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. The systems peak electron may send density radio occurswaves nearlybetween vertically 150 and upwards, 250 km height to be inrefracted the F-region. in the Ionosphericionosphere and radio returned systems to may earth, send as radiodepicted waves in Figurenearly 2a. vertically This phenomenon upwards, to is becalled refracted ‘Near Vertical in the ionosphere Incidence Skywave’ and returned (NVIS) to earth,propagation as depicted [5]. The in refractionFigure2a. Thisin the phenomenon ionosphere isdepends called ‘Near on the Vertical electron Incidence density Skywave’ in the ionosphere (NVIS) propagation [6]. Driven [ 5by]. Thethe radiationrefraction of in the the sun, ionosphere the electron depends density on thefollows electron a diurnal density cycle, in thethe ionosphereseasons and [6 the]. Driven 11-year by solar the cycleradiation [7]. 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. Research Goals organizationsionosphere, under refer theto the influence book of of Fiedler the Earth’s and magneticFarmer [5] field, on NVIS, are split which in two only circularly provides polarized generic information.characteristicOur previous A waves large research volume in opposite also of NVIS identified rotational research new directions, exists, area sbut where the it is ordinary scatteredimportant and over questions the 70extraordinary years remained. and a wide wave Firstly, range [8]. allofAppleton ourmedia. measurements To and improve his contemporaries were access performed to it, the [9,10 inauthors] the were Netherla ablepublis tondshed develop atan 52.7°N, overview this with ‘magneto-ionic article a magnetic on NVIS theory’dip thatangle basedrefers of 69°, onto on128work a NVIS north-to-south of Maxwell publications, and path. Thomson organized Nearly [11 perfectlyper], that subject explained circular [15]. To the polarization support polarization work was rotationfrom observed othe discoveredr groupsin 2 different bythat Faraday optimize NVIS in experimentsHigh1845 [12Frequency]. The spaced polarization (HF) seven antennas months of the characteristicfor apart mobile [25,28]. platforms waves To test depends [16–18]the generality on and the fixed angle of these installations between observations their [19,20]. direction and The of of theauthorspropagation isolation provided andbetween the elevation magnetic the characteristic angle field measurements vector. waves, In the measurements northern and raytracing hemisphere, over simulations multiple the polarization azimuths[21] to extend of are the needed, ordinary graphs preferablypublishedwave is right-hand byrepeated McNamara circular at a [22]second (RHCP) and location upwardsDavies [6].with and Th left-handesea different now circularinclude magnetic (LHCP)the influence dip downwards, angle. of frequencyFurthermore, as shown and in pronounced fading was observed in NVIS experiments in which the transmitted and received polarizationSensors 2019, 19 were, x; matched to one of the characteristic waves, which was unexpectedwww.mdpi.com/jou [28]. rnal/sensors Our current research therefore has three main objectives: (i) Verification of the isolation between both characteristic waves for different azimuths, distances and magnetic dip angles; (ii) Simultaneous measurement of the polarization dynamics on multiple radio wave paths with different azimuths and distances; (iii) Analysis of signal fading and its possible origins, by means of comparing the radio wave propagation on all paths. The first goal is the focus of the project. It will validate the previous measurements conducted in the Netherlands [25] and confirm the hypothesis that the achievable data rate can be doubled using characteristic wave propagation. The second goal is relevant for (possibly dynamic) optimization of antenna polarization, which will improve the isolation between the ordinary and the extraordinary waves. The third goal is perhaps the most ambitious: although signal fading is common in HF propagation, the proposed study will expand the knowledge on its mechanisms and drivers, and enable future mitigation techniques.

Sensors 2019, 19, x; www.mdpi.com/journal/sensors Sensors 2019, 19, 2616 3 of 16

Figure2b. 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, time delay and delay spread, Doppler shift and Doppler spread and fading.

1.1. Previous Work NVIS radio systems were used in New Orleans, but not to their full potential. Most practical organizations refer to the book of Fiedler and Farmer [5] on NVIS, which only provides generic information. A large volume of NVIS research exists, but it is scattered over 70 years and a wide range of media. To improve access to it, the authors published an overview article on NVIS that refers to 128 NVIS publications, organized per subject [15]. To support work from other groups that optimize High Frequency (HF) antennas for mobile platforms [16–18] and fixed installations [19,20]. The authors provided elevation angle measurements and raytracing simulations [21] to extend graphs published by McNamara [22] and Davies [6]. These now include the influence of frequency and 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 between transmitter and receiver [28].

1.2. Research Goals Our previous research also identified new areas where important questions remained. Firstly, all our measurements were performed in the Netherlands at 52.7◦N, with a magnetic dip angle of 69◦, on a north-to-south path. Nearly perfectly circular polarization was observed in 2 different NVIS experiments spaced seven months apart [25,28]. To test the generality of these observations and of the isolation between the characteristic waves, measurements over multiple azimuths are needed, preferably repeated at a second location with a different magnetic dip angle. Furthermore, pronounced fading was observed in NVIS experiments in which the transmitted and received polarization were matched to one of the characteristic waves, which was unexpected [28]. Our current research therefore has three main objectives: (i) Verification of the isolation between both characteristic waves for different azimuths, distances and magnetic dip angles; (ii) Simultaneous measurement of the polarization dynamics on multiple radio wave paths with different azimuths and distances; (iii) Analysis of signal fading and its possible origins, by means of comparing the radio wave propagation on all paths. The first goal is the focus of the project. It will validate the previous measurements conducted in the Netherlands [25] and confirm the hypothesis that the achievable data rate can be doubled using characteristic wave propagation. The second goal is relevant for (possibly dynamic) optimization of antenna polarization, which will improve the isolation between the ordinary and the extraordinary waves. The third goal is perhaps the most ambitious: although signal fading is common in HF propagation, the proposed study will expand the knowledge on its mechanisms and drivers, and enable future mitigation techniques. Sensors 2019, 19, 2616 4 of 16

1.3. The Need for Novel Probes and Sensors For each of these objectives, specialized equipment is needed to empirical obtain data of the radio wave propagation and the resulting radio channel. Duplicating our previous measurement system is not practical because of its size and the costs involved. System designs could not be adopted from other groups either: Hervás et al. [29] and Walden [30] did not measure polarization. Erhel et al. used large fixed antennas and did not achieve sufficient cross-polarization [24]. We therefore decided to design a novel system of probes and sensors that is portable (<20 kg), low-power (<5 W), quickly installed (<2 h) and autonomous. The sampling speed will be increased from 0.5/s to 3000/s. For this design we were able to build on the experience gained in our previous experiments. This publication covers the system design, practical realization and deployment criteria. The article is structured as follows: The experiment with one central sensor and multiple probes is described in Section2. Frequency selection for the experiment is described in Section3. The design of the central sensor is detailed in Section4. The sensor specifications and link budget calculations are provided in Section5 are then used to specify and design the probes in Section6. Section7 deals with deployment and Section8 validates the design in a field deployment. The article concludes with a discussion and suggestions for future work.

