Extracting Radar Parameters for Sky-Wave Over-The-Horizon Radar Using Ray-Tracing Through a Model of Ionosphere

Defence Research and Recherche et de´veloppement Development Canada pour la de´fense Canada

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Extracting Radar Parameters for Sky-Wave Over-The-Horizon Radar using Ray-Tracing through a Model of Ionosphere

Thayananthan Thayaparan Dale Dupont Yousef Ibrahim Ryan Riddolls DRDC – Ottawa Research Centre

Defence Research and Development Canada Scientiﬁc Report DRDC-RDDC-2019-R208 November 2019

CAN UNCLASSIFIED CAN UNCLASSIFIED

IMPORTANT INFORMATIVE STATEMENTS

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c Her Majesty the Queen in Right of Canada, Department of National Defence, 2019 c Sa Majesté la Reine en droit du Canada, Ministère de la Défense nationale, 2019

CAN UNCLASSIFIED Abstract

The Canadian Department of National Defence (DND) is developing an experimental over- the-horizon radar (OTHR) with the potential for surveillance of Canada. Because of dynamically changing ionospheric conditions in the Earth’s high-latitude and polar regions, the operating OTHR transmission frequency and elevation angle need to be adjusted regularly to maintain constant illumination of downrange targets. In this study, the feasible operating frequency, elevation angle and absorption radar parameters are determined for short- and longrange OTHR operation using three-dimensional ionosphere ray-tracing technique. The variation in these radar parameters is observed as a function of time of day (daytime and nighttime), season (winter, spring, summer and fall), range (short-range and long-range) and solar cycle (solar maximum and minimum). The range of operating frequencies and elevation angles obtained from this study will aid developing the transmitter and receiver antenna layout for experimental OTHR conﬁgurations in the poorly-understood high-latitude and polar regions. These methods will also help to form the basis of the frequency monitoring systems (FMS) that will control the conﬁguration of these polar OTHR systems in real-time.

Signiﬁcance for defence and security

Over-the-horizon radar (OTHR) is currently being investigated as a potential replacement for the North Warning System to monitor Canada’s North. It is imperative that an accurate frequency monitoring systems (FMS) is incorporated into the OTHR. This will ensure that the proper radio wave frequency and elevation angle are chosen to keep targets downrange continously visible.

DRDC-RDDC-2019-R208 i Résumé

Le ministère de la Défense nationale conçoit actuellement un radar transhorizon (OTHR) expérimental qui offre une capacité potentielle de surveillance du Canada. Compte tenu des changements des conditions ionosphériques en régions polaires et de hautes latitudes, il faut régulièrement rajuster la fréquence de transmission et l’angle d’élévation de l’OTHR exploité, de façon à maintenir l’illumination constante en aval des objectifs. Dans le cadre de l’étude, on définit les paramètres radar relatifs à la fréquence de fonctionnement, à l’angle d’élévation et à l’absorption réalisables en vue de l’exploitation de l’OTHR à courte distance et à longue distance au moyen d’une technique de tracé tridimensionnel par rayon de l’ionosphère. On estime que la variation dans ces paramètres radars est causée par l’heure (le jour ou la nuit), la saison (printemps, été, automne, hiver), la distance (courte ou longue) et le cycle solaire (le minimum et le maximum solaires). La gamme des fréquences de fonctionnement et d’angles d’élévation obtenue dans le cadre de l’étude servira à mettre au point la disposition des antennes de réception et de transmission dans les configurations de l’OTHR expérimental en régions polaires et en hautes latitudes, peu connues. En outre, ces méthodes contribueront à jeter les bases de systèmes de surveillance des fréquences (SSF), qui contrôleront en temps réel la configuration de ces systèmes d’OTHR en régions polaires.

Importance pour la défense et la sécurité

On étudie actuellement le radar transhorizon (OTHR) comme remplacement potentiel du Système d’alerte du Nord pour la surveillance du Nord canadien. Il est essentiel d’intégrer des systèmes de surveillance des fréquences (SSF) précis dans l’OTHR pour s’assurer de choisir la bonne fréquence radio et le bon angle d’élévation, de façon à maintenir la visibilité en aval des objectifs.

ii DRDC-RDDC-2019-R208 Table of contents

Abstract ...... i

Signiﬁcance for defence and security ...... i

Résumé ...... ii

Importance pour la défense et la sécurité ...... ii

Table of contents ...... iii

List of ﬁgures ...... iv

List of tables ...... vi

1 Introduction ...... 1

2 Three-Dimensional Ray-Tracing Technique ...... 3

2.1 Electron Density Model ...... 6

2.2 Earth Magnetic Model ...... 7

2.3 Absorption Model ...... 8

2.3.1 Electron Collision Frequency ...... 8

3 Monthly Variation ...... 11

4 Seasonal Variation ...... 15

5 A Case Study ...... 28

6 Conclusion ...... 30

References ...... 32

List of Symbols/Abbreviations/Acronyms/Initialisms ...... 37

DRDC-RDDC-2019-R208 iii List of ﬁgures

Figure 1: An overview of the operation of the ray tracer. The ray tracer ﬁrst creates an interpolated grid of geophysical and atmospheric parameters that uses several empirical models to model the ionosphere. This data is used along with speciﬁcations of the transmission and target location and the range of radar parameters to sweep while tracing the corresponding radar signals as rays...... 4

Figure 2: An illustration of sky-wave over-the-horizon radar propagation. The transmitter is aimed at the ionosphere so that the emitted signals will reﬂect oﬀ of this region of atmospheric plasma and thereby reach distant targets that are otherwise obstructed by the environment and the Earth’s surface...... 7

Figure 3: A plot of the sunspot number as a function of time. The sunspot number for the years 2014 and 2018 are indicated by black dots. These have been calculated using the 2016 version of the International Reference Ionosphere (IRI-2016) program...... 11

Figure 4: Available radar frequencies as a function of month for 2014 and 2018. The reﬂection height of the rays is given in the color bar. The IRI-2016 [8], EMM-2017 [10], and NRLMSISE-00 [16] empirical models were used. 12

Figure 5: Available elevation angles as a function of month for 2014 and 2018. The reﬂection height of the rays is given in the color bar. The IRI-2016 [8], EMM-2017 [10], and NRLMSISE-00 [16] empirical models were used. 13

Figure 6: Absorption as a function of month for 2014 and 2018. The reﬂection height of the rays is given in the color bar. The IRI-2016 [8], EMM-2017 [10], and NRLMSISE-00 [16] empirical models were used...... 14

Figure 7: Seasonal scatter plots of feasible radar parameters for the short-range target during 2014 at nighttime...... 16

Figure 8: Seasonal scatter plots of feasible radar parameters for the short-range target during 2014 at daytime...... 17

Figure 9: Seasonal scatter plots of feasible radar parameters for the short-range target during 2018 at nighttime...... 18

Figure 10: Seasonal scatter plots of feasible radar parameters for the short-range target during 2018 at daytime...... 19

iv DRDC-RDDC-2019-R208 Figure 11: Seasonal scatter plots of feasible radar parameters for the long-range target during 2014 at nighttime...... 20

Figure 12: Seasonal scatter plots of feasible radar parameters for the long-range target during 2014 at daytime...... 21

Figure 13: Seasonal scatter plots of feasible radar parameters for the long-range target during 2018 at nighttime...... 22

Figure 14: Seasonal scatter plots of feasible radar parameters for the long-range target during 2018 at daytime...... 23

Figure 15: Time as a function of usable radar frequencies for monitoring the ﬂight of a Boeing 777. The time intervals portrayed, beginning 04:35 and 16:35, start at the ground distance of 1170 km for the target and increase gradually by 450 km at every 30 min interval until reaching the ﬁnal target distance of 3000 km at 06:35 and 18:35. The IRI-2016 [8], EMM-2017 [10], and NRLMSISE-00 [16] empirical models were used. . . 28

DRDC-RDDC-2019-R208 v List of tables

Table 1: Available Parameter Ranges ...... 24

Table 2: Seasonal Average Feasible Parameters ...... 25

Table 3: Annual Average Feasible Parameters ...... 26

Table 4: Annual Average Feasible Conﬁgurations Count ...... 26

vi DRDC-RDDC-2019-R208 1 Introduction

Over-the-horizon radar (OTHR) systems operate in the high-frequency (HF) band 3-30 MHz and use either sky-wave propagation [1, 2, 3, 4, 5, 6, 7] or surface-wave propagation [8, 9, 10, 11]. Sky-wave OTHR systems are installed inland and use the ionospheric refraction of the radio waves several hundred kilometers above the Earth’s surface to overcome the line-of-sight limitation caused by the Earth’s curvature. For example, Australia’s Jindalee Operational Radar Network (JORN) is an HF skywave OTHR system which monitors both the surface and airspace of the surrounding oceans oﬀ the northern and western coasts [12, 13, 14, 15]. In contrast, surface-wave systems operate in the lower part of the HF spectrum and are installed on the coastlines using the electromagnetic coupling of radio waves to the sea surface to extend radio propagation over the horizon. An example of a maritime surface- wave OTHR system is the Wellen Radar (WERA) in northern Europe, which monitors the German Bight in the North Sea [16, 17, 18, 19]. More recently, the European "Radars for long distance maritime surveillance and Search and Rescue operations" (RANGER) project has led to the deployment of the Stradivarius radar system along the Ligurian Sea in the Mediterranean. Sky-wave radars have large detection ranges beyond the horizon, up to 3500-4000 km. In contrast, surface-wave radars have detection ranges limited to about 350- 400 km. Sky-wave propagation is best suited to the detection of fast targets like aircraft [2], while surface-wave propagation is best suited to the detection of larger, slower targets like surface vessels [8].

