The Anomalous Propagation of Radio and Microwaves in the Stratosphere

Atmospheric ducting has a significant effect on radio and microwave propagation, as signals trapped in atmospheric layers travel unexpected distances and may cause interference in radar measurements and radio communications. The ducts are generally caused by non-standard refractive index gradients, which are dependent on meteorological conditions, mainly temperature and humidity inversions.

Atmospheric ducts have been mainly studied in the troposphere, with an emphasis on surface ducts in maritime and coastal regions. Studies have often used radiosondes, Numerical Weather Prediction (NWP) models, and radar measurements. Elevated ducts, however, have received less attention, and particularly the occurrence of anomalous signal propagation in the stratosphere remains largely unexamined. Studying atmospheric ducts above the troposphere is challenging due to the scarcity of observations, and their insufficient vertical resolution to capture this phenomenon.

The meteorological conditions in the stratosphere can be studied using the atmospheric parameters retrieved from limb observations of propagation path bending between satellites functioning as transmitter-receiver satellite pairs for Global Navigation Satellite System (GNSS) signals. These observations may be inverted to obtain vertical profile information on temperature, humidity and pressure in the atmosphere. For propagation condition analysis, it should be noted that humidity and temperature gradients are relevant for tropospheric propagation, whereas temperature and pressure gradients drive stratospheric propagation conditions.

Here we present initial results for a study into the occurrence of the meteorological conditions allowing anomalous propagation in the lower stratosphere. We analyse refractivity profiles from radio occultation data from the Radio Occultation Meteorology Satellite Application Facility (ROM SAF) to understand both the frequency and distribution of atmospheric ducting as well as the data requirements to capture the phenomena, with a focus on the European region. The results are compared to reanalysis data from the European Centre for Medium Range Weather Forecasts Reanalysis v5 (ERA5) as well as radiosonde observations.