Satellite Communication ppt-3-1.pdf

Satellite Communication Engineering
Sayed Abozar Sadat (TCE Department)
Prepared by: Sayed Abozar "Sadat"

Overview of Satellite Communication
• The word Satellite is originated from Latin word Satellitem. It means attendant or follower.
• Any object which is moving around any other object is called Satellite.
• Natural satellite Like: Moon
• Man made satellite: The man made satellite orbiting around the earth is called passive or artificial Satellite.

Satellite Services and Applications
Communications:
• Voice Communication
• Video Communication
• Data Communication
• Rural Telephonery (Valley areas, where we can not provide physical links)
• Internet Trunking
• News gathering
• Distance learning
• Mobile telephone
• Broadcast and cable relay
• Multimedia core IP

GPS/Navigation:
• Position location
• Timing
• Searching area rescue operation
• Mapping
• Database access
• Emergency services
Remote Sensing:
• Pipeline Monitoring
• Infrastructure planning
• Forest fire prevention
• Urban planning
• Flood and storm watching
• Air pollution management

Satellite Orbits: is classified into four categories.
1. LEO (Low Earth Orbit) – around 46%
2. MEO (Medium Earth Orbit) – around 6%
3. GEO (Geosynchronous Earth Orbit) – around 43%
4. MOLNIYA – around 2%
5. Others - around 3%

Orbital Mechanics:
This deals with the launching of a satellite in a particular orbit.
Generally, space crafts get energy from sun through solar plates but during eclipse gets energy from
the batteries.
How to categorise the Communication Satellite:
I. Based on Coverage Area:
o Global
o National
o Regional
II. Based on service type:
o Fixed Service Satellite (FSS)
o Broadcast Service Satellite (BSS)
o Mobile Service Satellite (MSS)
III. General Usage:
o Military
o Experimental
o Commercial

Satellites Basics:
 In general orbits are elliptical or circular.
 The complete revolution time depends on distance between satellite and earth.
 Inclination: Angle between orbital plane and earth’s equatorial plane.
 Elevation: Angle between satellite and horizon ( the line at which earth’s surface and the sky appears to
meet).
 Line of sight (Los): Los to satellite is necessary for the connection.
 Uplink: From ground to spacecraft
 Downlink: From spacecraft to ground station
Uplink and downlink should have different frequencies of transmission to avoid crosstalk between two signals,
which is called mining of signals. it would be easy to differentiate both uplink and downlink.

Satellite vs Terrestrial Communication
Advantages:
• The coverage area of satellite tremendously increase over terrestrial communication.
• Transmission cost of satellite is independent of distance from centre to coverage area. Whereas in terrestrial cost depends on
distance.
• In satellite higher bandwidth is available . Whereas, in terrestrial we need to deploy more no. of cables.
• Satellite to satellite link is enough to cover the enough part of globe.
Disadvantages:
• launching satellite into space is so costly. We need money, technology and very difficult propagation.
• Delay is more.
• Lifetime of spacecraft is limited.

Types Satellites based on of Orbits
Geo stationary Earth Orbit (GEO) Satellite:
These satellites are in orbit 35863Km above the earth’s surface.
The objects in Geostationary orbit revolve around the earth at the same speed of earth’s rotation. So, this
means that Geo satellites remain in the same position relative to the surface of the earth. The inclination angle
of the satellite with respect to earth is zero.
The complete revolution takes one day. the antenna positions are fixed.
The main advantage of GEO is larger coverage area (Foot Field Area).
Theoretically around 34% of the earth’s surface is covered by this satellite.
The coverage angle is 120 degrees. The orbit of these satellites are circular.
Lifetime expectancy of these satellites is 15 years.

Geostationary satellite in practical is termed as geosynchronous as there are multiple factors which make these
satellites shift from the ideal geostationary condition:
• Gravitational pull of sun and moon makes these satellites deviate from their orbit. Over the period of time,
they go through a drag. (Earth’s gravitational force has no effect on these satellites due to their distance
from the surface of the Earth.)
• These satellites experience the centrifugal force due to the rotation of Earth, making them deviate from their
orbit.
• The non-circular shape of the earth leads to continuous adjustment of speed of satellite from the earth
station.
These satellites are used for TV and radio broadcast, weather forecast and also, these satellites are operating as
backbones for the telephone networks.

