## 9 Discussion and Conclusion  
The validation measurement and the field trials have shown that an exceptionally portable and very sensitive and accurate measurement system is created. The 1-Watt probe signal transmitters produce a 52.5 dB SNR in a 30 Hz bandwidth. This is 8.5 dB lower than predicted, caused by a higher than expected level of external radio noise. The radio noise level shows a diurnal variation consistent with NVIS ionospheric propagation, which could indicate that electromagnetic noise from the city of Barcelona, at 100 km distance, is received via ionospheric refraction. This would be consistent with the work of [PERSON] and [PERSON] [43] on radio noise propagation via the ionosphere and with our observations of significant radio noise reduction during a Sudden Ionospheric Disturbance [28], but cannot be proven without directional information. The polarization purity of the waves emitted by the probes is excellent. To the best of our knowledge, there are no other published concepts that reach comparably high cross-polarization  
Figure 12: Spectrogram of the LHCP end RHCP waves emitted by the probe and received by the sensor on a linearly polarized antenna, during the morning Happy Hour. Figure 13: Polarization of a sequence of probing signals captured by the sensor during normal NVIS propagation. values (25-35 dB) in HF transmission, which is much better than the cross-polarization achieved by [PERSON] et al. with large fixed antenna systems [24]. The fully digitally generated polarization, calibration and automated polarization sequence transmission provide unequalled experimental flexibility. The validation measurements of the sensor conform with expectations and show realistic propagation characteristics and variations. To objectively establish the cross-polarization of the sensor, a drone-based source would be required. The active antennas of the radio astronomy 'LOw Frequency ARray' (LOFAR) [44] provide \(>\)24 dB cross-polarization rejection for linearly polarized waves [44]. However, the measurements described by [PERSON], et al., which are the current state-of-the-art, can only provide cross-polarization for linearly polarized antennas and with some orientation uncertainty [45]. Drone measurement systems for circular polarization are not available yet, but are certainly on our wish-list. Compared with our previous measurement system, which was built around a professional measurement receiver, the measurement speed is \(>\)6000 times higher, the sensitivity is \(>\)10 dB better and the IFDR is \(>\)20 dB better. The sensor no longer needs switched input filters to avoid intermodulation. For comparison, the LOFAR active antennas produce intermodulation products from the strongest HF signals and high-pass filters are introduced to limit reception to frequencies above 20 MHz. Where the previous system could only alternatingly measure LHCP and RHCP, the novel system can reconstruct an infinite number of polarizations in parallel in each sampling moment. Quantitative comparisons with other systems were not undertaken; currently there are no systems that could do the same measurements. LOFAR could be used for the sensor side, but the presence of high-pass filters makes the reception of the low-power probes improbable. This work has led to the creation of a novel instrument and several new concepts are introduced that will contribute to future ionospheric propagation research. Interesting future extensions to the system could be time-of-flight measurements to investigate multipath and directional information to investigate origins of radio noise and of night-time scatter. This article was written by [PERSON] and [PERSON]. This includes the original draft preparation, preparation of the visualizations, review and editing. Funding acquisition and radio frequency licensing was done by [PERSON] and B.A.W. The original research idea was proposed by [PERSON].