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◦N, 6.4◦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 Figure3a. 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◦N, 5.04◦E) and Spain (41.68◦N, 1.48◦E). This is important as the inclination of the Earth’s magnetic field changes with latitude, as shown in Figure3b. 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◦ and 0.9◦. In Spain, they are 56.7◦ and 0.2◦. When varying the azimuth angle between 0◦ and 360◦ and the distance between 50 and 150 km, the angle between the downward waves and the Earth’s magnetic field will vary from 2◦ to 48◦ in the Netherlands and from 8◦ to 58◦ 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. Sensors 2019, 19, 2616 5 of 16 Sensors 2019, 19, 5 of 16

(a) (b)

Figure 3. Experiment topology: (a) An example of a star-shaped deployment (Catalonia, Spain); (b) Figure 3. Experiment topology: (a) An example of a star-shaped deployment (Catalonia, Spain); The inclination of the Earth’s magnetic field changes with latitude. Map courtesy of Free World Maps. (b) The inclination of the Earth’s magnetic field changes with latitude. Map courtesy of Free World 3. FrequencyMaps. Selection

3. FrequencyThe frequency Selection at which the experiments will take place has to be considered first, because of its significant impact on the design. On one hand, the sensor can only measure the signal of the probesThe when frequency NVIS propagationat which the isexperiments present for will a large take part place of has each to day.be considered This puts first, an upper because limit of onits thesignificant frequency impact used. on Lowering the design. the On frequency, one hand, on the the sensor other can hand, only will measure increase the the signal absorption of the probes in the D-regionwhen NVIS and propagation the ambient electromagneticis present for a noiselarge (radiopart of noise) each levelday. andThis therefore puts an requireupper significantlylimit on the morefrequency probe used. power. Lowering A lower frequencythe frequency, also implies on the aot largerher hand, antenna, will which increase will the make absorption the deployment in the moreD-region diffi andcult. the For ambient NVIS propagation electromagnetic the peaknoise plasma(radio noise) frequency level ofand the therefore ionosphere require must significantly be higher thanmore the probe frequency power. ofA thelower electromagnetic frequency also wave. implies The a larger local plasmaantenna, frequency which will is directlymake the related deployment to the localmore electron difficult. density For NVIS in the propagation ionosphere the [32 ]peak and plasma the highest frequency frequency of the at whichionosphere the ordinary must be wave higher is reflectedthan the verticallyfrequencyis: of the electromagnetic wave. The local plasma frequency is directly related to the local electron density in the ionosphere [32]s and the highest frequency at which the ordinary wave is 2 1 e Nmax p reflected vertically is: foF2 = 8.98 Nmax (1) 2π ε0me ≈ 1 𝑒 𝑁 3 where foF2 is the critical frequency in𝑓 Hz,=Nmaxthe peak electron ≈ 8.98 density𝑁 in electrons/m , e the charge(1) of 19 2𝜋 𝜀𝑚 12 an electron (1.602 10 C), ε the permittivity of free space (8.854 10 F/m), and me the mass of an × − 0 × − electron (9.109 10 31 kg). The critical frequency f of the extraordinary wave is slightly higher [32]: where foF2 is the× critical− frequency in Hz, Nmax the xF2peak electron density in electrons/m3, e the charge of an electron (1.602 × 10−19 C), ε0 the permittivitys of free space (8.854 × 10−12 F/m), and me the mass of !2 an electron (9.109 × 10−31 kg). The critical frequency fxF2fH of the extraordinaryfH wave is slightly higher fxF2 = foF2 1 + + (2) [32]: 2 foF2 2

where fH is the electron gyrofrequency in Hz, which ranges𝑓 from 1.0𝑓 to 1.2 MHz at 220 km height in the (2) Netherlands and Spain, so that Equation𝑓 (2)= 𝑓 can be1+ approximated + as: 2𝑓 2

where fH is the electron gyrofrequency finxF Hz,f oFwhich+ 0.55 rang MHzes from 1.0 to 1.2 MHz at 220 km height (3) in 2 ≈ 2 the Netherlands and Spain, so that Equation (2) can be approximated as: There is no propagation when fxF2 is lower than the probe frequency fprobe. Only the extraordinary 𝑓 ≈ 𝑓 +0.55 MHz (3) wave propagates when fxF2 > fprobe > foF2. And both waves propagate when foF2 > fprobe. The electron productionThere inis no the propagation ionosphere iswhen driven fxF2 by is solarlower radiation than the [probe4] and frequency therefore ffoF2probeand. OnlyfxF2 thefollow extraordinary a diurnal cycle,wave increasingpropagates in when the morning fxF2 > fprobe when > f theoF2. sunAnd starts both to waves illuminate propagate the ionosphere when foF2 and > fprobe decreasing. The electron in the afternoon,production as in shown the ionosphere in Figure4 is. Indriven this example, by solar radiation NVIS propagation [4] and therefore is possible foF2 from and f 09:00–18:00xF2 follow a diurnal h. The minimumcycle, increasing and maximum in the morning values depend when the on sun the locationstarts to onilluminate Earth, the the day ionosphere of the year and and decreasing the position in inthe the afternoon, 11-year solar as sunspotshown cyclein Figure [7]. The 4. experimentsIn this example, were planned NVIS inpropagation the winter tois takepossible advantage from 09:00–18:00 h. The minimum and maximum values depend on the location on Earth, the day of the year and the position in the 11-year solar sunspot cycle [7]. The experiments were planned in the Sensors 2019, 19, x; www.mdpi.com/journal/sensors SensorsSensors2019 2019, 19, 19, 2616, 66 of of 16 16

winter to take advantage of the higher electron densities caused by the ‘winter anomaly’ in the ofnorthern the higher hemisphere electron densities [33]. After caused analysing by the ionosphe ‘winter anomaly’ric sounding in the data northern from the hemisphere Royal Observatory [33]. After of analysing ionospheric sounding data from the Royal Observatory of Belgium (50.1 N, 4.6 E) and Belgium (50.1°N, 4.6°E) and the Ebre Observatory in Spain (40.8°N, 0.5°E), the critical◦ frequency◦ foF2 thewas Ebre expected Observatory to remain in Spainabove (40.8 7 MHz◦N, during 0.5◦E), theat leas criticalt 8 h frequencyof each day.foF2 Therefore,was expected an experimental to remain abovelicense 7 MHzwas obtained during at in least the 8Netherlands h of each day. to use Therefore, a clear anfrequency experimental of around license 6.99 was MHz. obtained In Spain, in the the Netherlandsexperiments to were use a clearconducted frequency around of around 7.04 6.99MHz MHz. using In Spain,an amateur the experiments radio license. were conductedTo enable aroundsimultaneous 7.04 MHz observation using an amateurof all probes, radio each license. of them To enable was simultaneousgiven a small observation(110 Hz) frequency of all probes, offset. eachUnmodulated of them was probes given signals a small were (110 used Hz) in frequency the meas ourements.ffset. Unmodulated With sufficient probes frequency signals werestability used the insensor the measurements. will separate them With by su ffifrequencycient frequency filtering. stability Narrow-band the sensor filtering will separate will also them increase by frequency the signal- filtering.to-noise Narrow-bandratio (SNR) and filtering allow willfor lower also increase probe po thewer. signal-to-noise We therefore ratiochoose (SNR) a sensor and allowbandwidth for lower of 30 probeHz. power. We therefore choose a sensor bandwidth of 30 Hz.