Disadvantages:
• High transmit power is needed.
• High latency is required (takes more time).
• Mobile communication is not possible.
• The drawback for the orientation of this satellite is that it’s difficult to cover the polar regions.

Low Earth Orbit (LEO) Satellite:
These satellites are much closer to earth compare to GEO.
They are typically 500-1500Km above the earth’s surface. So, it has to move at a faster rate or else it will fall
down due to gravitational force.
They are not at fixed position relative to earth.
Latency is 5-10ms, that is very less compared to GEO.
The coverage area is also small.
Better frequency reuse is possible.
We need the hand-over mechanism between the satellite to get the link connected.
LEO signal is stronger compare to GEO signal.
Mainly used for remote sensing and point to point communication.

Disadvantages:
• It’s so costly.
• Many satellites are required for global coverage.
• Higher number of satellites combined with fast movements resulting in a high complexity of satellite system.
• One general problem of LEOs is the short lifetime of about five to eight years due to atmospheric drag which
is causing shift in position and radiation from the inner Van Allen belt1.
• Doppler’s shift is also considered here.

Inner Van Allen Belt
A Van Allen radiation belt is a zone of energetic charged
particles, most of which originate from the solar wind, that
are captured by and held around a planet by that planet's
magnetosphere.
Doppler’s Shift
The Doppler effect, or Doppler shift, describes the changes
in frequency of any kind of sound or light wave produced by
a moving source with respect to an observer.

Medium Earth Orbit (MEO) Satellite:
These satellites are in orbit between 8000-15000Km above the earth’s surface.
Their functionality are similar to LEO.
Their coverage area is much more than LEO but less than GEO.
They are available for much longer time than LEO , around 2-8 hours.
Less satellites are required to get continuous link. So, hand-over mechanism is less.
Latency is 70-80ms.

MOLNIYA Orbit Satellite:
This orbit used by Russians for decades.
Shape of the orbital is elliptical. Where GEO and LEO are circular.
This satellite remain in fixed position relative to earth for 8-hours. Once 8-hours is done , other satellite takes
over.
A series of 3 MOLNIYA satellite acts as GEO satellite.
Useful near polar regions.

High Altitude Platform (HAP) Satellites:
They are mainly for small coverage areas and their signals are very strong.
Its very cheaper to put in position on orbit.
A plain around 20Km above the earth surface is used as a satellite position.

Orbital Mechanics
Newton’s Laws and Kinematics:
 S = 𝑢𝑡 + 1/2𝑎𝑡2
 𝑣2
− 𝑢2
= 2𝑎𝑠
 𝑣 = 𝑢 + 𝑎𝑡
 𝑓 = 𝑚𝑎
𝑠 − 𝑑𝑖𝑠𝑡𝑎𝑛𝑐𝑒 𝑡𝑟𝑎𝑣𝑒𝑙𝑙𝑒𝑑 𝑖𝑛 𝑡𝑖𝑚𝑒 𝑇, 𝑎 − 𝑎𝑐𝑐𝑒𝑙𝑒𝑟𝑎𝑡𝑖𝑜𝑛 , 𝑣 − 𝑓𝑖𝑛𝑎𝑙 𝑣𝑒𝑙𝑜𝑐𝑖𝑡𝑦, 𝑢 − 𝑖𝑛𝑖𝑡𝑖𝑎𝑙 𝑣𝑒𝑙𝑜𝑐𝑖𝑡𝑦.
𝑓𝑜𝑟𝑐𝑒 = 𝑚𝑎𝑠𝑠 × 𝑎𝑐𝑐𝑒𝑙𝑒𝑟𝑎𝑡𝑖𝑜𝑛
Newton is a force required to move an object of 1𝑘𝑔 mass with an acceleration of 1𝑚/𝑠2.
• Acceleration due to gravity is 𝑔 =
𝜇
𝑟2 ൗ
𝑘𝑚
𝑠2
• Where 𝑟 − 𝑟𝑎𝑑𝑖𝑢𝑠𝑜𝑓 𝑒𝑎𝑟𝑡ℎ, 𝜇 − 𝑢𝑛𝑖𝑣𝑒𝑟𝑠𝑎𝑙 𝑔𝑟𝑎𝑣𝑖𝑡𝑎𝑡𝑖𝑜𝑛𝑎𝑙 𝑐𝑜𝑛𝑠𝑡𝑎𝑛𝑡 (𝐺) × mass of the earth (𝑀)
𝑟 = 6371𝑘𝑚 𝐺 = 6.672 × 10−11
ൗ
𝑁𝑚2
𝑘𝑔2 𝑀 = 5.972 × 1024
𝑘𝑔
𝜇 = 𝐺 × 𝑀
𝑔 =
𝐺 × 𝑀
𝑟2

