PART-Amahalakshmiengineeringcollege.com/pdf/ece/VIIIsem/EC2045/UNIT 2,3.pdf · A satellite in a geostationary orbit appears to be stationary with respect to the earth, hence the name

SATELLITE COMMUNICATION-SRINIVASAN K, AP/ECE

MAHALAKSHMI ENGINEERING COLLEGE-TRICHY

QUESTION BANK

SATELLITE COMMUNICATION DEPT./SEM.:ECE/VIII

UNIT II & III

PART-A

1. What is meant by momentum wheel stabilization? [May 2008]

To provide stability for the satellite attitude gyroscopic effect of a spinning

flywheel can be used. And this approach is used in satellites with cube-like bodies

These are known as body-stabilized satellites. The complete unit, termed a

momentum wheel, consists of a flywheel, the bearing assembly, the casing, and an

electric drive motor with associated electronic control circuitry. The term

momentum wheel is usually reserved for wheels that operate at nonzero

momentum. This is termed a momentum bias.

2. What are geostationary satellites? [May 2008, June 2012]

A satellite in a geostationary orbit appears to be stationary with respect to the earth,

hence the name geostationary satellite.

3. What is a transponder? [May 2009, Dec 2011]

In a communication satellite, the equipment which provides the connecting

link between the satellite’s transmit & receive antennas is referred to as the

transponder.


4. Determine the angle of tilt required for a polar mount used with an earth

station at latitude 49O

north. Assume a spherical earth of mean radius 6371km

and ignore earth station altitude. [May 2009]

5. What is sun transit outage? [Nov 2009, Dec 2012]

Another event that happens during the equinoxes is the transit of the satellite

between earth and sun, such that the sun comes within the beamwidth of the earth-

station antenna. When this happens, the sun appears as an extremely noisy source

which completely blanks out the signal from the satellite. This effect is termed sun

transit outage, and it lasts for short periods—each day for about 6 days around the

equinoxes.

6. What is the difference between geostationary orbit and geosynchronous

orbit? [Nov 2009, May 2010]

An exact geostationary orbit is not attainable in practice, and the orbital parameters

vary with time. The satellites are geosynchronous, in that they rotate in

synchronism with the rotation of the earth. However, they are not geostationary.

The term geosynchronous satellite is used in many cases instead of geostationary

to describe these near-geostationary satellites. It should be noted, however, that in

general a geosynchronous satellite does not have to be near-geostationary, and

there are a number of geosynchronous satellites that are in highly elliptical orbits

with comparatively large inclinations (e.g., the Tundra satellites).


7. What is meant by Hohmann transfer orbit?[ May 2010]

This is an intermediate orbit having a highly elliptical shape. It is used by GEO

satellites to reach their final destination orbits. This orbit is connected to the LEO

orbit at the point of perigee forming a tangent and is connected to the GEO orbit at

the point of apogee again forming a tangent.

8. What is meant by spot beam antenna? [Dec 2011]

A beam generated by a communication satellite antenna of sufficient size that the

angular spread of sufficient size that the angular spread of the energy in the beam

is very small with the result that a region that is only a few hundred km in

diameter is illuminated on earth.

9. What is meant by station keeping? [June 2012, Dec 2012]

It is the process of maintenance of satellite’s attitude against different factors that

can cause drift with time. Satellites need to have their orbits adjusted from time to

time because the satellite initially placed in the correct orbit; natural forces induce

a progressive drift.

10. What is a polar antenna?

A single actuator is used which moves the antenna in a circular arc ie known as

polar mount antenna.

11. Wht is declination?

The angle of tilt is often referred to as the declination which must not be

confused with the magnetic declination used in correcting compass readings.


12. Define the terms in Eclipse.

During equinox periods, the earth the sun & the satellite are in alignment with the

result that earth’s shadow eclipses the satellite & the sunlight fails to reach the

satellite solar cells. The eclipse effect is noticeable for periods of about four weeks

& the maximum daily eclipse duration is about 1.20hours.

13. What is meant by payload?

The payload refers to the equipment used to provide the service for which the

satellite has been launched.

14. What is mean by Pitch angle?

Movement of a spacecraft about an axis which is perpendicular to its

longitudinal axis is called Pitch angle. It is the deg of elevation or depression.

15. What is a propellant?

A solid or liquid substance burnt in a rocket for the purpose of producing thrust.

16. What is a Yaw?

Yaw is the rotation of a vehicle about its vertical axis.

17. What is a Zero ‘g’?

Zero ‘g’ is a state when the gravitational attraction is opposed by equal &

opposite inertial forces & the body experiences no mechanical stress.

18. Describe the spin stabilized satellites.


In a, spin stabilized satellites, the body of the satellite spins at about 50 to

100rpm about the axis perpendicular to the orbital plane. The satellites are

normally dual spin satellites with a spinning section & a despun section on

which antennas are mounted. These are kept stationary w.r.to earth by counter

rotating the despunsection.

19. What is meant by frequency reuse?

The carrier with opposite senses of polarization may overlap in frequency

this technique is known as frequency reuse.

