U N IT INTRODUCTION TO SATELLITE pb COMMUNICATION & ORBITAL MECHANICS AND LAUNCHERS SIA GROUP PART-A SHORT QUESTIONS WITH SOLUTIONS Q1, Whats satellite? Define satellite communication. Ans: Satellite The he: object which moves around another object in space due to the mun known as “satellite” | gravitational fores Satellite Communication The communication between the Earth stations and Space by means of a radio spectrum is referred to as satellite communication. Q2. What are active and passive satellites? Ans: The satellite which receives the power of the transmitted signal from the ground station, amplifies it and retransmits by frequency shifting the modulated carrier is known as active satellite or transponder. The satellite which receives the electromagnetic radiation from the ground station and then reflects it back without modifying and amplifying is referred as passive satellite or refk ctor. Q3. Give the frequency ranges of VHF, UHF, L, S, and C, X, Ku, K and Ka Bands. Ans: Mode! Papers, Q1(a) | AprilMay-19, Set-3, 21(6)) ‘The various frequency bands used in satellite communication are listed below: [ Band Downlink (Hz) Uplink (Hz) ] I VHF 137138 Ml a8 150 MHz | UnF 387 - 390 MHz and 1541 - 1541.9 MHz 312-315 MHZ and 1624.5 - 1643.4 MHz | L 1.5 GHz 16 GHz 1 S 20 band (ansponders | CBand-Commercial [34-42Gand45-4.8G 3.725-7.075G |X Band-Military [725-7786 79-84 Gand 17176 T1,7= 12.2 G (Region 2 only) & 12.75 - 13256 K, Band-Commercial . . | Bands Commensiat 177-2126 275-3106 | K tantmitiary 202-2126 435-4556 Table SPECTRUM A@LLIN-ONE JOURNAL FOR ENGINEERING STUDENTS ————— SIA GROUP >1.2 Qa. List the applications of satellites. Ans: AprilMayt7, Sat, O(a) Satellite communications have applications such as, 1 Providing connectivity between large traifie nodes. 2. Serving coverage to large geographical areas. 3. Mobile communication 4. Broadcasting television and sound to the public. Providing end-to-end connectivity to users. Q5. Define orbit path and orbit radius. Ans: Orbit path: It isa path in which earth revolves around the earth, Orbit Radius: It isthe distance from the center of the earth to the satellite Q6, Write the Keppler's laws of planetary motion. Ans: (Model Papers, Qa) | Aprilt8, Set, 1(a) ‘The Keppler’s laws of planetary motion are, The orbit of any smaller body about a larger body is always an ellipse, with the center of mass of the larger body as one of the two foci. Itstates that, the path followed by a satellite around the planet will be an ellipse. The center of mass of the planet acts as one of the focus of the ellipse. Law-2 ‘The orbit of the smaller body sweeps out equal areas in equal time. It states thy me. i the satellite covers equal area in equal intervals of Stine Figure Law-3 The square of the period of revolution of smaller body about the larger body equals a constant multiplied by the third power of the semi major axis of the orbital ellipse. ana re Where, T — Period of the orbit 4 ~ Semi major axis of e! iptical orbit 4. — Keppler’s constant SATELLITE COMMUNICATION [JNTU-KAKINADA] Q7. Define Keplers laws and list the orbital elements of a satellit Apriitay-t7, Set3, a1(a) (or) What are the various orbital elements? ‘Apri-t8, Set, a1(0) (Refer only Orbital Elements) Ans: Keplers Laws For answer refer Unit-, Q6. Orbital Elements ‘The different orbital elements required to determine satellite orbit are, 1. Eccentricity (e) 2. Semi major axis (a) 3. Inclination (i) 4, Right ascension of an ascending node (2) 5. Argument of perigee (0) 6. Time of perigee (1). Q8. Define the terms Apogee and perigee. (or) Define perigee of a satellite. April-ta, Set2, a1(a) (Refer only Perigee) (or) Define apogee of a satellite. Apri-18, Ses, a1(a) (Refer only Apogee) the closest point of satellite to the earth on the orbit he farthest point of satellite from the earth on the Q9, Define the terms first point of Aries, Mean Anomaly and Anomalistic period. Ans: Anomalistie Period ‘The time period between successive perigee passages known as anomalistic period First Point of Aries The line joining the centre of the earth towards a fixed location in space is called first point of aries. Mean Anomaly Mean anomaly determines the average value of angular position of the satellite with respect to per Look for the SIA GROUP Loco {J on the TITLE COVER before you buyUNIT-1 (introduction to Satellite Communication & Orbital Mechanics and Launchers) 1.3 ato. a ci) Define look angle. (i) Look angles (ii) Elevation angle i) Azimuth angle (or) What is meant by azimuth angle? (Model Paper, 1a) | Apruhtay-19, Set-t, tfc) (Refer only: Azimuth Angle) Define the azimuth angle. ‘prit8, Sot, 10) (Refer only Azimuth Angle) (or) What is an elevation angle? April, Sot.2, 01/0) (Refer only Elevation Angle) Look Angles es are the angles at which the communication between carth station and a satellite is possible by pointing the transmitting antenna towards its respeetive satel Elevation Angle Elevation angle is defined as the angle at which the axis of antenna is rotated vertically facing towards the satellite in horizontal direction. Azimuth Angle Azimuth of a satellite is the angle which the satellite direction makes with the direction of the true north. It can be measured clockwise (east wards) from geographic north to the projection of the satellite path on a (locally) horizontal plane at the earth station as shown in figure. Nath pale “Oe Figure SPECTROM @LLIN-ONE JOURNAL FOR ENGINEERING STUDENTS SIA GROUP >1.4 SATELLITE COMMUNICATION [JNTU-KAKINADA] PART-B ESSAY QUESTIONS WITH SOLUTIONS 1.1 INTRODUCTION TO SATELLITE COMMUNICATION 4.1.1 Origin of Satellite Communications, Hi Q11. Write a short note on origin of satell Ans: AprivMay-19, Set, 02(a) Satellite communication has grown tremendously over the years. The idea of satellite communication was fist initiated by Arthur C.Clarke in the year 1945. He proposed a three satellite configuration in the geostationary orbit that provide world wide relay communication, ‘The evolution of satellite started in the year 1987 when a low orbit satellite SPUTNIK-I was launched by Russia. After ssfll launching of SPUTNIK-1, more satellites were launched in the following years as tabulated below’ jorical Back-ground fe communication. the suc year tite 1. | 1958 | SCORE (Signal Communicating by Orbiting Relay Equipment) 1960 | Echo 3. | 1962 [Tele star Table Clarke assumed that upto the year 1963, no satellites were launched into geostationary orbit. This is due to the inefficient and low power capabilities of rackets required to launch a satellite into that orbit, Finally, Clarke's idea became a reality when SYNCOMIL, the first geostationary satellite was launched on July 26, 1963. A federation called “International Telecommunication Satellite Consortium” with a membership of 1 countries was established in Washington in the year 1964 to enhance the importance of satellite communication. This membership was then extended to 150 countries including India in 1973 On 12% February 1973 this consortium was renamed as “Intemational Telecommunication Satellite Organization”, This organisation launched its first satellite on 6" April 1965 which was named as INTELSAT 1. also referred as Early bird. 28* June 1965 is recognized as the birthday of satellite communication because it marks the beginning of commercial communication between USA and Burope. Q12. Explain the brief history of satellite communications. Ans: (Aoriay-17, Sot, aN) | Apr-8, Set 2, a) ‘The studies on satellite communications started in 1945 after a science fiction written by A.C. Clarke. Based on this, a satellite at an altitude of 42,242 km moving in an equatorial orbit from the centre of the earth appears to be stationary, because it rotates with same angular velocity in same direction as that of earth and has zero relative velocity. It also proposed in such an arrangement, @ satellite covers almost all the parts of a hemisphere. Furthermore, it also proposed that global communication can be acheived by using three such satellites spaced at 120°away from each other. The first ever satellite in the history of satellite communication was launched by USSR on 4th october 1957. It was named sputnik - 1. It was then followed by an explorer satellite called SCORE (Signal Communication by Orbiting Relay Equipment), that was launced by USA on 18th December 1958 wi period of 101 minutes In 1960 and 1965, Echo - I and Echo - Il two passive satellites were launched by communication giant ATST, These satellites measured 110 ft in diameter and served as passive reflectors. In 1962, a broad band active satellite called TELSTAR -I(or LEO satellite of 50 MH bandwidth) was launched by Bell system laboratory. It contained on-board transponders with 6.389 GHy of uplink and 4.169 GHz of downlink frequen a period of 158 minutes and 225 minutes. ‘A year later, in 1963, NASA incollaboration with defence department of US launched a geostationary satellite called YNCOM (Synchronous Communication Satellite). It had FM/PSK transponder with 7 GHz of uplink and 1GHz of downlink frequencies with an ability to use two carriers at a time in full duplex operation. In 1965, the international telecommunication satellite organization of Furope and USA launched first ever communication satelite called INTELSAT -I (or Early bird). It had two transponders each of 25 MHz bandwidth. A frequency transponder of 6.301 Gilz and 6.390 GHz of entre frequency and 4.081 GHz and 4,161 Gl¥z of downlink frequency was used by Europe and USA respectively Look for the SIA GROUP Loco {J on the TITLE COVER before you buyUNIT-1 (Introduction to Satellite Communication & Orbital Mechanics and Launchers) LS In 1970's, a series 3 and 4 of 14 INTELSAT’s were launched for the purpose of communication and weather forecastin In 1980s, about fifteen $ and 6 INTELSAT series satellites were launched to help in weather forcast, TV transmission, satellite switched multiple access, etc. In 1990's, 7 and 6 INTELSAT series satellites using ku-band frequeney, 2 and 3 Arabsat series satellites and a commercial purpose Asia-sat of china were launched. In 2000, INTELSAT 9 series, third generation Arabsat were evolved. Q13. Write a brief history of Indian satellite commu- nication. ‘Apnitiy.19, et2, @2(2) (or) Explain the history of Indian satellite commu- ications. ‘Aprikt8, Sets, 2a) (or) Discuss the satellite development in India. Ans: (Apcivay 17, Set, Qa) | Aprittay 16, Set, Q1()) India’s Participation in other International Communication Satellite Systems 1. In August 1986, Department of Space (DOS) has con cluded a Memorandum of Understanding (MOU) with GEOSTAR corporation of USA for a joint feasibility study for implementing a Radio Determination Satellite Service (RDSS). A joint study with European Space Agency (ESA) on definition of an intemational civilian global Satellite ‘Negation (SATNAN) system has been set up, 3. With the support of ISRO, India is participant member of Future Air Navigation Systems (FANS) committee formulated by International Civil Aviation Organisation (ICAO) which is responsible for defining the basic functional specifications/characteristies for future satellite communication and navigation system/services tnd allied matters for civil aviation purpos 4. The Government of India is actively promoting, participating and contributing to the development of international Satellite-Aided Search and Resume system (SAS&R) for maritime, airborne and land distress alert detection and position location. The features of participation in (SAS&R) are listed below. 1. Use of existing USA-Russia-Franee and Canada COSPAS-SARSAT space segment and establishment ofa Local User Terminal (LUT) in India. Indigenous development of 406 MHz Emergeney Locator Becons (ELBS). Testing of a sample of existing 121.5 and 243 Miz ELBs in inventory and under indigenous production for COSPAS-SARSAT compatibility. 4. Contribution to the future intemational SAS&R space segment by carriage of SAS & R payloads for future Indian satellites, 5. Securing first-level participations for India in the international SAS&R system. Qi4, Discuss in detail about the development of satellite communication in the world scenario. An: ‘Aprinay-16,Set2, 1a) Important Mile Stones in the Development of Satellite Communication 1. In 1000 AD, Chinese invented the rocket. 2. In 1962, the first liquid propellant rocket was launched bby RM. Goddad in USA. 3. In 1942, Germany successfully launched the V-2 rocket 4. In 1958, for the first time voice communication was established through satellite when the first US satellite ‘was launched in this year. 5. In 1960, first communication satellite (Passive) was Jaunched into space. 6. In 1964, INTELSAT was founded and the first satelite was launched into geostationary orbit. 7. In 1975, first successful direct broadcast experiment was conducted jointly by India and USA. 8. In 1979, International mobile satellite organization (Inmarsat) was established. 9. In the year 1989-90, global mobile communication service was extended to land mobile and geonautical use. 10. In 1995, largest worldwide growth in number of VSATS was recorded and spectrum allocation for non-geostationary satellite system was carried out During this year, the first successful test of low data rate commeivial low earth orbit satellite system was conducted, (ORBCOM) 11, In 1990-2001 direct sound broadcasting system was introduced. 