Example: The radius of the moon is 1.74 × 106
𝑚. The mass of the moon is taken as 7.35 × 1022
𝑘𝑔. Find the
acceleration due to gravity on the surface of the moon.
𝐺 = 6.672 × 10−11
ൗ
𝑁𝑚2
𝑘𝑔2
Answer: 𝑔 = 1.620 Τ
𝑚
𝑠2

Kepler’s Law
Satellites (spacecraft) orbiting the earth follow the same laws that govern the motion of the planets around the
sun.
Kepler’s laws apply quite generally to any two bodies in space which interact through gravitation.
The more massive of the two bodies is referred to as the primary, the other, the secondary or satellite.
Kepler’s First Law:
Kepler’s first law states that the path followed by a satellite around the primary (Earth) will be an
ellipse.
F1 and F2 are Focal points.

Parameters associated with the 1st law of Kepler:
• Eccentricity (e): it defines how stretched out an ellipse is from a perfect circle.
• Semi-Major axis (a): It is the longest diameter, a line that runs through the center and both foci, its ends
being at the widest points of the shapes. This line joins the points of apogee.
• Semi-Minor axis (b): the line joining the points of perigee is called the Semi-Minor axis.
• The value of e could be determined by: 𝑒 =
𝑎2−𝑏2
𝑎

Kepler’s Second Law:
Kepler’s second law states that, for equal time intervals, a satellite will sweep out equal areas in its orbital
plane, focused at the barycenter.
The satellite speed, traveling s1
distance is more than satellite
speed traveling s2 distance.
s1

Kepler’s Third Law:
Square of period of the revolution equals to a constant multiplied by third power of the semi major axis.
𝑇2
∝ 𝑎3
𝑇2
=
4𝜋2
𝜇
𝑎3
𝜇 = 𝐺 × 𝑀 𝜇 = 3.986 × 1014 ൗ
𝑚3
𝑠2
The square of the orbital period of a planet is directly proportional to the cube of the semi-major axis of its
orbit.
This law shows the relationship between the distances of satellite from earth and their orbital period.

Example:
Find Geostationary orbit radius and height, nearly one period of revolution of geostationary orbit is 𝑇
= 23ℎ𝑟 56 min 4.1𝑠𝑒𝑐 .
Solution: based on Kepler’s third law we have,
𝑇2 =
4𝜋2
𝜇
𝑎3 𝜇 = 3.986 × 1014 ൗ
𝑚3
𝑠2
𝑎3
=
𝑇2 × 𝜇
4𝜋2
𝑇 = 23 × 60 × 60 + 56 × 60 + 4.1 = 86164.1
𝑎 = 42178.42 𝑘𝑚
Height , ℎ = 𝑜𝑟𝑏𝑖𝑡 𝑟𝑎𝑑𝑖𝑢𝑠 − 𝑒𝑎𝑟𝑡ℎ 𝑟𝑎𝑑𝑖𝑢𝑠
𝑒𝑎𝑟𝑡ℎ 𝑟𝑎𝑑𝑖𝑢𝑠 = 6378 km
ℎ = 42178.42 − 6378 = 35800 𝑘𝑚 ≈ 36000 𝑘𝑚

Orbital Parameters
• Apogee: A point for a satellite farthest from the Earth. It is denoted as ha.
• Perigee: A point for a satellite closest from the Earth. It is denoted as hp.
ha
hp

• Line of Apsides: Line joining perigee and apogee through center of the Earth. It is the major axis of the orbit.
One-half of this line’s length is the semi-major axis equivalents to satellite’s mean distance from the Earth.