Figure 4. Modelled diurnal variation of the critical frequencies of the F2-region of the ionosphere using Figure 4. Modelled diurnal variation of the critical frequencies of the F2-region of the ionosphere the International Reference Ionosphere [34]. NVIS propagation is possible when fxF2 > fprobe. using the International Reference Ionosphere [34]. NVIS propagation is possible when fxF2 > fprobe. 4. Sensor Design 4. Sensor Design A sensitive sensor will allow all the probes to emit weaker signals, and thereby significantly simplifyA theirsensitive design, sensor reduce will size allow and all cost the and probes improve to transportability.emit weaker signals, Typically, and atthereby frequencies significantly below 20simplify MHz, the their ambient design, electromagnetic reduce size and background cost and noise improve (or radio transportability. noise) limits theTypically, maximum at frequencies sensitivity thatbelow can 20 be MHz, achieved the ambient [35] (p.766). electromagnetic Electrical appliances background generate noise (or strong radio impulsive noise) limits electromagnetic the maximum noise,sensitivity but even that in acan quiet be rural achieved environment [35] (p.766). radio noiseElectrical will be appliances observed, causedgenerate by thestrong accumulative impulsive emissionselectromagnetic of distant noise, thunderstorms but even in a and quiet cities rural arriving environment via ionospheric radio noise reflection. will be observed, Therefore, caused for our by experimentthe accumulative it is important emissions to deployof distant the thunderstorm sensor in a quiets and rural cities environment, arriving viawhere ionospheric the radio reflection. noise levelTherefore, is low. for Furthermore, our experiment the noise it is important floor of the to sensordeploy has the tosensor be lower in a quiet than therural expected environment, radio noisewhere level,the radio which noise can belevel found is low. in ‘Recommendation Furthermore, the ITU-Rnoise P.372’floor of [36 the] (Equation sensor has (13)). to be For lower a quiet than rural the location,expected it radio predicts noise the level, radio which noise levelcan be as: found in ‘Recommendation ITU-R P.372’ [36] (Equation 13). For a quiet rural location, it predicts the radio noise level as:

Famb = 53.6 28.6 log f (4) 𝐹 = 53.6− − 28.6 10 𝑙𝑜𝑔 𝑓 (4) wherewhereF ambFamb isis thethe ambientambient noisenoise factor in dB dB and and ff isis the the frequency frequency in in MHz. MHz. Using Using this this Equation, Equation, we wefind find that that FambFamb is is29 29 dB dB at at 7 7MHz, MHz, from from which which the noise powerpower atat thethe antenna antenna terminals terminalsP Pn ncan can be be calculatedcalculated [36 [36]] (Equation (Equation (6)): (6)):

Pn = Famb + 10 log10(kT0b) (5) 𝑃 =𝐹 + 10 𝑙𝑜𝑔𝑘𝑇𝑏 (5) –23 where Pn is the noise power in dBW, k is Boltzmann’s constant (1.38 10 –23J/K), T is the reference where Pn is the noise power in dBW, k is Boltzmann’s constant (1.38× × 10 J/K), 0T0 is the reference temperature, of 290 K and b is the bandwidth in Hz. Assuming a lossless antenna, Pn is expected temperature, of 290 K and b is the bandwidth in Hz. Assuming a lossless antenna, Pn is expected to to be 160 dBW in 30 Hz bandwidth. To achieve maximum sensitivity, the noise floor of the sensor be −−160 dBW in 30 Hz bandwidth. To achieve maximum sensitivity, the noise floor of the sensor shouldshould be be at at least least 10 10 dB dB lower lower than than the the external external noise noise level. level. At At the the same same time time the the sensor sensor must must be be very very linear,linear, to to avoid avoid intermodulation intermodulation products products of strong of stro shortwaveng shortwave signals. signals. Transmissions Transmissions from high-power from high- power shortwave broadcast stations at distances between 500 and 2000 km may produce a

Sensors 2019, 19, x; www.mdpi.com/journal/sensors Sensors 2019, 19, 2616 7 of 16 shortwave broadcast stations at distances between 500 and 2000 km may produce a cumulative input Sensorspower 2019 of , 1970, dBW. Therefore, the intermodulation free dynamic range (IFDR) must exceed 1007 of dB.16 − This requirement is challenging even for most professional HF receivers. cumulativeThe sensor input consists power of of − a70 transducer dBW. Therefore, (antenna) the intermodulat and a samplerion (receiver). free dynamic The range antenna (IFDR) converts must exceedthe electromagnetic 100 dB. This requirement wave into electrical is challenging signals, even and for the most receiver professional converts theHF HFreceivers. signal to baseband andThe stores sensor 3000 consists samples of/second a transd onucer hard (antenna) disk. A and hardware a sampler platform (receiver). designed The antenna by the converts open source the ‘Highelectromagnetic Performance wave Software into electrical Defined signals, Radio’ and (HPSDR) the receiver group converts is used the [37 HF]. Itsignal has 2to coherent baseband 16-bit and storesanalogue-to-digital 3000 samples/second convertors on (ADC)hard disk. sampling A hardwa at 122re MS platform/s. Measured designed amplitude by the open and phase source drift ‘High are Performanceless than 0.01 Software dB and 0.2Defined◦ over Radio’ 48 h during (HPSDR) field group deployment. is used [37]. The It signals has 2 coherent are filtered 16-bit in theanalogue- digital to-digitaldomain,converted convertors to (ADC) baseband sampling and stored at 122 as MS/s. 32-bit Measured complex samples.amplitude Polarization and phase drift is calculated are less fromthan 0.01the datadB and of both 0.2° inputover ports48 h during and RHCP, field LHCP deployment and linear. The polarizations signals are filtered can be derived in the digital simultaneously domain, convertedon a per-sample to baseband basis. and stored as 32-bit complex samples. Polarization is calculated from the data of bothA full-sizeinput ports passive and antennaRHCP, hasLHCP been and chosen linear as polarizations a transducer, can to retain be derived linearity simultaneously and achieve a on high a per-sampleefficiency. Itbasis. consists of two resonant Inverted Vee dipoles made of stranded copper wire. Each of themA has full-size linear polarization.passive antenna By mountinghas been twochosen of them as a perpendiculartransducer, to andretain connecting linearity them and toachieve the two a highcoherent efficiency. receiver It inputs,consists full of two polarization resonant can Invert be measureded Vee dipoles on a per-samplemade of stranded basis. This copper design wire. yields Each a ofcross-polarization them has linear polarization. of 25–35 dB. TheBy mounting dipole leg two length of them L can perpendicular be calculated as:and connecting them to the two coherent receiver inputs, full polarization can be measured on a per-sample basis. This design c yields a cross-polarization of 25–35 dB. The dipoleL = leg.v length L can be calculated as: (6) 𝑐 4 f 𝐿= .𝑣 (6) 4𝑓 8 where c is the speed of light (2.9979 108 m/s), f is the frequency in Hz and v is the relative wave where c is the speed of light (2.9979 × 10 m/s), f is the frequency in Hz and v is the relative wave velocity along the antenna wire, which is approximately 0.95 for a wire diameter of 1 mm.mm. For For a a frequencyfrequency of 7.04 MHz, thethe legleg lengthlength isis approximatelyapproximately 10.1 10.1 m. m. TheThe antenna, antenna, shown shown in in Figure Figure5a, 5a, has has a footprint of 25 25 m. The antenna was modelled using NEC4.2 Method of Moments software with a a footprint of 25× × 25 m. The antenna was modelled using NEC4.2 Method of Moments software with arealistic realistic Sommerfeld Sommerfeld ground ground model mode [l38 [38].]. The The antenna antenna gain gain of of a singlea single element element towards towards the the zenith zenith is is3.7 3.7 dBi. dBi. Equal Equal length length coaxial coaxial feedlines feedlines connect connect the two the antenna two antenna elements elements to the dual-channelto the dual-channel receiver. receiver.The same The antenna same will antenna be used will for be the used probe, for but the now probe, connected but now to aconnected dual channel to transmittera dual channel unit transmittermounted in unit the masthead,mounted in as the shown masthead, in Figure as shown5b. in Figure 5b.