20. What is a TWTA?

Traveling-wave tube amplifiers (TWTAs) are widely used in transponders to

provide the final output power required to the transmit antenna.

21. What are the three conditions are required for an orbit to be

geostationary?

Three conditions are required for an orbit to be geostationary:

1. The satellite must travel eastward at the same rotational speed as the earth.

2. The orbit must be circular.

3. The inclination of the orbit must be zero.


PART-B

1. What are look angles and derive the expressions for azimuth and elevation?

[May 2008, June 2012]

The look angles for the ground station antenna are the azimuth and elevation

angles required at the antenna so that it points directly at the satellite. With the

geostationary orbit, the situation is much simpler because the satellite is stationary

with respect to the earth.

With the types of antennas used for home reception, the antenna beamwidth

is quite broad, and no tracking is necessary.

This allows the antenna to be fixed in position, as evidenced by the small

antennas used for reception of satellite TV that can be seen fixed to the sides of

homes.

The three pieces of information that are needed to determine the look angles for the

geostationary orbit are:

1. The earth-station latitude, denoted here by λE

2. The earth-station longitude, denoted here by φE

3. The longitude of the subsatellite point, denoted here by φSS


Latitudes north will be taken as positive angles, and latitudes south, as

negative angles. Longitudes east of the Greenwich meridian will be taken as

positive angles, and longitudes west, as negative angles. For example, if latitude of

40°S is specified, this will be taken as -40°, and if a longitude of 35°W is specified,

this will be taken as -35°.

With the geostationary orbit, variation in earth’s radius has negligible effect

on the look angles, and the average radius of the earth will be used.

Denoting this by R:

R =6371 km

In the figure below, ES denotes the position of the earth station, SS the

subsatellite point, S the satellite, and d is the range from the earth station to the

satellite. The angle σ is an angle to be determined.

The geometry used in determining the look angles for a geostationary satellite

Considering first the spherical triangle, the

sides are all arcs of great circles, and these

sides are defined by the angles subtended


by them at the center of the earth. Side a

is the angle between the radius to the

North Pole and the radius to the

subsatellite point, and it is seen that a = 90°.

A spherical triangle in which one side is 90° is called a quadrantal triangle. Angle

b is the angle between the radius to the earth station and the radius to the sub -

satellite point. Angle c is the angle between the radius to the earth station and the

radius to the North Pole.

From the figure it is seen that

c =90° - λE.

There are six angles in all defining the spherical triangle. The three angles A, B,

and C are the angles between the planes. Angle A is the angle between the plane

containing c and the plane containing b. Angle B is the angle between the plane

containing c and the plane containing a.

From above figure,

Angle C is the angle between the plane containing b and the plane containing a.

a=900

c =90° - λE

B =φE -φSS

The earth station is west of the subsatellite point, so B is negative, and when

east, B is positive. When the earth-station latitude is north, c is less than 90°, and

when south, c is greater than 90°.

Special rules, known as Napier’s rules, are used to solve the spherical triangle, and

these have been modified here to take into account the signed angles B and λE.

Napier’s rules gives angle b as


)cosarccos(cosE

Bb

and angle A as

b

BA

sin

sinarcsin

In above figure a, angle A is acute (less than 90°), and the azimuth angle is Az =A.

In above figure b, angle A is acute, and the azimuth is, by inspection, Az =360° -A.

In above figure c, angle AC is obtuse and is given by AC =180° - A

Again, by inspection, AZ = AC -180° - A.

In above figure d, angle Ad is obtuse and is given by 180° - A. By inspection ,

AZ=360° - Ad =180° + A.

In all cases, A is the acute angle.

Applying the cosine rule for plane triangles to the triangle of above figure allows

the range d to be found to a close approximation:

bRaaRd GSOGSO cos222


Applying the sine rule for plane triangles to the triangle of above figure allows the

angle of elevation to be found:

b

d

aEl GSO sinarccos

When the subsatellite point is east of the equatorial earth station (B < 0), the

azimuth is 90°, and when west (B > 0), the azimuth is 270°.

2. With neat block diagram, explain the attitude and orbit control system

present in the space segment.[May 2008, June 2009, May 2010,Dec 2011,Nov

2012]

The attitude of a satellite refers to its orientation in space. Much of the

equipment carried aboard a satellite is there for the purpose of controlling its

attitude.

A number of forces, referred to as disturbance torques, can alter the attitude,

some examples being the gravitational fields of the earth and the moon, solar

radiation, and meteorite impacts.

To exercise attitude control, there must be available some measure of a

satellite’s orientation in space and of any tendency for this to shift.

In one method, infrared sensors, referred to as horizon detectors, are used to

detect the rim of the earth against the background of space. With the use of four


such sensors, one for each quadrant, the center of the earth can be readily

established as a reference point.

Any shift in orientation is detected by one or other of the sensors, and a

corresponding control signal is generated which activates a restoring torque.

Controlling torques may be generated in a number of ways.

Any shift in orientation is detected by one or other of the sensors, and a

corresponding control signal is generated which activates a restoring torque.

