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QUESTION BANK FOR SATELLITE

COMMUNICATION

(EEE F472/ EEE C472)

SUBMITTED TO :

DR. M.K DESHMUKH

PREPARED BY

S.Aditya 2011A8PS350G

Aditya Chandratre 2011A8PS330G

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QUESTION BANK FOR CHAPTER 3 :

MAJOR TOPICS :

SATELLITE SUBSYSTEMS, ANTENNAS ,TRANSPONDERS,

RELIABILITY AND SPACE QUALIFICATION

Q1) What are the major subsystems in satellite communication?

Ans) The major subsystems required on the communications satellite are:

• Attitude & orbit control system (AOCS), consists of rocket

motors that are used to move the satellite back to the correct orbit

when external forces causes it to drift.

• Telemetry, Tracking & Command (TT&C), this is partly on the

satellite and partly on the controlling earth station. A dedicated

earth station is used for this purpose.

• Power system (mainly solar cells)

• Communications subsystem, these are the major components of a

communications satellite (transponders & antennas)

Q2) What are some of the functions of the TTC subsystem in satellite

communication?

Ans)

Tracking

• The determination of the current orbit and position of the

spacecraft.

• Velocity & acceleration sensors are employed.

• The control earth station can observe the doppler shift of the

telemetry carrier to determine the rate of change of the range.

• Triangulation can be used from measurements from several earth

stations observing the satellite.

Telemetry System

• Collects data from many sensors and sends them to the control

earth station.

• Pressure in fuel tanks

• Current drawn by each subsystem

• Critical voltages & currents

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• Temperatures.

• Status & positions of switches

• sighting devices used to maintain attitude

Q3) What are spin and 3-axis stabilized satellite system? Mention some

advantages and disadvantages of 3 axis stabilized satellite system.

Ans) A spin-stabilized satellite is a satellite which has the motion of one axis

held (relatively) fixed by spinning the satellite around that axis, using the

gyroscopic effect. The attitude of a satellite or any rigid body is its orientation in

space. If such a body initially has a fixed orientation relative to inertial space, it

will start to rotate, because it will always be subject to small torques. The easiest

form of attitude stabilization is to give the rigid body an initial spin around an

axis of minimum or maximum moment of inertia. The body will then have a

stable rotation in inertial space

• A three axis stabilized satellite is stabilized in motion along all three

directions. Done using an internal gyroscope and small thrusters jets or

rockets. Because its spin is so stable, the internal gyroscope can be used

as a sensor to tell the satellite when its attitude (direction) is changing.

The satellite can then correct the problem using thrusters. Thrusters

usually contain compressed gas that when sent out of the end of the

thrusters will move the satellite in space. The force of the compressed gas

(the action) causes the satellite to move in the opposite direction (the

reaction).

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The disadvantage : Thrusters, however, are heavy and use up lots of power

The advantage : The satellite as a whole can point stably in one direction; for

example, the solar panels can always point at the sun, and the instruments

can always point at their target

Q4) A spinner satellite has solar cells wrapped round a cylindrical drum 3.00

m in diameter, with a height of 5.0 m on station. The drum is rotated at 60

rpm to spin-stabilize the satellite. At the end of life, the solar cells are

required to deliver 4.0 kW of electrical power. Calculate the efficiency of the

solar cells at end of life. Assume an incident solar power of1.39 kW/m2, and

that the effective solar radiation absorbing area of the solar cells is equal to

the cross sectional area of the drum.

Ans) Area of solar cells absorbing sunlight is equivalent to cross sectional area of

drum. A = 3.0 m* 5.0 m = 15.0 m2.

At the end of life the solar cells are producing 4000 watts of electrical power.

Hence efficiency at end of life is ŋEoL = 4000 / (15 x 1390) = 19.2%

Q5) In Q4 if the solar cells degrade by 15 percent over the lifetime of the

satellite, so that the end-of- life output power is 85% of the beginning-of- life

output power, what is the output of the solar cells immediately after launch?

Ans) The beginning of life output of the cells is ŋBoL where

ŋBoL = 4000 / 0.85 = 4706 watts

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Q6) In Q4 If the drum covered in solar cells of the spinner design had been

replaced by solar sails that rotated to face the sun at all times, what area of

solar sails would have been needed? Assume that cells on solar sails generate

only 90% percent of the power of cells on a spinner due to their higher

operating temperature.

Using the end of life output of 4000 W, the efficiency of the solar cells on the

sails is ŋss = 0.9 ´ 19.18 = 17.26 %. The area of solar sails required is A where A

= 4000 / (0.1726 ´ 1390) = 16.67 m2

Q7) What are spot beam antennas ?

Ans) A spot beam, antenna emits a satellite signal that is specially

concentrated in power so that it will cover only a limited geographic area on

Earth.

Multiple Spot beams for satellite antenna.

