration accuracy. The dual-antenna setup allows extinction and attenuation retrieval even under cloudy or rainy conditions, with extended dynamic range. The system achieves elevation angles from 5° to 90°, a measurement precision better than 1 K, and a noise level below 0.5 K.

After its preliminary concept presentation in [4], MASTRad89 has been fully assembled in a weatherproof enclosure for continuous outdoor operation. Early tests confirmed a measurement accuracy better than 0.5 K for brightness temperatures above 300 K (even without nitrogen calibration). At lower temperatures (~20 K), an underestimation of ≈ 5 K was observed.

It is noteworthy that, to date, only two other single-antenna ST radiometers in the W band are operational worldwide (Rome, NY, US and Milan, IT). The deployment of MASTRad89 thus establishes a new reference site in Rome (IT) with advanced technology, offering valuable data for both meteorological and climatological applications in a region with different atmospheric characteristics from existing stations. Ongoing developments include the completion of the automatic pointing system, the start of an extended measurement campaign, and subsequent data analysis for the development of atmospheric models in the W band, further consolidating the role of MASTRad89 as a key instrument for propagation studies and for the validation of Earth observation and satellite communication systems at millimeter wavelengths.

1. V. Mattioli et al. “Retrieval of Sun Brightness Temperature and Precipitating Cloud Extinction Using Ground-Based Sun-Tracking Microwave Radiometry,” in IEEE Journal of Selected Topics in Applied Earth Observations and Remote Sensing, vol. 10, no. 7, pp. 3134-3147, July 2017, doi: 10.1109/JSTARS.2016.2633439.

2. L. Luini et al., “Development and Application of Advanced Experimental Techniques for Ground Microwave Radiometry in All Weather Conditions,” 2023 17th European Conference on Antennas and Propagation (EuCAP), Florence, Italy, 2023, pp. 1-5, doi: 10.23919/EuCAP57121.2023.10132964.

3. Yong Han and E. R. Westwater, “Analysis and improvement of tipping calibration for ground-based microwave radiometers”, IEEE Transactions on Geoscience and Remote Sensing, 38(3):1260-1276, May 2000, doi: 10.1109/36.843018

4. F. S. Marzano et al., “Sun-Tracking Ground-Based Microwave Radiometry: Challenges and Applications,” 2021 IEEE International Geoscience and Remote Sensing Symposium IGARSS, Brussels, Belgium, 2021, pp. 1437-1440, doi: 10.1109/IGARSS47720.2021.9553185.