(a) (b)

Figure 5. (a) The sensor; (b) One of the probes. Distance between probe and sensor is 50–200 km. Figure 5. (a) The sensor; (b) One of the probes. Distance between probe and sensor is 50–200 km. 5. Link Budget Calculations 5. Link Budget Calculations From previous section we know that sensitivity of the sensor, limited by radio noise, is 160 dBW. − For accurateFrom previous measurements, section a minimumwe know SNRthat of sensitivity 20 dB is needed. of the When sensor, we assumelimited that by fadingradio minimanoise, ofis −16035 dBdBW. may For occur, accurate the system measurements, has to be designed a minimum for a mean SNR inputof 20 powerdB is needed. of 160 When+ 20 + we35 =assume105 dBW that . fading− minima of −35 dB may occur, the system has to be designed for− a mean input− power of −160 + 20 + 35 = −105 dBW. The antenna gain at both ends of the link is 3.7 dBi. The ionospheric losses are low: we estimate them at 4 dB. The path length through the ionosphere can be approximated by assuming an ionospheric refraction height of 220 km and a horizontal distance of 100 km. A wave travelling in a straight line to the reflection point would cover a path length R of:

Sensors 2019, 19, x; www.mdpi.com/journal/sensors Sensors 2019, 19, 2616 8 of 16

The antenna gain at both ends of the link is 3.7 dBi. The ionospheric losses are low: we estimate them at 4 dB. The path length through the ionosphere can be approximated by assuming an ionospheric refraction height of 220 km and a horizontal distance of 100 km. A wave travelling in a straight line to the reflection point would cover a path length R of: s !2 150 km R = 2 220 km2 + = 451 km (7) 2

As the real path is a curved and not a straight line, approximately 1.2% must be added to its length, so that we arrive at a path length of 457 km. Probe frequency is 7 MHz. Using the Friis transmission equation for free space propagation [39] complemented with the ionospheric losses, we can calculate the required probe transmit power: ! c P = Psensor Gsensor G + 20 log R 20 log + A (8) probe − − probe 10 − 10 4π f i where Pprobe and Psensor are in dBW and Gprobe and Gsensor are the antenna gains in dBi and Ai are the ionospheric losses in dB. c is the speed of light (2.9979 108 m/s) and f is the frequency in Hz. × Substituting these values in (9), the minimum power that the probe must deliver to the antenna is:

8 !  3 2.9979 10 Pprobe = 105 3.7 3.7 + 20 log 457 10 20 log × + 4 = 5.9 dBW (9) − − − 10 × − 10 4π 7 106 − ∗ × According to these calculations, a minimum probe power of 0.26 Watt will produce 55 dB SNR at the sensor in a 30 Hz bandwidth. When the probe power is increased to 1 Watt, the sensor power will become 99 dBW, and the SNR in a quiet rural environment will be 61 dB. − 6. Probe Design For the investigation, a sequence of probing signals of precisely controlled polarization are needed. This can be realized by driving the two orthogonal antenna elements with two sine waves that have the same frequency, but a different amplitude and phase. The polarization of the emitted wave can be controlled by changing the power ratio ρ and the phase difference θ of the two outputs of the probe:

tv p 1 + ρ + 1 + ρ2 + 2ρ cos(2θ) kAR = p (10) 1 + ρ 1 + ρ2 + 2ρ cos(2θ) − where kAR is the axial ratio of the polarization ellipse [40]. To control the power ratio ρ and phase difference θ a novel dual-channel transmitter is designed around a direct digital synthesis (DDS) chip. The DDS generates two synchronous sinusoidal voltages, derived from a single 25 MHz temperature compensated crystal oscillator (TCXO) with a frequency stability of 2.5 10 6. The two outputs of × − the DDS are followed by two analogue 1-Watt amplifiers and low pass filters, as shown in Figure6. The relative amplitude and phase difference of the signals sent to antenna 1 and antenna 2 can be controlled by writing a digital word in a register of the DDS chip. To achieve circular polarization with >25 dB cross-polarization, the power error must be smaller than 0.3 dB and the phase error must be smaller than 2.5◦ [25]. Therefore, any amplitude and phase errors introduced by the analogue stages are measured and a calibration factor is stored to compensate for them. Similarly, the frequency of the probes is measured and calibrated by storing a digital multiplication factor. Sensors 2019, 19, 8 of 16

𝑅=2 220 𝑘𝑚 + = 451 𝑘𝑚 (7) As the real path is a curved and not a straight line, approximately 1.2% must be added to its length, so that we arrive at a path length of 457 km. Probe frequency is 7 MHz. Using the Friis transmission equation for free space propagation [39] complemented with the ionospheric losses, we can calculate the required probe transmit power:

𝑃 =𝑃 −𝐺 −𝐺 +20log 𝑅−20log +𝐴 (8)

where Pprobe and Psensor are in dBW and Gprobe and Gsensor are the antenna gains in dBi and Ai are the ionospheric losses in dB. c is the speed of light (2.9979 × 108 m/s) and f is the frequency in Hz. Substituting these values in (9), the minimum power that the probe must deliver to the antenna is:

. × 𝑃 =−105−3.7−3.7+20log 457 × 10 −20log +4=−5.9 dBW (9) ∗ × According to these calculations, a minimum probe power of 0.26 Watt will produce 55 dB SNR at the sensor in a 30 Hz bandwidth. When the probe power is increased to 1 Watt, the sensor power will become −99 dBW, and the SNR in a quiet rural environment will be 61 dB.