The Canadian Department of National Defence (DND) has pursued HF radar technology for military applications since 1984. In particular, the DND has investigated High-Frequency Surface-Wave Radar (HFSWR) technology for ocean surveillance of ships and aircraft, resulting in the establishment of operational HFSWR systems [8]. However, surface-wave technology cannot be readily applied to the long-range surveillance of fast air targets like aircraft over land due to the high attenuation of surface-waves over ground terrain [20]. Consequently, there has been a renewal of long-range HF sky-wave OTHR technology in Canada after its last major investigation during the Polar Cap III experiments of the early 1970s [21]. The desired OTHR system would provide a coverage area of 50 million km2 and could detect targets up to 30 km in altitude [22]. OTHRs use the ionosphere, the ionized part of Earth’s upper atmosphere from about 60-1000 km in altitude, to reﬂect HF waves in order to vastly increase the surveillance coverage area over traditional radar systems.

OTHRs have detected and tracked long-range air targets at mid-latitudes for many years, and have proved in these cases to be a cost-effective wide-area surveillance sensor. How- ever, OTHR technology has limited deployment at higher latitudes, including in much of Canadian airspace and in the polar regions, as it still faces significant challenges in these locations. At high-latitudes in Canada, the auroral zone passes through the center of the country, thus significantly altering the local conditions of the ionosphere. The backscatter from fast-moving irregularities in the convecting auroral plasma can can cause spread- Doppler clutter that inhibits the possibility of target detection. In addition, the polar ionosphere and the feasibility of OTHR systems in such environments are not sufficiently

DRDC-RDDC-2019-R208 1 understood to facilitate the design of full-scale OTHR in high-latitude and polar regions. Consequently, the transmission frequency and elevation angle of a polar or high-latitude OTHR must be periodically modiﬁed in order to keep the target region of the radar ﬁxed as the ionosphere changes. A frequency monitoring system (FMS) is also needed to manage the HF band on which the radar operates so as to avoid interference with existing HF operators.

In this study, feasible radar parameters for operating frequencies and elevation angles are calculated for OTHR operation using a three-dimensional (3-D) ray-tracing algorithm that interfaces with the 2016 International Reference Ionosphere (IRI-2016) model [23], the 2017 Enhanced Magnetic Model (EMM) [24], and an electron collision model. The electron collision model incorporates data from Naval Research Laboratory’s Mass Spectrometer Incoherent Scatter Extension 2000 (NRLMSISE-00) model [25, 26]. These models are eval- uated as a function of local time (comparing daytime and nighttime), season (comparing winter, spring, summer and fall), range (comparing mid-range distances up to 1300 km and long-range distances up to 2700 km) and solar cycle (comparing years which contain the solar maximum and minimum). Rays landing within 10 km of the target were considered close to the target and their corresponding frequency and initial elevation angle were considered feasible radar parameters (or available radar parameters) when used together for OTHR deployment within the circumstances described by the simulation. Though there are several HF propagation studies performed at northerly latitudes over a number of years for the purpose of HF communication and understanding the physics of the ionosphere, this is the first study to determine the range of operating frequencies and elevation angles for OTHR operation, especially for a northward-facing radar. The range of operating frequencies and elevation angles obtained from this study will aid in the development of the transmitter and receiver antenna layout for experimental OTHR configurations in the poorly-understood high-latitude and polar regions and the associated FMS. In the analysis that follows in this report, the profile of feasible parameters is plotted as ordered pairs of frequencies and elevation angles which are found to form a characteristic series of shapes, reflecting the structure of the ionosphere itself.

2 DRDC-RDDC-2019-R208 2 Three-Dimensional Ray-Tracing Technique

Ray-tracing is a numerical technique used to determine the path of a HF radio wave in anisotropic and inhomogeneous media different from the vacuum. Haselgrove derived the Hamiltonian (or canonical) equations of the raypath of HF radio waves propagating in the Earth’s ionosphere [27, 28]. The canonical equations, now commonly known as Haselgrove’s equations, have been used extensively over many years to study HF radio wave propagation [29, 30, 31, 32, 33, 34, 35, 36, 37, 38]. Due to the high computational overheads of numerical ray-tracing (NRT) for the full 3-D magnetoionic case, many researchers developed simplified formulations for their studies. These include analytical ray-tracing (ART) [39, 40, 41], 2-D NRT [42, 43], and 3-D ray-tracing where the effect of the Earth’s magnetic field is ignored [44, 45]. All of these techniques require compromises to be made, e.g., ART ignores the effect of the Earth’s magnetic field and assumes a spherical ionosphere whose profile is described by quasi-parabolic [39] or quasi-cubic [41] segments. Coleman’s 2-D NRT allows for a general ionospheric profile which varies downrange, but ray propagation is confined to a plane and again the effect of the Earth’s magnetic field is ignored. None of these techniques is able to separately calculate the raypaths of the O and X polarization modes. A recent comprehensive review of HF ray-tracing techniques is given in [46]. A unique point-to-point ray-tracing method is developed by Coleman [47].

Many previous researchers have used Jones and Stephenson [31] 3-D ray-tracing code for their raypath calculations [48]. The Defence Science and Technology Organization (DSTO) has developed a HF radio wave 3-D ray-tracing toolbox (PHaRLAP) in order to study the propagation of radio waves through the ionosphere [49].

Unlike optical ray-tracing, in which the refractive index is typically constant for a given medium, ray-tracing through the ionosphere must account for the complexities of a spatially and temporally varying refractive index, where changes in the ionospheric electron density, the Earth’s magnetic field and electron collision frequency correspond to changes in the refractive index. The ray tracing algorithm is developed based on the numerical solution of a system of Haselgrove differential equations [31, 32, 33]. An overview of the ray-tracing workflow is given in Figure 1. The refractive index used in the ray-tracing algorithm is determined by the Appleton-Hartree formula (1) [33],

2 X n = 1 − 1 , (1) Y 2 Y 4 2 1 − iZ − T ± T + Y 2 2(1−X−iZ) 4(1−X−iZ)2 L where the dimensionless quantities, X, Z, YT and YL, each of which respectively account for the effects of the electron density, Earth’s magnetic field, and the electron-particle collision frequency, are defined as,

ω2 υ X = ne ,Z = e , (2) ω2 ω

DRDC-RDDC-2019-R208 3 Figure 1: An overview of the operation of the ray tracer. The ray tracer ﬁrst creates an interpolated grid of geophysical and atmospheric parameters that uses several empirical models to model the ionosphere. This data is used along with speciﬁcations of the transmission and target location and the range of radar parameters to sweep while tracing the corresponding radar signals as rays.

4 DRDC-RDDC-2019-R208 ω cos θ ω sin θ Y = B ,Y = B , (3) L ω T ω where ωne is the angular frequency,

1 2 ! 2 nee ωne = , (4) 0me where ne is the electron density, e is the elementary charge, me is the mass of an electron and 0 is the permittivity of free space. ωB is the electron gyro-frequency, Be ωB = . (5) me B is the magnetic flux density of the Earth’s field, θ is the angle between the propagation direction of the ray and the geomagnetic field, υe is the electron-particle collision frequency, ω is the angular wave frequency and . The refractive index n in the Appleton-Hartree formula is complex-valued, with the real-part indicating the refractiveness of the medium, and the imaginary part indicating the medium’s absorption properties. Equation (1) has two solutions which differ by the ±-term in the denominator based on the O- and X- polarization modes. Addition is used to obtain the result for the O-mode and subtraction is used to obtain the result for the X-mode. The ray-tracing algorithm is applied to a 3-D ionosphere profile consisting of the electron density, geomagnetic field, and neutral particle collision frequency values that varies with altitude, latitude, longitude and time. These ionosphere parameters are calculated using the geophysical models described below. The refractive index n is updated via this ionosphere profile for the given location and time of the ray being drawn.