12. In 2000-2005, broadband personal communications were introduced and a number of low and medium orbit constellation systems were made operational. Growth of Satellite Communication (On the global scenario, the demand for trafic is likely to grow continuously in the area of telecommunication which is provided by fixed satellite service. A significant reduction in the call charges can be achieved by the use of personal VSATS can provide various broadband services. In the next decade, the mobile communication through satellites is expected to be a major growth area and hence the satellites will play a crucial role in the development of mobile communication. A number of mobile satellite systems using non-geostationary orbit will be offering voice and broadband personal servic ‘The satelite will also play an important roe in the direct- to-home broad casting, With the introduction of digital television employing highly efficient digital compression techniques, a number of innovative services such as download of large files, movies on demand ete., can be offered For VSATS or personal mobile terminal applications, simple inexpensive ground receivers are essential, One possible solution is the use of satellites with regenerative repeaters which are more intelligent than simple repeaters functions such as demodulation and sw: Other area being investigated includes the reduction in coding bit rate of speech signals resulting in grater bandwidth utilization, use of 20/30 GHz to reduce the frequency congestion problems of existing bands and inter-satellite links in space ice segment capacity and connectivity. Efforts are being made in defining the framework for global personal ‘mobile communication satellite system that is fully integrated with terrestrial mobile system, SPECTRUM ALL-IN-ONE JOURNAL FOR ENGINEERING STUDENTS SIA GROUP {31.6 SATELLITE COMMUNICATION [JNTU-KAKINADA] 1.1.2 Basic Concepts of Satellite Communications Q15. Draw a basic block diagram of satellite communication system and explain each block in detail. (Model Papert, 02(a) | Aprilitay-1 4, a2(b)) (or) Explain about the basic components of satellite in detail. (or) Explain the architecture of a satellite communication system. Apil-18, Set, 020) (or) Draw the block diagram for satellite communication system. Explain the function of each block. AprtMay-17, Set, a2(a) (or) Explain the basic principle of satellite communication system with a neat diagram. —_Anik1s,Sot3, at(a) (or) Draw the general structure of a satellite communications system and explain. Apri. Sete, a) Ans: Satellite ‘The heavy object which moves around another object in space due to the mutual gravitational force is known as “satellite” The block diagram representing the general structure of a satellite communication is as shown in figure. Sutlite A —_ Eath ath Station Staton mn Teresa Downlink Uplin Teresa System transission transmission System Use User igure: General Structure of Satellite Commu From the figure, it can be observed that satellite communication system consists of two links ie., uplink and dowalink Each link consists of three blocks ie., uset, Terrestrial system and earth station. The user transmits the signal to earth station through terrestrial system, which transmits the signal to satellite through transmitting antenna, Satellite retransmit the processed alto the receiving antenna, which is further transmitted to the other user at the receiving end, Look for the SIA GROUP Loco {J} on the TITLE COVER before you buyUNIT-1 (Introduction to Satelite Communication & Orbital Mechanics and Launchers) 17 ‘According to the guidelines of WARC - 1979 commercial communication satellites, the frequency bands assigned to uplink and dowalink transmission are as shown in table ‘Transmission Bandwidth I Uplink 500 MHz (near to 6 GHz) 2. Downlink — | 500 Mrz (near to 4 GHz) Table ‘These frequency bands assigned to uplink and downlink transmissions are divided into 12 channels, Total Bandwidth Total Channels 00 ive.,Channel Bandwidth = = 40 Miz ‘Thus, each channel contains 40 MHz of bandwidth and these channels ean transmit power upto 5 - 10 watts. In order to handle a single TV channel or 1500 analog voice signals, these 12 channels desire for 12 transponders in their communication system. To overcome this drawback, a digital modulation technique has been developed, which employs Single Side Band (SSB) modulation scheme and serve upto 10,000 voice channels, over a single satellite transponder. Thus, higher transponder data rates ean be obtained ic. upto 50 to 100 MB. Q1G. Give the classification of Satellite Systems. Ans: The satellite types. They are, 1. Ground to ground 2. Ground eross link ground and 3. Ground to relay platform. 1. Ground to Ground In this type of satellite system, the earth station p first transfers the signal to the satellite (S,). After processing the received signal, the satellite retransmit the signal to another earth station g, as illustrated in figure (1). Satelite P 4 Figure (1): Ground to Ground 2. Ground Cross Link Ground This satellite system is used to link the two earth stations, ‘when they are out of line-of-sight of a common satellite. Figure 2) illustrates the concept of ground-cross-link-ground satellite system. anh alon Figure (2: Ground Cross Ground The links of satellites (S, and S,) are maintained in such, a way that, when the earth station p transmits the signal, it ‘goes to satellites S, and then the processed signal is transferred to satellite S., which after further processing, retransmits it to another earth station g. 3. Ground Relay Platform In ground relay platform satellite system, the earth station transmits the signal to sa llte S,. After processing the signal, it ‘gets transmitted various relay platforms, as shown in figure (3), 8, Earth station Relay platform Figure (3: Ground to Relay Platform Q17. What are the different segments in satellite architecture? Explain. Aprilt8, Set-2, Q2(a) (or) Explain the functions of space segment and ground segment of a satellite system. Ans: Aprivifay-17, Set2, Q2(a) The different segments in satellite architecture are, 1. Space segment Ground segment, SPECTRUM ALL-IN-ONE JOURNAL FOR ENGINEERING STUDENTS ————————— sia GRouP YDSATELLITE COMMUNICATION [JNTU-KAKINADAI space Segment The main funetion of space segment is to provide ‘communication services toall other subsystems of communication satellite system through a satellite bus and communication payload. (a) Satellite Bus A modem satellite bus include many subsystems such as structural, thermal, power, propulsion, telemetry, tracking and command systems. The thermal and structural components integrated in the satellite to minimize the heat radiated from sunlight. The power systems available in the satellite bus are solar energy and on board batteries in which the solar energy being the primary source. During the eclipse period, batteries are used to provide the required power supply. The propulsion system in the satellite bus is used forattitude control and to place the satelite into a desired orbit. The telemetry, tracking and command (TTC) system is used to transfer the information of satellite to grand (6) Communication Payload Communication satellite system consists of many subsystems. All the subs using communication payload. The communication ean be done in hetween the satellites present in space segment out of 7 satellites, 3 satellites are placed in GEO at angles of 32.5, 83° and 18,5° east and other 4 satellites are in GEO synchronous orbit with an inclination of 29° with longitude consisting of 83° and 13.5° e east. The schematic diagram of communication payload is as shown in below figure. | fran ‘Communication payload Figure 2. Ground Segment ‘The main function of the ground segment is to operate and maintain the satellite system, The ground segment consists of fixed or mobile transmission, reception features The satellite system is operated by ground segment by using following elements, 1. Space craft control facility 2, Ranging stations 3. Timing center 4. TTC and uplink station, The space craft control is used to maintain atinude control system to place the space craft in the desired direction (or) at the desired point, ‘The ranging stations are used in ground segment (0 provide the information related to range of the satellite in its orbit. ‘The timing station in ground segment consists of highly stable clocks to give the details about time. ‘The TTC and uplink station is used to control the space segment and to maintain satellite control facility in the space segment. Q18, Explain the functions of each element of a Mobile Satellite Service (MSS). Ans: ‘The mobile satellite communication is @ long distance ‘communication which is used to overcome the limitations of conventional and based communica jon system. The terminal of mobile satellite may be considered as Very Small Aperture Terminal (VSAT) on wheels (i.e., controlled VSAT).. ‘The network structure of mobile satellite communication is as shown in figure, JN, “ee ate ‘oncom Da = co en Figure: Structure of MSAT Network Figure depicts that in an MSAT Network, “Telephone exchange” is connected to the satellite using gateway earth station. Thisallows to complete the eals from remote terminals to terrestrial subscribers and then to call subscribers using Federal Telecommunication System (FT) network “Operations center” communicates with satellite using control earth station, It provides the funetions such as configuration control, performance assessment, security and fault prevention management. This operations center or control carth station is used to control both satellite and network of the MSAT system. ‘The “Dispatch console” controls a remote base station located at a long distance from communication center, Look for the SIA GROUP Loco {J on the TITLE COVER before you buyUNIT-1 (Introduction to Satellite Communication & Orbital Mechanics and Launchers) 19 In MSAT system, the Effective Isotropic Radiated Power (EIRP), which is based on onboard power of satellite and antenna gain, is high in the direction of coverage area, The system employs steered and non-steered mobile antennas with circular polarization in their mobile stations. The link between satellite and mobile in MSAT system involves frequeney range of L-band ie., Frequency. Range 1.626 GHz. to 1.66 GHz 1.55 GHz to 1.559 GHz Uplink frequency Downlink frequency MSAT spacecraft allows 3200 radio channels based on the type of mobile antenna and bandwidth used in the system. ‘The three major services offered by an MSAT system 1. Mobile Radio Base Station operations (MRS) 2. Mobile Data Service (MDS) 3. Mobile Telephone Service (MTS) MSAT system also finds its applications in searching and rescue operations, emergency fire fighting, ambulance, public safety, aeronautical and marine purposes. Q19. Give the classification of satellites. Explain them. (or) Explain the basic differences between an active and passive satellites. (Apriumay-17, Se, 22(0)| ApeiMay-19, Set2,Q1(0) (Refer only Differences between Active and Passive Suellite) Ans: Satellites are generally categorized depending on the following two parameters, 1. Satellite orbit 2. Communication relay. 1. Depending on the type of orbit path, satellites are classified as, For answer refer Unit, QU6, Depending on the type of communication relay used, satellites are classified as, Gi) Active satellite Gi) Passive satellite Differences between Active and Passive ites The basie difference between an active and passive satellite is due to the type of communication relay used in the satellite system. @ Active Satellite The satellite which receives the power ofthe transmitted signal from the ground station, amplifies it and retransmits by frequency shifting the modulated cartier is known as active satellite or transponder. ‘The modern communication satellites are referred as active satellites. The first active satellite launched by USA is SCORE" Active satellites also serve as “repeaters” by increasing the strength of the signal at the receiving terminal, The transmitter at the ground station consumes less amount of power for its operation, As the height of the satellite increases from the ground, its communication capabilities with directional antennas also increases rapidly. (ii) Passive Satellite ‘The satellite which receives the electromagnetic radiation from the ground station and then reflects it back without modifying and amplifying is referred as passive satellite or reflector. In passive satellites, the reflector of the relay system is a metal coated plastic spherical balloon. The most popular passive communication satelite was launched in 1960 by the ic system, the transmitter atthe ground station consumes large amount of power whereas only a fract of power is received at the receiving station. Due to the impenetrable property of passive satellites, these are most commonly used in military applications. The communication capability of passive systems is less when compared to active systems. Q20. Explain various reasons for preferring satellite communication over optical fiber communication. Ans: Aprivitay Advantages of Satellites Over Optical Fibers ‘The main advantage of satellite over fiber optic cable is its cost is independent of distance. Further, the quality of signal transmitted is independent of the locations of stations sending and receiving information. In other words, quality of signal is independent of the distance it travels unlike fiber optic cable. So, as long as the two stations fall within the coverage area of the satellite antenna, they will have the same quality of information transfer. For thin route remote area communications in the plains as wells hilly stations and communication between the islands and the main land, only satellite communication is the cost effective option 9, Set-4, 2(a) ‘SPECTRUM ALL-IN-ONE JOURNAL FOR ENGINEERING STUDENTS SIA GROUPS1.10 SATELLITE COMMUNICATION [JNTU-KAKINADA] ‘Satellite are used not only for communication purposes but also used in the following applications, 1. Surveillance 2. Navigation 3. TV distribution 4. Satellite broadband internet. 1. Surveillance Satellite can capture the images of potential enemies, nuclear weapons present on the earth and send them back to earth, ‘Surveillance is important in the fact that, by using this, the natural resources, mines, rivers, lakes etc.,.on the earth can be located. 2. Navigation ‘The major application of satellite is Global Positioning System (GPS). Using GPS, any location on the earth can be located. ‘The directions leading to the desired location can be obtained using GPS. 3 TY Distribution The direet broadcast satellite provides direct home broadcasting of television. DBS is capable of handling almost 32 TV channels 4. Satellite Broadband Internet ‘The satellite broadband internet is available in the bandwidths of $12 kbps, 1024 kbps, 2048 kbps, 4 Mbps ete. These all are some applications, which cannot be provided by optical fibers. Hence, satellites are preferred more than optical fibers. Similarly, for research, rescue and navigation purposes satellites offer the advantages which no other system can offer. Q21. Compare the sat je and optical fiber communications. Ans: ‘The comparison satellite and optical fiber communications is given below. Satellite Communication Optical Fiber Communic: 1, | Electromagnetic waves carry the signal information. | 1 | Photons carry the signal information. 