6. Probe Design For the investigation, a sequence of probing signals of precisely controlled polarization are needed. This can be realized by driving the two orthogonal antenna elements with two sine waves that have the same frequency, but a different amplitude and phase. The polarization of the emitted wave can be controlled by changing the power ratio ρ and the phase difference θ of the two outputs of the probe: 𝑘 = (10)

where kAR is the axial ratio of the polarization ellipse [40]. To control the power ratio ρ and phase difference θ a novel dual-channel transmitter is designed around a direct digital synthesis (DDS) chip. The DDS generates two synchronous sinusoidal voltages, derived from a single 25 MHz temperature compensated crystal oscillator (TCXO) with a frequency stability of 2.5 × 10−6. The two outputs of the DDS are followed by two analogue 1-Watt amplifiers and low pass filters, as shown in Figure 6. The relative amplitude and phase difference of the signals sent to antenna 1 and antenna 2 can be controlled by writing a digital word in a register of the DDS chip. To achieve circular polarization with >25 dB cross-polarization, the power error must be smaller than 0.3 dB and the phase error must be smaller than 2.5° [25]. Therefore, any amplitude and phase errors introduced by the analogue stages are measured and a calibration factor is stored to compensate for them. Similarly, Sensorsthe frequency2019, 19, 2616 of the probes is measured and calibrated by storing a digital multiplication factor.9 of 16

. Figure 6. The block diagram of the probe signal transmitter. Figure 6. The block diagram of the probe signal transmitter. A low-power programmable controller with non-volatile memory is integrated in the probe. With it, a sequenceA low-power of diff erentprogrammable polarizations controller can be with pre-programmed. non-volatile memory An example is integrated of such in a sequence,the probe. inWith which it, LHCP, a sequence RHCP andof different linear polarized polarizations (LIN) probingcan be wavespre-programmed. are alternated, An is shownexample in of Figure such7. a Switchingsequence, from in which one polarization LHCP, RHCP to theand other linear is polarized done with (LIN) a smooth probing slope waves to avoid are widebandalternated, spurs is shown or ‘clicks’in Figure in the 7. adjacent Switching channels. from one The polarization duration of theto the polarization other is intervalsdone with is deriveda smooth from slope the to TCXO avoid Sensors 2019, 19, 9 of 16 wideband spurs or ‘clicks’ in the adjacent channels. The duration of the 6polarization intervals is frequency, making the interval length extremely precise and stable (2.5 10− ). As the frequency of derived from the TCXO frequency, making the interval length extremely× precise and stable (2.5 × the10− transmitted6). As the frequency wave is of also the derived transmitted from wave the same is also TCXO, derived the from frequency the same is precisely TCXO, the known frequency at the is receiverpreciselySensors 2019 and known, 19 with, x; at it thethe intervalreceiver length.and with The it the sensor interval can nowlength. be The synchronized sensor canwww.mdpi.com/jou with now the be synchronized polarizationrnal/sensors intervalswith the of polarization the probe by intervals detecting of thethe ‘o probeff’ interval by detecting that is inserted the ‘off’ in interval each sequence. that is inserted The controller in each willsequence. also be usedThe controller to activate will the also probes be onused a programmable to activate the day probes and timeon a andprogrammable deactivate themday and at night time toand save deactivate battery power. them at Remote night to control save battery via the mobilepower. phoneRemote network control hasvia the been mobile investigated phone network but the resultinghas been power investigated consumption but the would resulting be toopower high consumption for this application. would be too high for this application.

Figure 7. Example of a sequence of polarized probing signals. This example shows left-hand circular Figure 7. Example of a sequence of polarized probing signals. This example shows left-hand circular polarization (LHCP), right-hand circular polarization (RHCP), transmitter identification (ID) and linear polarization (LHCP), right-hand circular polarization (RHCP), transmitter identification (ID) and polarization (LIN). The ‘off’ interval is used to synchronize probe and sensor. linear polarization (LIN). The ‘off’ interval is used to synchronize probe and sensor. The entire probe transmitter is mounted in a 23 cm 8 cm 8 cm water-tight box at the masthead, The entire probe transmitter is mounted in a 23 cm× × 8 cm× × 8 cm water-tight box at the masthead, in the centre of the antenna system. This eliminates the use of coaxial antenna cables and calibration of in the centre of the antenna system. This eliminates the use of coaxial antenna cables and calibration their length and also reduces weight. A single lightweight telescoping mast is used to support the of their length and also reduces weight. A single lightweight telescoping mast is used to support the masthead, from which the antenna wires slope downward in four directions. A mast-foot makes the masthead, from which the antenna wires slope downward in four directions. A mast-foot makes the mast self-supporting during installation. The antenna wires also act as guy wires. The battery at the mast self-supporting during installation. The antenna wires also act as guy wires. The battery at the base of the antenna mast provides counterweight and a 12 VDC cable runs up the mast to feed the base of the antenna mast provides counterweight and a 12 VDC cable runs up the mast to feed the transmitter. The complete probe can be installed by one person in 2 h, which is important for the transmitter. The complete probe can be installed by one person in 2 h, which is important for the deployment of all the probes in a short period of time in a wide area. Eight probes are realized in total. deployment of all the probes in a short period of time in a wide area. Eight probes are realized in Together with an equally portable sensor system, an accurate and deployable system is realized to total. Together with an equally portable sensor system, an accurate and deployable system is realized investigate the influence of the ionosphere and the Earth’s magnetic field on radio wave propagation to investigate the influence of the ionosphere and the Earth’s magnetic field on radio wave below the plasma frequency of the ionosphere. propagation below the plasma frequency of the ionosphere. 7. Deployment Criteria 7. Deployment Criteria As the polarization purity of the waves emitted by the probes and the cross-polarization of the sensorAs the are polarization essential in purity the intended of the waves experiments, emitted by the the probes probes and and sensors the cross-polarization have to be deployed of the insensor environments are essential that in do the not intended distort the experiments, emitted or receivedthe probes waves. and sensors This means have that to be the deployed near-field in ofenvironments the antenna, that up todo 1not or distort 2 wavelengths the emitted (40–80 or received m at 7 MHz),waves. shouldThis means be devoid that the of near-field metal wiring of the (wireantenna, fences, up powerto 1 or 2 cables, wavelengths telephone (40–80 wire) m andat 7 MHz) large, conducting should be devoid objects. of Furthermore, metal wiring (wire the ground fences, underpower the cables, antenna telephone wires, which wire) isand also large in the conducting near-field andobjects. influences Furthermore, the antenna the diagram,ground under must bethe homogeneousantenna wires, and which flat. Rooftops is also arein generallythe near-field not suitable, and influences as the structure the antenna directly underdiagram, the antennamust be mayhomogeneous contain conducting and flat. Rooftops supports are and generally wiring, which not suit willable, distort as the the structure antenna di diagram.rectly under In the the far antenna field, resonantmay contain objects conducting (e.g., HF antennas)supports and or multi-resonant wiring, which objectswill distort (e.g., the telephone antenna and diagram. power In lines) the far should field, preferablyresonant objects not be within(e.g., HF line-of-sight. antennas) or In multi-resona practice, conformingnt objects to (e.g., all these telephone criteria and is not power always lines) possible, should butpreferably the more not strictly be within these criteria line-of-sight. are adhered In practice, to, the higherconforming the quality to all of these the measurements. criteria is not always possible, but the more strictly these criteria are adhered to, the higher the quality of the measurements. Additionally, for the sensor, a location where the radio noise level is very low has to be selected. City locations generally exhibit very high radio noise levels. Photovoltaic sites may also emit high levels of radio noise. Radio noise is generally lowest in remote rural areas, with a very low density of buildings and human activity. As the waves that we observe in the NVIS experiments arrive at very steep angles, typically 70°–90°, a measurement site located in a valley and shielded from far-away noise sources by mountains may be preferred over hilltop sites with a good view all around. Radio noise measurements with calibrated equipment are preferred to assess the local radio noise level. An example will be given for the measurements in Spain. Firstly, the central low noise sensor location was selected and verified by measurement. Secondly, potential probe locations were identified at distances between 50 and 150 km from the sensor location via friends and colleagues. Screening of potential sites was done firstly via Google Earth, then secondly by personal inspection. Figures 8 and 9 provide an impression of such sites. The site in Cal Cerdanyola was located in a

