Methods to Estimate Gas Attenuation in Absence of a Radiometer to Support Satellite Propagation Experiments

The experimental characterization of tropospheric effects on electromagnetic waves relies on the reception of a beacon signal transmitted by a satellite. The total tropospheric attenuation can be derived from these measurements in combination with additional information, specifically the attenuation due to atmospheric gases, in turn derived from ancillary instruments or data sources. When available, this information is obtained from a cosited radiometer. This article presents three alternative procedures to estimate the gas attenuation that can be used in the absence of this instrument. The first one makes use of zenith total delay data obtained from the global navigation satellite system (GNSS) receivers and additional meteorological data. The second one relies on atmospheric profiles gathered from radiosonde observations and/or numerical weather prediction (NWP) products. The third one only makes use of standard atmospheric profiles. The accuracy of the procedures, when used in this application, is compared with the reference total tropospheric attenuation derived with the support of radiometric data, exploiting the Alphasat beacon measurements collected in Milan, Italy, in 2017. The results indicate that a better accuracy (average root mean square (rms) error below 0.1 dB when compared with the use of a radiometer) is achieved by using GNSS data because of their finer temporal resolution. Nonetheless, the three procedures can be equally recommended, their use being conditioned to the availability of the appropriate data in the area around the experimental site.