UNIT 18 SPACE

Structure

18.1 Introduction Objectives 18.2 Propagation of Waves in the Earth's Atmosphere 18.3 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 , 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 [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 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 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. These electro-magnetic waves can easily propagate through the atmosphere and are reflected back by the ionosphere. Since these waves go from transmitter antenna to receiver antenna while travelling through sky, their propagation is known as sky wave propagation (Fig. 18.4). Above 40 MHz the ionosphere bends the electromagnetic waves incident on it but does not reflect them back to the earth.

Fig.18.4: Sky wave propagation

The ability of the ionosphere to return a radio wave to the earth depends upon the ion density, the fmquency of the incident radio wave and its angle of incidence. Ion density shows seasonal variation along with variations due to solar activity. a certain frequency, waves transmitted vertically into the ionosphere, continue Space Communication e into the ionosphere, penetrating all layers and out into space. This frequency d the critical frequency. The highest angle at which a wave of a specific cy can be propagated and still be returned from the ionosphere is called the angle for that particular frequency.

explain to your students you may give this example:

pose a girl is travelling by car and there is no traffic on the road in a particular on 'R' which she has to cross. Her car will easily pass through 'R'. If there is c and many pedestrians in the region R, she will drive slowly and in the process and energy (fuel) will be consumed. If her car does not have sufficient energy kdown or out of fuel) she may get stranded in that region only (if densities of nd electrons are high, the radio wave energy is absorbed by them).

e may be another case that there is a traffic jam. Then she has to reverse and drive (if the energy of the wave is not sufficient to enter the ionosphere, it is reflected

s sky waves up to around 30 MHz are reflected back from the ionosphere and are d in short wave radio communication.

ace-wave propagation

e space waves are the VHF radio waves (30 MHz to 300 MHz).The two types of ce waves are shown in Fig. 18.5. They are the direct wave and ground reflected ve. You ,should point out that these are different fiom the ground waves just cussed. The direct wave is by far the most widely used mode of antenna mmunication. The propagated wave is direct from transmitting to receiving antenna d does not travel along the ground. The earth's surface, therefore, does not

Transmitter Receiver Direct wave

wave Earth

Flg.18.5: Space-wnve propagatlon

that the direct space wave has one severe limitation - it is limited to the so-called line -of-sight transmission distances. Due to the of the earth, the range of reception is the region from Pi to Pz line of sight. Therefore, this is also called line-of-sight the curvature of the earth are limiting

order to get wider coverage it is mferred that the broadcast be made fiom tall ntennae. The signal transmitted from antenna weakens as the inverse square of

he reflected wave shown in Fig. 18.6 cah cause reception problems. If the phase of he two (direct and reflected) received components is not the same, some degree of 1ienal fadinn andlor distortion will occur. This can also result when both a direct and Communication Physics ground wave are received or when any two or more signal paths exist. We present as an example, a special case involving TV reception.

VHF, UHF transmitted t

HF retlected II

Example 1: Ghcusting in TV reception

Any tall or massive construction obstructs space waves. This results in diffraction (and subsequent shadow zones) and reflections. Reflections pose a specific problem since, for example, reception of a TV signal may be the combined result of a direct space wave and a reflected space wave. This condition results in ghosting, which manifests itself in the form of a double image interference pattern. This is due to the two signals amving at the receiver at two different times - the reflected signal has a farther distance to travel. The reflected signal is weaker than the direct signal because of the inverse square-law relationship of signal strength to distance and because of losses incurred during reflection.

A possible solution to the ghosting problem is to reorient the receiving antenna so that the reflected wave is too weak to be displayed. Of course, the direct wave must exceed the receiver's sensitivity limit as it will undoubtedly be reduced in level when the antenna is reoriented. Note that ghosting can also be caused by transmission line reflections in the link between the antenna and set.