Also, where a shift in attitude is desired, an attitude maneuver is executed.

The control signals needed to achieve this maneuver may be transmitted from an

earth station.

Passive attitude control refers to the use of mechanisms which stabilize the

satellite without putting a drain on the satellite’s energy supplies. Examples of

passive attitude control are spin stabilization and gravity gradient stabilization.

The other form of attitude control is active control.

With active attitude control, there is no overall stabilizing torque present to

resist the disturbance torques. Instead, corrective torques is applied as required in

response to disturbance torques.

Methods used to generate active control torques include momentum wheels,

electromagnetic coils, and mass expulsion devices, such as gas jets and ion

thrusters.

The electromagnetic coil works on the principle that the earth’s magnetic

field exerts a torque on a current-carrying coil and that this torque can be

controlled through control of the current. However, the method is of use only for

satellites relatively close to the earth.


Fig(a) Roll, pitch, and yaw axes. The yaw axis is directed toward the earth’s

center, the pitch axis is normal to the orbital plane, and the roll axis is

perpendicular to the other two.

Fig(b) RPY axes for the geostationary orbit. Here, the roll axis is tangential to the

orbit and lies along the satellite velocity vector.

The three axes which define a satellite’s attitude are its roll, pitch, and yaw (RPY)

axes. All three axes pass through the center of gravity of the satellite.

For an equatorial orbit, movement of the satellite about the roll axis moves

the antenna footprint north and south; movement about the pitch axis moves the

footprint east and west; and movement about the yaw axis rotates the antenna

footprint.

Spinning satellite stabilization

Spin stabilization may be achieved with cylindrical satellites.

The satellite is constructed so that it is mechanically balanced about one

particular axis and is then set spinning around this axis.


For geostationary satellites, the spin axis is adjusted to be parallel to the N-S

axis of the earth. Spin rate is typically in the range of 50 to 100 rev/min. Spin is

initiated during the launch phase by means of small gas jets.

In the absence of disturbance torques, the spinning satellite would maintain

its correct attitude relative to the earth.

Disturbance torques is generated in a number of ways, both external and

internal to the satellite.

Solar radiation, gravitational gradients, and meteorite impacts are all

examples of external forces which can give rise to disturbance torques. Motor-

bearing friction and the movement of satellite elements such as the antennas also

can give rise to disturbance torques.

The overall effect is that the spin rate will decrease, and the direction of the

angular spin axis will change.

Impulse-type thrusters, or jets, can be used to increase the spin rate again

and to shift the axis back to its correct N-S orientation.

Nutation, which is a form of wobbling, can occur as a result of the

disturbance torques and/or from misalignment or unbalance of the control jets.

This Nutation must be damped out by means of energy absorbers known as

nutation dampers.


Spin stabilization in the geostationary orbit

The antenna in the satellite, which points along the pitch axis, also rotates

with the satellite. Where a directional antenna is used, which is more common for

communications satellites, the antenna must be despun, giving rise to a dual-spin

construction.

An electric motor drive is used for despinning the antenna subsystem.

The antenna subsystem consists of a parabolic reflector and feed horns

mounted on the despun shelf, which also carries the communications repeaters

(transponders).

The control signals and power must be transferred to the despun section, and

a mechanical bearing must be provided. The complete assembly for this is known

as the bearing and power transfer assembly (BAPTA).

Certain dual-spin spacecraft obtain spin stabilization from a spinning

flywheel rather than by spinning the satellite itself. These flywheels are termed

momentum wheels, and their average momentum is referred to as momentum bias.


Momentum wheel stabilization

Stability is achieved by utilizing the gyroscopic effect of a spinning

flywheel, and this approach is used in satellites with cube-like bodies. These are

known as body-stabilized satellites.

The complete unit, termed a momentum wheel, consists of a flywheel, the

bearing assembly, the casing, and an electric drive motor with associated electronic

control circuitry.

The flywheel is attached to the rotor, which consists of a permanent magnet

providing the magnetic field for motor action.

The stator of the motor is attached to the body of the satellite. Thus the

motor provides the coupling between the flywheel and the satellite structure.

Speed and torque control of the motor is exercised through the currents fed

to the stator. The housing for the momentum wheel is evacuated to protect the

wheel from adverse environmental effects, and the bearings have controlled

lubrication that lasts over the lifetime of the satellite.

The term momentum wheel is usually reserved for wheels that operate at

nonzero momentum. This is termed a momentum bias.

Such a wheel provides passive stabilization for the yaw and roll axes when

the axis of rotation of the wheel lies along the pitch axis, as shown in figure.


When a momentum wheel is operated with zero momentum bias, it is

generally referred to as a reaction wheel.

Reaction wheels are used in three-axis stabilized systems. Here, as the name

suggests, each axis is stabilized by a reaction wheel, as shown in figure c.

Reaction wheels can also be combined with a momentum wheel to provide

the control needed.

Random and cyclic disturbance torques tends to produce zero momentum on

average.

However, there will always be some disturbance torques that causes a

cumulative increase in wheel momentum, and eventually at some point the wheel

saturates. In effect, it reaches its maximum allowable angular velocity and can no

longer take in any more momentum.