2. | High directional antennas are employed. 2. | Amtenna is not required 3. | The life time ranges from 5 to 18 years. 3. | The life time ranges from 20 to 25 years. 4. | Point © multi point communication is possible. } 4. | Point to point communication is possible. 5. | Installation and operating cost is very high. 5, | Installation and operating cost is very less compared to satellite communication. 6, | Best suited on low density routes and wide area 6, | Best suited for high density point-to-point routes, broadcast service networks. Table Q22. Discuss the advantages and disadvantages of satellite communication over other types of communication methods, Ans: ‘Apritttay-19, Set-4, 02(0) Advantages of Satellite Communication 1, High Transmission Capability Before the advent of communication satellite, the long distance communication over space is accomplished by means of cascaded ratio relays, very low frequency radios and high frequeney or short wave radio, which are low capacity media, suitable for specialized applications where as the communication satellites have very high transmission capacity over long. 2. Point-to-Multipoint Communication Satellite has unique geometry which helps in transmitting signals from one point to n umber of points within its coverage area as shown in figure. Look for the SIA GROUP LoGo {QB on the TITLE COVER before you buyUNIT-1 (Introduction to Satellite Communication & Orbital Mechanics and Launchers) 1.11 Swtte Figure Quick Installation ‘Once the satellite is stationed in appropriate or predefined position, the installation of earth stations and establishment of communication link between it and satellite can be done quickly. Also an earth station can be repositioned quickly. The mobile communication can be carried out simply by satelite. Thus the satellite has become an alternative to short wave radio. It is also advan ageous for telecommunication services between or with isolated communities. So, several countries, where populated areas are separated by vast distances or difficult terrain, use satellite on trunk routes. Also, satellite communication is used extensively on international routes, where the distances involved are usually very large. 6 Asatellite mobile system is capable of providing full international roaming to mobiles. 7. And most important, satellites are used to provide voice and data communications to air crafts ships, land vehicl 8. It can be used for relief operations in inaccessible areas. 9. Through satellite communication it is possible to offer coverage to wide areas in a quite short timeperiod. Disadvantages of Satellite Communication 1. High Cost This is one of the main disadvantage of a satellite communication network. Hence, a careful techno-economic study necessary before the introduction of this system. 2. This isa loss of service to a large area if'a satellite meant to serve the region fails. 3. Itisnearly impossible to correct any defects or to do any needed repair, afer launching the satellite. Hence the reliability is less, So, to avoid this problem of orbit eatasirophic failure, an in orbit spare satellite is deployed. This spare satellite is often leased or sometimes used to augment the communication capacity of the network, 4. Another disadvantage is the propagation delay, introduced by long propagation path, which gives rise to echo. Q23. Mention the advantages of digital satellite communication over its analog counter part. Ans: ‘The analog transmission has been substituted with the digital transmission due to the fast growth in technology. 1, Itis observed that the transfer of audio and other information for terrestrial purposes is through digital communications, also covering satellite networks. Therefore, the best choice for satellite communications is the digital modulation for handling digital signals which digital systems use 2. Achannel consisting of digital data can be portioned to transfer analog signals digitally in the digital communication as it uses similar procedure for each digital signal and is not concemmed with the matter contained in it, 3. Depending on the required congestion, the telephone and data signal are combined and transmitted through a digital satellite link SPECTRUM ALLIN-ONE JOURNAL FOR ENGINEERING STUDENTS. ———— SI GROUP ZS1.12 SATELLITE COMMUNICATION [JNTU-KAKINADA] arth stations are limited. This limi Substitution of eable TV stations with digital receivers could transmit innumerable TV signals over one band oF k, A digital speech signal can be compressed to bit stream at a rate of 4.8 Kbits/sec. By using voice compression methods, which is taken as a merit by digital LEO and MEO mobile communication systems, The transmission of video signals using MPEG-2 (Moving Picture Coding Expert Group) or various video compression methods will yield accuracy at a rate less than 6.2 kbps. The differe advantages of digital satellite communications over analog are listed below. Multiplexing of the data from several sources yields simple and effective transmission, management of digital information in the form of “packets”, provides favourable transformation. Digital systems when compared fo analog systems are insensitive to retransmission noise Digital systems are capable of handling errors especially of reasonable percentage and also find erors and rectify them very precisely. Digital systems render safe communication. Digital systems are adaptable to usage of any component such as microprocessors, miniprocessors, digital switches and Large Seale Integrated (LSI) systems, In satellite laser, microwave communications there is an increased demand for digital transmission methods. The FDM-FM-FDMA scheme used in analog communication systems bestow standard satellite links. But the number of jon is overcome in QPSK-TDMA digital satelite system that can handle them with minimum, transponder capacity loss. 24. Ans: The differences between terrestrial and satellite communi Explain how terrestrial communication is different from satellite communication. jon are mentioned below. Ferrestrial Communication Satellite Communication 1. [In terrestrial communications, the exchange of | 1. | In satellite communications, the exchange of information is done through microwave signals, information is done through RF signals 2. | Communication at ionosphere is not effective, | 2, | In the regions of ionosphere, the communication as ionosphere does not reflect microwaves, as it through satelites is effective, when compared to does RF signals. terrestrial communication. 3. | Transponders (repeater) used in terrestrial | 3. | Transponders used in satellite communication contains circulators does not contain circulators, 4, | The antenna used for transmittingyreceiving the | 4. | Basic antennas are used for ransmitting/receiving signals isa hog horn, which als (RF with tappered horn, ‘a parabolic antenna the sig nals), Inis the broadeast from antenna towers o the earth Itis the broadeast by spacecraft from outer space. 6. | The area of coverage is less when compared to | 6, | The area of coverage of satellite communication that of satellite communications, system is more. Table 1.4.3 Frequency Allocations for Satellite Services. Explain the frequency allocations for satellite services. (Model Papersit, Q2(a) | April-18, Set-1, Q2(a)) (07) Describe the various frequencies used for satellite communications. April-18, Set-3, Q2(b) (or) List the various frequency bands being used in satellite communication. Compare the advantages and disadvantages of different bands considering the effects of propagation media. ApeitMay-17, Set-4, 2 (or) Look for the SIA GROUP Loco {J} on the TITLE COVER before you buyUNIT-1 (Introduction to Satelite Communication & Orbital Mechanics and Launchers) 1.13 ‘and explain the various frequency band allocations used for satellite services. Apri-s, et2, 010) (or) What are the different frequencies used for satellite communications? Explain. ‘Aprit5, Soe, 6) (or) Write about satellite frequency allocations and band spectrum. Apiitay-4,Set-4, 210) (or) Explain the various frequency band allocations used for satellite services. ‘Apiltay 17, Sot, 26) (Refer excluding the effects) Ans: ‘The various frequency bands used in satellite communication are listed below. Band Downlink (GH) Uplink (GH2) 34-42 and 45-48 3.725-7.075 X Band-Military 7.28-7.78 7.9-8.410.7- 11,7 Ku Band-Commercial | 11.7 - 12.2 (Region 2 only) & | 12.75 - 13.25 12.5 - 12.75 (Region | only) | 14.0 - 14.5 Ka Band-Commercial 275-310 Ka Band-Military 435-455 The channel affects the radio wave propagation in several ways, The first and foremost affected parameters are path attenuation, polarization and noise. ‘The aspects to be contemplated are gaseous absorption in atmosphere, absorption and scaticring by clouds, fog, precipitation, atmospheric turbulence and ionospheric effects. ‘The most important impairments of radio wave propagation take place in the troposphere and ionosphere. The frequencies being used for satellite communication range between 100 MHz to 30 GHz. The frequency window of 3 to 10 GHz is least affected and therefore most satellite systems operate within this band. lonospheric effects are significant between 30 MHz and 7 GHz with major effects confined below 3 GHz, Above 10 GHz, absorption in the troposphere begins to become significant. The main constituents which absorb RF nergy in troposphere are water and oxygen. Condensed water ‘vapour existing in the atmosphere as rain, hail, ice, fog, cloud or snow produces the most significant impairment to radio wave propagation. In mobile satellite communication systems, a continuous variation is observed in the earth-satellite path profile whenever there is a motion in the mobile unit, ‘The environment in the vicinity of the mobile plays an important role in the propagation ccharacterist For example, a land mobile satelite path may become shadowed by a building or may pick up scattered signals from trees, buildings etc. as the vehicle travels. Let us consider main effects of troposphere and ionosphere on the bands. atellite communication Gaseous Absorption The first absorption band caused by water vapour is centered around 22.2 GHz while the second band caused by oxygen, is centered around 60 GH. Attenuation due to Hydrometers Condensed water vapours existing in atmosphere in the form of ick Ihydrometers. All types of hydrometers produce transmission impairments wud ete., are generally known as Rain attenuation becomes significant above 10 GHz and therefore when a satellite system is planned to operate above 10 GHz, extensive measurements are obtained at several locations in the coverage area of satelite system, Link reliability of 99.95 % is provided by 11 GHz link with a 6 dB margin SPECTRUM ALLIN-ONE JOURNAL FOR ENGINEERING STUDENTS. ———— SIA GROUPS1.14 SATELLITE COMMUNICATION [JNTU-KAKINADA] For frequencies below 30 Glzthe larger attenuation from clouds in fair weather is equivalent to the attenuation caused by high rain (10 mm/l1g). However, in satellite communications attenuation caused by fog can be neglected. Tonospheric Effet The polarization angle of a linearly polarized wave is rotated in the ionosphere owing to interaction of the electromag- netic wave with the earth’s magnetic field. ‘This phenomenon is known as Faraday effect. Faraday effect decreases as and restricts the f important effects to frequencies lower and around 2 Giz. Faraday rotations at4 and 6 GHz are 9 and 4° respectively. The effect is insignificant above 10 GHz 1 Ionospheric scintillation decreases as — as the radio f frequency is increased, the main scintillation is restricted to frequencies lower than 4 GHz. However, in exceptional cases like magnetic storm, scintillation may create problems upto 7 GH. 26. Discuss the various satellite services in brief. AprilMay-12, S0t-2, 0216) ‘The various services provided by a satelite are Mobile Communication Mobile communication is the area of communication which has seen a large and fastest growth in satellite communication services. This can establish communication between two continents, over oceans or to any mobile units in any part of the world Audio and Video Broadcasting The main entertainment services provided by satellite communication is the audio and video broadcasting. The recently introduced DTH (Direct to Home) technology is an example of this satellite service. It is capable of providing television services to all remote areas irrespective of the terrain contour that are present there. The only thing the customer has to do is arrange a receiving antenna on top of his house. Military Services Every country has to maintain security at its borders. And also has to keep an eye on the enemies. This is also accomplished by means of satellite communication. We have few satellites working for this purpose. ‘The data which is communicated in this services is generally encrypted so that it ean be received by any one, but the receiver that has the corresponding decryption technique can only make out the correct meaning ofthe received data 4 Weather Services Also we need to look after the weather conditions all over the world so that, w calamities, if any, in advance so that safety measures can be taken, Moreover, we can also extend services t0 remote areas, if they are in trouble, because they cannot intimate via, the conventional mode of communication. We should keep a constant observation on remote areas via satellites 1.1.4 Applications . Explain the various applications of satellite communications. e well aware of natural (Mode! Paper. 