Sensors 2019, 19, x; www.mdpi.com/journal/sensors Sensors 2019, 19, 2616 10 of 16

Additionally, for the sensor, a location where the radio noise level is very low has to be selected. City locations generally exhibit very high radio noise levels. Photovoltaic sites may also emit high levels of radio noise. Radio noise is generally lowest in remote rural areas, with a very low density of buildings and human activity. As the waves that we observe in the NVIS experiments arrive at very steep angles, typically 70◦–90◦, a measurement site located in a valley and shielded from far-away noise sources by mountains may be preferred over hilltop sites with a good view all around. Radio noise measurements with calibrated equipment are preferred to assess the local radio noise level. An example will be given for the measurements in Spain. Firstly, the central low noise sensor location was selected and verified by measurement. Secondly, potential probe locations were identified at distancesSensors 2019, between 19, 50 and 150 km from the sensor location via friends and colleagues. Screening10 of of16 potential sites was done firstly via Google Earth, then secondly by personal inspection. Figures8 and9 Sensors 2019, 19, 10 of 16 providecanyon in an the impression lower Pyrenees, of such with sites. steep The sitemountain in Cal Cerdanyolaridges all around. was located This site in aprovided canyon inanthe excellent lower demonstrator of the performance of NVIS radio systems in areas seemingly cut off from the horizon. Pyrenees,canyon in with the steeplower mountainPyrenees, ridgeswith steep all around. mountain This ridges site provided all around. an This excellent site provided demonstrator an excellent of the performancedemonstrator of of NVIS the performance radio systems of in NVIS areas radio seemingly systems cut in o ffareasfrom seemingly the horizon. cut off from the horizon.

Figure 8. The central sensor system in in Sant Martí Sesgueioles. Figure 8. The central sensor system in in Sant Martí Sesgueioles. Figure 8. The central sensor system in in Sant Martí Sesgueioles.

(a) (b)

Figure 9. One of the six(a probe) transmitters in Cal Cerdanyola. (a) The complete(b system) including the Figure 9. One of the six probe transmitters in Cal Cerdanyola. (a) The complete system including the wire antennas; (b) A close up of the compact (23 cm 8 cm 8 cm) probe transmitter on top of the mast. wire antennas; (b) A close up of the compact (23 cm× × 8 ×cm × 8 cm) probe transmitter on top of the Figure 9. One of the six probe transmitters in Cal Cerdanyola. (a) The complete system including the mast. 8. Validationwire antennas; Measurements (b) A close up of the compact (23 cm × 8 cm × 8 cm) probe transmitter on top of the 8. ValidationMeasurementmast. Measurements data obtained during a field deployment of six probes and one sensor in Spain was analysed to verify the performance of the design described here. This is not the science performed with 8. ValidationMeasurement Measurements data obtained during a field deployment of six probes and one sensor in Spain was analysed to verify the performance of the design described here. This is not the science Measurement data obtained during a field deployment of six probes and one sensor in Spain performed with the system—which requires a different type of analysis and will be subject to separate was analysed to verify the performance of the design described here. This is not the science publications—but a validation of the realized system of sensor and probes. performed with the system—which requires a different type of analysis and will be subject to separate In our design, we assumed a radio noise level of -160 dBW in a 30 Hz bandwidth, and expected publications—but a validation of the realized system of sensor and probes. a signal level of -99 dBW and a resulting SNR of 61 dB. We processed 45 min of data from the field In our design, we assumed a radio noise level of -160 dBW in a 30 Hz bandwidth, and expected deployment using a Fast Fourier Transformation (FFT) [41] with a block size of 1000 samples and a a signal level of -99 dBW and a resulting SNR of 61 dB. We processed 45 min of data from the field Blackman-Harris window [42] and a bin size of 3 Hz. We than aggregated the processed data in a deployment using a Fast Fourier Transformation (FFT) [41] with a block size of 1000 samples and a statistical spectrum plot, in which the colour represents the relative occurrence of each of the values, Blackman-Harris window [42] and a bin size of 3 Hz. We than aggregated the processed data in a see Figure 10. The 6 peaks correspond with the six probe signals. Other amateur radio signals can be statistical spectrum plot, in which the colour represents the relative occurrence of each of the values, seen around 7009.57, 7009.86 and 7010.05 kHz. As they do not interfere with our measurements, no see Figure 10. The 6 peaks correspond with the six probe signals. Other amateur radio signals can be effort is undertaken to identify them. From this graph, we can see that the maximum probe signal seen around 7009.57, 7009.86 and 7010.05 kHz. As they do not interfere with our measurements, no power is approximately −99 dBW, with a few dB difference per probe. However, the radio noise level effort is undertaken to identify them. From this graph, we can see that the maximum probe signal is approximately −161.5 dBW in a 3 Hz bandwidth, or −151.5 dBW in a 30 Hz bandwidth. This is 8.5 power is approximately −99 dBW, with a few dB difference per probe. However, the radio noise level Sensorsis approximately 2019, 19, x; −161.5 dBW in a 3 Hz bandwidth, or −151.5 dBW in a 30www.mdpi.com/jou Hz bandwidth.rnal/sensors This is 8.5

