Subject Code Name of the of the Subject and Title
BECDEC602 Satellite Communication and 2 mark Questions With Answers 13 04 2010 16 04 2010 (10:20 am to 11:10 am)
Date of Issue of Issue Date of Conduct of Conduct of Exam of Exam
01. Ans.
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Write the advantages and disadvantages of Satellite of Satellite Communication. Advantages:‐ Wide Area of Coverage. of Coverage. Point to Multipoint Links whereas many terrestrial links are point to point. Mobile Communication can be established. Economical when long distance is involved. For Geo‐Stationary Satellite Doppler shift is negligible. Disadvantages:‐ Propagation delay is very high. Impedance mismatch + Propagation delay produces echo in telephone systems. Echo Suppressors or Echo chancellors are to be added so that complexity Increases. Propagation Delay reduces the efficiency of the of the data transmission over satellite communication. • • • • •
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02. Ans.
03. Ans.
Distinguish between active and passive satellites. A Satellite in which the Electronic system of the of the Satellite receives power from the ground based transmitting system amplifies this power, frequency shifts the modulated carrier and re‐radiates this power. The power received by the earth receiving station is high. Such a satellite is termed as Active Satellite. In a Passive Satellite the earths transmitting system beams power at the reflector of the of the Passive Satellite while the receiving system receives very small fraction of the of the transmitted power. What are the permanent organisations of ITU? of ITU? The permanent organisations of ITU of ITU are General Secretariat. IFRB (International Frequency Registration Board.) CCIR (Consultative Committee for International Radio.) CCITT (Consultative Committee for International Telegraphy and Telephony.) What are the three regions defined for frequency allocation plan in Satellite Systems? • • • •
04. Ans.
Regions defined for the frequency allocation are Region 1:‐ Europe, Africa, VSSR, Magnolia. Region 2:‐ North and South America and Green Land. Region 3:‐ Asia (except VSSR & Mongolia.), Australia, South West Pacific. Write the advantages of Geo of Geo Stationary orbit. Advantages of Geo of Geo Stationary orbit are Tracking equipment avoided Earth stations at constant distance and remain at line of sight of sight Larger coverage area Global coverage with less no. of satellites. of satellites. Same quality of service of service at all places‐no Doppler shift‐cost effective. • • •
05. Ans.
06.
Compare LEO, MEO, HEO, GEO. 1
Ans.
LEO Less than 1000 km 1 ½ hour Mobile Communication.
Height from surface of earth of earth Rotation Period Uses
07. Ans.
MEO 2000km – 10,000Km 5 – 12 hours Telephone, Email, Fax.
HEO Above 20,000 Km
GEO Above 36,000Km
Used by polar countries.
Global Coverage with less no: of Satellite’s.
Sate Kepler's laws of planetary of planetary motion. st Kepler’s 1 law : The orbit of every of every planet is an eclipse with sun as the focus. nd Kepler’s 2 law : The Line Joining a Planet and the Sun sweeps out equal areas during equal intervals. Kepler’s 3rd law : The Square of the of the orbital period of the of the planet is directly proportional to the cube of Semi Major axis of its of its orbit. ‐
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08.
Define orbital Parameters.
Ans.
In order to mention the position of the of the Earth orbiting satellites some parameters are used and these are termed as orbital parameters. They are 1. Semi Major Axis. 2. Eccentricity. 3. Mean Anomaly. 4. Inclination. 5. Argument of Perigee. of Perigee. 6. Right Ascension of Ascending of Ascending Node Problem in Orbital Parameter Calculation.
09. 10. Ans.
What are Look angles? Define Them. The co‐ordinates to which the earth station antenna must be pointed to communicate with a satellite are called look angles. These are most commonly specified as Azimuth and Elevation angles. Azimuth Angle: It is defined as horizontal pointing angle of an of an earth station antenna. Elevation Angle:‐ It is the vertical angle formed between direction of travel of travel of an of an EM wave radiated from and earth station antenna pointing directly towards a satellite and the horizontal plane. What are the parameters which may affect the orbital position of the of the satellite? ‐
11. Ans.
The parameters that affect the orbital position of the of the satellite are Non Symmetrical nature of the of the earth. Gravitational pulls from the sun and the earth. Atmospheric Drag. Objects form other planets. Earth’s magnetic field. What do you understand by station Keeping? What are the methods used for that? • • • • •