• Ascending Node: The point where the orbit crosses the equatorial plane going from north to south.
• Descending Node: The point where the orbit crosses the equatorial plane going from south to north.
• Inclination: the angle between the orbital plane and the Earth’s equatorial plane. Its measured at the
ascending node from the equator to the orbit, going from East to North. Also, this angle is commonly
denoted as i.
• Line of Nodes:
the line joining the ascending and
descending nodes through the center of Earth.

Prograde Orbit: an orbit in which satellite moves in the same direction as the Earth’s rotation. Its inclination is
always between 0 to 90. Many satellites follow this path as Earth’s velocity makes it easier to lunch these
satellites.
Retrograde Orbit: an orbit in which satellite moves in the same direction counter to the Earth’s rotation.

• Argument of Perigee: An angle from the point of perigee measure in the orbital plane at the Earth’s center,
in the direction of the satellite motion.
• Right ascension of Ascending node: The definition of an orbit in space, the position of ascending node is
specified. But as the Earth spins, the longitude of ascending node changes and cannot be used for reference.
Thus for practical determination of an orbit, the longitude and time of crossing the ascending node is used.
For absolute measurement, a fixed reference point in space is required. It could also be defined as “right
ascension of the ascending node.

• Mean Anamoly: It gives the average value to the angular position of the satellite with reference to the
perigee.
• True Anamoly: It is the angle from point of perigee to the satellite’s position, measure at the Earth’s center.

Eclipse
Near the time of spring and autumnal equinoxes, when the sun is crossing the equator, the satellite passes into
sun’s shadow.
These eclipses begin 23 days before the equinox and end 23 days after the equinox.
They last for almost 10 minutes at the beginning and end of equinox and increase for a maximum period of 72
minutes at a full eclipse.
The solar cells of the satellite become non-functional during the eclipse period and the satellite is made to
operate with the help of power supplied from the batteries.

A satellite east of the earth station enters eclipse during daylight busy) hours at the earth station.
A Satellite west of earth station enters eclipse during night and early morning hours (non busy time).

Sun Transit Outage
Sun transit outage is an interruption in or distortion of geostationary satellite signals caused by interference
from solar radiation.
Sun appears to be an extremely noisy source which completely blanks out the signal from satellite. This effect
lasts for 6 days around the equinoxes. They occur for a maximum period of 10 minutes. Generally, sun outages
occur in February, March, September and October, that is, around the time of the equinoxes.
At these times, the apparent path of the sun across the sky takes it directly behind the line of sight between an
earth station and a satellite.
As the sun radiates strongly at the microwave frequencies used to communicate with satellites (C-band, Ka
band and Ku band) the sun swamps the signal from the satellite.
The effects of a sun outage can include partial degradation, that is, an increase in the error rate, or total
destruction of the signal.

Launching Procedures
Low Earth Orbiting satellites are directly injected into their orbits.
This cannot be done incase of GEOs as they have to be positioned 36,000kms above the Earth’s surface.
Launch vehicles are hence used to set these satellites in their orbits.
These vehicles are reusable. They are also known as Space Transportation System(STS).
When the orbital altitude is greater than 1,200 km it becomes expensive to directly inject the satellite in its
orbit.
For this purpose, a satellite must be placed in to a transfer orbit between the initial lower orbit and destination
orbit. The transfer orbit is commonly known as Hohmann-Transfer Orbit.

Orbit Transfer:
The transfer orbit is selected to minimize the energy required for the transfer. This orbit forms a tangent to the
low attitude orbit at the point of its perigee and tangent to high altitude orbit at the point of its apogee.