Mass expulsion devices are then used to unload the wheel, that is, remove

momentum from it. Of course, operation of the mass expulsion devices consumes

part of the satellite’s fuel supply.

3. Discuss about near Geo-stationary orbits.[June 2009, Dec 2011]


There are a number of perturbing forces that cause an orbit to depart from

the ideal Keplerian orbit.

For the geostationary case, the most important of these are the gravitational

fields of the moon and the sun, and the nonspherical shape of the earth. Other

significant forces are solar radiation pressure and reaction of the satellite itself to

motor movement within the satellite.

As a result, station-keeping maneuvers must be carried out to maintain the

satellite within set limits of its nominal geostationary position.

An exact geostationary orbit therefore is not attainable in practice, and the

orbital parameters vary with time.

The period for a geostationary satellite is 23 h, 56 min, 4 s, or 86,164 s. The

reciprocal of this is 1.00273896 rev/day, which is about the value tabulated

for most of the satellites.

Thus these satellites are geosynchronous, in that they rotate in synchronism

with the rotation of the earth. However, they are not geostationary.

The term geosynchronous satellite is used in many cases instead of

geostationary to describe these near-geostationary satellites.

In general a geosynchronous satellite does not have to be near-geostationary,

and there are a number of geosynchronous satellites that are in highly elliptical

orbits with comparatively large inclinations (e.g., the Tundra satellites).

To determine orbital motion, the small inclination makes it difficult to locate

the position of the ascending node, and the small eccentricity makes it difficult to

locate the position of the perigee

Because of the small inclination, the angles ω and Ω can be

assumed to be in the same plane.


From above figure it is assumed that the subsatellite point will be east of the line of

Aries.

The longitude of the subsatellite point (the satellite longitude) is the easterly

rotation from the Greenwich meridian. The Greenwich sidereal time (GST) gives

the eastward position of the Greenwich meridian relative to the line of Aries, and

hence the subsatellite point is at longitude

and the mean longitude of the satellite is given by

The true anomaly can be approximated as


4. Determine the limits of visibility for an earth station situated at mean sea

level, at latitude 48.420 north, and longitude 89.26

0 west. Assume a minimum

angle of elevation of 50. [June 2009]

Solution:

min

0

min

0

min

00 90;6371;42164;5;26.89;42.48 ElkmRkmaEl GSOEE

000

min 66.8590sin42164

6371arcsinsinarcsin

GSOa

RS

00000

min

0 34.7666.8590180180 Sb

0

0

0

15.6942.48cos

34.76cosarccos

cos

cosarccos

E

bB

The satellite limit east of the earth station is at

approxBE

000 2015.6926.89

And the west of the earth station at

approxBE

000 15815.6926.89

5. Discuss about sun transit outage. [June 2009]

Another event which must be allowed for during the equinoxes is the transit

of the satellite between earth and sun, such that the sun comes within the

beamwidth of the earth-station antenna.

When this happens, the sun appears as an extremely noisy source which

completely blanks out the signal from the satellite.

This effect is termed sun transit outage, and it lasts for short periods—each

day for about 6 days around the equinoxes.

The occurrence and duration of the sun transit outage depends on the latitude

of the earth station, a maximum outage time of 10 min being typical.


6. What is the chief advantage of TWTA used aboard the satellites compared

to the other types of high power amplifiers? What are the main disadvantages

of TWTA? [June 2009]

Traveling-wave tube amplifiers (TWTAs) are widely used in transponders

to provide the final output power required to the transmit antenna.

The advantage of the TWT over other types of tube amplifiers is that it can

provide amplification over a very wide bandwidth.

Disadvantage:

The comparatively large size and high power consumption of solenoids make them

unsuitable for use aboard satellites

7. What is thermal control? Why is it required? [June 2009]

Satellites are subject to large thermal gradients, receiving the sun’s radiation

on one side while the other side faces into space.

In addition, thermal radiation from the earth and the earth’s albedo, which is

the fraction of the radiation falling on earth which is reflected, can be significant

for low-altitude earth-orbiting satellites, although it is negligible for geostationary

satellites.

Equipment in the satellite also generates heat which has to be removed.

The most important consideration is that the satellite’s equipment should

operate as nearly as possible in a stable temperature environment. Various steps are

taken to achieve this.

Thermal blankets and shields may be used to provide insulation.

Radiation mirrors are often used to remove heat from the communications

payload.


These mirrored drums surround the communications equipment shelves in

each case and provide good radiation paths for the generated heat to escape into the

surrounding space.

One advantage of spinning satellites compared with body-stabilized is that

the spinning body provides an averaging of the temperature extremes experienced

from solar flux and the cold background of deep space.

In order to maintain constant temperature conditions, heaters may be

switched on (usually on command from ground) to make up for the heat reduction

which occurs when transponders are switched off.

8. Write brief note on TT&C subsystem.[May 2010]

The TT&C subsystem performs several routine functions aboard the

spacecraft.