To illustrate the effect of electron density on the refractive properties of the ionosphere, first consider ignoring the influence of Earth’s magnetic field and electron-particle collisions. It follows that the refractive index of the ionosphere is given by (substituting Z = YT = YL = 0 in Equation (1)), f 2 n2 = 1 − X = 1 − ne , (6) f 1 81n 2 n = 1 − e , (7) f 2 where f is the wave frequency and fne is the plasma frequency which is proportional to the square root of the electron density. The plasma frequency fne is related to the angular frequency iωne , via the relation, ωne = 2πfne . It follows therefore that the plasma frequency is proportional to the square root of the electron density by substituting this ex- pression into Equation (4). Equation (7) implies that as the density of the electrons in the ionosphere increases, the maximum frequency that is reflected increases. Note that below the ionosphere, for example at ground level, the refractive index is unity. The refractive index decreases with altitude until it is zero, at which point the local plasma density is 2 ne = f /81, and a vertically incident radio wave would undergo reflection. If the ionosphere is tenuous enough that the refractive index never reaches zero, then a vertically

DRDC-RDDC-2019-R208 5 incident radio wave will escape into space. For example, if the peak plasma density in the 12 −3 ionosphere is ne(max) = 10 m , then waves at frequencies above 9 MHz will escape at vertical incidence. In the case of an obliquely incident wave at an elevation angle θ with respect to the horizon, Snell’s law predicts that reflection will occur when the plasma den- 2 2 2 ◦ sity is ne = (f sin θ)/81. Since sin θ < 1 when θ < 90 , reflection at oblique incidence will occur at lower altitude than at vertical incidence. Similarly, for a given value of peak plasma density ne(max), it is possible to reflect higher radio wave frequencies at oblique ◦ 12 −3 incidence than at vertical incidence. For example, if θ = 20 , and ne(max) = 10 m , then reflection occurs for wave frequencies up to 26 MHz. Furthermore, for a given elevation angle in the range 20-90◦, reflection will occur for wave frequencies up to a corresponding value in the range 9-26 MHz [1].

The physical constraint on maximum coverage range is about 3800 km due to obstruction by the curved surface of the Earth. The constraint on minimum range depends on the refractive properties described above. Since a given frequency reﬂects only up to a maximum elevation angle, it follows that radar coverage starts at a certain minimum range away from the transmitter. Figure 2 visualizes these range limitations for a sky-wave OTHR system. At higher elevation angles, the radar signals escape into space, and no target illumination is possible. The region of no target illumination between the radar and the minimum range is called the skip zone [1].

2.1 Electron Density Model

The dominant factor contributing to the Appleton-Hartree formula is a dimensionless quantity X, which depends on the electron density, ne. Electron density information can be used in the design stage for simulating OTHR system performance and it can be used opera- tionally for coordinate registration and frequency selection. For example, empirical electron density models are typically used to assess future radar placement and orientation, system frequencies, and the expected surveillance area observed by these instruments. The ef- fectiveness of these applications are therefore constrained by the accuracy of the electron density measurements they use. Thus, this motivates the use of an accurate model of the ionosphere.

The ionosphere can be characterized directly by a network of ionosondes or by a long-term ionospheric model ﬁtted to empirical measurements, such as the IRI. The IRI model is an empirical standard ionospheric model internationally sponsored by the Committee on Space Research (COSPAR) and the International Union of Radio Science (URSI). For a given location, time, and date, IRI provides the electron density in the altitude range from 50 km to 2000 km. The major data sources are the worldwide network of ionosondes, the powerful incoherent scatter radars, and in situ instruments on several satellites and rockets across the globe. The IRI-2012 model [51] was used in the previous study [50]. The updated IRI-2016 model was used in this study to obtain the electron density ne for the calculations of the X quantity in Equation (1) in the ray-tracing simulations described here [23]. IRI-2016 includes an improved representation of the ionosphere during the very

6 DRDC-RDDC-2019-R208 Figure 2: An illustration of sky-wave over-the-horizon radar propagation. The transmitter is aimed at the ionosphere so that the emitted signals will reflect off of this region of atmospheric plasma and thereby reach distant targets that are otherwise obstructed by the environment and the Earth’s surface. low solar activities that were reached during the last solar minimum in 2008/2009 [23], and improves the fit to observations by 10% on average compared to IRI-2012, and by up to 30% at high and low latitudes.

2.2 Earth Magnetic Model

The second factor which contributes to the Appleton-Hartree formula is a dimensionless quantity, Y , that depends on Earth’s magnetic field, B. The Earth’s internal magnetic field is a superposition of the field generated by the geodynamo in the liquid outer core (the main field) and the field of magnetized rocks in the crust and upper mantle. The main field dominates the long wavelengths, whereas the crustal field dominates at wavelengths smaller than 2500 km. In the ray-tracing program described in this report, Earth’s magnetic field, B, is calculated using the 2017 version of EMM [24]. The standard World Magnetic Model (WMM) 2015 [52], on which EMM is based, uses a spherical harmonic representation to an order of 12, resolving the magnetic field down to 3000 km in wavelength resolution. In contrast, the EMM extends this series expansion to an order of 720, resolving magnetic anomalies down to 56 km in wavelength resolution. The higher resolution of the EMM results in significantly improved accuracy. The EMM model was compiled from numer- ous magnetic surveys, including data from the European Space Agency’s Swarm satellite mission. The Swarm constellation of satellites currently represents the best source of data about the evolution of the Earth’s main magnetic field. The EMM model provides the magnetic field vector at any desired location and altitude close to and above the Earth’s surface. The B from the EMM model is used to calculate the Y quantity in Equation (1) for the simulations described in this report.

DRDC-RDDC-2019-R208 7 2.3 Absorption Model

HF radio waves experience absorption during ionospheric propagation which can significantly influence the received signal strength. The calculation of ionospheric absorption is thus vital for OTHR applications. HF radio signals propagating through the ionosphere experience power loss of the signal due to multiple causes, including free-space attenuation and the nonlinear effects of multipathing. An important factor in radio wave attenuation is the absorption of the signal by the ionosphere itself, which is dependent on the electron density and collision frequency with neutral particles and ions along the signal’s propagation path. There are two approaches to calculate ionospheric absorption: 1) the Sen Wyller ray-tracing formulation is generally cited as the best approximation in the D- and E-regions of the ionosphere [53] and 2) the Appleton-Hartree formulation which is more consistent with the theory in the F-region of the ionosphere [54].

Either ray trace formulation can be used to calculate the ionospheric absorption if the cor- rect collision frequencies are utilized. Another frequently overlooked aspect of the attenuation calculation is the variation in the electron-neutral and electron-ion collision frequencies as a function of local time, season, latitude and solar cycle. This variation results in diﬀer- ences on the order of 30% in the total ionospheric attenuation and should be included in absorption calculations. In this study, the Appleton-Hartree formulation is used to calculate the ionospheric absorption.

The total ionospheric absorption in decibels can be described by the integral of the imaginary part of the complex propagation function over the distance along a path as [54] Z La = −8.68 κ ds, (8) where ds is the distance along the path and κ is the imaginary part of the complex propagation function, k, which is defined as k = 2πfn/c where f is the input wave frequency, c is the speed of light in a vacuum and n is the complex index of refraction n = µ + iχ, (9) where µ is the real part of the index of refraction and χ is the imaginary part of the index of refraction. The approximation of the absorption coefficient, κ, is defined by [55] e2 1 n v κ = e e , (10) 2 2 20mec µ ve + (2πf ± ωecf cos θ) where ve is the electron collision frequency, ωecf is the electron cyclotron frequency and θ is the angle between the direction of propagation and the Earth’s magnetic field, similar to the one applied in the ray-tracing technique.