22{a) | April-t8, Sot-4, 2() | ‘Apres, Sees, Na) | Aprii-5, Set, Nb) (or) List the applications of satellites. Apritay 6, Sa-t, a) (or) What are the applications of satellites? Explain. Ans: Aprilay-1,Set.3, 10) Satellite communications have applications such as, 1. Providing connectivity between large traffic nodes Serving coverage to lange geographical arcas 3. Mobile communication 4. Broadcasting television and sound to the public 5. Providing end-to-end connectivity to users. In fixed telecommunications, the different kind of transmission systems used are optical fibre cable, satellite links and line of site radio relays. Present technologies are employing optical fibre eable in subscriber loop and junction lines for having routes over tens of kilometers or even more. Many transmission systems are employed in applications where the length of trunk routes are large. The transmission systems can be radio relays, optical fibre ete. Instead of using long transmission mediums, a satellite can be placed to cover the complete distance, itis also a bit economical. A satelite ‘usually connects the Intemational Switching Centre (ISC) of one country to the international switching centre of other countries. Therefore, satellites can provide communications even wien the distances are very large. With the help of satellite communication, the telecommunication services can be brought into action between two separate communities. Thus, over trunk routes satellites are usually preferred. They even serve the current terrestrial services, Look for the SIM GROUP Loco {Yj} on the TITLE COVER before you buyUNIT-1 (Introduction to Satellite Communication & Orbital Mechanics and Launchers) The VSAT'S (Very Small Aperture Termin terminals that can be easily mounted on e and helps in avoiding the public switched networks completely. The VSAT networks are more economic than public switched networks and they even remove the delays that are related to PSN (Public Switched Networks). Due to these reasons the AT network is emerging more quickly and are imagined to get reduced to desktop size, Satellite communications are growing rapidly day-by-day. Telecommunication services are well setup to serve mobile by terrestrial cellular radio. The different facilitis of satellite communication are, ) are the mers premises 1, They serve coverage to large areas such as oceans, intercontinental, fight, corridor and vast land mass. ‘They provide international roaming services to mobiles. 3. They serve voice and data communications to ships, aircrafts land vehicles and handsets. 4, They use advance picture compression technique to broadcast television to ships. 5. They provide message and voice communication to pocket size and desktop terminals. 6. Tracing the movements of truck fleets or yatch, 7. Provides worldwide paging service. Satellite communications are used for serving coverage ‘over wide areas within a short period of time. It takes 4-5 years to plan and implement a satellite. Once the satellite is brought into action then the services can be provided throughout, Ifthe services are provided with the help of terrestrial means then it ‘consumes much time for services by even leaving many areas as conserved. Satellite communications has been opted by many upcoming countries such as, India to serve immediate television coverage. In developed areas such as USA and Europe the direct broadcast service of satellite has gained much populacity Q28. Give and explain the 3 different types of applications with respect to satellite systems. ‘Aprlittay-19, Set-4, (a) Areas of Application for Satellite Communications Q27. The list of applications of satellites are, For answer refer Unit 1. Communica 1 purpose (TV and radio broadcasting) Surveillance 3. Navigation 4, TV distribution Satellite broadband internet 6, End-to-end connectivity ‘Telephony and data transmission, 41.1.5 Future Trends of Satellite Communications Q29. Discuss the future trends and advanced concepts relating to the satellite communication. ‘Apritay-19, Set-1, 22(a) (or) What are the future trends of satellite commu- nications? April-t8, Sot, Qt(o) (or) Describe the future trends of satellite commu- nications. Api, Sat, 16) (or) Write about the future trends of satellite com- munications. Ans: ApilMay-44, Set2, N00) Future Trends of Satellite Communication The satellite communication is the fastest developing mode of communication, It has a wider range o ippications. Its main applications include navigation, surveillance, weather prediction, exploitation into space ete, The usage of satellite communication will be increased drastically in the coming future, Ina coming decade, the mobile communication by satellites will reach the new heights. Direct is 9 trademark of modern satellite communication, Home (DTH) technology introduced recently, Intelligent satellites grouped with multiple beam coverage and intersatellite links are likely to play an increasing role in the future, Also, study is being done to reduce the coding bit rate of speech signals to efficiently utilize the available band width, Also the unused frequency bands such as 20/30 GHz ate being considered to remove the frequency congestion problems of existing bands, ‘The satellite communication must be used in education, data collection, meteorology, search and rescue work in near future, As satellite technology is unaffected by geographical conditions, it must be used effectively where cable networks have not yet been established. SPECTRUM ALLIN-ONE JOURNAL FOR ENGINEERING STUDENTS. SIA GROUP:1.16 SATELLITE COMMUNICATION [JNTU-KAKINADA] q The overall global development is possible with the advent of satellite communication and the development of 0% country is possible, as the services provided by satellite communication are identical for both small towns and metropolitan citi Global positioning satellites are much helpful in the field of astronomical studies ‘The future predictions show that there could be a three-tier system in space. (i) Low Altitude Constellation (LAC) (ii) Semi-synchronous Constellation (SC) (iii) Geo Synchronous Constellation (GSC). ‘The advent of single time successful innovation and test models of small satellites has ended and the systematic development of regular space systems based on ultra-sinall vehicles has been started. Earlier, small space-crafts are extensively used for earth remote sensing, ecological monitoring, prediction of earth quake and analyses of ionosphere. ‘The rapid advancement of fiber optic-technology and broadband terrestrial wireless systems suggests that satellite technology needs to pursue some new break-through technologies ‘There are many possibilities to be considered. They are the scope of modeling new orbital systems. However, recent ‘market events conveys that low-earth orbit and medium-earth orbit systems, in particular, would be delayed owing tothe technical constraints and challenges with broadband switching and uncertain business models. Advanced Concepts Relating to Satellite Communications ‘The need of satellite communication is increased in toda tion has increased enormously. Satellite communication isa wireless mode of communication that is able to serve large areas without any hazards, Because the existing transmission wires have limited scope of providing services and also the recently introduced fiber optic communication is very efficient but laying of these wires or fibers over large area is very discomfortable, time consuming the whole world though costly is moving towards satellite communication 's world because the range of communic Satellite communication offers diverse services like communication between continents, military services, naval or defence ices, weather forecasting ete Due to terrestrial drawbacks many remote areas are deprived of these services. But these areas can now be provided with these services by means of satellite communications. This will update them to the current affairs going through the world (One such service recently launched was DTH (Direct to Home) broadcasting services. Also the satellite communication is very fast at its services, since speed is one of the main parameter for today’s, communication field, satellite communication is very necessary. Also satellite ean study the weather conditions all over the world, these reports warn us on the natural calamities so that safety measures ean be taken. Though the whole world is shifting from analog to digital communications, satellites can still support gital communication. Moreover all digital services are implemented using satellites, Exploration of mincrals world wide is also done by satellites. Thus, wastage of manpower and time are prevented. Mobile communication which is mostly used now is also served by satellite, The GSM technology enabled has evolved the direct connectivity between far ends in the world, hus, satellite communication is an essential mode of communication in modern world, Q30. Write a detailed summary about the satellite communication in the 21* century. Ans: Satellite Communication in the 21" Century SPUTNIK-1 was the first satellite sent by USSR in October 1957. This satellite failed but ilustrat launch the satellites into orbit by powerful rockets. Then explorer | was launciied by United successful message was transmitted back to earth by project score satellite in December 1958. that itis possible to tes in January 1958, but the first ‘The first successful satellite was Telstar {and II built by Bell Telephone Laboratories. Its uplink frequency was 6389 MHz and downlink frequency of 4169 MHz with SO MHz bandwidth, ‘These satellites were launched in Lower Earth Orbit (LEO) and Medium Earth Orbits (MEO) due to the incapabilities of the launcher. INTELSAT-I was launched on April 1965 in Geostationary Earth Orbit (GEO) which weighed 36 kg and consisted of two 6/4 GHz transponder, each with 25 MHz bandwidth, Anik 1A was launched in May 1974 by Canada for telecommunication In 1975 and 1980s, all the telephone traffic and video distribution was carried out by GEO satellite systems. GEO satellites can carry data upto 4 Gbps or 16 high power DBS-TV [Direct Broadcast Satellite Television] signal which can deliver many video channels, Look for the SIA GROUP Loco {J on the TITLE COVER before you buyUNIT-1 (introduction to Satellite Communication & Orbital Mechanics and Launchers) 1.17 1.2000, GEO satellites were sent which weighted 10000 kg which can generate 12 KW power and can carry 60 transponders. Apstar-V, a GEO stationary satellite of space system was launched in 2001, which weighed 4845 ke with expected lifetime of 13 years. Itean generate 10.6 kW power and can carry $4 transponders. Satellites are being planned which can generate 25 kW power and can carry antennas with hundreds of heams in the time frame 2005-2010, Some of the GEO, MEO and LEO communication cn inthe tables (1) and (2). satellites and their organizations in 2000 are gi Number of Orbit Application Satell Organisation ' Transponders | Location 1. AMSC-1 American mobile satellite 16 L band 101 W Mobile comporation Reston, VA ‘communication 2. Columbia $1 Columbia Communication | 12 € band Pw Telecommunication corporation MD. 12 Ka band Balhada, 3. Garuda 1 Asia Cellular Satellite (ACES), | 140 with 123° EMobile Indonesia, Spot beams | communication 4. Amabsat 24 Arab Satellite communication |22C band | 26° E Broadeasting organization, SA ID KV band Riyadh, 5. Lstar 1 Asia Broadeasting and 32kV band | 126° E Broadcasting communications Network, Ltd Bangkok, Thailand. Table (1): GEO Satelite Systems Number of Organisation Transponders Constellation Orbit Application Satellites 48LEO | Globalstar, San | 16 spot beams and 6 orbital planes at 52° | Mobile and Satellites | Jose, CA, USA. | each beam can deliver | with 8 satellites per | Satellite 14 channels. CDMA plane at an altitude | communication is used as multiple of 1413 km ss technique and Land S-bands are linked to mobiles. 66 LEO | Iridium LLC, 48 spot beams with 6 orbital planes at 84.6° | Mobile and Satellites | Washington, DC | 7RF channels in 8 MHz, | with 11 satellites per | Satellite USA L-band links to mobiles | planeatan altitude of | communication and Ka-band links to 898 km, Gateways FDMA/TDMA, is used as multiple acces technique. Table (2): LEO and MEO Si Hite System SPECTROM ALLIN-ONE JOURNAL FOR ENGINEERING STUDENTS —— SIA GROUP}8 SATELLITE COMMUNICATION [JNTU-KAKINADA] ‘The revenues earned by the GEO satellite has been increasing since 2000 and just in USA there were 14 million customers of DBS-TV. High-power transmitters and high gain antenna of earth stations have increased the eapacity of GEO satellite and inturn the revenues. The revenues eamed from 1980 through 2010 are shown in the figure 200 3 | s | Revenue (Billions ) or a Figure: Revenues during 1980-2010 In the beginning, satellites were operating at C band and as the number of satellites using C band increased, the next frequency band Ku-band was used, With the advent of internet even Ku-band was filled which forced the satellites to use Ka- band, Astra-k satellite launched in 1998 was used for internet access. GEO satellites use the frequency bands ranging from 3.2 to 50 GHz. Satellites are also used for complex communication architecture and the examples are Very Small Aperture Terminals (VSAT), Witeless Local Loop (WLL), Local Multipoint Distribution Systems (LMDS). 