The telemetry, or telemetering, function could be interpreted as

measurement at a distance.

Specifically, it refers to the overall operation of generating an electrical

signal proportional to the quantity being measured and encoding and transmitting

this to a distant station, which for the satellite is one of the earth stations.

Data which are transmitted as telemetry signals include attitude information

such as that obtained from sun and earth sensors; environmental information such

as the magnetic field intensity and direction, the frequency of meteorite impact,

and so on; and spacecraft information such as temperatures, power supply voltages,

and stored-fuel pressure.

During the transfer and drift orbital phases of the satellite launch, a special

channel is used along with an omnidirectional antenna. Once the satellite is on

station, one of the normal communications transponders may be used along with its


directional antenna, unless some emergency arises which makes it necessary to

switch back to the special channel used during the transfer orbit.

Telemetry and command may be thought of as complementary functions.

The telemetry subsystem transmits information about the satellite to the

earth station, while the command subsystem receives command signals from the

earth station, often in response to telemetered information.

The command subsystem demodulates and, if necessary, decodes the

command signals and routes these to the appropriate equipment needed to execute

the necessary action.

Thus attitude changes may be made, communication transponders switched

in and out of circuits, antennas redirected, and station-keeping maneuvers carried

out on command.

It is clearly important to prevent unauthorized commands from being

received and decoded, and for this reason, the command signals are often

encrypted.


Thus, the telemetry, tracking, and command functions are complex

operations which require special ground facilities in addition to the TT&C

subsystems aboard the satellite.

Transponders

A transponder is the series of interconnected units which forms a single

communications channel between the receiver and transmitter antennas in a

communications satellite.

Some of the units utilized by a transponder in a given channel may be

common to a number of transponders.

Thus, this must be thought of as an equipment channel rather than a single

item of equipment.

The bandwidth allocated for C-band service is 500 MHz, and this is divided

into subbands, one for each transponder.

Atypical transponder bandwidth is 36 MHz, and allowing for a 4-MHz

guardband between transponders, 12 such transponders can be accommodated in

the 500-MHz bandwidth. By making use of polarization isolation, this number can

be doubled.

Polarization isolation refers to the fact that carriers, which may be on the

same frequency but with opposite senses of polarization, can be isolated from one

another by receiving antennas matched to the incoming polarization. Because the

carriers with opposite senses of polarization may overlap in frequency, this

technique is referred to as frequency reuse.


Section of an uplink frequency and polarization plan

Satellite transponder channels


The incoming, or uplink, frequency range is 5.925 to 6.425 GHz.

The input filter passes the full 500-MHz band to the common receiver while

rejecting out-of-band noise and interference such as might be caused by image

signals.

The frequency conversion shifts the carriers to the downlink frequency band,

which is also 500 MHz wide, extending from 3.7 to 4.2 GHz. At this point the

signals are channelized into frequency bands which represent the individual

transponder bandwidths.

The wideband receiver


The wideband receiver is shown above. A duplicate receiver is provided so that if

one fails, the other is automatically switched in.

The combination is referred to as a redundant receiver, meaning that although two

are provided, only one is in use at a given time.

The first stage in the receiver is a low-noise amplifier (LNA). This amplifier

adds little noise to the carrier being amplified, and at the same time it provides

sufficient amplification for the carrier to override the higher noise level present in

the following mixer stage. The equivalent noise temperature of a satellite receiver

may be on the order of a few hundred kelvins.

The LNA feeds into a mixer stage, which also requires a local oscillator

(LO) signal for the frequency-conversion process. The power drive from the LO to

the mixer input is about 10 dBm.

A second amplifier follows the mixer stage to provide an overall receiver

gain of about 60 dB.

Splitting the gain between the preamplifier at 6 GHz and the second

amplifier at 4 GHz prevents oscillation, which might occur if all the gain were to

be provided at the same frequency.

The wideband receiver utilizes only solid-state active devices. In some

designs, tunnel-diode amplifiers have been used for the preamplifier at 6 GHz in

6/4-GHz transponders and for the parametric amplifiers at 14 GHz in 14/12-GHz

transponders.

The input demultiplexer

The input demultiplexer separates the broadband input, covering the

frequency range 3.7 to 4.2 GHz, into the transponder frequency channels.

Referring to Fig. 7.13, for example, the separate channels labeled 1 through 12 are

shown in detail in Fig. 7.15. The channels are usually arranged in even-numbered


and odd-numbered groups. This provides greater frequency separation between

adjacent channels in a group, which reduces adjacent channel interference. The

output from the receiver is fed to a power splitter, which in turn feeds the two

separate chains of circulators. The full broadband signal is transmitted along each

chain, and the channelizing is achieved by means of channel filters connected to

each circulator. Each filter has a bandwidth of 36 MHz and is tuned to the

appropriate center frequency. Although there are considerable losses in the

demultiplexer, these are easily made up in the overall gain for the transponder

channels.