Launch Vehicle and Propulsion
The rocket injects the satellite with the required thrust into the transfer orbit. With the STS, the satellite carries
a perigee kick motor which imparts the required thrust to inject the satellite in its transfer orbit.
Similarly, an apogee kick motor (AKM) is used to inject the satellite in its destination orbit.
Generally it takes 1-2 months for the satellite to become fully functional. The Earth Station performs the
Telemetry Tracking and Command function to control the satellite transits and functionalities.
Thrust: It is a reaction force described quantitatively by Newton's second and third laws. When a system expels
or accelerates mass in one direction the accelerated mass will cause a force of equal magnitude but opposite
direction on that system.
Kick Motor: refers to a rocket motor that is regularly employed on artificial satellites destined for a
geostationary orbit.
As the vast majority of geostationary satellite launches are carried out from spaceports at a significant distance
away from Earth's equator. The carrier rocket would only be able to launch the satellite into an elliptical orbit
of maximum apogee 35,784-kilometres and with a non-zero inclination approximately equal to the latitude of
the launch site.

ANTENNA LOOK ANGLES
The coordinates at which earth station communicate with satellite is called look angle.
The look angles for the ground station antenna are Azimuth and Elevation angles. They are required at the
antenna so that it points directly at the satellite. Look angles are calculated by considering the elliptical orbit.
These angles change in order to track the satellite.
For geostationary orbit,
these angels values does not change
as the satellites are stationary with
respect to earth.

Azimuth Angle:
The angle between local horizontal plane and the plane passing through earth station, satellite and center of
earth is called as azimuth angle.
Elevation Angle:
The angle between vertical plane and line pointing to satellite is known as Elevation angle. Vertical plane is
nothing but the plane, which is perpendicular to horizontal plane.
Sub-satellite points:
Is the place where the line drawn from center of earth
to satellite passes through the earth’s surface.

Sub-satellite points:
Latitude 𝐿𝑠
Longitude 𝑙𝑠
Earth station locations:
Latitude 𝐿𝑒
Longitude 𝑙𝑒
Central Angle:
Angle between line connects the earth-center to satellite and earth-center to earth station.
Central angle shown by 𝛾 .
cos 𝛾 = cos(𝐿𝑒) cos(𝐿𝑠) cos 𝑙𝑠 − 𝑙𝑒 + sin 𝐿𝑒 sin 𝐿𝑠
𝜶

Since the local horizontal plane at earth station is perpendicular to 𝑟𝑒 vector, elevation angle is
𝐸𝐿 = 𝛹 − 900
The magnitude of the vectors joining the center of the earth, the satellite and the earth station are related by
law of cosine.
𝑑 = 𝑟𝑠 1 +
𝑟𝑒
𝑟𝑠
2
− 2
𝑟𝑒
𝑟𝑠
cos(𝛾)
By sine law we have
𝑟𝑠
sin(𝛹 )
=
𝑑
sin(𝛾)
So, we have : sin 𝛹 = sin(𝐸𝐿 + 90) = cos(𝐸𝐿)

By replacing the values we have,
cos(𝐸𝐿) =
𝑟𝑠 × sin(𝛾)
𝑑
Now we have,
cos 𝐸𝐿 =
sin 𝛾
1 +
𝑟𝑒
𝑟𝑠
2
− 2
𝑟𝑒
𝑟𝑠
cos(𝛾)
And if we consider 𝑟𝑠 = 42164 𝑘𝑚 & 𝑟𝑒 = 6378.14 𝑘𝑚
cos 𝐸𝐿 =
sin(𝛾)
1.0228826 − 0.3025396 cos(𝛾)

the intermediate angle is shown by (𝜶)
𝛼 = tan−𝟏
tan 𝑙𝑠 − 𝑙𝑒
sin 𝐿𝐸
Thus, the azimuth angle is calculated as shown below
𝑎𝑧 = 1800
− 𝛼
Azimuth angle can lie anywhere between 0 -360.
Azimuth angle for GEO satellites.
Case 1: Earth station in northern hemisphere with
• Satellite to the SE of the earth station. 𝑎𝑧 = 1800
− 𝛼
• Satellite to the SW of the earth station. 𝑎𝑧 = 1800
+ 𝛼
Case 2: Earth station in northern hemisphere with
• Satellite to the SE of the earth station. 𝑎𝑧 = 𝛼
• Satellite to the SW of the earth station. 𝑎𝑧 = 3600
− 𝛼