1.2 ORBITAL MECHANICS AND LAUNCHERS 4.21 Orbital Mechanics Q31. What are the various forces a fing on the satellite? Explain. (Model Papers, Q2(b) | April-t8, Set-4, Q4(a) (on What are the various forces acting on the satellite in the orbit? Explain with necessary expressions. Aoritsy-17, 802, a6) Ans: ‘The various forces ating onthe satellt in an orbit are, 1. Centrifugal force 2. Cenripetal fore. Centrifugal force is the force which acts away from the center of a circle, The centrifugal force is occurred due to orbital velocity of satellite moving in a cirele, The expression for centrifugal force is given by, reo a Where, in Mass of satelite y— Velocity of satellite in orbit Radius of a circle, Look for the SIA GROUP Loco {J on the TITLE COVER before you buyUNIT-1 (Introduction to Satellite Communication & Ort ‘The following figure shows the various forees acting on the satellite in an orbit Figure Centripetal force is the force which acts inwards (or) towards the center of the earth as shown in figure. Centripetal force is occurred due to gravitational force of the earth, The expression for centripetal foree is given by, _ Git Where, G ~ Gravitational constant MM ~ Mass of earth, ‘The two opposing forces acting on the satellite are equal ‘Then from equations (1) and (2), we get, _ GMm - = = vet 3) r ‘The above expression is referred as Kepler’s third law. tal Mechanics and Launchers) 1.19 (Q32. Derive the expression for the time period of satellite's orbit. Ani ‘Apb-8, St, 0) The time required for one complete revolution of satellite around a planet is referred as “Time period’ of the satellite and is denoted with “7”. Its unit is ‘second’. Ifa satellite of mass, m revolving around the planet of ‘mass, Mata height, b with velocity, vthen, the centripetal force required for the satellite to revolve around the earth in circular orbit is offered by the gravitational foree between earth and satellite On equating gravitational force and centrifugal force, GMm _ mw? “ -) Where, r ~ Orbital radius GMm _ mor? ; GMm P ou ae e r |e Vlcm an 7- +6) vom ‘ On replacing GM = 4 and r= a (semimajor axis) in equation (3), = “ Therefore, equation (4) represents the time period of satellite's orbit SPECTRUM ALL-IN-ONE JOURNAL FOR ENGINEERING STUDENTS. SIA GROUPS1.20 SATELLITE COMMUNICATION [JNTU-KAKINADA] Q33. Derive the equation of the satellite orbi coordinate system. in polar Ans: ‘The satellite rotates round the earth in a path called orbit This orbit could be a circular path or an elliptical path, Letus consider the geocentric coordinate system, in which earth is center and horizontal and vertical reference pl \ith the equatorial and polar planes as shown in figure (1). res coincide Let the distance between the center of earth and the satellite is". ‘The gravitational foree that is exerted by the earth on satellite is given by, -GMpm F 7 Where, 7 indicates, direction of gravitational fo radial and ve" sign indicates force is radially inwards. Where, G Universal gravitational constant = 6.672 « 10°" Nmm'hkg? M, ~ Mass of earth m ~ Mass of satellite From the general definition of force, ar dt F @) Equating equations (1) and (2), we get, GM, = Constant (40)] a) Equation (3) isa second order differential, but it involves differentiation of a unit vector F. Thus, to solve this we need to consider another coordinate system (with its unit vector constant) x,,.¥9, =, a8 shown in figure (2). Figure (2) Here, xy.) is called orbital plane because the orbit is being taken as reference. Thus, substitute, F = xpiy + yyy in equation (3), we get, : Wei 9h) — (4,8, + PF, )+ Sr ttaky +8) 4) co) Expressing the above equation into polar coordi system, we get, 5708 fy =f, 00805 ~4) sing Sy cosdy +h sindy ‘Substitute these in equation (4) and comparing the vector components of r, and 6,, We get, (Hale) From above two equations, we get, dry He de it wi 1" Tr eeos(, —0,) (5) Lecos(y ~Oy) Look for the SIA GROUP LoGo {J} on the TITLE COVER before you buyUNIT-1 (Introduction to Satellite Communication & Orbital Mechanics and Launchers) 1,21 Where, ilatus rectum P of the elliptical orbit Here, ~ Orbital angular momentum of the satellite M, bt —Kepler’s constant 98 * 10° km’ e— Becentticity of the elliptical orbit 6, ~ Constant, it gives the orientation of ellipse ‘with respect to orbital plane axes x, andy. Equation (5) is the satellite orbit equation, P determines the semilatus rectum value ofthe ellipse, ‘e*the eccentricity and 6, the orientation. By changing these parameter values number of different elliptical orbits can be derived. For a circular orbit e = 0 thus equation (5) reduces to, r= Pl lit Q34. State the kepler's laws. Discuss its importance in satellite communications. AprilMay-18, Set, 03a) (or) Define Kepler's laws of planetary motion with relevant mathematical expressions. Ans: Apr May-14, Set-1,02(a) ‘The Kepler’s laws are defined as follows, Kepler's First Law It states that the path followed by a satellite around the primary is an ellipse, The center of mass of the two body system termed as heavy center is located at one of the focus of the ellipse, Earth being the massive body center is at one of the focus of the elliptical orbit of the satellite revolving around it. ‘The eccentricity of the orbital path with semimajor axis a and semiminor axis b is given by, a for ellipt orbit O= 5h Q By conservation of angular momentum, jua(l—e") Substituting */ in equation (2) we get, dA a @) di Equating the equations (1) and (3), the time period “7” can be expressed as, This proves that Kepler's third law is valid for any orbiting satellite Q36. Describe the orbit and explain how the satellite is located with respect to earth. Ans: Orbit The path ofa celestial body or an artificial satelite as it revolves around another body is known as “orbit”, Location of a Satelite in an Orbit with Respect to Earth The lo carth can be determined by considering a geocentric equatorial coordinate system as shown in figure (1), tion of a satell in an orbit with respect to Figure (1k: Geocentric Equatorial Coordinate System In figure (1), z,axis represents the rotational axis of the earth which is through the geographical north pole, The x, axis represents a line joining the centre ofthe earth towards a fixed location in space ealled first point of aries, First point of aries is the direction of line joining the center of the earth to the sun at the vernal equinox (21" March in the Northern Hemisphere) The coordinate system translates in space when earth revolves round the sun and it doesn’t rotate with the rotation of the earth, The x, direction always points along the first point of aries irrespective of earth's position around the sun. The (x, ) plane is called an equatorial plane due to the presence of the equator. Any object (satellite) in the above coordinate system be located by its Right Ascension (RA) and its declination (6), Right Ascension (RA) is the angular distance measured eastward in the equatorial plane from the X, axis, The two points ‘where the orbital plane intersects the equatorial plane are called nodes, If the satellite moves upwards (south to north) through equatorial plane itis called as ascending node and if it moves downwards (north to south) through equatorial plane itis called 1s descending node. The right ascension of the ascending node is denoted by ©. ‘The inclination angle (i) is the angle measured between the orbital plane and equatorial plane, itis shown in figure (2). Perigee |_—Saelite exit Look for the SIA GROUP Loco {Yj} on the TITLE COVER before you buyUNIT-1 (introduction to Satellite Communication & Orbital Mechanics and Launchers) 1.23 ‘The location ofthe orbital plane with respect to the equatorial plane is done by © and /, but for the location of the orbital coordinate system with respect to the equatorial coordinate system requires the argument of perigee west (0). The angle measured along the orbit from the ascending node to the perigee is termed as argument of perigee (0). To completely specify the orbit of satellite at time 1, requires six parameters eccentricity (e), semimajor axis (a), right ascension of ascending node (Q), inclination (i), angument of perigee («) and time of perigee (/,) or mean anomaly (M).. Q37. What are the various orbits in which satellite can exist? Discuss. Ans: Satellite orbits are defined as the specific paths in which satellite will revolve around the earth. It is not fixed that the earth should have only one satellite, These satellites have their own specific paths called orbit paths and these orbits are placed around the earth with a fixed small radius or large radius depending on the functioning of the satellite. By this it is clear that different types of orbits exist. The various types of orbits used in satellite communication are, 1. Low earth orbit [LEO] 2. Medium earth orbit [MEO] 3. High earth orbit [EO] 4. Geosynctronous orbit [GEO] 1. Low Earth Orbit [LEOJ: An orbit whose limits are between 750 km and 1500 km ic., upper orbit altitude 1500 km and lower orbit altitude ~750 km is known as low earth orbit, It is abbreviated as LEO. The power required for the satellite to orbit in LEO is low. Since propagation delay is directly related to orbital height, it is also low for LEO, 2. Medium Earth Orbit [MEO]: The abbreviation “MEO” stands for medium earth orbit. The orbital height of MEO is limited between 1500 km and 36000 km ic., lower orbit altitude is at 1500 km and upper orbit altitude is at 36000 km. It is used in navigation applications and propagation delay due to this is moderate. 3. Geosynehronous Orbit [GEO|: "GEO" stands for geostationary orbit. It is an orbit with circular shape. In other words, the orbit with zero inclination and eccentricity. The height of geo sationary orbit is 35,786 km, igh Earth Orbit [HEO}: The full form of +HEO" is highly elliptical orbit. It isan elliptical orbit Characterized by a low altitude perigee and a quite high altitude apogee. Dwelling (staying) at a point in the sky for ¢ long time is a benefit for these elongated orbits during the approach and descent from apogee. Examples: Molniya orbit, Tundra orbit. Apes, Figure: Representing Four Types of Orbits Q38. Compare the three types of satellite orbits. (or) Compare LEO and MEO satellites. (Mode Paper, 22(0)| Aprii-t5, Set-1, 7(b)) (Refer Only LEO and MEO) (or) Compare GEO and MEO satellites. ‘April-15, et-2, Q7(b) (Refer Only GEO and MEO) SPECTRUM ALLIN-ONE JOURNAL FOR ENGINEERING STUDENTS ——— SIA GROUP{/>1.24 SATELLITE COMMUNICATION [JNTU-KAKINADA} Ans: The comparison between the orbits is listed as follows, Geostationary Orbit Low Earth Orbit (LEO) Medium Earth Orbit (MEO) 1. | An orbit that is circular in 1. | An onbit that is limited between | 1. | An orbit that is limited between shape, located at the equatorial ower orbit altitude of 500 km. lower orbit altitude of 1500 km plane, where the satellites travel and upper orbit altitude of and upper orbit altitude of eastwards with the speed equal 1500 km is known as Low Earth 36,000 km (ie., GEO) is to the earth’s rotation is known Orbit abbreviated as LEO. known as Medium Earth as Geostationary orbit. Orbit abbreviated as MEO. 2. | The height of orbit is 35,786 km | 2, | It’s range of orbital height is | 2. | It’s range of orbital height is, 750 km-1,800 km 10,000 km-15,000 km 3. | The one-way delay of GEO 3. | The one-way delay of LEO 3. | The one-way delay of MEO satellite is 119.3 ms satellite placed at a height of lite placed at a height of £800 km is 2.7 ms typically. 10,355 km is typically 34.5 ms. 4, | A satellite placed in this orbit | 4. | Number of satellites placed in| 4. | Number of satellites placed in has an orbital period of 23 h this orbit will give accurate this orbit will give accurate 56 min 4.1 see which is equal to global coverage. global coverage. one sidereal day and covers those regions that are abundant populated in the world 5. | Propagation delay is large by | 5. | Propagation delay is low about | 5. | Propagation delay is which voice and time sensitive Sto 10 ms moderate about 30 to 100 ms data protocols are disturbed Compared to LEO and MEO | 6. | Spectrum is utilized efficiently. | 6, | Spectrum is utilized efficiently, spectrum efficiency is less. 7. | Ithas comparatively immense | 7. | It is feasible to utilize smaller | 7. | It's feasible to utilize smaller path loss terminals as it has low path loss terminals as it has low path Foss, 8. | Life span ofa battery is lengthy | 8. | The duration of the life of 8. | Compared to GEO, life time of since there are few number of battery is less due to more battery is less in MEO since, sun eclipses. number of sun eclipses, which there are more sun eclipse increase battery charge/ period extending the battery's discharge eycles. charge/discharge cycle 9, | Doppler effect is negligible. 9. | Doppler effect is not negligible | 9. | Doppler effect is not negligible therefore it should be accounted. |__| and therefore should be accounted. 10,] It has fixed strength of signal | 10.] Unstable signal strength results | 10.] Unstable signal strength results siving a uniform range of in fluctuations in range and in fluctuations in range and round satellite elevation angle elevation angle Table Look for the SIA GROUP Loco {J} on the TITLE COVER before you buyUNIT-1 (Introduction to Satellite Communication & Orbital Mechanics and Launchers) 1.25 Q39. What is a geosynchronous orbit? Discuss the advantages and disadvantages of these orbits. Ans: Geosynchronous Orbit A geosynchronous orbit is defined as the orbit, which allows the satellite to remain stationary for an observer on earth. To make satellite orbit stationary the satellites orbital period must be equal to the rotational period of the earth. However, the rate of rotation of earth is similar to the rate at which satellite orbit moves. According to the principle of working, any orbit with a period ‘equal to carth’s rotational period is referred as geasynchronous orbit. The distance ofa geosynchronouis satellite from the centre of the earth ean be ealeulated by means of Kepler's third law. This law is relation between orbital period, orbital radius and mass ofthe earth. By considering earths rotation period in seconds and Earth's mass in kg. distance of 42,200 km is obtained from centre of the earth to the orbit, Subtracting earth radius from above distance we get the distance from earth surface as 35,900 km or 22,300 mi Application of Geosynehronous Orbit The satellites present in geasynchronous orbit can be easily tracked by the receiving antennas. However, they remain pointing towards the satelite atthe same position in the sky. Advantages Geosynchronous orbits have many advantages as follows, The operational demands on the terminals are reduced by the stationary satellite within its coverage region due to the ‘minimum tracking requirements and invariant transmission parameters, 2, Ithas minimum Doppler shift. 3. The constant geometry is responsible for less predictable interference to and from other radio systems. Disadvantages 1. Launching of satellites in these orbits is complex and expensive 2, Due to large satellite range, the propagation delays are of 250 ms duration which are quite significant. 