The power amplifier

A separate power amplifier provides the output power for each transponder

channel. This is necessary to permit the input drive to each power amplifier to be

adjusted to the desired level. The attenuator has a fixed section and a variable

section. The fixed attenuation is needed to balance out variations in the input

attenuation so that each transponder channel has the same nominal attenuation, the

necessary adjustments being made during assembly. The variable attenuation is

needed to set the level as required for different types of service. Because this

variable attenuator adjustment is an operational requirement, it must be under the

control of the ground TT&C station.


Traveling-wave tube amplifiers (TWTAs) are widely used in transponders to

provide the final output power required to the transmit antenna.

In the TWT, an electron-beam gun assembly consisting of a heater, a

cathode, and focusing electrodes is used to form an electron beam.

A magnetic field is required to confine the beam to travel along the inside of

a wire helix. For high-power tubes, such as might be used in ground stations, the

magnetic field can be provided by means of a solenoid and dc power supply.


The comparatively large size and high power consumption of solenoids

make them unsuitable for use aboard satellites, and lower-power TWTs are used

which employ permanentmagnet focusing.

The rf signal to be amplified is coupled into the helix at the end nearest the

cathode and sets up a traveling wave along the helix. The electric field of the wave

will have a component along the axis of the helix.

In some regions, this field will decelerate the electrons in the beam, and in

others it will accelerate them so that electron bunching occurs along the beam. The

average beam velocity, which is determined by the dc potential on the tube

collector, is kept slightly greater than the phase velocity of the wave along the

helix. Under these conditions, an energy transfer takes place, kinetic energy in the

beam being converted to potential energy in the wave.

The wave actually will travel around the helical path at close to the speed of

light, but it is the axial component of wave velocity which interacts with the

electron beam.

This component is less than the velocity of light approximately in the ratio

of helix pitch to circumference. Because of this effective reduction in phase

velocity, the helix is referred to as a slowwave structure.

The advantage of the TWT over other types of tube amplifiers is that it can

provide amplification over a very wide bandwidth. Input levels to the TWT must

be carefully controlled, however, to minimize the effects of certain forms of

distortion.

9. Discuss in detail various units of a transponder.[May 2010]

or

Explain transponder in detail. [ Dec 2011, June 2012]


A transponder is the series of interconnected units which forms a single

communications channel between the receiver and transmitter antennas in a

communications satellite.

Some of the units utilized by a transponder in a given channel may be

common to a number of transponders.

Thus, this must be thought of as an equipment channel rather than a single

item of equipment.

The bandwidth allocated for C-band service is 500 MHz, and this is divided

into subbands, one for each transponder.

Atypical transponder bandwidth is 36 MHz, and allowing for a 4-MHz

guardband between transponders, 12 such transponders can be accommodated in

the 500-MHz bandwidth. By making use of polarization isolation, this number can

be doubled.

Polarization isolation refers to the fact that carriers, which may be on the

same frequency but with opposite senses of polarization, can be isolated from one

another by receiving antennas matched to the incoming polarization. Because the

carriers with opposite senses of polarization may overlap in frequency, this

technique is referred to as frequency reuse.

The incoming, or uplink, frequency range is 5.925 to 6.425 GHz. The input

filter passes the full 500-MHz band to the common receiver while rejecting out-of-

band noise and interference such as might be caused by image signals.

The input filter passes the full 500-MHz band to the common receiver while

rejecting out-of-band noise and interference such as might be caused by image

signals. There will be many modulated carriers within this 500-MHz passband, and

all of these are amplified and frequency converted in the common receiver.


The frequency conversion shifts the carriers to the downlink frequency band,

which is also 500 MHz wide, extending from 3.7 to 4.2 GHz. At this point the

signals are channelized into frequency bands which represent the individual

transponder bandwidths.


At this point the signals are channelized into frequency bands which

represent the individual transponder bandwidths.

A transponder may handle one modulated carrier, such as a TV signal, or it

may handle a number of separate carriers simultaneously, each modulated by its

own telephony or other baseband channel.

10. Discuss with neat diagram the Anik-E C band transponder.[May 2010]

The Anik series of satellites are designed to provide communications

services in Canada as well as cross-border services with the United States.

Anik-E was the first of the body-stabilized satellites used in the series, a

dual-band satellite which had an equivalent capacity of 56 television channels, or

more than 50,000 telephone circuits. Attitude control was of the momentum-bias,

three-axes-stabilized type, and solar sails were used to provide power, the capacity

being 3450 W at summer solstice and 370

0 W at vernal equinox. Four NiH2

batteries provided power during eclipse.


Anik-E spacecraft configuration

Anik-E C-band transponder functional block diagram


This is seen to use solid-state power amplifiers (SSPAs) which offer

significant improvement in reliability and weight saving over traveling-wave tube

amplifiers.

The antennas are fed through a broadband feeder network (BFN) to

illuminate the large reflectors. National, as distinct from regional, coverage is

provided at C band.

The TWTAs aboard a satellite also may be switched to provide redundancy.

The scheme shown is termed a 4-for-2 redundancy, meaning that four channels are

provided with two redundant amplifiers.

11. Explain in detail about station keeping.[Dec 2011]

In addition to having satellite’s attitude controlled, it is important that a

geostationary satellite be kept in its correct orbital slot.

