## 2 Description of the Experiment  
A network of probes and sensors will be created for these investigations. The probes will emit electromagnetic waves of constant frequency towards the ionosphere. When these waves are refracted back to earth, the sensor will measure their polarization and signal strength and store these data for postprocessing. From them, the mean path loss and fading dynamics can be calculated, as well as mean polarization, polarization dynamics, Doppler shift and Doppler dispersion. To investigate polarization effects, the probing signals will be switched periodically between LHCP, RHCP and linear polarization.  
Our previous experiments were all performed on a north-south path in The Netherlands (52.7\({}^{\circ}\)N, 6.4\({}^{\circ}\)E) over a ground distance of 105 km. For more generalized conclusions, the experiment must be extended to cover multiple azimuths and distances, to obtain different angles of the radio waves with the magnetic field. Therefore, in this experiment, measurements will be performed on waves emitted by 4-8 probing signal transmitters deployed in a star-like arrangement around a single very accurate sensor, as shown in Figure 3a. To avoid interference between groundwave and skywave, the probes are located at a distance of between 50 and 200 km from the sensor. At distances beyond 20-30 km distance, at frequencies above 5 MHz, the ground waves will be sufficiently attenuated and the sky waves will dominate [21].  
The experiment will be repeated at two different locations in the Netherlands (52.21\({}^{\circ}\)N, 5.04\({}^{\circ}\)E) and Spain (41.68\({}^{\circ}\)N, 1.48\({}^{\circ}\)E). This is important as the inclination of the Earth's magnetic field changes with latitude, as shown in Figure 3b. The International Geomagnetic Reference Field (IGRF) model describes the direction and magnitude of the Earth's magnetic field [31]. In the Netherlands, at a height of 220 km in the ionosphere, the inclination and declination are respectively 66.9\({}^{\circ}\) and 0.9\({}^{\circ}\). In Spain, they are 56.7\({}^{\circ}\) and 0.2\({}^{\circ}\). When varying the azimuth angle between 0\({}^{\circ}\) and 360\({}^{\circ}\) and the distance between 50 and 150 km, the angle between the downward waves and the Earth's magnetic field will vary from 2\({}^{\circ}\) to 48\({}^{\circ}\) in the Netherlands and from 8\({}^{\circ}\) to 58\({}^{\circ}\) in Spain.  
The repetition of the experiment in two countries requires transportable probes and sensors. The installation of six probes in an area with a radius of 100 km requires 900 km of driving. Therefore, the installation of the entire system on a single winter day is only feasible when the work is divided between two teams and the installation time is less than 2 h per probe. To eliminate the need for staff supervision of the equipment, automation is essential. And as the best measurement locations are away from buildings and structures, battery operation and low power consumption are required. Furthermore, as multiple probe signal transmitters are needed, cost of production and calibration become important. As high accuracy is required as well, novel solutions are necessary.