Sensors 2019, 19, x; www.mdpi.com/journal/sensors Sensors 2019, 19, 2616 11 of 16 the system—which requires a different type of analysis and will be subject to separate publications—but a validation of the realized system of sensor and probes. In our design, we assumed a radio noise level of -160 dBW in a 30 Hz bandwidth, and expected a signal level of -99 dBW and a resulting SNR of 61 dB. We processed 45 min of data from the field deployment using a Fast Fourier Transformation (FFT) [41] with a block size of 1000 samples and a Blackman-Harris window [42] and a bin size of 3 Hz. We than aggregated the processed data in a statistical spectrum plot, in which the colour represents the relative occurrence of each of the values, see Figure 10. The 6 peaks correspond with the six probe signals. Other amateur radio signals can be seen around 7009.57, 7009.86 and 7010.05 kHz. As they do not interfere with our measurements, no effort is undertaken to identify them. From this graph, we can see that the maximum probe signalSensors power2019, 19,is approximately 99 dBW, with a few dB difference per probe. However, the11 radio of 16 Sensors 2019, 19, − 11 of 16 noise level is approximately 161.5 dBW in a 3 Hz bandwidth, or 151.5 dBW in a 30 Hz bandwidth. − − ThisdB ishigher 8.5 dB than higher was than predicted was predicted for a quiet for arural quiet environment. rural environment. As a result, As a result, the SNR the SNRis only is only−99 –99 (− –151.5) ( 151.5) = 52.5= 52.5 dB. dB The. The resulting resulting SNR SNR is is suffic sufficientient for for the intendedintended experiments,experiments, butbut the the −99 −– (−151.5)− = 52.5 dB. The resulting SNR is sufficient for the intended experiments, but the bandwidthbandwidth may may be be decreased decreased to to 10 10 Hz, Hz, which whichich will willwill improve improveimprove the thethe SNR SNRSNR to toto 57.3 57.357.3 dB. dB.dB.

FigureFigure 10. 10.Statistical Statistical spectrogram spectrogram showing showing the the six six probe probe signals signals as as received received by by the the sensor. sensor. The The colour colour Figure 10. Statistical spectrogram showing the six probe signals as received by the sensor. The colour showsshows the the relative relative occurrence occurrence in in a a 0.15 0.15 dB dB amplitude amplitude bin bin and and a 3a Hz3 Hz frequency frequency bin. bin. shows the relative occurrence in a 0.15 dB amplitude bin and a 3 Hz frequency bin. Cross-polarization of the probes is verified during ‘Happy Hour’ propagation [25], during which Cross-polarization of the probes isis verifiedverified duringduring ‘Happy‘Happy Hour’Hour’ propagationpropagation [25],[25], duringduring whichwhich the ionosphere acts as a polarization filter and only the extraordinary wave propagates. Figure 11 the ionosphere acts as a polarization filter and only the extraordinary wave propagates. Figure 11 shows the spectrogram of the waves received by the sensor on a linearly polarized antenna during shows the spectrogram of the waves received by the sensor on a linearly polarized antenna during normal NVIS propagation, showing that the waves emitted in LHCP and RHCP are received equally normal NVIS propagation, showing that the waves emitted in LHCP and RHCP are received equally strong. Figure 12 shows reception during the Happy Hour interval, and 25–35 dB suppression of the strong. Figure 12 shows reception during the Happy HourHour interval,interval, andand 25–3525–35 dBdB suppressionsuppression ofof thethe ordinary wave (RHCP transmission of the probe). This proves that the cross-polarization of the probe ordinary wave (RHCP transmission of the probe). This provesproves thatthat thethe cross-pocross-polarizationlarization ofof thethe probeprobe is very good. During the field tests, one of the probes developed a faulty amplifier in one channel. isis veryvery good.good. DuringDuring thethe fieldfield testtests, one of the probes developed a faulty amplifier in one channel. This was immediately visible at the sensor, as the transmit cross-polarization was lost. This was immediately visible at the sensor, as the transmit cross-polarization was lost.

Figure 11. Spectrogram of the LHCP end RHCP waves emitted by the probe and received by the sensor Figure 11. Spectrogram of the LHCP end RHCP waves emitted by the probe and received by the onFigure a linearly 11. Spectrogram polarized antenna, of the duringLHCP normalend RHCP NVIS waves propagation. emitted by the probe and received by the sensor on a linearly polarized antenna, during normal NVIS propagation. sensor on a linearly polarized antenna, during normal NVIS propagation.

Figure 12. Spectrogram of the LHCP end RHCP waves emitted by the probe and received by the 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. sensor on a linearly polarized antenna, during the morning Happy Hour.

Sensors 2019, 19, x; www.mdpi.com/journal/sensors Sensors 2019, 19, x; www.mdpi.com/journal/sensors Sensors 2019, 19, 11 of 16

dB higher than was predicted for a quiet rural environment. As a result, the SNR is only −99 – (−151.5) = 52.5 dB. The resulting SNR is sufficient for the intended experiments, but the bandwidth may be decreased to 10 Hz, which will improve the SNR to 57.3 dB.

Figure 10. Statistical spectrogram showing the six probe signals as received by the sensor. The colour shows the relative occurrence in a 0.15 dB amplitude bin and a 3 Hz frequency bin.

Cross-polarization of the probes is verified during ‘Happy Hour’ propagation [25], during which the ionosphere acts as a polarization filter and only the extraordinary wave propagates. Figure 11 shows the spectrogram of the waves received by the sensor on a linearly polarized antenna during normal NVIS propagation, showing that the waves emitted in LHCP and RHCP are received equally strong. Figure 12 shows reception during the Happy Hour interval, and 25–35 dB suppression of the ordinary wave (RHCP transmission of the probe). This proves that the cross-polarization of the probe is very good. During the field tests, one of the probes developed a faulty amplifier in one channel. This was immediately visible at the sensor, as the transmit cross-polarization was lost.

