sensors Article Design and Validation of Probes and Sensors for the Characterization of Magneto-Ionic Radio Wave Propagation on Near Vertical Incidence Skywave Paths Ben A. Witvliet 1,2,* , Rosa M. Alsina-Pagès 3 , Erik van Maanen 4 and Geert Jan Laanstra 5 1 Centre for Space, Atmospheric and Oceanic Science (CSAOS), Department of Electronic and Electrical Engineering, University of Bath, Claverton Down, Bath BA2 7AY, UK 2 Telecommunication Engineering (TE), Faculty of Electrical Engineering, Mathematics and Computer Science, University of Twente, P.O. Box 217, 7500AE Enschede, The Netherlands 3 Grup de recerca en Tecnologies Mèdia (GTM), La Salle-Universitat Ramon Llull, Quatre Camins, 30, 08022 Barcelona, Spain; [email protected] 4 Spectrum Management Department, Radiocommunications Agency Netherlands, P.O. Box 450, 9700AL Groningen, The Netherlands; [email protected] 5 Data Science–Data Management & Biometrics (DS/DMB), Faculty of Electrical Engineering, Mathematics and Computer Science, University of Twente, P.O. Box 217, 7500AE Enschede, The Netherlands; [email protected] * Correspondence: [email protected]; Tel.: +44-7952-637-667 Received: 14 April 2019; Accepted: 6 June 2019; Published: 9 June 2019 Abstract: This article describes the design and validation of deployable low-power probes and sensors to investigate the influence of the ionosphere and the Earth’s magnetic field on radio wave propagation below the plasma frequency of the ionosphere, known as Near Vertical Incidence Skywave (NVIS) propagation. The propagation of waves that are bent downward by the ionosphere is dominated by a bi-refractive mechanism called ‘magneto-ionic propagation’. The polarization of both downward waves depends on the spatial angle between the Earth’s magnetic field and the direction of propagation of the radio wave. The probes and sensors described in this article are needed to simultaneously investigate signal fading and polarization dynamics on six radio wave propagation paths. The 1-Watt probes realize a 57 dB signal-to-noise ratio. The probe polarization is controlled using direct digital synthesis and the cross-polarization is 25–35 dB. The intermodulation-free dynamic range of the sensor exceeds 100 dB. Measurement speed is 3000 samples/second. This publication covers design, practical realization and deployment issues. Research performed with these devices will be shared in subsequent publications. Keywords: deployable; magneto-ionic; magnetic field; polarization; fading; ionosphere; radio wave propagation; Near Vertical Incidence Skywave (NVIS); circular polarization 1. Introduction When a natural disaster occurs, relief work is severely hampered by telecommunication infrastructure damage. This is best illustrated with the landfall of Hurricane Katrina and the subsequent flooding of New Orleans in 2005 [1]. Most of the telecommunication infrastructure was destroyed by the storm [2]. Given that the electricity distribution was also disrupted, the few remaining cell towers operated on backup batteries for another 4 h, after which they ceased to work as well [3]. The roads into the disaster were flooded, which left the entire disaster zone, an area of 200 km 200 km, without access and telecommunications. In such circumstances, as an alternative, × Sensors 2019, 19, 2616; doi:10.3390/s19112616 www.mdpi.com/journal/sensors Sensors 2019, 19, 2 of 16 Sensors 2019, 19, 2616 2 of 16 km × 200 km, without access and telecommunications. In such circumstances, as an alternative, ionospheric radiocommunication systems could be used to provide instantaneous access to the entire area.ionospheric The ionosphere radiocommunication is a natural plasma systems between could be 60 used and 1000 to provide km height, instantaneous sustained access by solar to theradiation entire [4].area. Several The ionosphere regions can is a be natural distinguished plasma between [4], as shown 60 and 1000in Figure km height, 1. The sustained peak electron by solar density radiation occurs [4]. betweenSeveral regions150 and can 250 be km distinguished height in the [ 4F-region.], as shown Ionospheric in Figure radio1. 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For of theNVIS sun, propagation, the electron the density frequency follows of the a diurnalradio waves cycle, must the seasonsbe smaller and than the the 11-year maximum solar plasmacycle [7 ]. Forfrequency NVIS propagation, of the theionosphere, frequency offor the radiomid-latitudes waves must typically be smaller between than the maximum3 and 10plasma MHz. frequency of the ionosphere, for mid-latitudes typically between 3 and 10 MHz. Sensors 2019, 19, 3 of 16 sunspot number. Furthermore, the authors showed that the bi-refractive properties of the ionosphere could be used to transmit two isolated waves on the same frequency for diversity and Multiple-Input Multiple-Output (MIMO) [23]. Erhel et al. demonstrated their application in an HF MIMO system [24]. Precise measurements of the authors showed that the isolation of the characteristic waves in the ionosphere is at least 25–35 dB [25] and they described the ‘Happy Hour’ propagation phenomenon for the first time. Nearly perfectly circular polarization was observed on a 105-km-long NVIS path from north to south at 52.7°N. Night-time observations on frequencies above the maximum usable frequency (MUF) were reported earlier by Wheeler and McNamara [26,27]. The authors showed that, at night-time above the critical frequency, the received waves become unpolarized and exhibit fluttery fading [28]. Very similar characteristics were observed when a solar X-ray flare inhibited the NVIS path betweenFigureFigure transmitter 1. TheThe ionosphere ionosphere and receiver is is a a natural natural [28]. plasma, plasma, sustained sustained by solar radiation [4]. [4]. The ionosphere is bi-refractive. Appleton and Builder showed that radio waves entering the ionosphere, under the influence of the Earth’s magnetic field, are split in two circularly polarized characteristic waves in opposite rotational directions, the ordinary and the extraordinary wave [8]. Appleton and his contemporaries [9,10] were able to develop this ‘magneto-ionic theory’ based on work of Maxwell and Thomson [11], that explained the polarization rotation discovered by Faraday in 1845 [12]. The polarization of the characteristic waves depends on the angle between their direction of propagation and the magnetic field vector. In the northern hemisphere, the polarization of the ordinary wave is right-hand circular (RHCP) upwards and left-hand circular (LHCP) downwards, as shown in Figure 2b. In the Southern hemisphere, the sense of rotation is reversed. For NVIS propagation at mid-latitudes and at frequencies above 5 MHz, the polarization of the characteristic waves is almost circular [6]. Both characteristic waves follow a different path through the ionosphere, which can be shown using ray-tracing techniques [13,14]. And as the ionosphere is not homogeneous, they suffer different attenuation,(a) time delay and delay spread, Doppler shift(b) and Doppler spread and fading. Figure 2. (a) Near Vertical Incidence Skywave (NVIS) propagation; (b) Magneto-ionic propagation: Figure 2. (a) Near Vertical Incidence Skywave (NVIS) propagation; (b) Magneto-ionic propagation: ordinary (red) and extraordinary wave (green). Polarization is shown for the northern hemisphere. 1.1. Previousordinary Work(red) and extraordinary wave (green). Polarization is shown for the northern hemisphere. NVISThe ionosphere radio systems is bi-refractive. were used in Appleton New Orleans, and Builder but not showed to their that full radiopotential. waves Most entering practical the 1.2. 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