3. Itdoesn’t provide adequate coverage to the locations above +76° altitude. The signal quality beyond these altitudes is very poor due to the appearance of geostationary satellites at very low elevation angles 4, Large propagation losses are produced from the areas with low elevation angles of satellites due to shadowing from obstructions i., buildings, tree etc. Q40. Write short notes on Geosynchronous orbit and Geostationary orbit. Ans: ‘ApruMlay-49, et2, atte) Geosynchronous Orbit For answer refer Unit-1, Q39, Topic: Geosynchronous orbit stationary Orbit ‘An orbit that is circular in shape, located at the equatorial plane, where the satellites travel eastwards with the speed equal to the earth’s rotation is known as Geostationary orbit. The height of orbit is 35,786 km. The one-way delay of GEO satellite is, 119.3 ms. A satellite placed in this orbit has an orbital period of 23 h 56 min 4.1 see which is equal to one sidereal day and covers those regions that are abundant populated in the world. Propagation delay is large by which voice and time sensitive data protocols isturbed. Compared to LEO and MEO spectrum efficiency is less. It has comparatively immense path loss. Life span of a battery is lengthy since there are few number of sun eclipses. Doppler effect is negligible. It has fixed strength of signal giving a uniform range of ground satelite. SPECTRUM ALLIN-ONE JOURNAL FOR ENGINEERING STUDENTS ————— SIA GROUP} are1.26 SATELLITE COMMUNICATION [JNTU-KAKINADA] Q41. What are the differences between the geosynchronous and geostationary orbits? Ans: Apil-18, Sets, Qc) The differences between geosynchronous and geostationary orbits are mentioned in the below table Geosynchronous Orbit Geostationary Orbit 1 | ttisa circular orbit 1 | Iisa non-circular orbit 2. | This orbit exists in equatoril plane. 2. | This orbit also exists in equatioril plane 3. | There is no inclination in orbit 3. | Orbit is inclined according to the equatioril plane. 4. | Eecentricity is always zero Eccentricaity exists and is not equal to zero, 5. | Time period of satellite is 23 hrs 56 min 4.1 sec Time period of satellite is 23 hrs 56 min 4.1 see 6. | A planet will have only one geosynchronous orbit. } 6. | A planet can have several geostationary orbits Q42. Discuss the factors which determine the choice of orbit for a communication satellite. (or) Aprivay-17, Set-4, 03(a) What are the orbital parameters required to determine a satellite's orbit? Name and explain them. Ans: AprivMay-14, Set-3, 02(6) According to the first Kepler’s law, satellite moves along a conic on the orbital plane and follows the equation. P 1 ecosy Where, P Parameter eB icity and. v—Central angle or true anomaly. An elliptical satellite orbit is shown in figure (1). The different orbital parameters required to determine the satellite orbit are, 1 2 3 4 5 6, Eccentricity (e) Semiminor axis (a) Inclination (i) The right ascension of an ascending node (Q) The argument of perigee (() ‘Time of perigee (t,) Look for the SIA GROUP Loco D on the TITLE COVER before you buyUNIT-1 (Introduction to Satellite Communication & Orbital Mechanics and Launchers) 2 Eccentricity (@) The type of conic is determined by the value of the eccentricity as, 0 Grete <1 Ellipse 1 Parabola >I Hyperbola| It shows the ellipticity of the orbit. Semimajor Axis (a) ‘The size of the conic orbit is described by the semi major axis (a). ation (i) The angle between the plane of the satelite orbit and the equatorial plane measured at the ascending node is termed as inclination angle (i). An ascending node is the point at which the satellite erosses the equatorial plane ‘moving fom south to north and the point at which it crosses the equatorial plane moving from north to south is termed as descending node. ‘The line joining the ascending node and descending node is termed as line of nodes. This parameter can be understood by considering a geocentrie-equatorial coordinate system as shiown, in figure (2). Figure (2): Geocentric-equator Right Ascension of an Aseending Node (©) The angle between the x-axis and the ascending termed as the right a in figure (2). Argument of Perigee (@) The angle between the line of nodes and the line joining the perigee is termed as the argument of perigee («). It lies in the orbital plane of the satellite as shown in figure O. ime of Perigee (t,) Itisthe amount of time elapsed since the satellite passed the perigee. Sometime f, is expressed in terms of mean anomaly (MM) and they are related as tu 2n Where, T Satellite period. 043. Ans: 44, Ans: 1.27 ‘A satellite is in an elliptical orbit with a perigee of 1000 km and an apogee of 4000 km. using a mean earth radius of 6378.14 km, find the period of the orbit. Aprit8, Sot, 0300) (or) Asatellite is in an elliptical orbit with a perigee of 1000 km an apogee of 4000 km. Find the period of the orbit and eccentricity of the orbit. ‘AprvMayet7, Set, 110) Given that, For a satellite in an elliptical orbit, Perigee ~ 1000 km Apogee ~ 4000 km Radius of Earth, a = 6378.14 km Period of orbit, T=? ‘Then, the expression for period 0 orbit is given by, In the above expression ‘a’ can be expressed as, _ 2r+Perigee + Apogee 2 __ 2637814 +1000 +4000 a * w= 3.986 10°km/s!] = 6.93087 x10 8325.18 see, = 8325.18 see The period of the orbit, 1 = $325.18 sec ALEO satellite orbits at an altitude of 250 km above the earth surface. Calculate the period of the satellite orbit, if the orbit is circular. April, Set, 02{6) Given that, For a LEO satelite orbit, Distance from earth surface (r) = 250 km Period of satellite orbit (7) = ? SPECTRUM ALL-IN-ONE JOURNAL FOR ENGINEERING STUDENTS SIA GROUP.1.28 Then, expression for time period of satellite orbit is given by, rok wo a . Substituting *r’ and “1 values in equation (1), then, (25010899? 678 125000000 19949937 34 735000000 _ 45 5 19949937 38 a8 39.3 see 1.2.2 Look Angle Determination Q45. Define the terms: () Sub-satellite point (li) Apogee and perigee (il) Ascending and descending nodes (iv) Sun-synchronous orbit (v) Angle of inclination. (or) Define subsatellite point of a satellite. Aprilt8, Set, Q1(a) (Refer only Sub-satellite Point) Ans: Sub-satellite Point Sub-satellite point is defined as the point where astraight line drawn from a satellite to the center of the earth intersects the earth’s surface, The location of sub satellite point can be expressed in two ways, (a) (b) In terms of latitude and longitude. In terms of co-ordinates of rotating system as, | In general, sub satellite points are preferred for 2D map display of satelite path, «i Apogee and Perigee “Apogee: Apogee is defined as the point on the satellite orbit, whichis long distance away from the middle of the earth ‘The distance from point to the earth is known as apogee distance. Inis denoted by “4” ‘The apogee distance “A” can be calculated in two ways. They a Perigee: By wing perigee distance ‘p” and velocity at perigee ‘p’ value, it is possible to determine the apogee distance as follows, 2y2 Pv 2 psp SATELLITE COMMUNICATION [JNTU-KAKINADA] iii) Ascending and Descending Nodes An ascending node is the point at which satellite crosses the equatorial plane moving from south to north, An descending node is the point at which satellite crosses the equatorial plane moving from North to South, (iy) Sun-synehronous Orbit Sun synchronous orbit is defined as the orbit in which satellite passes every location at the same time. It maintains angle in between the orbital plane and earth-sun direetion during the complete year. Based on the distance from the earth, itis treated as LEO orbit. These sun synchronous orbits are mosily used in remote sensing and earth monitoring applications. () Angle of Inclination The angle between the plane ofthe satellite orbit and the equatorial plane measured at the ascending node is termed as inclination angle (/). An ascending node is the point at w the satellite crosses the equatorial plane moving from south to north and the point at which it erosses the equatorial plane ‘moving from north to south is termed as descending node. ‘The line joining the ascending node and descending node is termed as line of nodes. This parameter can be understood by considering a geocentric-equatorial coordinate system as shown in figure Figure: Geocentric equatorial Coordinate System 46. Define Ascending node and Descending node. Mention the apogee and perigee height. Ans: ‘Aprittday-19, Set, O10) Ascending and Descending Nodes Por answer refer Unit-l, Q45, Topic: Ascending and Descending nodes. Apogee and Perigee Height When a satellite in an orbit is at its furthest point from the earth is called height and when it is at its closet point is called perigee height. Apogee and perigee heights are expressed as, r=h,+r=all +e) rah+r alle) Where, +r, — Distance of apogee from earth rr, ~ Distance of apogee from earth 1 — Radius of earth i, — Apogee height 4, Perigee height Semi major axis e—Fecentricity, Look for the SIA GROUP Loco {J on the TITLE COVER before you buyUNIT-1 (Introduction to Satellite Communication & Orbital Mechanics and Launchers) 1.29 Q47. Draw the geometry of a geostationary link showing elevation, azimuth and range. Ans: Geometry of a Geostationary Link ‘The geometry of geostationary link is as shown in figure(3). Tagore age 7 Figure (1) Elevation Angle (EL) Itis the angle which a satellite makes with the tangent (ic. the local horizontal) at the earth station, It is given as c0s(y)-o siny Where, vy, — Coverage angle cos y= cos 0, e0s(6, — 4) For geostationary link = wr evs feos @, cos(6, — 4] Azimuth Arimuth ofa satellite isthe angle which the satellite direction makes with the direction of the true north, It can be measured clockwise (east wards) from geographic north to the projection of the satellite path on a (locally) horizontal plane at the earth station as shown in figure (2) Sub-satellite points Figure (2) It can also be illustrated as shown in figure (3). Local vertical ast Progeetion of path on leeal horizontal plane Figure (3) SPECTROM ALLIN-ONE JOURNAL FOR ENGINEERING STUDENTS ——— SIA GROUP{>1.30 SATELLITE COMMUNICATION [JNTU-KAKINADA] Range The region where the satelite is visible to the earth can be considered as its range The elevation angle should be always greater than the minimum value such that the satellite is visible 10 an earth station, generally Range ‘c" can be given as, nly) cos(n) d= 6.6235 (For a geostationary satellite) 48, What is slant range and explain its importance in satellite communications. Ans: Slant Range Slant range *A’ is the range between an earth station aand a satellite, Range *A’ in the figure (1) is the slant range of a satellite. — —— Figure (1: Slant Range of Satelite Importance of Slant Range The tange(slant range) & from an earth station to a eosynchronous satellite is given as, R= Ry + 2RAR, R) (1 €05 6 c0s 0) In terms of elevation “2°, it can be given as, R=(R+RY+R? e+sin'| Be cos LR Ry Where, AR, ~ Radius of earth (R= 6378 km) k E-Blevation angle 2RAR, +R) * sin Altitude of the satellite (R, = 35786 km) The radiation between elevation angle and range from earth sation to satellite (R) is shown in the following figure (2), Satellite Earth station Figure (2) Hence, the value of *R’ is maximum, °° is minimum and °R” has a minimum value when *E” is maximum, Se, Rg. = (R,+ Ry 2R(R, +R) fF The minimum elevation angle required to visible the satellite (o the earth station is 5°. ie. £,,, = 5°. Hence, substituting £,,. = 5° in equation (1) we R= 41,127 km Which gives satellite round-trip delay of, 2d 214s (c= 3% 10° m/s) Now, the maximum value of £ is 90", from the obvious reasons. For which will have minimum value of range. 2 [8 =(R, + RP + R2 —2R(R+R, sin] E+ sin | RR se] R, 35786 km This corresponds to the satellite round-trip delay of, dd _ 2R ar 0238's, Hence the value of slant range determines significant parameters in satellite communication like, elevation ang! satellite round-trip delay and also slant range plays vital role in determining path losses of a satellite. Look for the SIA GROUP Loco wD on the TITLE COVER before you buyUNIT-1 (Introduction to Satellite Communication & Orbital Mechanics and Launchers) Q49. State and derive the expressions for the look angles. Give necessary diagrams. AprilMayt9, Set, 03(0) (or) What are look angles? How do you determine? Explain with the help of neat diagrams. (AprilMay-17, Set-1, Q3(b) | AprivMay-14, Set-3, 02(3)) (or) Define the azimuth angle and de sion for it, the expres- ‘Apri, $2, a3(a) (Refer only Determination of Azimuth Angle) (or) Define elevation angle and derive the expression for (Aprit-5, Sot-2, 23(a) | AprilMay-17, Set-4, 23(0) ‘Apriliay-18, Sot3, 03(3)) (Refer only Determination of Elevation Angle) Ans: Look Angles Look angles are the angles at which the communication between earth station and a satellite is possible by pointing the transmitting antenna towards its respective satellite. These les includes both Azimuth (A) and elevation angles (E), h are measured with the help of latitudes and longitudes arid with respect to the carth station. Generally, these angles communication is opted between the stations. Geostationary satellite refers to the satellites having circular orbit along the earth's equator such that all points in orbit are equidistant from surface of earth. are set, such that effectiv Figure (1) depicts the determination of look angles and the positions of the satellite and the earth station. toa Howocad iene Figure (1): Look Angles Determination 1.31 Determination of Elevation Angle Elevation angle is defined as the angle at which the axis of antenna is rotated vertically facing towards the satellite in horizontal direction, Consider the figure (2), Local horizontal I: For Calculating Elevation Angle From the figure (2), we can write, 90° +E =w B= y 90° cos{E] = cos(y = 90°) cosfE] = sin y a According to sine’s law, Rg ad siny Sing siny = a Now. Apply cosine rule for figure (2) to get the relation between the vectors joining the centre of the earth, satellite and, earth station. ie, =a +B? 2ab cosy Similarly, R44} 2Rr, cos y wf fi. Rw 2E cos R | R 7 pom] a) Substitute equation (4) in equation (3), we get, Pam] cos SPECTRUM ALL-IN-ONE JOURNAL FOR ENGINEERING STUDENTS SIA GROUP.1.32 In case of geostationary satellite (ideal), the cosy and siny Values can be calculated from the point (line joining from the centre of the earth to the satellite), which is on the