Unit 18 Space Communication
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UNIT 18 SPACE COMMUNICATION Structure 18.1 Introduction Objectives 18.2 Propagation of Waves in the Earth's Atmosphere 18.3 Radio Wave Communication 18.4 Satellite Communication Orbits of Satellites Frequencies Used in Satellite Communication Remote Sensing: Application of Satellites Satellites in India 18.5 Summary 18.6 Terminal Questions 18.7 Solutions and Answers I 18.1 INTRODUCTION In Unit 17 you have studied the basics of the communication systems and We have introduced the idea of communication channels and mentioned how electromagnetic waves serve to cam signals in the audio-visual region. You have learnt about two types of communication channels: one in which the signals from the transmitter are guided along a physical path (or line) to the receiver. The mode of communication using guided media is called line communication and we take it up in the next unit. The communication process in which the signal is transmitted freely in the open space (atmosphere) is called 'space communication'. In this communication we use radio frequency waves as carriers. Communication through space refers mainly to propagation of etectromagnetic waves in the earth's atmosphere and satellite communication. This is what we shall discuss in this unit. Objectives After studying this unit, you will be able to: teach better to your students the concepts of propagation of electromagnetic waves in the atmosphere, principles of satellite communication and its usefulness, communication bands, and applications of satellites and satellite communication in India; and devise strategies to help your students to learn these concepts and assess how well these have worked. 18.2 PROPAGATION OF WAVES IN THE EARTH'S ATMOSPHERE The earth's atmosphere affects the propagation of electromagnetic waves from one place to another on the surface of the earth. You may like to begin by explaining about the earth's atmosphere to understand how it affects the communication process. Based on the variation of temperature, air density and electrical conductivity with altitude, the atmosphere may be thought of as made up of several layers (Fig. 18.1). Troposphere is the lowest region in the immediate vicinity of earth. It is characterized by a negative temperature gradient (6 K km-')leading to temperatures between 290K (at the equator) to 220K (at high lditudes) at tropopause. The air density is maximum but electrical conductivity is the least compared to other layers. is the nearly isothermal region beyond the tropopause and extends from 50 km. There is an ozone layer in this region, which absorbs the UV y rays, etc. coming from the outer space. ospheve extends from about 50 km to 90 km. The temperature ofthe air shows a trend in this layer with minimum temperature around 180K. 7I-.. I,, f Fig.1 1: Layers in the earth's atmosphere. 1; ' ere has been defined by the Institute of Radio Engineers as "the part of the r atmosphere where ions and electrons are present in quantities sufficient he propagation of radio waves". It extends from about 90 km to 350 km and rature increases with height to around 1000 K. Hence it is also called the here. The ionosphere is divided into regions called D, E and F (which is ided into F1 and F2). This division is based on the number density of the ionosphere, increasing with height from about 1 o9 m-3 in D region to he E region and to 1012 m" at F2 peak (Fig. 18.2). vy;; vy;; Layer - D Region Earth's surface Electron density !I Fig.18.2: Electron concentration in the earth'r atmosphere of these regions is attributed chiefly to the following causes: firstly,ahe deposits its energy, which depends on the absorption chwacteristics of at various heights. Secondly, the process of recombination depends on tlie various gases present in the atmosphere and lastly, the composition changes with height. 29 Before discussing the propagation of electromagnetic waves in theeearth's atmosphere, you may like to revisit the nature of electromagnetic waves from the standpoint of communication. Nature of electromagnetic waves Electromagnetic waves, as you know, consist of oscillating electric and magnetic fields at right angles to each other. The direction of propagation of these waves is perpendicular to both the fields (Fig. 18.3). Fig.18.3: Electromagnetic waves These waves are transverse in nature and travel with the speed of light through vacuum. We are all familiar with the fact that visible light is a part of electromagnetic spectrum and different frequencies of electromagnetic waves have different velocities in a medium. Source of electromagnetic waves The si~nplestsource of electromagnetic waves is an accelerated charge which produces varying electric and magnetic field that constitute