The equatorial ellipticity of the earth causes geostationary satellites to drift

slowly along the orbit, to one of two stable points, at 75°E and 105°W.

To counter this drift, an oppositely directed velocity component is imparted

to the satellite by means of jets, which are pulsed once every 2 or 3 weeks.

This results in the satellite drifting back through its nominal station position,

coming to a stop, and recommencing the drift along the orbit until the jets are

pulsed once again.

These maneuvers are termed east-west station-keeping maneuvers.

Satellites in the 6/4-GHz band must be kept within ±0.1° of the designated

longitude, and in the 14/12-GHz band, within ±0.05°.

A satellite which is nominally geostationary also will drift in latitude, the main

perturbing forces being the gravitational pull of the sun and the moon.


These forces cause the inclination to change at a rate of about 0.85°/year. If left

uncorrected, the drift would result in a cyclic change in the inclination, going from

0° to 14.67° in 26.6 years and back to zero, at which the cycle is repeated. To

prevent the shift in inclination from exceeding specified limits, jets may be pulsed

at the appropriate time to return the inclination to zero. Counteracting jets must be

pulsed when the inclination is at zero to halt the change in inclination.

These maneuvers are termed north-south station-keeping maneuvers, and they are

much more expensive in fuel than are east-west station-keeping maneuvers. The

north-south station-keeping tolerances are the same as those for east-west station

keeping, ±0.1° in the C band and ±0.05° in the Ku band.

Orbital correction is carried out by command from the TT&C earth station, which

monitors the satellite position. East-west and north-south station-keeping

maneuvers are usually carried out using the same thrusters as are used for attitude

control.

Satellite altitude also will show variations of about ±0.1 percent of the nominal

geostationary height. If, for sake of argument, this is taken as 36,000 km, the total

variation in the height is 72 km.

A C-band satellite therefore can be anywhere within a box bound by this height

and the ±0.1° tolerances on latitude and longitude. Approximating the

geostationary radius as 42,164 km, an angle of 0.2° subtends an arc of

approximately 147 km. Thus both the latitude and longitude sides of the box are

147 km.


The -3dB beamwidth of a 30-m antenna is about 0.12°, and of a 5m antenna, about

0.7° at 6 GHz. Assuming 38,000 km for the slant range, the diameter of the 30-m

beam at the satellite will be about 80 km. This beam does not encompass the whole

of the box and therefore could miss the satellite. Such narrow-beam antennas

therefore must track the satellite.

The diameter of the 5m antenna beam at the satellite will be about 464 km, and this

does encompass the box, so tracking is not required.

The positional uncertainty of the satellite also introduces an uncertainty in

propagation time, which can be a significant factor in certain types of

communications networks.


By placing the satellite in an inclined orbit, the north-south station keeping

maneuvers may be dispensed with.

The satellite is placed in an inclined orbit of about 2.5° to 3°, in the opposite sense

to that produced by drift. Over a period of about half the predicted lifetime of the

mission, the orbit will change to equatorial and then continue to increase in

inclination.

12. Explain Antenna subsystem in detail.[June 2012]

The antennas carried aboard a satellite provide the dual functions of receiving the

uplink and transmitting the downlink signals.

They range from dipole-type antennas where omnidirectional characteristics are

required to the highly directional antennas required for telecommunications

purposes and TV relay and broadcast.

Directional beams are usually produced by means of reflector-type antennas—the

paraboloidal reflector being the most common. The gain of the paraboloidal

reflector, relative to an isotropic radiator, is given by

2

DnG I

where l is the wavelength of the signal, D is the reflector diameter, and nI is the

aperture efficiency.

The -3dB beamwidth is given approximately as

reesD

dB deg703

The ratio D/λ is seen to be the key factor in these equations, the gain being

directly proportional to (D/λ)2 and the beamwidth inversely proportional to D/λ.

Hence the gain can be increased and the beamwidth made narrower by

increasing the reflector size or decreasing the wavelength.


In comparing C-band and Ku-band, the largest reflectors are those for the

6/4-GHz band.

Satellites used for mobile services in the L-band employ much larger

antennas.

The largest reflectors are for the 6/4-GHz hemisphere and zone coverages,

these are fed from horn arrays, and various groups of horns can be excited to

produce the beam shape required.

Separate arrays are used for transmit and receive. Each array has 146 dual-

polarization horns.

In the 14/11-GHz band, circular reflectors are used to provide spot beams,

one for east and one for west. These beams are fully steerable.

Each spot is fed by a single horn which is used for both transmit and receive.

Wide beams for global coverage are produced by simple horn antennas at 6/4 GHz.

These horns beam the signal directly to the earth without the use of reflectors.

A simple biconical dipole antenna is used for the tracking and control

signals.

The same feed horn may be used to transmit and receive carriers with the

same polarization.

The transmit and receive signals are separated in a device known as a diplexer,

and the separation is further aided by means of frequency filtering.