2.3.1 Electron Collision Frequency

The third factor contributing to the Appleton-Hartree formula is a dimensionless quantity, Z, which depends on the electron collision frequency. As HF radio waves propagate through

8 DRDC-RDDC-2019-R208 the ionosphere, the ions remain effectively stationary due to their mass, but the lighter electrons respond to the oscillating electromagnetic field. Since the electrons collide with both ions and neutral particles, it is necessary to consider both the electron-neutral ven collisions as well as electron-ion vei collisions. The total electron collision frequency is defined as the sum of these two components [55],

ve = ven + vei. (11)

The electron-neutral collision frequency dominates at low altitudes (up to about 120 km), and the electron-ion collision frequency dominates at higher altitudes. The electron-ion collision frequency can be reliably calculated from theoretical considerations, as the potential function (Coulomb) for electron-ion collisions is well-defined. The equation for the effective election-ion collision frequency, as described in Schunk and Nagy [1978], is defined as [56],

1 22 4(2π) 2 zie ln Λ vei = ni 1 3 , (12) 3 2 2 me (kBTe) where zi is the charge number of the ion, kB is the Boltzmann constant, Te is the electron temperature, ni is the density of the ions and ln Λ is deﬁned as

 1  2 2 2 2 2 4kBTe ke + ki ki + ke ln Λ = ln 2 2 − 1 ln  , (13) γ zie ke 2 ke me

2 2 2 2 4πnizi e 2 4πnee ki = , ke = , (14) kBTi kBTe where Ti and ni are the ion temperature and the ion density respectively and γ is the Euler-Mascheroni constant (γ ≈ 0.5572). Electron-neutral ven collision frequency is deﬁned as the summation of frequencies of a number of neutral particles [55],

ven = veN2 + veO2 + veO + veHe + veH , (15) where, −11 −4 veN2 = 2.33 × 10 n (N2) 1 − 1.21 × 10 Te Te, (16)

1 1 −10 −2 2 2 veO2 = 1.82 × 10 n (O2) 1 + 3.6 × 10 Te Te , (17)

−11 −4 1 veO = 8.9 × 10 n (O) 1 + 5.7 × 10 Te Te 2 , (18)

−10 1 veHe = 4.6 × 10 n (He) Te 2 , (19) −9 −4 1 veH = 4.5 × 10 n (H) 1 − 1.35 × 10 Te Te 2 , (20)

3 where n is the density per cm of the neutral particle. Below 120 km, N2 is the dominant neutral particle in the electron-neutral collision frequency calculation. Another frequently overlooked aspect of the total electron collision frequency calculation is the variation in the electron-neutral and electron-ion collision frequencies as a function of latitude, longitude,

DRDC-RDDC-2019-R208 9 local time, season and solar cycle. This variation results in diﬀerences on the order of 30% in the total electron collision frequency calculations [55]. In order to calculate the total electron collision frequency ve as a function of local time, season, latitude, longitude, and solar cycle, in addition to altitude, the neutral density from NRLMSISE-00 model and the electron density, ion density, electron temperature and ion temperature from IRI-2016 were used [23]. NRLMSISE-00 is an empirical model of the atmosphere based on mass spectrometer and incoherent scatter radar data as well as atmospheric drag data from the trajectories of satellites [25]. It describes the atmospheric densities and temperatures from the ground to the base of the exosphere. The total electron collision frequency ve is used to calculate the Z quantity in Equation (1) for the simulations analyzed in this report.

Ray-tracing simulations were run by performing parameter sweeps across frequencies from 3-30 MHz with a step size of 0.1 MHz, and for elevation angles from 3-60◦ with a step size of 0.5◦.

10 DRDC-RDDC-2019-R208 Figure 3: A plot of the sunspot number as a function of time. The sunspot number for the years 2014 and 2018 are indicated by black dots. These have been calculated using the 2016 version of the International Reference Ionosphere (IRI-2016) program.

3 Monthly Variation

The variation in available radar frequency, elevation angle and absorption is observed as a function of local time (daytime 1500 LT, i.e. 2000 UT, and nighttime 0300 LT, i.e. 0800 UT), month and recent solar cycle (solar maximum in 2014 and minimum in 2018). The eﬀects of the solar cycle are controlled in this study by simulating the years 2014 and 2018, which are respectively the most recent local solar maximum and minimum as evidenced in Figure 3. The transmission location Ottawa, Ontario (45◦N, 76◦W) and target location Hall Beach, Nunavut (69◦N, 82◦W) were selected in Canada. The distance is approximately 2700 km. Ray-tracing simulations were run for frequencies 3 to 22 MHz with a step size of 0.1 MHz, and for elevation angles from 3◦ to 60◦ with a step size of 0.5 degrees.

Figure 4 illustrates the available radar frequencies as a function of month for 2014 and 2018. The reflection height of rays is given in the color bar. During the winter daytime, the solar minimum year 2018 manages to reflect a wider range of frequencies compared to the solar maximum year 2014, while during winter nighttime, the solar maximum year 2014 is able to cover a greater range of frequencies than the solar minimum year 2018. During the summer, the range of frequencies are somewhat similar in both solar maximum and minimum years. There are 2 major frequency ranges of reflection during the nighttime of

DRDC-RDDC-2019-R208 11 Figure 4: Available radar frequencies as a function of month for 2014 and 2018. The reflection height of the rays is given in the color bar. The IRI-2016 [8], EMM-2017 [10], and NRLMSISE-00 [16] empirical models were used. each year: a range from 3-13 MHz during the winter and fall of a solar maximum, a range from 6-15 MHz during the summer and spring of a solar maximum, a range from 3-8 MHz for the winter and fall of a solar minimum and a range of 8-12 MHz for the summer and spring of a solar minimum, where the higher range of frequencies are reflected at higher altitudes, while the lower frequency ranges are reflected at lower altitudes, corresponding to the altitudes of the different layers in the ionosphere.

During the daytime lower frequencies are not available throughout both years due to the effect of D-layer absorption and its dominant presence during the daytime. The frequency range during the winter daytime varies between 8-22 MHz in 2018 and 10-22 MHz in 2014, whereas during the summer daytime it covers a shorter range of frequencies varying between 14-22 MHz in 2018 and 15-22 MHz in 2014. There also is a trend in the absolute numbers of rays reflected by the ionosphere, where the maximum solar cycle year always dominates in the range of frequencies it is able to reflect compared to the solar minimum year. This domination is due to the existence of multiple prevalent ionospheric layers that reflect various frequencies, whereas at a solar minimum, the general density of the layers are much lower, while the D- and E-layers are significantly lower. During the winter daytime, the solar minimum year 2018 reflects higher frequencies (18-22MHz) at higher altitudes than present in 2014, while reflecting the same frequencies at much lower altitudes in 2018 than 2014 during the summer. The X-mode covers about 1-2 MHz higher frequencies than O-mode frequencies. Such trends and relationships may be of use for determining future relations for OTHR applications.

12 DRDC-RDDC-2019-R208 Figure 5: Available elevation angles as a function of month for 2014 and 2018. The reﬂection height of the rays is given in the color bar. The IRI-2016 [8], EMM-2017 [10], and NRLMSISE-00 [16] empirical models were used.

Figure 5 illustrates available elevation angle as a function of month for 2014 and 2018. At the solar minimum in 2018 during the daytime (2000 UT) of the winter, reflected rays cover higher altitudes as well as a greater spread of elevation angles, ranging from 7.5 to 23.5 degrees, in comparison to the ones available in 2014 at the solar maximum, which covered a range of 6 to 12 degrees. The results also displayed a similar range of elevation angles during the summer of both years, ranging between 5 and 24 degrees. In this case, the maximum altitudes during the daytime of the winter months in 2018 reach 280 km while the ones in 2014 reach a maximum of 180 km. These observations illustrate the weak existence of D- and E-layers in the solar minima due to a lack of ionization, which results in the F- layer being the main source of reflection during the daytime of the winter in solar minima. During the solar maxima, the D- and E-layers are still prevalent, resulting in lower altitude reflections as well as some higher reflections if the rays manage to penetrate through.

During the nighttime throughout the year, both solar limits seem to display similar height profiles with no significant differences, exhibiting elevation angles ranging from 3 to 20 degrees, with the solar maximum year having a slightly more frequent presence in the higher bounds of this range of elevation than the solar minimum. This occurred as a result of the domination of the E- and F-layers during the nighttime in terms of ionospheric density.