equator. Determination of Azimuth Angle Azimuth angle (A) is defined as the angle at which the antenna pointing horizontally is rotated in clockwise direction around its axis vertically. ‘The calculation proceeds in finding the exact location of the point (ie., a line drawn from the centre of the earth to the satellite). Consider the following figure (3). re (3): Geometry for Azimuth Calculations The polar angle Cis given by, c=\t, Where, 1 1, — Latitude at point 1, —Latitude at point 2. ‘The angles X at vertex A and angle ¥ at vertex B are related as, an siy_ay) - £M0.5Csi 0.50 Ip) (an[0.50"— 4] ‘snl 0.5(l+/s)] c0t(0.5C} 6010.54 In) sin[ 0.50, +19) ‘The Azimuth angle and elevation angle should be in range of 0° to 360° and above the minimum value respectively, for the visibility of satellite from the earth station. Q50. A satellite is orbiting round the earth at 42124 km. The earth station is looking at this satellite at an elevation angle of 35° . Calculate the stant range, Make suitable assumptions and give reasons for making such an assumption. tan{.s(y+ X)]= Ans: Given that, For a geostationary satellite, ‘The distance at which satell d= 42124 km orbiting round the earth, Elevation angle, 0, = 35°, Slant range, d,= SATELLITE COMMUNICATION [JNTU-KAKINADA] The expression for slant range of a geostationary satelite, a) = Q) 4 6378cos3: 42124 _if 9378x0819] 4, 2124 ° soos" =cos ' (0.124) 523.582 42124 35 2.87735 y=47 87 @) By Substituting R_, d,y in equation (1), we get, d, = 4214 | 1+] m4 = 2124 (14+(0.151 ° -0.8 0.671] = 22124 (1.023 -0.208y'2 y = 2i (0.80)? = 42124 « 0.906 38164.344 km. Slant range,d, = 38164344 kim Considering the radius of earth (R) as 6378 km Look for the SIA GROUP Loco on the TITLE COVER before you buyUNIT-1 (Introduction to Satellite Communication & Orbital Mechanics and Launchers) 1.33 Q51. The longitude and latitude of an earth station are 73°E and 19°N. Calculate azimuth and elevation angles from the earth station to a satellite launched at 135°E. Ans: Given that, ‘The longitude of an earth station (/.) = 73°E ‘The latitude of an earth station (L,) = 19°N The subsatellte point (1) ie., the satellite is launched at 13°F Caleulate, 1. Azimuth angle 2. Blevation angle Consider a geostationary satellite (ic., £, 0) 1, Azimuth Angle As the given satellite is launched to the east ofthe earth, in northern hemisphere, the expression for the azimuth angle is stiven by, A,= 180° 0 -() Here, is the intermediate angle and is calculated as, tan(l, a) oe an Sia, sje 2) sin 19 , ‘| 188 © E8"[0325 = tan" [5.776] = 80.1789 +a =80,18 Q) Substituting a value in equation (1), we get, A= 180° 0 180° — 80,18, = 99,82° Azimuth angle, A, = 99.82°. 2. Elevation Angle The expression for the elevation angle £7 is given by, if (6:5107345 — cosy) siny Elta -y @) Where, yis the central angle, which is obtained from the expression, 0s y= 0s J, cost, ~1) A) = cos 19 cos(135 — 73) = 0.9455 * 0.4694 0.44389 cosy = 0.44389 y= cos (0.44389) 63.64 Substituti value in equation (3) we get, (6,6107345 =a} soul tan [: 6107345 —cos(63.647) 63.647 in( 63.047) 6.6107345 0.44389 mw [Spar | oe 6.1668445 (Sasa ] -63.607 = tan (6.882033) -63.647 = 81,73245 ~ 63.647 = 18.085 Q52. An earth station has a longitude of 99.5 west and latitude of 29.5° north. The satellite has a longitude of 143° west. Find the azimuth and elevation angle. Given that, For an carth station, Longitude, /, = 99.5° West 29.5° North 1 Latitude, L, Longitude of satellit 143° West Azimuth angle = Flevation angle =? =0) Consider a geostationary satellite (i. Azimuth Angle As the given satelite is launched at the west of the earth, in northern hemisphere, the expression forthe azimuth angle is given by, 180° +o, a Where. ris the intermediate angle and is ealeulated as, ian [1.929] 2.596 SPECTRUM ALL-IN-ONE JOURNAL FOR ENGINEERING STUDENTS SIA GROUP.1.34 SATELLITE COMMUNICATION [JNTU-KAKINADA] Substituting a value in equation (1), we get, 80° + 62.596 = 242.596 142.596 = Azimuth angle, A, Elevation Angle The expression for the elevation angle Eis given by, oy [sai o siny Where, yi the central angle, which is obtained from the expression, cos y= cosh, cos(l,~ 1.) = c0s(29.5*) eos (143° 99.5°) ~ c0s(29.5°) cos(43.5°) = 08704 « 0.7254 = cos 7= 0.6314 = 050.6314) > #5085 Substituting y value in equation (2), we get, Pa, sates Set) s . [ose os | sons 0.7755 = tan [3% | soss ~ 82.61 ~ $0.88 > BI=31.76 = Elevation angle = 31.76 1.2.3. Orbital Perturbations Q53, Explain in detail about of Orbit perturbations. (ode! Paper, @2()|ApilMay9, Se, 2310) (on) Write short notes on orbital perturbations. (Aorist, Set2, 420) | Apeiay-17,Set-t, 16) (or) Explain the various orbital perturbations for a satellite. Aorta 16, S02, 021) (or) What are orbital perturbations in satellite communication. Aaa. 16,Se4, 020) Ans: There are several gravitational forces such as gravitation attraction, asymmetry of earth's gravitational field acting upon the satellite. Under initial conditions with only gravitational attraction as the foree on satellites, the true orbit is referred to as an elliptical orbit whose parameters are constant with time, this orbit is called the “Keplerian orbit’, But the different gravitational effects such as gravitational field of sun and moon, asymmetry of earth's gravitational field and solar radiation pressure to which satellite and earth responds and leads to rate of change of subsatellite point of geosynchronous satellite Look for the SIA GROUP Loco J} on the TITLE COVER before you buyUNIT-1 (Introduction to Satellite Communication & Orbital Mechanics and Launchers) 1.35 “Many techniques have been implemented so that an orbit can be defined by including all orbital perturbations, For ‘communication satellites the process of incorporation of orbital perturbations is as follows, Initially the perturbations are assumed to be ineffective for the spacecraft following the Keplerian orbit. Thus the parameters will be (a, €, 1,9, 1,0) Where, a Semi major axis Eccentricity 1, ~Time of perigee Ascending node Inclination © Argument of perigee. Ata time instant“, the effect of perturbation fo assumed to be nill and the parameters are denoted as, (44, €5 fy Mp pO) Considering, the forces exist and the parameters change linearly with time at uniform rate, These parameters become as, ade diy dQ di de dt de” dt” dt de’ dt By utilizing these parameters, the position of satellite at a time instant *,”is calculated by a Keplerian orbit with above parameters it de This technique or method is more applicable in practical applications as it allows the usage of theoretical calculations de 4, =the? GW of empirical values depending on satellite observations, However, there are many other considerations that needs to be followed as, The orbit considered should be perfectly elliptical but, in ly an ellipse. Hence, orbital period should be defined carefully general the orbit is not ex: 2. The orbit is not a Keplerian orbit hence, there must be other forces that can change the position of geosynchronous satellite with respect to time, 3. ‘The time period between successive perigee passages ic period must be oftenly defined as. the satellite doesn’t come back to the same point in one revolution, known as anomalis Effect of Earth Oblateness ‘There are several imegularities present on the surface of the earth. Due to these the earth is neither an ellipse nor a sphere, itis referred asa triaxial ellipsoid. The irregularities present on earth ate, 1. The carthis fattened at poles ic., the equatorial diameter is more than polar diameter by 20 kms. 2. The average density is high at many regions of earth, commonly known as Mascons. 3. Even though the noncireularity is small the equatorial radi is not constant. Due to these irregularities the gravitational field of earth is not same in all regions of earth. Thus, the force on orbiting satellite depends upon position of earth, A geosynchronous satelite in its orbit is completely different from a satellite in LEO (Low Earth Orbit). The weight of geosynchronous satellite is nill, when it is in its orbit, Therefore, even a small force can result in drifting of geosynchronous satellite from its mean position. It is essential for the satellite to maintain its longitudinal position in the orbit but, the forees that exist near equator will not allow to shift its position either in eastern or western side of the orbit plane, The way or may not be in line with the gravitational force at the center of the earth, Thus, based on this there will be acceleration or deceleration that leads to the change of longitudinal position of the satellite. resulting Fore The presence of Mascons and equatorial bulges leads in two stable and two unstable points in the geostationary orbit, These points are called as equilibrium points. The values of stable points are 75°E and 252°E and the unstable points 162°E and 348°E. The stable and unstable points can be treated as bottom of valley and top of the hill respectively. When a ball placed on hill is given a small force, it rolls down into the valley. Similarly the satellite present at unstable position, when given a small force will get back to the stable position. The satellite at unstable point is perturbed a little, then it gets accelerated and shifts itself towards the nearest stable point, oscillates at that point until it gets stabilized at that point, The stable points are referred as grave yard location of the geosynchronous orbit, 54. Explain the effects of the sun and the moon on satellite. Ans: ApriuMay-19, Set-3, 3(5) The effect of gravitational force due to sun and moon on the orbital motion of satellite in the low earth onbitis small. In contrast, these forces produce undesirable perturbations on the orbital motion of satellites in the geostationary orbit. SPECTRUM ALL-IN-ONE JOURNAL FOR ENGINEERING STUDENTS SIA GROUP {31.36 SATELLITE COMMUNICATION [JNTU-KAKINADA] A geostationary satellite experiences a change in the range of 0.75° to 0.94° in its inclination duc to the gravitational pull of sun and moon, When the satellite is near to the sun and moon, it experiences a greater gravitational force, pulling it in their direction, As a result of this force the orbit normal i.e, the line perpendicular to the place of orbit is moved in the direction of ‘vernal equinox from its original position The inclination of satellite's orbit due to gravitational pull of moon is not constant and is dependent on the position of moon. While, the changes in the orbital path due to gravitational pull of sun is almost fixed as the relative position of both sun and satellite remains same. Moon causes the path of satellite's orbit to change cyclically inthe range of ~0.48° to ~0.67°. The inelination in satellite's path due to moon is maximum, when the orbit normal of moon and satellite are farthest away from each other. 1.2.4 Orbit Determination, Launches and Launch Vehicles Q55. Describe the history of satellite launch vehicles. Ans: Aprit8, Set2, 03(0) Satellite launch vehicles are those vehicles that are used in launching satellites into the earth’s orbits (LEO, MEO, GEO), During World Ws vehicles based on missile technology. This led to the desire to launch large satellites into higher earth orbits. Later, the objective and the technology used in building launch vehicles made them economically dependent. The advanced technology used in designing these vehicles made them popular and use for other countries to provide same services. 1 to gain supremacy in space technology, the countries US and Russia designed and developed laurtch Russia is the first country to design launch vehicles, Sputnik- is the first satellite that was launched using single stage R-7 rocket called Semyorka by Russia. Russia has also developed several series of launch vehicles such as, Vostok, Molniya, Soyuz, Proton, Zenith and Energia, The launch vehicle, Energia has the capacity to launch 65 to 200 tonnes of payload onto Low Earth Orbit(LEO), Buran is the reusable launch vehicle designed by Russia and itis deployed into the orbit using launch vehicle, Energia, (because it does not have propulsion system). The various series launch vehicles designed by US are the Atlas, the Titan, the Pegasus are re-usable space shuttle. Ariane is the reusable launch vehicle designed by Europe. It’s fifth generation ie., Ariane-5 deploys 12 tonnes of payload into geostationary orbit Long March by China, PSLV and G shuttles in aerospace technology. ‘LV by India and H-2 series form Japan are some most popular and successful space Q56. What are the various types of satellite launch vehicles? Explain. Ans: Apr There are two types of satellite launch vehicles. They are, 1. Expendable Launch Vehicles (ELY) 2, Reusable Launch Vehicles (RLV) 1. Expendable Launch Vehicles (ELV) Expendable Launch Vehicles are one time user vehicles and are not included in the payload. While launching the satellite into the desired orbit, in each stage these vehicles are expended i.e., used. So, these are known as Expendable Launch Vehicles. There are many countries which have their own launch vehicles and the most popular ELV are, (US Expendable Launch Vehicles For answer refer Unit-1, QS7. (ii) European Expendable Launch Vehicles For answer refer Unit-1, Q58. ‘Set-3, a3(b) Reusable Expendable Launch Vehicles As the name indicates, these launch vehicles are reused after launching satellite. These vehicles reduces the cost in launching satellites. The advantages of these vehicles are inexpensive, dependant and on-demand space access Space shuttle of USA is the first RLV that has launched several times but complete reusable Iaunch vehicle is not yet designed, Look for the SIA GROUP Loco {J} on the TITLE COVER before you buyUNIT-1 (Introduction to Satellite Communication & Orbital Mechanics and Launchers) 1.37 Q57. Write a summary of U.S. expendable launch vehicles. Ans: While several types of militariy derived rockets have served as launch vehicles for communication satellites, the work horse of the recent years and the main competitor for STS and Ariane is the MC Donnells Douglas Delta as shown in the following figure. aw oo a Dats Delia BG Dela LsM.N ——Deltu 2800,2900 Delta 3920 (1960) 10955) (70) W973) 92) 12 Missions 28 Missions 2 Missions 43 