Example:
Find the central angle and all other angles for the Earth station latitude for 520
North , longitude for 00
and Satellite
latitude 00 , longitude 660 East.
Step 1:
Find the central angle (𝛾)
cos 𝛾 = cos(𝐿𝑒) cos(𝐿𝑠) cos 𝑙𝑠 − 𝑙𝑒 + sin 𝐿𝑒 sin 𝐿𝑠
𝛾 = 75.5
Step 2:
Find elevation angle 𝐸𝐿
cos 𝐸𝐿 =
sin(𝛾)
1.0228826 − 0.3025396 cos(𝛾)
𝐸𝐿 = 5.85

Step 3:
Find the intermediate angle 𝛼
𝛼 = tan−𝟏
tan 𝑙𝑠 − 𝑙𝑒
sin 𝐿𝐸
𝛼 = 70.6668
Step 4:
Find the azimuth angle 𝑎𝑧
𝑎𝑧 = 1800 − 𝛼
𝑎𝑧 = 109.333
Thus, the look angle of the satellite are:
Elevation angle = 5.85
Azimuth angle =109.333

Limits of visibility
The east and west limits of geostationary are visible from any given Earth station.
These limits are set by the geographic coordinates of the Earth station and antenna elevation.
The lowest elevation is zero (in theory) but in practice, to avoid reception of excess noise from Earth.
Some finite minimum value of elevation is issued. The earth station can see a satellite over a geostationary arc
bounded by +- (81.30) about the earth station’s longitude.

NEAR GEOSTATIONARY ORBITS
• There are a number of perbuting forces that cause an orbit to depart from ideal Keplerian orbit. The most
effecting ones are gravitational fields of sun and moon, non-spherical shape of the Earth, reaction of the
satellite itself to motor movements within the satellites.
• Thus the earth station keeps manoeuvring the satellite to maintain its position. Within a set of nominal
geostationary coordinates. Thus the exact GEO is not attainable in practice and the orbital parameters vary
with time. Hence these satellites are called “Geosynchronous” satellites or “Near-Geostationary satellites”.

DESIGN CONSIDERATIONS
Communication Considerations:
For telecommunication satellite, the main design considerations are:
i. Type of service to be provided
ii. Communication capacity
iii. Coverage area
iv. Technological limitations
Depending upon the type of service to be provided by the satellite, basic specifications are laid down.
• For domestic fixed satellite services, the main parameters are EIRP per carrier, number of carriers and the
assigned coverage area.
Equivalent isotropic radiated power (EIRP): is the total radiated power from a transmitter antenna times
the numerical directivity of the antenna in the direction of the receiver, or the power delivered to the
antenna times the antenna numerical gain.

• For direct broadcast satellites, the number of television channels and coverage area is specified.
• Based on these parameters, satellites are designed to fulfill the areas needs and at the same time it should
be made in the specified cost fulfilling all the technical constraints.
• While developing a satellite, the earth station’s previous experience and in-house capabilities are also taken
into account.
• Often, for same set of requirements, different types of configurations are often proposed.

Environmental Conditions:
Different environmental conditions are encountered by a satellite during its mission. Some of them are
mentioned below.
Zero Gravity:
• In geostationary earth orbit, effect of earth’s gravity is negligible thus making the “zero gravity” effect.
• Disadvantage: This causes a problem for liquids to flow. The major issue of fuel is encountered. Thus an
external provision has to be made to force the liquids to flow.
• Advantage: Absence of gravity leads to operation of deployment mechanism used for stowing antennas and
solar panels during the launch.

Atmospheric pressure and temperature:
• At geostationary earth orbit, atmospheric pressure is very low, thus making the thermal conditions negligible
which further leads to the increase in friction between surfaces.
• Thus additional lubricants are required to keep the satellite parts in motion.
• Due to the presence of electronic components inside the satellite, pressure us the satellite is higher making
the functioning of the inner components of the satellite more manageable.
• Sun’s heat also affects the external components of the satellite.

