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Attenuation in a Ionospheric Radio Link

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Attenuation in a Ionospheric Radio Link ATTENUATION IN A IONOSPHERIC RADIO LINK Inês Martins Manique Instituto de Telecomunicações, Instituto Superior Técnico Av. Rovisco Pais 1, 1049-001 Lisboa, Portugal assumptions of topics one and two are used in the Abstract - This work addresses in the study calculation of the attenuation in a communication of some relevant propagation phenomena in between two points, where the signal suffers ionospheric radio links. The main objective is the successive reflections in the ionosphere and the calculation of the attenuation of a signal in a HF ground. Finally, topic five addresses the communication link between two points at the phenomenon of fading applied to the ionospheric Earth's surface. Throughout the work, some links, with the different types and their influence simulations and calculations are made for the on signal transmission as well as the appropriate representation and analysis of some parameters. treatment to each of these, making use of statistical distributions. I. INTRODUCTION II. SCATTERING ON ROUGH SURFACES The ionosphere, not being nowadays one of the mainstream media, it is still widely used in II.1 Specular reflection military and emergency communications. It is also seen as an alternative if the main media means are to fail. In the event of a natural disaster, terrorist attack or any other event that affects the communication systems, conventional communications via the ionosphere are able to restore communication. Thus, phenomenology Fig. 1 – Specular reflection. associated with the ionosphere and how this affects the signal is still an important matter to be Due to the presence of the Earth, phenomena studied nowadays. like ground reflection and scattering of waves This work aims to calculate the attenuation of occur. The interference of the direct ray with the the signal in a connection between two earth reflected ray causes fluctuations in the electric points, when there are successive reflections in field and it can strongly change the signal at the the layers of the ionosphere and the ground, receiving antenna, compared to the received assuming rough ground. signal in free space. This paper is divided by four topics, all having The equation that represents the total theoretical bases, as well as some graphical electrical field as a sum of the direct ray with the simulations and calculations. The first topic reflected ray is addresses reflection in a rough surface. As a first approach, the effects of specular reflection in the � = �![1 + |�|exp (���)] ground are shown. Then, the reflection in a rough surface (diffuse reflection) and how this influences !"! ! where � = ! ! represents the free-space the signal obtained at the reception. The second ! ! topic focuses on the propagation via the field, Γ the ground reflection coefficient and �� ionosphere, assuming a model of layers, the the phase difference. The reflection coefficient constitution of the ionospheric plasma and also depends on the polarization, which can be vertical the measures that characterize the propagation in or horizontal as represented in Fig. 2. the ionosphere. On topic three the theoretical 1 2��� − arg � = �� � II.2 Diffuse reflection (scattering) Fig. 2 – Reflection in horizontal and vertical polarizations [1]. The reflection coefficients for horizontal and vertical polarizations are given respectively by ��� � − �! − ���!� ��: Γ! = ��� � + �! − ���!� Fig. 4 – Diffuse reflection. �!��� � − �! − ���!� ��: Γ! = �!��� � + �! − ���!� The electric field corrected for the case of a rough surface is given by The phase difference is related with the path difference between the direct ray and the !!! !∆! reflected ray (Fig. 2) and is represented by � = �! 1 + � � � �� !! �� = ��� � − 2� where � = ℎ sin � which translates the phase � ! � difference relative to any two points on the ground. where �� = �! − �!. The ℎ! parameter is related to the roughness of the ground. According to the Rayleigh criterion, the ground will behave as flat if the amplitude of the surface roughness is small in terms of wavelength. This criterion defends that the ground is flat if the following condition holds � ≪ � Fig. 3 – Representation of the direct and reflected rays. The power received at the observation point due to the surface element �� of the rough In the interference zone, the electric field ground is given by oscillates between maxima and minima. These occur when exp ��� = 1 and exp ��� = −1, respectively. From the electric field equation, 1 1 these are given by ��! = �!�! � ! � �, � �� ! �! � 4��! 4��! � = 1 + Γ , ���� � where the scattering cross section is given by �! !"# � = 1 − Γ , ��� � ! �! !"# 1 ��� � � ≃ ��� − �! �! and they occur when 2 where � defines the ground’s roughness and it’s �!