8) What is space qualification?

Ans) Space qualification involves detailed procedures undertaken to ensure

that all, components, systems and the satellite as a whole are invoked after

careful screening and testing. The satellite so constructed is measured for it’s

reliability using the reliability theory.

9) What are the different kinds of antennas used for satellite communication?

Ans)

1) A Horn antenna or microwave horn is an antenna that consists of a flaring

metal waveguide shaped like a horn to direct radio waves in a beam. Horns are

widely used as antennas at UHF and microwave frequencies, above 300 MHz

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They are used as feeders (called feed horns) for larger antenna structures such

as parabolic antennas, as standard calibration antennas to measure the gain of

other antennas.

2) An Reflector antenna is an antenna that reflects electromagnetic waves.

Antenna reflectors can exist as a standalone device for redirecting radio

frequency (RF) energy, or can be integrated as part of an antenna assembly.

The function of a standalone reflector is to redirect electro-magnetic (EM)

energy, generally in the radio wavelength range of the electromagnetic

spectrum.

3) An Array antenna is a set of individual antennas used for transmitting

and/or receiving radio waves, connected together in such a way that their

individual currents are in a specified amplitude and phase relationship. This

allows the array to act as a single antenna, generally with improved directional

characteristics (thus higher antenna gain) than would be obtained from the

individual elements

4) Wire antennas are also used widely in satellite communication

Q10) The earth subtens an angle of 17o when viewed from geo stationary orbit.

What should be dimensions of such an antenna? Assume frequency of

operation to be 4GHz.

Ans) D/ʎ= 75/Ø3dB = 4.4 Hence at 4GHz we may choose ʎ=0.075m so D=0.33m.

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Q11) Explain the bath-tub curve in light of satellite reliability. Explain its three

regions of operation.

Ans) The bathtub curve is a plot of failure rate vs. time. It is characterized by

three regions in time: an initial region of decreasing failure rate, and

intermediate region of relatively constant failure rate, and a final region of

increasing failure rate. The bathtub curve is actually a composite curve, made

up of the sums of three smaller curves: infant mortality, random failures, and

wear out. Infant mortality failures are caused by defects in the product which

cause it to fail early in its lifetime. They are also called intrinsic failures, since

they are due to causes internal to the product. This type of failures decrease

sharply with time. Random failures occur at a somewhat constant rate over the

entire life of the product. Ideally, in a mature product, where the design and

processes are good, the failures are usually due to forces external to the

product, such as mishandling, external interface failures, or accidents. They are

therefore called extrinsic failures. In the time period after infant mortality, but

before the beginning of wear out, random failures dominate. This region is also

called the useful life region of the product.

The final region of the bathtub curve represents the time when the product

begins to wear out because it has reached the end of its useful life. There is

usually only a few wear out failure mechanisms, which results from the

stresses accumulated over the life of the product. These failures are intrinsic,

and the failure rate increases in this region.

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Q12) A geostationary satellite provides service to a region which can be covered

by the beam of an antenna on the satellite with a beamwidth of 1.8o. The

satellite carries transponders for Ku band and Ka band, with separate antennas

for transmit and receive. For center frequencies of 14.0/11.5 GHz and 30.0/20.0

GHz, determine the diameters of the four antennas on the

satellite. Find the diameters of the two transmitting antennas. Specify the

diameter and calculate the

gain at each frequency.

Ans) beamwidth: q3 dB = 75 ʎ/ D.

Hence D = 75 ʎ / q3 dB. Gain = 33,000 / (q3 dB) 2

The transmitting antennas on the satellite operate at the lower frequency

(downlink) in each

band.

For 11.5 GHz: l = 0.02609 m, D = 75 ´ 0.02609 / 1.8 = 1.087 m

For 20 GHz: l = 0.015 m, D = 75 ´ 0.015 / 1.8 = 0.625 m

G = 33,000 / 1.82 = 10.185 or 40.1 dB

Q13) What is meant by the term MTBF ? What is difference between MTBF and

MTTF?

Ans) Mean time between failures (MTBF) is the predicted elapsed time between

inherent failures of a system during operation. MTBF can be calculated as the

arithmetic mean (average) time between failures of a system. The MTBF is

typically part of a model that assumes the failed system is immediately repaired

(mean time to repair, or MTTR), as a part of a renewal process. This is in contrast

to the mean time to failure (MTTF), which measures average time to failures

with the modelling assumption that the failed system is not repaired (infinite

repair time).

Q14) Suppose in a satellite communication system it is observed that in a time t

36 out of 800 components have failed. Calculate failure rate

Ans ) Failure rate = No. of components Failed during time t / No.of Components

Survived = 36/764= 0.0471.

Q15) In the above problem also calculate MTBF.

Ans) MTBF of a satellite = 1/ Failure rate = 1/0.0471 = 21.23