12. Ans.
13. Ans. 14. Ans.
The process of Maintaining of Maintaining the satellite in its orbit by overcoming the orbital perturbations is known as station keeping. The methods used are East‐ West station keeping & North‐South Station Keeping. Define Slant Range. The Line of Sight of Sight distance from a particular point on the earth to the satellite is called Slant Range. Why is Uplink frequency greater than the downlink frequency? The gain of an of an Antenna is α operating frequency. 2
1/Beam Width. If the If the high band were used for downlink any given antenna would have a narrower BW thereby increasing the problems of maintaining of maintaining antenna alignment as the satellite drifts around in space about its mean position to overcome this a high cost servo control Steering System would be required. By using the high band for uplink the extra gain can be usefully employed to make up for the extra path length attenuation. As the losses increase with frequency with this arrangement the overall system signal to noise ratio can be better managed. The large dish ground transmitting station will require a servo controlled Tracking system. So there is no added cost in this case. What are the types of Launch of Launch Vehicles used for Satellite Launching? α
15. Ans.
16. Ans.
17. Ans. 18.
Types of Launch of Launch vehicles used for satellite launching are 1. Expendable Lunch Vehicle (ELV). 2. Reusable Launch Vehicle or Space Transportation System (STS). What do you understand by Hohmann Transfer? While launching satellites in orbits greater than 200 km it is not economical in terms of launch power to perform direct injection, and satellite must be placed into transfer orbit between the initial low earth orbit and the final high altitude orbit. In most of the of the cases, the transfer orbit is selected to minimize the energy required for transfer and such and orbit is known as Hohmann Transfer Orbit. What are the methods used to improve the reliability of Satellite of Satellite System? What is a Regenerative Repeater?
Ans. 19.
What is a Payload of a of a Communication Satellite?
Ans. 20.
The Payload of a of a communication Satellite Consist of Transponder of Transponder and Antenna. What is the difference between active and passive attitude control?
Ans.
Passive attitude control refers to the use of mechanisms of mechanisms which stabilize the satellite with out putting a drain on the Satellite’s energy supplies. Active attitude control refers to the application of corrective of corrective torques required in response to disturbance torques instead of overall of overall stabilizing of torque. of torque. What are the methods used for attitude control?
21. Ans.
22. Ans. 23. Ans.
The methods for attitude control are Active attitude control: ‐ Momentum Wheels, Electromagnetic coils, mass explosion devices. Passive attitude control:‐ Spin Stabilization and gravity gradient stabilization. What ate the functions of Telemetry, of Telemetry, Tracking and Command Subsystem? The functions of Telemetry, of Telemetry, Tracking and Command functions are complex operations which require special ground facilities in addition to the TT&C sub system aboard the satellite. What are the types of redundancy of redundancy connections used in Spacecraft? Different types of redundancy of redundancy connections used in Spacecraft are Series Connections. Parallel Connections. Series/Parallel Connections. Switched Connection. What are the types of antennas of antennas used in Spacecraft? Mainly 4 types of antennas of antennas are used in Spacecraft. They are Wire Antenna. Horn Antenna. Reflector Antenna. Array Antenna. How the Capacity of a of a Satellite Communication System can be increased? • • • •
24. Ans.
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Ans. 26.
Capacity of Satellite of Satellite Communication System can be increased by employing frequency reuse technique etc. What are the types of Antennas of Antennas used in Earth Stations? Write their advantages and disadvantages.
Ans.
Different types of Antennas of Antennas used in Earth Stations are 1. Paraboloid Antenna with a focal point feed. 2. Cassegrain Antenna.
27.
What are the requirements of an of an Earth Station antenna?
Ans.
The requirements of Earth of Earth Station Antenna are High Directive Gain. Low Noise Temperature. Easily Steerable Problems in T G/T Calculation. • • •
28. 29. Ans.
30. Ans.
31. 32. Ans.
