Code: D-17                                 Subject: ELEMENTS OF SATELLITE COMMUNICATION

Time: 3 Hours                                      June 2006                                                               Max. Marks: 100

NOTE: There are 9 Questions in all.

· Question 1 is compulsory and carries 20 marks. Answer to Q. 1. must be written in the space provided for it in the answer book supplied and nowhere else.

· Out of the remaining EIGHT Questions answer any FIVE Questions. Each question carries 16 marks.

· Any required data not explicitly given, may be suitably assumed and stated.

Q.1       Choose the correct or best alternative in the following:                                         (2x10)

a. When the orbit eccentricity equals unity, the shape of the orbit would be

(A)  an ellipse                                      (B)  a circle

(C)   a parabola.                                  (D)  a hyperbola

b. The height of the geostationary orbit from the earth’s surface is

(A) 900 kms                                       (B)  1800 kms

(C)  42,500 kms                                  (D)  35860 kms

             c.   The figure of merit for a satellite transmitter is the

(A) ratio.                                    (B) ratio.

(C)  EIRP.                                           (D) ratio.

             d.   The factor which expresses in decibels, the improvement afforded by the FM system in return for a sacrifice of bandwidth, with usual notations, is 

                   (A) .                          (B) .

(C) (D) .

             e.   In the present day standard digital voice communication, the amplitude of the voice signal is sampled at a rate of around

(A) 800 . (B)  2000 .

(C)  8000 . (D)  16000 samples/sec.

             f.    The bandwidth in M-ary FSK, for a symbol frequency of  is 

(A) 4 M (B)  8 M

(C)  16 M (D)  2 M

             g.   The multiple satellite access system that suffers from the presence of intermodulation products is

(A) TDMA.                                       (B)  FDMA.

(C)  CDMA.                                       (D)  MACA

             h.   If the apogee distance from the earth’s center is eight times the perigee distance from the earth’s center, then the orbit eccentricity would be

(A) 0.12.                                            (B) 0.78.

(C) 0.31.                                             (D) 0.53.

             i.    If a FET power amplifier is used in the design of an earth station transmitter, the power obtainable will be in the range of

(A) 500 to 5000 W.                           (B) 100 to 1000 W.

(C) 5 to 50 W.                                    (D) 100 to 300 W.

             j.    The video carrier frequency (in MHz) usually employed for the T-9 cable TV channel is around

(A)  7.                                                 (B)  25.

(C)  13.                                               (D) 19.

Answer any FIVE Questions out of EIGHT Questions.

Each question carries 16 marks.

Q.2     a.   How many frequency bands have been allocated for use with satellite communications and which are they?  Illustrate in the form of a table the frequency allocations for mobile satellite service. (8)

             b.   What are the factors that determine the optimum radio frequency range of satellite communication?  Starting from fundaments, deduce the equation for the power received by the receiving antenna of a satellite link.                                                           (5)

             c.   The range between a ground station and a satellite is 42000 Km. Calculate the free-space loss at a frequency of 6 gigahertz.                                      (3)

Q.3     a.   What do you mean by the term ‘multiplexing’ as used in communication engineering?  What is the multiplexing technique used for analog communications?  Explain the technique in brief with necessary illustrations.                                                                                                      (10)

             b.   Why frequency modulation is used in satellite analog communication links?  What is the relationship between the and at FM detector in the satellite receiver, expressed in decibels?          (4)

             c.   An FM satellite signal has a deviation of 3 KHz with a baseband frequency of 1 KHz.  What is the bandwidth of the signal?                       (2)

Q.4     a.   Briefly explain the following:                

(i)   Bit           (ii)   Byte          (iii)  Baud.                                                            (6)

b.   Name the additional units that you come across in a digital communication system as compared to its analog counter-part.  What is a digital baseband signal?  What is its advantage?  Name the typical digital baseband signals commonly used.                                                                                      (7)

c.   Binary data is transmitted over a link with usable bandwidth of 2400 Hz using the FSK signalling scheme.  The transmit frequencies are 2025 Hz and 2225 Hz and the data rate is 300 bits/sec.  What is the frequency of the unmodulated carrier and its deviation from the basic value for the binary signals transmitted?                                                             (3)

Q.5     a.   Briefly explain the principle of TDMA. (5)

b.   Write a note on TDMA burst structure.                                                              (6)

c.   Write a note on satellites for weather forecast.                                                    (5)

  Q.6     a. Which are the orbital aspects of importance?                                                      (6)

             b.   A satellite in an elliptical orbit has an apogee of 30000 Km and a perigee of 1000 Km. Determine the semi major axis of the orbit.                         (2)

             c.   Briefly explain the effects of eclipse on the working of satellites.                           (8)

Q.7     a.   What are the main functional units of a communication satellite?  Mention their primary function.  Mention the main subsystems of a geostationary satellite.  For each sub system, what are the functions performed and the parameters of importance?                                                                              (13)

b.   A Ku-band 20-m paraboloidal antenna has been designed for a satellite earth station.  Calculate the gain of the antenna for the Uplink.  Assume an aperture efficiency of 0.648. Assume Ku-band frequency of 14-25 GHz for uplink.                                                                                                  (3)

Q.8     a.   What is an earth station?  With a neat schematic, explain the operation of an earth station transmitter.                                                                     (8)

             b.   Why are transmitters called the expensive part of an earth station?                       (5)

             c.   Mention the salient features of a small earth station.                                             (3)

Q.9           Write a brief note on any TWO of the following:

(i) Direct Broadcast satellite (DBS).                                                   

(ii) INSAT-II.                                                                                    

(iii) ASK and PSK modulation.

(iv) Principle used in TDM and its comparison with FDM.            (2x8)