-- SAQ 2

How will you expl .*-' a) myis metro TV visible in-qd eoudo not have a cable connection?

long range communi s is because ground waves suffer from conducti line of sight and sky waves penetrate the ionosphere at frequen~ies'beyondf;~?J *The launching of communication satellites in the 1950s paved the way for the Communication Revolution, which allows us to talk to any one mund the globe at any time and view events at any place in real time. Satellite communication has led to higher communication muencies, large number of communication channels and a large frequency bandwidth. llites in space. The added advantage was that the artificial satellite could be made

Communication

Fig.18.7: Satellite communlcatlon Communication Physics Table 18.2: Early communication satellites

F 1 Sputnik I October 1957 U.S.S.R. Telemetry information (2 1 days) Explore - I January 1958 U.S.A. Telemetry information (5 months) Score December 1958 U.S.A. Voice Communication Echo I 1960 U.S.A. Medium altitude signals were Reflected from metal surface Courier 1960 U.S.A. To store and forward Information Telstar I July 1962 U.S.A. Medium altitude Bell system Relay I December 1962 NASA Transmission of voice, video Syncom I February 1963 NASA Lost Early Bird April 1965 Commercial use (Intelsat I) The important features of satellite communication are: An orbit to be decided for the satellite. The fiequency of the received and the transmitted beams.

18.4.1 Orbits of Satellites Most communication satellites are placed in geostationary orbits. Explain !o your students that a geostationary satellite must have an orbital period of one sidereal day in order to appear stationary to an observer on the earth. One sidereal day is defined as the time required for the earth to travel 360' about its axis. This time is slightly less than the solar day because of the movement of the earth around the Sun. It is actually 23 hours 56 minutes and 4.1 seconds. You can show that the orbital period of the satellite matches that of the sidereal day at an altitude of 37,786 km. The velocity of the satellite in this orbit is 3075 mls. Such an orbit is called the geo-synchronous orbit. When the inclination and the eccentricity of this orbit are zero, the satellite appears to be stationary to an observer on the ground and is said to be in geo- stationary orbit. Three such satellites can cover the entire globe (Fig. 18.8). This orbit is well suited for communication because of many advantages: Advantages and disadvantages of geostationary satellites As the satellite is stationary with respect to a point on the earth, the ground stations within its coverage require minimal tracking. A wide coverage is offered by satellites in such an orbit which is quite good for populated areas. Doppler shift is minimal. You could remind students that Doppler effect is the phenomenon of apparent change in frequency of sound waves at the receiver when the sound source moves with respect to the receiver. This phenomenon is also observed at radio frequencies. The fiequency of satellite transmissions received on the ground increases as the satellite is approaching the ground observer and reduces as the satellite moves away. This Doppler shift in frequency has to be taken into account while dealing with satellite communication systems. However for a geostationary satellite, this shift is negligible because the band widths of signals are much larger. Space Communication .

II, Fig.18.8: Geostationary orbit ostationary orbits also suffer from certain disadvantages. For instance, this orbit is emely difficult to achieve from northern latitudes. Secondly, poles cannot be red adequately by these orbits. A variety of other orbits can also be used for mmunication, like synchronous and medium altitude polar and equatorial orbits ig. 18.9). For northern altitudes, particularly, Molniya orbit is used which involves a hly elliptical orbit with apogee remaining over the northern hemisphere. The other r disadvantage is that propagation delays are significant, about 250 ms, because e large satellite distance. 'This results in long transmission paths causing pagation delays as large as 250 ms in one direction. We can notice these delays in ephonic over long distances. There is degradation in the unication quality for short duration, which can also be predicted. It is generally to some kind of noise caused by natural or other sources, for example, when the n appears within the beam width of an earth station antenna.

II Fig.18.9: Orbits of sateltites (Source: www.sunblock99.org.uW... I rocket/orbit_sm.gif) Ike radio spwtnrm is a limited natural resource which has to Be shadby all types of dEo semis, be it mestrial or via satellites. As mentioned earlier, there is an intc&m&onal Mycalled the International Teleoommuaications Union (ITU), which monitors alWmof hquewy on a global and regional basis. Individud countries tegulate the use of~~ksFor domestic applications and assign frequencies &in$ to radio regulations to ensm that tlYe radio transmissions originating in &eir respective countries bo not cause inktference with domestic or international netw&.