Figure 11. Spectrogram of the LHCP end RHCP waves emitted by the probe and received by the Sensors 2019sensor, 19 ,on 2616 a linearly polarized antenna, during normal NVIS propagation. 12 of 16

Figure 12. Spectrogram of the LHCP end RHCP waves emitted by the probe and received by the sensor Figure 12. Spectrogram of the LHCP end RHCP waves emitted by the probe and received by the on a linearly polarized antenna, during the morning Happy Hour. Sensorssensor 2019, 19 on, a linearly polarized antenna, during the morning Happy Hour. 12 of 16

TheThe performance performance of of the the probe probe can can to to some some extent extent be be extrapolated extrapolated to to the the sensor, sensor, as as identical identical antennasantennasSensors 2019 are are, 19 used, usedx; and and the the receiver receiver has has been been characterized characterized using using laboratory laboratory equipment.www.mdpi.com/jou equipment. Furthermore, Furthermore,rnal/sensors thethe field field measurement measurement makes makes use use of of hourly hourly calibrations calibrations with with a a precise precise dual dual channel channel source source and and shows shows veryvery little little drift. drift. The The results results obtained obtained with with stable stable NVIS NVIS propagation propagation are are promising, promising, see see Figure Figure 13 13.. The The first two intervals show almost identical power in both polarization planes, but +90 and 90 phase first two intervals show almost identical power in both polarization planes, but +90°◦ and− −90°◦ phase didifferencefference respectively, respectively, corresponding corresponding with with received received RHCP andRHCP LHCP and polarization. LHCP polarization. This is consistent This is withconsistent the extraordinary with the extraordinary and ordinary and wave ordinary propagation wave propagation and the reversal and the of thereversal polarization of the polarization when the waveswhen are the refracted waves are downward refracted [6 ,downward32]. The interval [6,32]. in The which interval the probe in which sends linearlythe probe polarized sends linearly waves clearlypolarized shows waves rapidly clearly changing shows polarization rapidly changing caused polari by interferencezation caused of the by ordinary interference and of extraordinary the ordinary wave,and extraordinary which also concurs wave, withwhich expectations. also concurs with expectations.

Figure 13. Polarization of a sequence of probing signals captured by the sensor during normal Figure 13. Polarization of a sequence of probing signals captured by the sensor during normal NVIS NVIS propagation. propagation. 9. Discussion and Conclusion 9. Discussion and Conclusion The validation measurement and the field trials have shown that an exceptionally portable and very sensitiveThe validation and accurate measurement measurement and the systemfield trials is created. have shown The 1-Wattthat an probeexceptionally signal transmitters portable and producevery sensitive a 52.5 dB and SNR accurate in a 30 measurement Hz bandwidth. system This is is 8.5 created. dB lower The than 1-Watt predicted, probecaused signal bytransmitters a higher thanproduce expected a 52.5 level dB SNR of external in a 30 radio Hz bandwidth. noise. The This radio is noise 8.5 dB level lower shows than a predicted, diurnal variation caused consistentby a higher withthan NVIS expected ionospheric level of external propagation, radio whichnoise. The could radio indicate noise thatlevel electromagnetic shows a diurnal noise variation from consistent the city ofwith Barcelona, NVIS ionospheric at 100 km propagation, distance, is received which could via ionospheric indicate that refraction. electromagnetic This would noise from be consistent the city of withBarcelona, the work at of100 Pederick km distance, and Cervera is received [43] via on radioionospheric noise propagation refraction. This via thewould ionosphere be consistent and with with ourthe observations work of Pederick of significant and Cervera radio [43] noise on reduction radio noise during propagation a Sudden via Ionospheric the ionosphere Disturbance and with [28 our], butobservations cannot be provenof significant without radio directional noise reduction information. during a Sudden Ionospheric Disturbance [28], but cannotThe be polarization proven without purity directional of the waves information. emitted by the probes is excellent. To the best of our knowledge,The polarization there are no purity other of published the waves concepts emitted that by reachthe probes comparably is excellent. high cross-polarizationTo the best of our knowledge, there are no other published concepts that reach comparably high cross-polarization values (25–35 dB) in HF transmission, which is much better than the cross-polarization achieved by Erhel 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 Paonessa, 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

Sensors 2019, 19, x; www.mdpi.com/journal/sensors Sensors 2019, 19, 2616 13 of 16 values (25–35 dB) in HF transmission, which is much better than the cross-polarization achieved by Erhel 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 Paonessa, 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.

Author Contributions: This article was written by R.M.A.-P.and B.A.W. This includes the original draft preparation, preparation of the visualizations, review and editing. Funding acquisition and radio frequency licensing was done by R.M.A.-P. and B.A.W. The original research idea was proposed by B.A.W. This includes the concept of the low-power portable probes, of which the dual channel programmable transmitters were designed by G.J.L. under supervision of B.A.W. All other hardware was realized by E.v.M. and B.A.W. Experiment site acquisition was done by R.M.A.-P. Data analysis was done by B.A.W. All four authors participated in the field experiments in Catalonia, assisted by volunteers. Project management was done by B.A.W. Funding: The realization of probes, sensor and antennas was partly paid by the University of Twente and partly paid by E.V.M and B.A.W. The European Association on Antennas and Propagation (EurAAP) covered travel expenses of the Spanish field measurements. R.M.A.-P. would like to thank the Secretaria d’Universitats i Recerca del Departament d’Economia i Coneixement (Generalitat de Catalunya) and Universitat Ramon Llull, Barcelona, Spain, for projects 2014-URL-Trac-018 and 2015-URL-Proj-041. Acknowledgments: The authors thank Joe Martin and Doug Wigley for the modifications to the PowerSDR software to make our scientific measurements possible with the HPSDR hardware. For the experiments in Spain, the authors thank Enric Fraile i Algeciras for hosting the operation of the probe transmitters under the amateur radio license of the LaSalle Radio Club. We thank David Badia Folguera for the selection of the quiet rural sensor location and his radio noise measurements in Sant Martí de Sesgueioles, and David Altadill of the Ebre Observatory for hand-scaled ionosonde measurements. We thank the landowners who hosted our probe transmitters and the sensor system in Spain: the Bishop of Vic, Martí Caraona, La Salle Aula de Natura, Joan Lluís Pijoan Vidal, Maria Mercè Alsina, Ramon Pahí and Josep Ma Perramona. We also thank the staff of La Salle Ramon Llull University of Barcelona that helped installing and recollecting the probes: Silvia Aybar Carrasco, Ferran Orga Vidal, Joan Lluís Pijoan Vidal, Enric Fraile i Algeciras and David Badia Folguera. For the experiments in the Netherlands, the authors thank the Ministry of Defence for the use of one of their frequencies and the Radiocommunications Agency Netherlands for issuing an experimentational license. We thank the Royal Observatory of Belgium for ionosonde measurements. We thank the landowners who hosted our probe transmitters and the sensor system in the Netherlands: Caya and Goos Visser of Bouvier Kennel Caya’s Home, Frank Holl, Hans Wierink, Jan Simons, Albert and Ans Westenberg, George Petersen and Peter Adegeest of the Royal Dutch Navy. Conflicts of Interest: The authors declare no conflict of interest. Sensors 2019, 19, 2616 14 of 16

Acronyms

DDS Direct Digital Synthesis EurAAP European Association on Antennas and Propagation FFT Fast Fourier Transform HF High Frequency (3-30 MHz) HPSDR High Performance Software Defined Radio IFDR Intermodulation Free Dynamic Range IRI International Reference Ionosphere ITU International Telecommunication Union IGRF International Geomagnetic Reference Field LHCP Left-Hand Circular Polarization NEC Numerical Electromagnetics Code NVIS Near Vertical Incidence Skywave RHCP Right-Hand Circular Polarization SNR Signal-to-Noise Ratio TCXO Temperature Compensated Crystal Oscillator

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