Polarization discrimination also may be used to separate the transmit and

receive signals using the same feed horn. For example, the horn may be used to

transmit horizontally polarized waves in the downlink frequency band, while

simultaneously receiving vertically polarized waves in the uplink frequency band.

The polarization separation takes place in a device known as an

orthocoupler, or orthogonal mode transducer (OMT). Separate horns also may be


used for the transmit and receive functions, with both horns using the same

reflector.

13. With mathematics, illustrate the limits of visibility. [Dec 2012]


There will be east and west limits on the geostationary arc visible from any given

earth station.

The limits will be set by the geographic coordinates of the earth station and the

antenna elevation.

The lowest elevation in theory is zero, when the antenna is pointing along the

horizontal.

A quick estimate of the longitudinal limits can be made by considering an earth

station at the equator, with the antenna pointing either west or east along the

horizontal, as shown in figure.

The limiting angle is given by

o

GSO

E

a

a

3.81

42164

6378arccos

arccos

Thus, an earth station could see satellites over a geostationary arc bounded by

±81.3° about the earth-station longitude.

In practice, to avoid reception of excessive noise from the earth, some finite

minimum value of elevation is used, which will be denoted here by Elmin. A typical

value is 5°.

The limits of visibility will also depend on the earth-station latitude.


As in figure, let S represent the angle subtended at the satellite when the angle

min =90°+Elmin.

Applying the sine rule gives

minsinarcsin

GSOa

RS

A sufficiently accurate estimate is obtained by assuming a spherical earth of mean

radius 6371 km as was done previously. Once angle S is known, angle b is found

from the equation

Sb o min180

And B can be found by equation,

E

bB

cos

cosarccos

Once angle B is found, the satellite longitude can be determined.

14. How do you place satellites into geo stationary orbit? [Dec 2012]

Satellites may be directly injected into low orbits from a launch vehicle. Launch

vehicles may be classified as expendable or reusable. Typical of the expendable

launchers are the U.S. Atlas-Centaur and Delta rockets and the European Space

Agency Ariane rocket. Japan, China, and Russia all have their own expendable

launch vehicles.

Where an orbital altitude greater than about 200 km is required, it is not

economical in terms of launch vehicle power to perform direct injection, and the


satellite must be placed into transfer orbit between the initial LEO and the final

high-altitude orbit.

In most cases, the transfer orbit is selected to minimize the energy required for

transfer, and such an orbit is known as a Hohmann transfer orbit. The time required

for transfer is longer for this orbit than all other possible transfer orbits.

The Hohmann elliptical orbit is seen to be tangent to the low altitude orbit at

perigee and to the high-altitude orbit at apogee.

At the perigee, in the case of rocket launch, the rocket injects the satellite with

the required thrust into the transfer orbit.

With the STS (Space Transportation System ), the satellite must carry a

perigee kick motor which imparts the required thrust at perigee.


At apogee, the apogee kick motor (AKM) changes the velocity of the satellite to

place it into a circular orbit in the same plane. As shown in figure, it takes 1 to 2

months for the satellite to be fully operational. Throughout the launch and

acquisition phases, a network of ground stations, spread across the earth, is

required to perform the tracking, telemetry, and command (TT&C) functions.

Velocity changes in the same plane change the geometry of the orbit but not its

inclination. In order to change the inclination, a velocity change is required normal

to the orbital plane. Changes in inclination can be made at either one of the nodes,

without affecting the other orbital parameters. Since energy must be expended to

make any orbital changes, a geostationary satellite should be launched initially

with as low an orbital inclination as possible. It will be shown shortly that the

smallest inclination obtainable at initial launch is equal to the latitude of the launch

site. Thus the farther away from the equator a launch site is, the less useful it is,

since the satellite has to carry extra fuel to effect a change in inclination. Russia

does not have launch sites south of 45°N, which makes the launching of


geostationary satellites a much more expensive operation for Russia than for other

countries which have launch sites closer to the equator.

Prograde orbits have an easterly component of velocity, so prograde launches gain

from the earth’s rotational velocity. For a given launcher size, a significantly larger

payload can be launched in an easterly direction than is possible with a retrograde

launch. In particular, easterly launches are used for the initial launch into the

geostationary orbit.

The relationship between inclination, latitude, and azimuth may be seen as follows.

This is a right spherical triangle, and Napier’s rule for this gives

cosi=cosλsinAZ

For a prograde orbit ,0≤i≤900 , and hence cosi is positive.

Also,-900≤λ≤90

0 , and hence cosλ is also positive.

It follows therefore from above equation that 0≤AZ≤1800 , or the launch azimuth

must be easterly in order to obtain a prograde orbit.

For a fixed λ, to minimize the inclination i, cosi should be a maximum, which

requires sin AZ to be maximum, or AZ=90°. Under these conditions the above

equation becomes


cosimin=cosλ or imin=λ

Thus the lowest inclination possible on initial launch is equal to the latitude of the

launch site.

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PART-Amahalakshmiengineeringcollege.com/pdf/ece/VIIIsem/EC2045/UNIT 2,3.pdf · A satellite in a geostationary orbit appears to be stationary with respect to the earth, hence the name