Probing Earth’s ionosphere with novel ionosonde and ray-tracing solutions

Near-real-time observations of Earth's ionosphere electron density are an important input for a variety of technologies and services that depend on HF radio or space-based communication. An ionosonde is a radar that performs ionospheric sounding and provides measurements of the current state of the ionosphere. The critical parameters (foF2, foF1, foE) derived from ionosonde data define the maximum frequencies of the ionospheric E- and F-layers for estimates of their electron densities. Users of space weather services, e.g. in aviation, use these parameters to search for optimal frequencies in over-the-horizon communication. Ray-tracing codes as combined with statistical ionospheric electron density models are often used in more accurate radio wave propagation assessments.

Finnish Meteorological Institute (FMI) started ionospheric measurements in Southern Finland with a new ionosonde in February 2025. The ionosonde, manufactured by RF-shamans Ltd, is based on a software-defined radio (SDR) implementation, which enables its small, both transmitter and receiver antenna size (≈ 1m^3), quick and flexible operation, and low transmitting power level (< 0.5W). In addition, the ionosonde antennas are easy to install and relocate when needed. The ionosonde currently produces vertical ionograms using a 10 second sweep five times per minute. The measurements are filtered, and the critical parameters are detected in real-time. The more detailed real-height analysis is currently operated manually and an automated process is under development.

In addition, FMI analyzes ionograms with a numerical ray-tracing software, AU-Ray [1]. The polynomial analysis software (POLAN) is also used in real-height analysis to verify the scaling results from the AU-Ray method. AU-Ray calculates the radio wave propagation path step-by-step based on Appleton-Hartree (AH) or Booker Quartic (BQ) Hamiltonians. AU-Ray allows ray-tracing not just with statistical ionospheric models but also with background conditions customized by the user. The code is based on freely available programming languages and open-source models. The software package includes also a mapping tool, which allows simulations with large quantities of rays, for propagation maps in user-defined ionospheric conditions. In the presentation, we describe the main specifications of the FMI ionosonde and the AU-ray code, analyze ionosonde performance during a recent space weather storm, and demonstrate the usage of AU-Ray in the real-height analysis, while comparing the simulation results to the ionosonde measurements.

[1]: E. A. O. Hirvonen, K. Kauristie and E. Kallio. “AU-Ray Program for Modelling Radio Wave Propagation in the Ionosphere“. Radio Science. Manuscript submitted May 2025.