Figure 6 illustrates absorption of the ray path as a function of month for 2014 and 2018. The daytime throughout both years displays a higher absorption, ranging between 3 to 30 dB, than the nighttime, varying between 0.7 and 16.5 dB. Furthermore, a general relationship

DRDC-RDDC-2019-R208 13 Figure 6: Absorption as a function of month for 2014 and 2018. The reﬂection height of the rays is given in the color bar. The IRI-2016 [8], EMM-2017 [10], and NRLMSISE-00 [16] empirical models were used. was observed during both years where the absorption values during the summer (ﬂuctuating between 9.7 and 27 dB in the daytime of 2014, 11 and 23 dB in the daytime of 2018, 2.5 and 9.6 dB for the nighttime of 2014, 4.5 and 13 dB in the nighttime of 2018) are larger than in the winter months (varying from 6.6 to 20 dB in the daytime of 2014, 3 to 23 dB in the daytime of 2018, 0.7 to 10 dB in the nighttime of 2015, 0.9 to 6.5 dB in the nighttime of 2018), with the exception of the winter nighttime of 2014. Higher absorption values were also noticed to occur at lower altitudes during both years throughout all seasons as a result of lower ionospheric layers.

14 DRDC-RDDC-2019-R208 4 Seasonal Variation

The goal of this ray-tracing simulation study is to compare the differences in available frequencies and elevation angles between short-range (up to 1300 km away from the transmitter) and long-range (up to 2700 km away from the transmitter) targets for northward-facing over-the-horizon radar while factoring the effects of differences in local time (LT) (daytime at 1500 LT, i.e. 2000 universal time (UT), and nighttime at 0300 LT, i.e. 0800 UT), season (winter, spring, summer, and fall), and the solar cycle (2014 solar maximum and 2018 solar minimum). For each year, simulations were ran for the 15th day of every month, and the results were grouped by season. Finally, for each day selected, ray-tracing calculations were done for both nighttime (0800 UT) and daytime (2000 UT). By controlling for the time of day, the extent of the effects of solar illumination may be accounted for in ionospheric radar propagation by comparing the daytime versus nighttime results of the same simulation. The ray-tracer described in this section has been used to perform simulations for each combination of these factors across the target location, the time of day, the season, and the year of the solar cycle. Although the study was performed for both ordinary- and extraordinary- modes, the results presented in this report are only for the ordinary-mode signals. While there are some differences between both modes, they generally exhibit similar behavior. In the Results and Discussion section, the outcomes of these simulations are given as a series of plots and tables which are then compared. All these simulations use the same transmission site location, which was selected to be Ottawa, Ontario (45◦N, 76◦W). Two different target locations in Canada were considered: 1) a short-range target, which was chosen to be Peawanuck, Ontario (55oN, 85oW) and 2) a long-range target, which was chosen to be Hall Beach, Nunavut (69◦N, 82◦W). The distance is approximately 1300 km between Ottawa and Peawanuck and 2700 km between Ottawa and Hall Beach. Rays landing within 10 km of the target were considered close to the target and their corresponding frequency and initial elevation angle were considered feasible parameters when used together for OTHR deployment within the circumstances described by the simulation. Simulations were run for each target, with the transmitter pointed azimuthally towards the destination site while varying the radar parameters, local time, season, and year.

Figure 7 is a plot of the feasible frequency-elevation angle profile for the short-range target across the various seasons of 2014 at nighttime. Figure 8 is the same plot but under the conditions of daytime. Figures 9, and 10 are analogous to Figures 7 and 8, but for the year 2018 instead. Finally, Figures 11, 12, 13, and 14 present the same classes of data, but performed for the long-range target. These scatter plots predict how a viable OTHR system would change its operating parameters in order to maintain illumination of the desired target while the ionospheric conditions and the target location change. Such a system would have to select parameters within the plotted region for radar signals to land sufficiently close to the target. Furthermore, the maximum height that each signal achieves above the Earth’s surface is indicated by the color of plot. From these colors, the ionospheric layer that each ray reflected against can be deduced, depending on which layer corresponds to the height depicted.

DRDC-RDDC-2019-R208 15 Figure 7: Seasonal scatter plots of feasible radar parameters for the short-range target during 2014 at nighttime.

The characteristic feasible parameter proﬁle can be seen from each scatter plot. The resulting distribution of feasible radar parameters is largely continuous with very few discrete outliers. Hence, over-the-horizon radars must operate within these distinct regions in order to illuminate their desired target, and outside these areas the radar system will fail to maintain illumination of its target and lose track of the desired object. Understanding the characteristics of these regions and how they evolve over time is thus necessary for functional polar over-the-horizon radar deployments. In particular, Figure 8 is exemplary of the distinct features common to each plot. Such a proﬁle generally has two parts: 1) a primary crescent-shaped region which opens concave to the upper-left of the graph towards lower frequencies and higher elevation angles and 2) a lower horizontal band that spans a narrower range of lower elevation angles but a wider spectrum of frequencies. This band tends to have a hook-shaped region at its higher frequency terminal towards the right which terminates at higher elevation angles.

Given the two features for the profile of feasible parameters, the lower band region is found to be entirely dark blue in the plot for its height profile, indicating that the rays for systems configured in this way will reflect at heights near 90 km in altitude, which is the within the

16 DRDC-RDDC-2019-R208 Figure 8: Seasonal scatter plots of feasible radar parameters for the short-range target during 2014 at daytime.

D-region. This implies that the rays for this set of radar configurations will successfully illuminate the target by reflecting off the D-layer. This region of configurations is also the region which takes the lowest elevation angles, and thus for a radar system to illuminate a target using the most shallow elevation angle possible, the D-layer must be used.

For the other configuration profile which takes a crescent shape and which is based at higher elevation angles, unlike the lower blue band, the crescent region has a diverse height profile as indicated by its variety of colors in the figures. This sickle-shaped region has two arms which project towards the upper-left near higher elevation angles and lower frequencies. Each arm is colored differently: The left arm is colored a light blue indicating ray reflections at heights of around 120 km, while the right arm is colored orange, indicating ray reflections at heights of around 240 km. Thus, the left arm serves as a profile for configurations which reflect off the E-region and illuminate the target, while the right arm provides the profile for rays which reflect off the F-region and illuminate the target. The gap in elevation angles which fail to illuminate the target indicate the discreteness of the layers: Fixing the frequency while increasing the elevation angle generally will cause the radar signal to lose track of the target, since it still reflects the rays at the same altitude, since it reflects off the same layer.

DRDC-RDDC-2019-R208 17 Figure 9: Seasonal scatter plots of feasible radar parameters for the short-range target during 2018 at nighttime.

Only at sufficiently high elevation angles will the ray be able to break through the current ionosphere layer at which it reflects, as increasing the angle implies that the ray will spend less time in a given ionosphere layer and thus refract less. When the refraction becomes weak enough, the ray will not longer undergo reflection and thus escape into the next ionosphere layer. At this new higher layer, a steeper elevation angle will be necessary to continue illuminating the target. Thus, higher layers will generally need steeper elevation angles in order to illuminate the same target as was illuminated by using lower layers. This is exhibited in the figures in which the profiles generally shift from blue to orange (or from lower reflection heights to higher reflection heights, and thus to higher ionospheric layers) as elevation angle increases.

In general, the lower the elevation angle, the bluer the plot becomes. This reflects the correlation between lower elevation angles and reflection off lower regions of the ionosphere. These two parameters are combined to offset each other and keep a constant landing distance of the ray, since achieving lower maximum altitudes before ionospheric reflection limits the ray’s ability to navigate over obstructions and prevents the ray from exploiting the effects of Earth’s curvature over long distances.

18 DRDC-RDDC-2019-R208 Figure 10: Seasonal scatter plots of feasible radar parameters for the short-range target during 2018 at daytime.

However, decreasing the elevation angle allows the ray to travel further along the Earth, assuming that the ray will achieve roughly the same height regardless of the elevation angle chosen before reflecting back down to Earth. Conversely, there is a correlation between higher elevation angles and higher reflection heights. This is observed in the plots by the regions becoming more yellow as the elevation angle increases. Similar to the previous explanation, the higher elevation angle reduces the range, which is offset by the higher reflection height, thus achieving essentially constant ground distance. Thus, the simulations indicate that the changing elevation angle in response to changes in the reflection height is effective in maintaining target illumination for OTHR.