the wave. Radio waves may be produced by charges accelerating in AC circuits with an inductor and a capacitor. Electric circuits with oscillating currents produce microwaves. Infrared waves are emitted by the atoms and molecules of hot bodies. Some atoms under suitable conditions emit visible light. The sun emits large amount of ultraviolet rays. X-rays are produced when fast moving electrons are incident on a metallic target. These different types of electromagnetic waves are useful in our day to day applications in communications, radar systems, microwave oven, infrared therapy and medical diagnosis. Frequency-range of electromagffetic waves Electromagnetic waves have a broaq frequency spectrum, having wavelength as small as 1 fm (1 fin L 10-lS m) to as large as few km.This broad spectrum has been classified into different regions for convenience, although these regions do not have any sharply defined boundaries. These regions are starting from the shortest wavelength of gamma rays and X-rays increasing to ultraviolet, visible light , infm~d,microwaves and radio waves. Electromagnetic waves with frequency greater than 10 KHz and less than 300 GHz are classified as radio waves. These are further subdivided into smaller ranges (as shown in the Table 18.1). Space Communication Table 18.1: Radio Spectrum Frequency Range Wavelength range Principal application 3-30 KHz 10-100 km Direct long range communication I l ow Frequency 30-300 KHz 1-10 km Marine, navigational aids Medium frequency 300 -3000 KHz 100-1000 m Broadcasting [MF) High frequency 1 3-30 MHz 10-100 m All types of [)IF) Cominunication Mery High 1 30-300 MHz 1-10 m TV,FM, Radar, ectromagnetic waves in the visible range can easily pass through the atmosphere d that is why we can see objects. But all radiations are not offered free passage h the atmosphere. For example, most of the radiations in infrared range are ed.by the atmosphere. The ultraviolet radiations are absorbed by the ozone ere are five main layers in the atmosphere, which play a role in communication, ese are C, D, E, F1 and F2. C layer is at about 60 km height from the earth's surface and reflects low and very low frequencies. D layer at a height of about 80 km reflects VLF and LF electromagnetic waves but absorbs medium frequency (MF) and high frequency (HF)waves. E layer at a height of about 1 10 km helps MF wave propagation but reflects HF waves in the day time. F1layer (1 80 km) reflects some HF waves but allows most to pass through. Fzlayer (300 km in day and 350 km at night) reflects back upto 30 MHz electromagnetic waves but lets 40 MHz waves pass through. e behaviour of the earth's atmosphere in the radio frequency range is of special I erest in space communication. This is referred to as radio communication. I I#w did you use the information given in Sec.18.2 to teach your students about I elhctromagnetic wave propagation in the earth's atmosphere? I I lb.3 RADIO WAVE COMMUNICATION essential feature of space communication is that a signal emitted from the antenna e transmitter has to reach the antenna of the receiver. Depending on the frequency Communication Physics of the radio wave it can happen in the following ways: Ground wave, Space wave, Sky wave and Satellite communications. You can use the example of two persons playing with a ball given in the Class XI1 NCERT text book to illustrate these processes. We shall now discuss these in detail. Ground wave propagation A ground wave or a surface wave is a radio wave that travels along the earth's surface. Ground waves can travel around curves and can go right around the globe. These waves can bend around the corners of the objects on the earth and are necessarily affected by changes in the terrain. Their intensity falls with distance as per the inverse square law. That is why ground waves cannot travel very long distances on the ground. If the earth's surface has high conductivity, then the absorption of wave energy and its attenuation are both greatly reduced. Ground wave propagation is much better over salt water as opposed to dry desert terrain owing to its poor conductivity. Ground waves are not effective at frequencies higher than 2 MHz as the ground losses increase rapidly with inqeasing frequency. It is suitable for low and medium frequency. That is why it is called medium wave propagation. The maximum range of ground wave propagation depends on: the frequency of the radio waves, the power of the transmitter. Ground wave propagation is the only way to communicate into the ocean with submarines and for this purpose extremely low frequencies (30 to 300 Hz) are used. For transmission of higher frequencies, sky wave propagation is used. Sky wave propagation Radio waves of frequency between 2 MHz (Medium) xld 30 MHz (Short) are called sky waves.