What are the functions of Monitoring of Monitoring and Control Unit? The Monitoring and control system must have the capability to Collect status data for classification. Convey status data to network operator. Interpret fault isolations. Switch over redundant equipment on command. Convey control data to the baseband equipment for traffic assignment, antenna pointing, and so forth maintain surveillance of equipment of equipment shelter facilities. What is the need for demodulation and remodulation in TVRO? A major difference between DBS TV and conventional TV is that with DBS Frequency Modulation is used where as in conventional TV amplitude modulation in the form of Vestigial of Vestigial Side band is used. Hence the received FM wave is demodulated and remodulated in amplitude. Compare TVRO, MATV & CATV. What is VSAT? Write its Applications and Advantages. VSAT stands for Very Small Aperture Terminal. Applications: Data Broadcasting Service. 2 way data Service. Advantages: 1. VSAT find advantage over terrestrial packet networks due to simpler flow and congestion control. Write the applications of Mobile of Mobile and Transportable Earth Stations? Applications of What do you understand by intermodulation distortion in TWT A? A TWT also may be called on to amplify two or more carriers simultaneously, this being referred to as multicarrier operation. The AM/PM conversion is then a complicated function of carrier of carrier amplitudes, but in addition, the nonlinear transfer characteristic introduces a more serious form of distortion of distortion known as intermodulation distortion. What is the advantage of double of double conversion over single conversion in up and down convertors? ‐
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33. Ans. 34. Ans.
35. Ans. 36.
What are the common HPA’s and LNA’s used in earth station?
Ans.
Commonly used HPA’s in the earth station are Travelling Wave Tube amplifier (TWT) and 4
37. Ans. 38. Ans.
39. Ans.
Klystron Amplifier. Commonly used LNA’s in the earth station are Parametric Amplifier and GaAs FET amplifier. What are the parameters to be considered for earth station site selection? Write the difference between Single access and Multiple Access A Transponder channel aboard a Satellite may be loaded by a single transmission from and Earth station. This is referred as Single access mode of operation. of operation. Similarly if the if the Transponder channel is loaded by multiple carriers originated by multiple Earth stations which are geographically separate. Then this is referred as Multiple access mode of operation. of operation. Distinguish between Demand assigned and Pre assigned Multiple Access. Pre assigned Demand assigned These are allocated All circuits are on fixed or partially available to all users fixed basis to certain and are assigned users according to the demand. It is simple to implement but It results in more efficient only for efficient over all use circuits only with of the of the circuit but continuous traffic. more costly and complicated to implement. List out the Advantages and Disadvantages of FDMA. of FDMA. •
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40. Ans.
Advantanges: • Uses existing hardware. There is no need for N/w Timing. • There is no restriction regarding the type of Base of Base band or type of modulation. of modulation. • Disadvantanges: Intermodulation in the Transponder leads to interference with other links sharing the • TWT & consequently reduces the Satellite Capacity. Lack of flexibility of flexibility In channel allocation. Since for each change in allocation the receiver • filter need to be retuned. This disadvantage applies to the MCPC type of FDMA. of FDMA. Demand assigned SCPC removes this limitation. • It requires uplink power control to maintain the link quality. In a mix of traffic of traffic containing strong and weak carriers especially in MCPC the weak • carriers tend to be supressed. List out the merits and Demerits of TDMA. of TDMA. Advantages: • Uplink power control is not necessary. • The digital format of TDMA of TDMA permits utilisation of all of all the advantages of digital of digital techniques i.e. digital speech interpolation, source and channel coding etc. • Maximum use can be made of the of the available satellite power since intermodulation noise is minimum. • Transmission plans are easier to construct & modify. • Capacity management is simple and flexible. Disadvantages: • analogue Signals must be converted to Digital form. • It needs network wide timing synchronization. Hence it is relatively complex. Interface with Analogue Terrestrial Plant is expensive. • ‐
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41. Ans.
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42. Ans. 43. Ans.
Draw the Frame Format for a TDMA system [Pre assigned and Demand Assigned] Define Frame Efficiency in TDMA.
44.
Frame Efficiency is a measure of fraction of fraction of time of time used for the transmission of the of the traffic. It may be defined as ηf = traffic bits/total bits. Compare FDMA and TDMA.
45.
What is the Concept of SS of SS‐TDMA?
Ans.
46.
The spread spectrum is one in which the transmitted signal is spread over a wide band of frequency much wider than minimum band width required to transmit the information being sent. The spreading is accomplished by means of a of a spreading signal called as code signal which is independent of data. of data. What is Spread Spectrum Multiple Access? Write its advantages.
47.
Define Process Gain and Jam Margin.
48.
Write the advantages and disadvantages of CDMA of CDMA & FHSS?
49.
What is meant by Multiple Access?
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