�! ! ��� − ! ! given by �! � 1 − � = ! ! �� �! � !! 1 − � 2ℎ! � = � ! ! ! where � = ! and � = ! are both The total power received at the observation ! ! ! point due to the dispersion in the rough ground is dimensionless parameters. � is seen as an given by indicator of the level of the ground’s roughness , since it is inversely proportional to �. Fig. 6 represents the contribution of the �! = ��! different regions of the ground for the scattered power. To obtain these results, it is computed the ! integrand function of ! !! The effective dispersive area (EDA) defines the area that effectively contributes to the total �!�! ��� − scattered power collected by the receiving � 1 − �! ! �(�, �) = antenna. � 1 − �! ! Fig. 5 – Effective dispersing area. Fig. 6 – Representation of �(�), as a function of t. The transversal dimension �! of the EDA is given by Analysing the graph, it is noted that for high values of t, the contributions to the integral are �! = � � ℎ centered at � =0, which corresponds to the specular point. Whenever � < 1, the largest contribution to the integral comes from the region whereas the longitudinal dimension �! is in the proximity of the antennas. Based on this expressed by result, we can now classify the ground in two types, depending on the values of t: reflecting ground (� > 1) and diffusing ground (� < 1). � ! ℎ � � , ��� ≫ � �(�) � � = 2 2 � � Fig. 7 represents �!, where ! is the ! � ℎ ℎ − , ��� ≪ � maximum value of the scattering cross section, 2 2 � � 2 � � according to After obtaining the previous expressions, we �(�) �!�! can finally calculate the total power received by = ��� − � 1 − �! ! scattering on the ground, and it’s given by ! 3 ionosphere. There are various models for the analytical expression of the electron density. The first model was developed by Chapman, where the electron density is given by !!!!!"# ! !!! ! �! = �!!"#� �(�) Fig. 7 – Representation of , with varying t. �! !!! where � = !, and � represents the angle ! The conclusions are similar to those of Fig. 6, between the sun and the vertical of the earth. the ground contribution is concentrated around There are also the linear, exponential and the specular point when the ground is flat and parabolic models, which were developed in order along all the paths between the antennas when to simplify Chapman’s mathematical expression. the ground is rougher. When there is a separation of free electrons and ions, a plasma is originated and the plasma frequency is given by III. IONOSPHERIC COMMUNICATIONS ! �! 1 � �! �! = = The ionosphere has the ability to reflect and 2� 2� �!�! absorb electromagnetic waves, depending on the frequency and the ionization degree of the region. where � represents the electron’s charge and �! In terms of radio communications, the most the electron’s mass. important feature of the ionosphere is its capacity The Fig. 9 illustrates the plasma frequency to reflect. The wave can be reflected between the variation as a function of altitude and time of day, ionosphere layers and the surface of the earth during April 2015, as measured by the Ebro over thousands of kilometers, enabling the Observatory ionosonde in Spain. intercontinental transmission of short waves. The ionosphere is structured in layers (Fig.7), which are discretized by the electron density and the collision frequency. As the sun is the major source of ionization, the layers vary from day to night. Fig. 9 – Plasma frequency variation with altitude and time of day. From around 07:00 to 21:00, at an altitude of Fig. 8 – Ionosphere stratification. 300 km, the plasma frequency takes its highest values, ranging from 7 MHz and 9 MHz. This The ionosphere is mainly composed of a cold altitude corresponds to the F2 layer, which plasma, which shows variations in the electron reaches the maximum value of their reflective density depending on the amount of characteristics around 12h. electromagnetic energy received from the sun. The electromagnetic waves, when penetrating Knowing the solar flux, it is possible to calculate the ionosphere, transfer a considerable amount of both the ion and electron density of the energy to the electrons. This energy affects the 4 moving components of the ionospheric plasma by a process of collision between the electron and heavier particles. The higher the collision frequency (�), the greater the attenuation. To obtain the attenuation constant, a very simple model is considered, in which are despised the action of the magnetic field and the collision losses. Fig. 10 – Ionospheric waves, jump representation. Manipulating some key expressions, one comes to the expression of the propagation constant, given by the expression Once returned to the Earth, the wave can be reflected on the ground and return to the ! ! ionosphere. Thus, there are several successive �
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