The lrtJ has cakegmid dioservices according to their broad functions. Frequency Jhtimsate mrtbe !iw wch mice.At present, several radio sewices have ken Mh@dby tk IRJ. Mostiy we are concerned with:

f&) Ih FidWlgk Semices (FSS),which apply to systems which intmmmllcct fixed @ntrs swh a In-I 'Pewone Ex&anges. (b) I& Sewice (W)wtsich wfm to ~gmmwesdirectly to ltke public. (c) The M&R WlriSewice (MSS) which p~videsamm mhh.

afremm smhgcis rfl)ldtqrtlphyM'bewl of 1Pellp in mZUaty uses, wivqsoPfnWra1 m~~cesand wn and country planning. Now-a-days this type of photography is done with the Space Communication Ip of satellites. Satellite sensors take pictures and collect other data and transmit it ck to earth stations. These satellites generally fly in near polar orbits at an altitude 91 8 km in such a way that it passes over a given location on the earth at the same al time.

remote sensing satellite takes repeated photographs of a particular location of the during its repeated journey over a location from which a comparative study can ade. For taking the photographs of any object, we need the reflection of ic waves from the object (visible light in case of normal photography). d near infra-red bands which can be displayed as colows are chosen to ion and water. Microwaves are of relevance for study of soil moisture n. Related pictures can be taken at night time and in cloudy conditions

ow that when a satellite is placed in a polar orbit, it can provide global e. Because of their low orbit they can scan larger areas of the earth's surface eir movement. Geostationary satellites are at very high altitudes and are used meteorological observation platforms. These provide continuous data but

solution of the remote sensing data depends on the type of sensor employed. I, satellites in higher orbit are not able to give good spatial resolution. The also depends upon the wavelength of radiation used. Low altitude satellites olution of the order of 10m while geostationary satellites have a resolution imilarly, the temporal resolution of a geostationary satellite can be about r while that of polar orbits could be 15 days.

nsing is extremely useful and is being continuously applied to study the ent of weather systems and weather forecasting. Climatic changes, oil ,sewage, industrial waste, pollution and sea surface temperature etc. are being monitored by remote sensing. Natural resources like water, minerals, and forestry, archeology and geological surveys are some of the important ich are being studied through remote sensing.

8.4. 1 Satellites in India 1 t\i transmission is one important of revolution in communication . Besides transmitting pictures and sound over long distances, it has telecommunication, , telegraph etc. We can now make long one calls not only to other towns within the country but also to several countries through direct dialing, i.e.; without the help of a telephone t, for remote places like Leh, Port-Blair and Aizawl which are y sea or difficult terrain, satellite transmission offers the only viable and

Instructional Experiment (SITE) conducted in our country 76 was possible only with the help of an American Satellite but in 1978, ent of India decided to launch its own programme of multi-purpose xpanding the communication network in the entire country. INSAT-1A in 1982, but it developed technical snags. MSAT-1 B was then 83, and INSAT -lC in 1988. These satellites provided widespread he media, in addition to many other services like in the fields of urce surveys, telecommunication, and research etc. MSAT 2B was The National Information Centre at Delhi has linked all its ugh MSAT 2B. INSAT 2C, another Indian Communication Satellite, 1995. Recently, in May, 2003 the latest Indian Communication n installed in a geo-synchronous orbit. We all know that we are very Communication Physics well connected to the entire world through information technology receiving all international broadcasts at our homes.

The Indian remote sensing satellites are IRS-IA, IRS-1B and IRS-1C.

You can use the following. figure- to show space-wave and satellite communication in a combined manner.

Ionosphere

Fig.18.10: Space communication

We now summarise the contents of the unit.

18.5 SUMMARY

In this unit you have learnt about

how radio waves communication is possible through ionosphere and satellite technology. the merits and demerits of different orbits for satellite. the importance and utilities of remote sensing. frequencies used in satellite communication. the Indian communication and Remote-Sensing Satellites.

18.6 TERMINAL OUESTIONS

1. How did you explain to your students the importance of the ionosphere in communication? 2. Explain why ground wave propagation is better on ocean surface. 3. Collect information about Indian communication and remote sensing satellites and present it in an attractive manner for your students.

18.7 SOLUTIONS AND ANSWERS

Please bring your work to the ECP or submit it in a folder at the study centre.