In most plots shown, the lower band extends fully to frequencies smaller than 3 MHz, the minimum frequency considered for this study. In all cases, this band is generated at signiﬁcantly lower altitudes (typically 80-120 km) than the upper-crescent area (typically 160-260 km). This corresponds roughly to the distinction between the D- and E-regions versus the F-region, where the lower band is present in both the D- and E-layers, while the upper crescent is present in the F-layer. In Figures 8 and 10, during the daytime for the short-range target, the dark-blue lower band region can be seen spanning a wide frequency

DRDC-RDDC-2019-R208 19 Figure 11: Seasonal scatter plots of feasible radar parameters for the long-range target during 2014 at nighttime. range of roughly 3-20 MHz. In Figures 7 and 9, this band is largely absent, but it can be seen during the summer at low frequencies between 3-4 MHz, suggesting that it has undergone a frequency shift to lower values. For Figures 11, 12, 13, and 14, there is a complete absence of this region which may indicate that reflection off the lower ionospheric layers does not achieve sufficient accuracy to be viable for long ranges, even under similar local times.

Furthermore, the increased distance traveled northwards into the Arctic means that solar illumination levels will be weaker and hence the D-region in particular may be weaker. Therefore, the lower band of viable radar parameters may only be available during the daytime under conditions strong enough to ionize the D-layer by solar illumination.

For the upper region of available parameters which resembles a slanted sickle-shaped region, Figures 8 and 10 show that this area is fully observed during the day across the entire frequency range and nearly the entire elevation angle range considered for this study (up to about 50◦ maximum). However, this region undergoes a shift towards lower frequencies (leftwards in the ﬁgures) when going from daytime to nighttime propagation. This frequency shift is evidenced when comparing Figures 7 and 8, or Figures 9 and 10. Furthermore,

20 DRDC-RDDC-2019-R208 Figure 12: Seasonal scatter plots of feasible radar parameters for the long-range target during 2014 at daytime. this shape can vary in its width, in which the ﬁgure becomes compressed or stretched in frequency depending on the season. This pattern can be strongly seen in Figures 8 and 10 in which the available region is generally stretched the furthest around winter and compressed the most strongly in the summer, with fall and spring taking on intermediate shapes.

A series of tables are given detailing various statistics about the plots. Table 1 details the available ranges of feasible radar parameters for each combination of factors considered in this study, namely the target range, time-of-day, season, and year. Table 2 shows the average feasible frequency and elevation angle that each parameter set centralizes around. Table 3 provides these same averages, but taken over all seasons and presented with respect to the entire year. Finally, Table 4 represents the number of available frequency-elevation parameter combinations found in each category averaged over all seasons. From the Figures, within a given year for a specific target, the total number of the points in each profile across seasons is largely the same, hence the table examines only differences due to year and target location and not season. Thus, from Table 4, there is a peak in the number of feasible radar parameters (7324) in the daytime in 2014 when considering the short-range target, whereas

DRDC-RDDC-2019-R208 21 Figure 13: Seasonal scatter plots of feasible radar parameters for the long-range target during 2018 at nighttime. the radar conﬁguration is highly restricted with the least number of available parameters (392) when considering long-range targets in 2018 at daytime.

Comparing the long-range plots to the short-range plots, we that Figures 11 and 13 closely resemble their corresponding short-range plots given in Figures 7 and 9. The most notable diﬀerence is a shift downwards around 17-18◦ on average towards lower elevation angles when considering the long-range target over the short-range target, as evidenced by Table 3. In both cases, the nighttime regions move slowly each month so that the plot for a given season over its corresponding three months appears to be a seemingly continuous region, in which the individual months can only be distinguished by the breaks in coloration.

The peaks separate into three distinct curves in some cases, as evidenced in Figure 8. These three regions correspond to each sample taken on the 15th day for each of the three months of a given season. However, in the daytime, the individual months within a season separate more strongly, leading to the appearance of multiple peaks or crescent-shaped regions which overlap in diﬀerent areas, and otherwise are separated by distinct gaps. This suggests that daytime regions are more variable than their nighttime counterparts on a monthly basis. Hence, a radar operator would have to make stronger adjustments for daytime operation

22 DRDC-RDDC-2019-R208 Figure 14: Seasonal scatter plots of feasible radar parameters for the long-range target during 2018 at daytime. than nighttime operation, and take stronger consideration of longer time-scale variations in the ionosphere. Otherwise, fixing the daytime parameters could lead to system failure in later months should the region undergo a strong shift. Similar effects do occur for the nighttime, but they are much weaker and have significant overlap. However, the lower band that appears at shorter ranges during the day could help higher elevation angles during night.

Furthermore, the distinct lack of overlap between day and night regions across all plots suggests that the radar operator would always have to adjust the frequency of the radar system on daily basis, switching to lower frequencies at night and to higher frequencies during the day.

At nighttime, winter consistently achieves significantly high reflection heights in the ionosphere than during than other seasons. The higher altitudes achieved by radar signals during the winter night before being reflected back to the Earth’s surface could increase the great-circle distance traversed and assist in long distance propagation.

From Table 4, daytime is found consistently to oﬀer a smaller available region of radar parameters than found during nighttime when considering the long-range target. However,

DRDC-RDDC-2019-R208 23 Table 1: Available Parameter Ranges Season Frequency Elevation (MHz) (degrees) Range Short Long Short Long 2014 Winter 3.0-8.9 3.0-13.7 4.5-48.0 3.0-20.0 Night Spring 3.0-9.7 3.3-14.9 3.5-47.0 3.0-20.5 Summer 3.0-10.0 6.7-15.8 3.0-46.5 3.0-19.5 Fall 3.0-9.6 3.0-13.9 4.5-48.5 3.0-20.0 2014 Winter 3.0-26.9 10.4-30.0 3.0-47.0 5.5-20.5 Day Spring 3.0-22.4 16.8-30.0 3.0-53.5 4.0-26.0 Summer 3.0-22.2 16.9-25.2 3.0-50.5 5.0-24.5 Fall 3.0-29.3 12.6-30.0 3.0-50.5 4.0-25.5 2018 Winter 3.0-6.0 3.0-8.9 15.5-44.5 3.0-18.5 Night Spring 3.0-6.1 3.0-10.3 12.0-41.0 3.0-20.5 Summer 3.0-6.6 5.5-11.6 3.0-46.0 3.0-15.0 Fall 3.0-5.8 3.0-8.5 17.0-52.5 3.0-14.5 2018 Winter 3.0-16.7 8.6-21.3 3.0-47.5 7.5-24.5 Day Spring 3.0-19.2 13.3-22.9 3.0-58.5 4.0-22.0 Summer 3.0-19.5 12.7-22.3 3.0-47.0 4.5-23.0 Fall 3.0-17.5 10.5-22.2 3.0-50.0 8.0-18.5 the trend is completely reversed for the short-range target. In particular, the short-range target has a band of frequencies for low elevation angles that are able to reach near the target. Hence, the onset of the D-layer during the day is useful when considering shorter range targets, but otherwise may lack practical use for long-range propagation.

Radar operators may have to make the fewest adjustments to the radar operating parameters by choosing the base of the upper crescent region as the configuration. In this way, should there be any shifts or deformations that occur over a period of months, radar parameters will still generally not fall into any of the resulting gaps that form for viable parameters. Furthermore, located immediately beneath this region is the horizontal band of low-elevation angle settings near the terminal hook where the band is generally thickest. Consequently, during daytime for short-range targets, radar operators will also be able to take advantage of reflection off the D-region by lowering the elevation angle appropriately with little change to the frequency. The base area of the upper crescent region also generally offers the most flexibility in radar parameters: it has the most even distribution in elevation angle and frequency which is also among the greatest, ignoring the lower elevation angle horizontal band, which restricts the angle a radar can operate, and the peaked areas, which allow very little variation in frequency.