Missions Figure: Evolution of Delta ELV The models currently in use form what is called the 3900 series, developed in a program that was jointly funded by MC Donnell - Douglas and RCA. The last stage ofthe delta is usually a PAM fitted with a PAM-D. A delta 3920 rocket can put a 1200 kg pay load into transfer orbit at a 1983 cost of $18.5M, with a PAM-D2, the same rocket can deliver 1500 kg to transfer orbit. ‘The PAM-A now under development is expected to carry 2000 to 2500 kg. While NASA plans to stop using delta in 1986, the rocket may continue to be available commercially. ‘The Titun missile can be used for launching very heavy satellites, when used with the centaur upper stage, itis capable of delivering 4000 kg to geosynchronous orbit. Titans nominally cost $35-40 m per launch, but 1983 bids fora Titan launch for Intel Sat VI were as high as $75 m. Q58. Explain Expendable Launch Vehicle(ELVs). Ans: Commercially available European ELY is Ariane. The operational versions of Ariane are Ariane-I, Ariane-Il and Aviane - IL and more recently a large Ariane - IV, Arianes are designed to launch two satellites atthe same time from an enclosure called ‘System de-launcement Double Ariane (SYLDA), The structure of Ariane-I, II and III are shown in figure below. _ ‘byl-3m giving oO me | Additional sei Figure SPECTRUM ALLIN-ONE JOURNAL FOR ENGINEERING STUDENTS SIA GROUP QD1.38 SATELLITE COMMUNICATION [JNTU-KAKINADA]} ‘On-orbit capabilities of Ariane-I are total pay load of 1700 kg and of Ariane-II with one or two satellite of 2100 ka payload and of Ariane-II with two satellite is 2580 kg. The Ariane-IIL is expected to have on-orbit payload of 4300 kg. Ariane are launched from France Guiana injecting satellite into transfer orbit with inclinations of 5° permitting more payload ‘weight due to lesser AKM fuel. Q59. What are the various satellite launch vehicle selection factors? Explain them. ‘Apritt8, Set-4, 03a) for) What are the different launch vehicle selection factors? Explain. Ans: (April, Set-4, 02(b) | AprilMay-17, Set-3, 23(a)) The various selection factors of launch vehicle are, 1. Reliability of launcher 2. Launcher performance and suitability 3. Launcher price 4. Availability and schedule 5S. Safety 6. Customer relationships and partnerships. 1, Reliability of Launcher Reliability is one ofthe main factor for selecting a launch Vehicle. As per customer satisfaction survey, maximum number of customers are interested to select the launch vehicle with high reliability. Most of the companies are willing to pay (or) spend more money on the high reliability vehicles. However, the reliability of launcher is critical in increasing the chances of pay load reaching the orbit. 2. Launcher Performance and Suital Performance and suitability is another main factor for selecting a launch vehicle. The performance of vehicle specifies the capability of Titing a certain payload mass tothe specified (oF) desired orbit. The suitability of vehicle refers to the vehicles ‘compatibility (or) comfort with various types of payloads and payload margins, The vehicle with good performance and suitability gives good results 3. Launcher Price Price is another top factor, which plays a key role while selecting the launch vehicle, Launching a satellite involves very costly process. So, the launching vehicle with flexible price including insurance is considered, 4, Availability and Schedule In availability and schedule factor, availability specifies whether the launch vehicle ean meet the customer need (or) not. The launch vehicle is selected only if it meets the requirements of the station, Most of the companies are preferred to schedule the launching of satellites independently because of less contral ‘over launch schedule in partnership. S Safety Safety is the another factor in selecting a launch vehicle. Many of the customers select the launcher with more safety. 6 Customer Relationship and Partnerships The quality of relationship that they established with launch service provider and the maintenance (or) development cof partnership with providers are also considered while selecting launch vehicle. Q60. What is transfer orbit and how is the orbit correction for the launch of satellite at latitude other than equator is obtained? Ans: During the orbit-raising phase the geostationary satellite is kept in an intermediate orbit before itis placed in ‘geostationary orbit, This intermediate orbit is known as transfer orbit, The motor of the rocket is ignited at apogee when the orbital elements are steady after a few orbits in the GTO. GTO is elevated till it becomes a circular geostationary orbit. ‘The earth spins towards the east. At the equator the rotational velocity ofa sea level site in the plane of the equator is 0.4651 km/s. The rotation of earth increments the velocity of equator by 0.465 kis. In general, the equatorial launch of satellite in circular orbit at an altitude of 900 km needs an orbital velocity of around 7.4 kmvs tangential to the carth’s surface. A rocket launched from the equator should impart an extra velocity of 7.4-0.47=6.93 kins, easterly launch from th Hence, the energy necessary for equatorial launch is decreased nearly by 6% . The payload capabilities of any rocket decreases ‘with increase in inclination if the satellite launch is not into circular equatorial orbit ‘The inclination is determined by the latitude of the launch station and is given by, cost) = sin(¥,) c03(0,) Where, i Inclination E\— Azimuth of launch 6, ~ Latitude of launch site. To achieve an altitude other than equator necessary correction is needed. This is achieved by applying a velocity increment in a direction perpendicular to the orbital plane. The velocity increment is minimal if itis applied at the point of ‘minimum velocity ic., at the apogee of the transfer orbit, The correction is performed when the satellite is being imparted from transfer orbit to geostationary orbit Look for the SIA GROUP Loco @ ‘on the TITLE COVER before you buyUNIT-1 (Introduction to Satellite Communication & Orbital Mechanics and Launchers) Q61. Explain the launching procedure of geo- stationary satellites using launch vehicles. Give diagrams. Apriuttay-18, Set-t, 03(a) (or) Explain as to how a satellite is placed into geostationary orbit from earth. (spriiay-14, Set, 22) | April May-47, Set2, 23(0)) (or) Explain the mechanism of launching a satellite. ‘Aprilay-16, St, G2(a) (or) Discuss the procedure for launches and launch vehicles for space crafts. ‘ApuitiMay-17, Set-2, 03(b) (Refer only Procedure to Launch Satellite into Orbit) Ans: Launching of a Satellite into Orbit Newior 's law of gravitation plays vital role in launching a satellite into the orbit, The expression which describes the Newton's law of gravitation is given by, p= olln As the force 0 traction is inversely proportional to the square of distance, the orbital height chosen should be more. ‘After selecting the height, the satellite should orbit around the planet. To achieve this, the energy required to mave should counteract the force of attraction due to gravity As velocity is inversely proportional to orbital heigl less height requires more orbital velocity and more height requires less orbital velocity. Thus, the two parameter orbital height and orbital velocity are important for launching a satelite Procedure to Launch a Satellite into Orbit The procedure to launch a satellite in Geostationary orbit is as follows, Two parameters that are calculated to place a satellite into a stable orbit are velocity vector and orbital height. A geostationary satellite must be in an orbitata height of 35,786.03 km above the surface of the earth, with an inclination of zero degrees on ellipticity of zero, and a velocity of 3074.7 mis, tangential to the earth in the plane of the orbit, which is the earth's equatorial plane. To make efficient use of the fuel itis 1.39 moves ‘eommon to drop-out exeess mass from the launcher a upwards, this is called staging, Most launch vehicles have multiple stages and as each stage is completed, that portion of launcher is droped out until the final stage places the satelite into desired trajectory. The first step in launch of satellite into geostationary orbit was placing the space craft with the final stage of rocket still connected into low earth orbit (LEO) . ‘The space eraft is then launched into geostationary transfer orbit (GTO) afier the final stage is ignited when the orbital elements become steady after a few orbits, The perigee and apogee are similar to original LEO and GEO altitudes respectively. The apogee point is positioned very near to the orbital longitude likely in orbit test location of satellite before it is shifted to its effective location, The motor of the rocket is ignited at apogee when the orbital elements are steady after a few orbits in the GTO, GTO is clevated till it becomes a circular geostationary orbit. The rocket motor is usually called apogce kick motor since the rocket is ignited at apogee. The inclination error i eliminated and the orbit at GEO is circularized by AKM in order to make orbit of satellite completely to geostationary. Direct insertion into geostationary orbit is also possible, which is similar to the GTO technique but, the satellites ean bee directly placed into GEO without using its own propulsion system to g0 to GEO from GTO with the help of final stages of rocket. GTO/AKM approach to geostationary orbit is shown in figure. Q62. Explain how the synchronous orbit of a geostationary satellite is determined. Also explain why these satellites are not capable of illuminating polar regions. (or Explain the procedure of orbit determination. AprilMay-19, Set-4, 3(b) (Refer only Determination of Synchronous Orbit for Geostationary Satellites) SPECTRUM AILLIN-ONE JOURNAL FOR ENGINEERING STUDENTS SIA GROUP:1.40 SATELLITE COMMUNICATION [JNTU-KAKINADA] Ans: Determination of Synchronous Orbit for Geostationary Satellite The synchronous orbit ofa geostationary satellite is determined from the geostationary orbit. Geostationary orbit is circular in shape and it lies in the equatorial plane of the earth, All the satellites inthis orbit complete their rotation around the earth in a period of 23 hours and 56 minutes, which is equal to the period of earth’s revolution around its own axis. Thus, the satellites move together with earth and seem to be stationary with respect to a point on the earth, Figure (1) illustrates a geostationary orbit, Direction of | satelite metion Satelite Figure (1): Geostationary Circular Orbit Geosynchronous orbit is discovered by taking an inclination angle of 64° with respect to equatorial plane of geostationary orbit. The geosynchronous orbit thuss obtained is elliptical in shape and satelite inthis orbit seems to be oscillating with respect toa point on the earth, However, the period of rotation around the earth for this satellites is same as geostationary orbit ie., they complete their rotation around the earth in a period of 23 hours and 56 minutes. Figure (2) illustrates a geosynchronous orbit, for this orbits one of the focal point of ellipse coincides with center of the earth, Drestion of stele ion Figure (2l: Synchronous Elliptical Orbit Thus, at the time of satellites revolution the distance between satellite and earth changes continuously. The location of satellite in the orbit, which is at a minimum distance from earth is called perigee. The location of satellite inthe orbit, which is nce is called apogee. These distan rth’s gravitational fore ‘ata maximum di ‘sare evaluated from the center of Geosynehronous Satellites are not Capable of Illuminating Polar Regions Figure (3) illustrates how geosynchronous orbit is inclined at an angle with equatorial plane. From this, we observe that satellites in the geosynchronous orbit are unable to view the polar regions. Hence they cannot illuminate the polar regions, For illuminating the polar regions special navigational satellites are launched into the polar orbit Polar region Figure (3: S to Orbits Look for the SIA GROUP Loco {> on the TITLE COVER before you buyUNIT-1 (introduction to Satellite Communication & Orbital Mechanics and Launchers) 1.41 ion for the radius of geosynchro- Apilllay-4, Set2,02(a) ler 2 geostationary satellite revolving around the earth as shown in figure. 9) ¢) Satellite Figure As earth is a sphere, the centripetal force (F,) required for a satellite to be in the circular orbit is given by, my? a a Where, ‘m-~ Mass of the satellite v- Velocity of orbit 1— Radius of orbit, ‘The gravitational force (F,) between satellite and earth is expressed as, GMm => Q Where, M- Mass of the earth, According to newton’s gravitational law, centripetal force is equal to the gravitational force. Equating equations (1) and (2), we get, RF GMm Ifr is the radius of the circular orbit, then the time taken “T to complete one orbit is given by, 2a 4 = 3 @ Substituting equation (4) in equation (3), we get 2m GM rT) Gur? > as, =| OMT! an If period of a geosynchronous orbit is 1 day i. T= | day - 24 hours/day ~ 86000 sec G= 6.67 * 10" Na M= 5.98 10 ky Then equation (5) becomes, - [seine 081086000 = 42.1 = 10 210m 42x10" m 1.2.5 Orbital Effects in Communication ‘Systems performance Q64. What are orbit effects? Which effects the per- formance of satellite? Explain in detail. Aprnty-19, S23 (or) Explain the various orbital effects in satellite communication systems performance. April-t8, Set, a3(b) (or) Explain the different orbital effects in satellite communication system performance. -ApevMay-17, Set-4, a1(0) SPECTRUM ALL-IN-ONE JOURNAL FOR ENGINEERING STUDENTS SIA GROUPS1.42 SATELLITE COMMUNICATION [JNTU-KAKINADA] Ans: Orbital Effeets in Satellite Communication System Performance ‘There can be four orbital effects, 1. Doppler shift 2. Range variation Solar eclipse 4. Sun transit outage. 1, Doppler Shift Consider that 2 satellite is moving with a relative velocity of v, and transmitting the signals of frequency /,. The earth station which is at rest will receive the frequency f, that depends on the direction of the movement of satellite is shown in the following figure (1). arth sation Figure (1) satellite is moving towards earth sta onthen/,>f,and f,

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