Thus, operating in the upper base region should give radar operators the most time on average, considering both frequency and elevation angle to be equally important parameters, to respond to diurnal or range-based parameter shifts. Generally, changes in the season or

24 DRDC-RDDC-2019-R208 Table 2: Seasonal Average Feasible Parameters Season Mean Frequency Mean Elevation (MHz) (degrees) Range Short Long Short Long 2014 Winter 6.04 9.19 23.64 5.76 Night Spring 6.53 11.85 22.74 6.47 Summer 6.53 13.25 21.04 6.09 Fall 6.44 9.03 24.31 6.92 2014 Winter 16.76 24.81 15.30 7.94 Day Spring 14.98 24.91 14.33 9.85 Summer 13.77 22.21 12.97 10.83 Fall 17.04 24.64 14.89 8.13 2018 Winter 4.29 6.24 22.73 4.89 Night Spring 4.32 7.66 20.66 5.58 Summer 4.61 9.78 23.58 5.20 Fall 4.24 5.77 23.50 5.31 2018 Winter 11.10 17.73 14.03 11.16 Day Spring 11.68 19.10 11.55 9.56 Summer 11.67 18.40 10.48 10.08 Fall 11.50 18.32 13.27 10.20 year (factoring solar activity changes) do not affect the location of the center of this region, and thus radar operators should be able to largely ignore these effects by choosing the upper base location. From the plots, this region may be identified for the short-range target to occur within 15-20 MHz in frequency and 18-28◦ in elevation angle during the daytime, and within 3-8 MHz in frequency and 17-25◦ in elevation angle during the nighttime. For the long-range target, this region occurs from roughly 5-10 MHz in frequency and from 3-10◦ in elevation angle for the nighttime. In daytime, this region becomes 18-22 MHz in frequency and 9-11◦ in elevation angle. From these observations we can further remark that it becomes increasingly difficult to operate an over-the-horizon radar at longer distances, as the range of viable parameters drops off. This is observed strongly in Figures 12 and 14 by the thinning of the crescent regions into narrow curves.

Seasons largely have similar characteristics and ranges when ﬁxing the year, target, and time of day. However, in the daytime, the deformation of the upper crescent-shaped region changes based on which group of seasons in data falls under. Generally, fall and winter will spread in the frequency range covered, while spring and summer will lengthen in the elevation angles covered. This can be most clearly witnessed in Figures 8 and 10 for the short-range target. The eﬀects are most pronounced when comparing the summer and winter. Additionally, from Figure 8 and 10, the lower elevation horizontal band that appears during the day is shifted towards lower frequencies during the fall and winter, but shifts towards higher frequencies in the spring and summer.

DRDC-RDDC-2019-R208 25 Table 3: Annual Average Feasible Parameters Mean Frequency Mean Elevation Angle (MHz) (degrees) Range Short Long Short Long 2014 Night 6.42 10.44 22.83 6.35 2014 Day 15.84 24.12 14.51 9.43 2018 Night 4.39 7.06 22.21 5.24 2018 Day 11.48 18.41 12.41 10.23

Table 4: Annual Average Feasible Conﬁgurations Count Average Feasible Conﬁgurations per Season Range Short Long 2014 Night 3027 1237 2014 Day 7324 590 2018 Night 1306 581 2018 Day 4541 392

In Table 3 for both the short-range and long-range targets, regardless of the year, the desired elevation angle is roughly 23◦ during the night and 10◦ during the day. This is to be expected, as the emergence of the D-layer means that the radar signal will be reflected at lower altitudes and so a shallower elevation angle is needed to achieve the same distance. When considering frequency for the short-range target, an expected frequency of roughly 5 MHz is used at nighttime, while at day-time an expected frequency of about 11 MHz is used. Between different years, the expected elevation angle varies by less than a degree, while the expected frequency varies by less than 1 MHz. A similar pattern is found in the long-range target: during the nighttime, an elevation angle of around 6◦ is needed while during the day, an elevation angle of 10◦ is needed. At nighttime, roughly 8.5 MHz is expected while during the day, 19 MHz is the expected viable frequency. Between different years, the elevation angles vary by about 2◦, while the frequencies vary by roughly 2 MHz. Between the short-range and long-range statistics, the expected elevation angle can differ by 18◦, while the expected frequency can differ by over 8 MHz.

Table 4 gives the number of available radar configurations each year possesses on average for a given season as it varies between daytime and nighttime. For the short-range target, in Table 4, there there is a consistent increase in the number of parameters during the daytime, indicating there is a greater choice of radar parameters. Thus, it should be easier in this case to operate an over-the-horizon radar assuming the desired parameters are located sufficiently near the expected radar parameters. The trend is reversed when considering the long-range target, in which there are more possible configurations during nighttime. This is to be expected, since the appearance of the D-layer during the daytime means that radar signals will experience refraction at lower altitudes and hence the range of the radar will be reduced. During the nighttime, this layer weakens, and signals are able to obtain

26 DRDC-RDDC-2019-R208 higher altitudes before being reﬂected back towards the ground. This increase in the feasible area in parameter space may be due to obtaining favorable elevation angles, as in principle, one could simply attempt to increase or decrease the elevation angle in the absence of the D-layer to make contact with the ionosphere and yet still be aligned with the target.

DRDC-RDDC-2019-R208 27 Figure 15: Time as a function of usable radar frequencies for monitoring the ﬂight of a Boeing 777. The time intervals portrayed, beginning 04:35 and 16:35, start at the ground distance of 1170 km for the target and increase gradually by 450 km at every 30 min interval until reaching the ﬁnal target distance of 3000 km at 06:35 and 18:35. The IRI-2016 [8], EMM-2017 [10], and NRLMSISE-00 [16] empirical models were used.

5 A Case Study

In this study, we observed the path of a Boeing 777 flight from New York to Beijing during the summer (August) and winter (January) daytime and nighttime of 2017, at 30-minute intervals, between the ground ranges of 1170 km (55◦N, 77◦W) and 3000 km (71◦N, 94◦W), at a constant altitude of 9.1 km, to determine frequency availability and its corresponding properties. Time intervals portrayed, starting at 04:35 and 16:35, start at the ground distance of 1170 km and increase gradually by 450 km at every 30-minute interval until reaching the final distance of 3000 km at 06:35 and 18:35. In Figure 15, a larger range of frequencies, 3-22 MHz, is covered during the daytime in both winter and summer than that of the nighttime, covering 3-10 MHz in the winter and 3-12 MHz in the summer. Comparing the two seasons, the winter nighttime covers a shorter range of frequencies at a lower frequency value than that of the summer nighttime, largely due to ionization level differences between the summer and winter. When observing reflection heights, during the winter nighttime, the ray altitudes are much higher due to the dominant higher F-layers and a weaker existence of lower D- and E-layers during such times, while during the daytime, there are more prevailing lower reflection heights due to the dominance of the D- and E- layers during the increased ionization levels of the daytime. As frequency increases, the reflection altitude of the ray increases corresponding to higher ionosphere layers, with the

28 DRDC-RDDC-2019-R208 exception of certain times, including 16:35 LT and 17:05 LT, where the higher frequencies happen to reﬂect at lower altitudes corresponding to lower D- and E-layers. Figure 15 suggests that the larger the ground range, the higher the radar frequency can be selected to detect the target.

DRDC-RDDC-2019-R208 29 6 Conclusion

DRDC Ottawa is developing an experimental OTHR with the potential for surveillance of Canada. Because of dynamically ionospheric conditions, the OTHR parameters such as transmission carrier frequency and elevation angle need to be adjusted regularly to maintain constant illumination of targets downrange. In this study, the feasible OTHR operating frequencies and elevation angles are determined using three-dimensional ray-tracing simulations, taking into account the effects of electron density using the IRI-2016 model, the geomagnetic field with the 2017 EMM model, and electron-particle collisions using NMRLSISE-2000. This study is then controlled for target distance (long- and short-range), time of day (daytime and nighttime), month, seasonality (winter, spring, summer and fall) and solar activity (solar maximum and minimum). A variety of patterns are described which arise in the resulting profiles of the feasible parameters which are determined.

• Monthly Variation: During the nighttime, there are two major frequency ranges of reflection during the nighttime: a range from 3-13 MHz during the winter and fall in 2014, a range from 6-15 MHz during the summer and spring in 2014, a range from 3-8 MHz for the winter and fall in 2018 and a range of 8-12 MHz for the summer and spring in 2018, where the higher range of frequencies are reflected at higher altitudes, while the lower frequency ranges are reflected at lower altitudes, corresponding to the altitudes of the different layers in the ionosphere. During the daytime, lower frequencies are not available throughout both years due to the effect of D-layer absorption and its dominant presence during the daytime. The frequency range during the winter daytime varies between 8-22 MHz in 2018 and 10-22 MHz in 2014, whereas during the summer daytime it covers a shorter range of frequencies varying between 14-22 MHz in 2018 and 15-22 MHz in 2014. Available elevation angles varies during the daytime and nighttime in 2014 and 2018. In 2018 during the daytime of the winter, reflected rays cover higher altitudes as well as a greater spread of elevation angles, ranging from 7.5 to 23.5 degrees, in comparison to the ones available in 2014, which covered a range of 6 to 12 degrees. The results also displayed a similar range of elevation angles during the summer of both years, ranging between 5 and 24 degrees. In this case, the maximum altitudes during the daytime of the winter months in 2018 reach 280 km while the ones in 2014 reach a maximum of 180 km. These observations illustrate the weak existence of D- and E-layers in the solar minima due to a lack of ionization, which results in the Flayer being the main source of reflection during the daytime of the winter in solar minima. The absorption varies during the daytime and nighttime in 2014 and 2018. The daytime throughout both years displays a higher absorption, ranging between 3 to 30 dB, than the nighttime, varying between 0.7 and 16.5 dB. Furthermore, a general relationship was observed during both years where the absorption values during the summer (fluctuating between 9.7 and 27 dB in the daytime of 2014, 11 and 23 dB in the daytime of 2018, 2.5 and 9.6 dB for the nighttime of 2014, 4.5 and 13 dB in the nighttime of 2018) are larger than in the winter months (varying from 6.6 to 20 dB in

30 DRDC-RDDC-2019-R208 the daytime of 2014, 3 to 23 dB in the daytime of 2018, 0.7 to 10 dB in the nighttime of 2015, 0.9 to 6.5 dB in the nighttime of 2018), with the exception of the winter nighttime of 2014. Higher absorption values were also noticed to occur at lower altitudes during both years throughout all seasons as a result of lower ionospheric layers.

• Seasonal Variation: A characteristic profile of the feasible parameters is obtained across daytime and nighttime conditions, the local solar minimum and maximum, the seasons, and across different target ranges. Such a profile contains an upper region shaped as a crescent with two terminals and opens towards lower frequencies and higher elevation angles. A separate band existing of a low elevation angles appears during the daytime for short-range targets. These regions shift to lower frequencies during the solar minimum, and they shift to lower elevation angles as target distance increases. The ideal region for operating is located within the base area of the crescent-shaped region of the profile, as it exhibits the lowest variability over the factors examined in this study. This region is roughly estimated to be 15-20 MHz in frequency range and 18-28◦ in elevation angle for the short-range target during the day, shifting to 3-8 MHz and 17-25◦ at night. For the long-range target this is 18-22 MHz and 9-11◦ during the day, and 5-10 MHz and 3-10◦ at night.

• A Case Study: In a case study, we observed the path of a Boeing 777 ﬂight from New York to Beijing during the summer (August) and winter (January) daytime and nighttime of 2017. The study suggests that the larger the ground range, the higher the radar frequency can be selected to detect the target.

The dominant factor contributing to the Appleton-Hartree formula is a dimensionless quantity X which depends on electron density. Accurate electron density is necessary for the purpose of improving operational OTHR and OTHR planning/design. The high-latitude and polar ionosphere pose signiﬁcant challenges for empirical modeling through its highly dynamic nature, via coupling with the interplanetary magnetic ﬁeld, and the scarcity, and traditionally poor quality of data in these regions. International standards, such as IRI, have been repeatedly shown to perform poorly at high latitudes. Several ionosondes have currently been scheduled to be installed in high-latitude regions to overcome this short- coming. DRDC is now developing the Empirical Canadian High Arctic Ionosphere Model (E-CHAIM) which is designed to replace the use of IRI in its use at high-latitudes that will make use of all available high-latitude electron density observations. The E-CHAIM model will constitute a substantial improvement over existing standards. The ECHAIM model can be used to investigate system planning for future developments of OTHR systems to the Canadian Arctic. By having a more realistic climatology, system planners can more heavily rely on simulations to aid OTHR system design, thus reducing the investment risk in exploratory/experimental OTHR systems.

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36 DRDC-RDDC-2019-R208 List of Symbols/Abbreviations/Acronyms/Initialisms

COSPAR Committee on Space Research CPI Coherent Pulse Integration DND Department of National Defence DRDC Defence Research and Development Canada E-CHAIM Empirical Canadian High Arctic Ionospheric Model EMM Enhanced Magnetic Model FMS Frequency Monitoring System HF High Frequency HFSWR High-Frequency Surface-Wave Radar IRI International Reference Ionosphere JORN Jindalee Operational Radar Network LT Local Time NRLMSISE-00 Naval Research Laboratory’s Mass Spectrometer Incoherent Scatter Extension 2000 NRT Numerical Ray-Tracing OTHR Over-the-Horizon Radar PRF Pulse Repetition Frequency RANGER RAdars for loNG distance maritime surveillancE and SaR opeRations URSI International Union of Radio Science UT Universal Time WERA Wellen Radar WMM World Magnetic Model

DRDC-RDDC-2019-R208 37 DOCUMENT CONTROL DATA *Security markings for the title, authors, abstract and keywords must be entered when the document is sensitive 1. ORIGINATOR (Name and address of the organization preparing the 2a. SECURITY MARKING (Overall security marking of document. A DRDC Centre sponsoring a contractor’s report, or a the document, including supplemental markings if tasking agency, is entered in Section 8.) applicable.) DRDC – Ottawa Research Centre CAN UNCLASSIFIED 3701 Carling Avenue, Ottawa ON K1A 0Z4, Canada 2b. CONTROLLED GOODS NON-CONTROLLED GOODS DMC A

3. TITLE (The document title and sub-title as indicated on the title page.) Extracting Radar Parameters for Sky-Wave Over-The-Horizon Radar using Ray-Tracing through a Model of Ionosphere

4. AUTHORS (Last name, followed by initials – ranks, titles, etc. not to be used. Use semi-colon as delimiter) Thayaparan, T.; Dupont, D.; Ibrahim, Y.; Riddolls, R.

5. DATE OF PUBLICATION (Month and year of publication of 6a. NO. OF PAGES (Total 6b. NO. OF REFS (Total document.) pages, including Annexes, cited in document.) excluding DCD, covering and verso pages.) November 2019 43 56

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11b. FUTURE DISTRIBUTION OUTSIDE CANADA (Approval for further dissemination of the document. Security classiﬁcation must also be considered.) Public release 12. KEYWORDS, DESCRIPTORS or IDENTIFIERS (Use semi-colon as a delimiter.) High-Frequency Radar, Sky-wave Radar, Over-The-Horizon Radar, OTHR, Ray-Tracing, IRI, In- ternational Reference Ionosphere

13. ABSTRACT/RÉSUMÉ (When available in the document, the French version of the abstract must be included here.)

The Canadian Department of National Defence (DND) is developing an experimental over-the- horizon radar (OTHR) with the potential for surveillance of Canada. Because of dynamically changing ionospheric conditions in the Earth’s high-latitude and polar regions, the operating OTHR transmission frequency and elevation angle need to be adjusted regularly to maintain constant illumination of downrange targets. In this study, the feasible operating frequency, elevation angle and absorption radar parameters are determined for short- and longrange OTHR operation using three-dimensional ionosphere ray-tracing technique. The variation in these radar parameters is observed as a function of time of day (daytime and nighttime), season (winter, spring, summer and fall), range (short-range and long-range) and solar cycle (solar maximum and minimum). The range of operating frequencies and elevation angles obtained from this study will aid developing the transmitter and receiver antenna layout for experimental OTHR configurations in the poorly-understood high-latitude and polar regions. These methods will also help to form the basis of the frequency monitoring systems (FMS) that will control the configuration of these polar OTHR systems in real-time. Le ministère de la Défense nationale conçoit actuellement un radar transhorizon (OTHR) ex- périmental qui offre une capacité potentielle de surveillance du Canada. Compte tenu des changements des conditions ionosphériques en régions polaires et de hautes latitudes, il faut régulièrement rajuster la fréquence de transmission et l’angle d’élévation de l’OTHR exploité, de façon à maintenir l’illumination constante en aval des objectifs. Dans le cadre de l’étude, on définit les paramètres radar relatifs à la fréquence de fonctionnement, à l’angle d’élévation et à l’absorption réalisables en vue de l’exploitation de l’OTHR à courte distance et à longue distance au moyen d’une technique de tracé tridimensionnel par rayon de l’ionosphère. On estime que la variation dans ces paramètres radars est causée par l’heure (le jour ou la nuit), la saison (printemps, été, automne, hiver), la distance (courte ou longue) et le cycle solaire (le minimum et le maximum solaires). La gamme des fréquences de fonctionnement et d’angles d’élévation obtenue dans le cadre de l’étude servira à mettre au point la disposition des antennes de ré- ception et de transmission dans les configurations de l’OTHR expérimental en régions polaires et en hautes latitudes, peu connues. En outre, ces méthodes contribueront à jeter les bases de systèmes de surveillance des fréquences (SSF), qui contrôleront en temps réel la configuration de ces systèmes d’OTHR en régions polaires.