Space Particles:
• Besides planets, natural and artificial satellites, many other particles like cosmic rays, protons, electrons,
meteoroids and manmade space debris exists in space.
• These particles collide with the satellites causing permanent damage to it and sometimes degrading the
solar cells.
• Space debris, also known as orbital debris, space junk and space waste, is the collection of objects in orbit
around Earth that were created by humans but no longer serve any useful purpose. These objects consist of
everything from spent rocket stages and defunct satellites to explosion and collision fragments.
• The debris can include slag and dust from solid rocket motors, surface degradation products such as paint
flakes, clusters of small needles, and objects released due to the impact of micrometeoroids or fairly small
debris onto spacecraft. As the orbits of these objects often overlap the trajectories of spacecraft, debris is a
potential collision risk.

• The vast majority of the estimated tens of millions of pieces of space debris are small particles, like paint
flakes and solid rocket fuel slag. Impacts of these particles cause erosive damage, similar to sandblasting.
The majority of this damage can be mitigated through the use of a technique originally developed to protect
spacecraft from micrometeorites, by adding a thin layer of metal foil outside of the main spacecraft body.
• Impacts take place at such high velocities that the debris is vaporized when it collides with the foil, and the
resulting plasma spreads out quickly enough that it does not cause serious damage to the inner wall.

Magnetic Fields:
• Due to the magnetic field of earth, charged particles which are trapped in the surrounding region of the
earth get deflected.
• This effect is more seen in the layers around the equator where the magnetic power of the earth is of
maximum effect. This region is called the Van Allen’s Belt.
• Even though satellites in geostationary earth orbit are not really affected by the earth’s magnetic field, they
have to pass through the Van Allen’s belt during orbit raising (launching).
• The electric charges present in this belt affect the electronic components against radiation.
• To overcome this effect, large coils are used by satellites.

LIFETIME AND RELIABILITY
Lifetime:
• The useful lifetime of a geostationary satellite is determined by the highest tolerable deviation in inclination
and orbit location together with reliability of satellite’s critical sub-system.
• A lifetime could be improved by increasing the fuel capacity and by saving fuel by accepting orbital deviation
to the maximum extent that is possible. Saving fuel couldn’t be implemented to a great level. So for this
purpose propulsion is used.
• Propulsion: It is a method used to accelerate spacecraft and artificial satellites.
Reliability
• Reliability is counted by considering the proper working of satellites critical components. Reliability could be
improved by making the critical components redundant. Components with a limited lifetime such as
travelling wave tube amplifier should be made redundant.

Travelling Wave Tube Amplifier (TWTA): travelling wave tube amplifiers have applications in both receiver and
transmitter systems, and come in all shapes and sizes, but they all consist of three basic parts-the tube, the
tube mount (which includes the beam focusing magnets) and the power supply.
The main attraction of these devices is their very high gain (30-60 dB), linear characteristics and 1-2 octave
bandwidth. They are quite widely used professionally.
Octave Bandwidth: A band is said to be an octave in width when the upper band frequency is twice the lower
band frequency.

When used as receiver RF amplifiers they are characterized by high gain, low noise figure and wide bandwidth,
and are known as low noise amplifier (LNA).
Operating frequency: 300MHz – 50GHz
Power level: few watts to mega watts.
Amplification can be done through continuous interaction between electron beam and RF field over entire
tube length. Electron gun is used to emit an electron beam with uniform velocity toward the tube. Anode
plates used to focus the beam and increase the velocity.
Magnets produce an axial magnetic field to prevent the spreading of electron beam.
Helix is a slow-wave structure, helix creates an electric field at the center of the helix with velocity of light.
Velocity of RF wave is higher than the phase velocity of electron beam. So, to decrease the speed we multiply it
with ratio of helix pitch to the helix circumference.
We use attenuator in order to restrict the generation of unwanted oscillation inside the tube.
Speed of the wave depends on number of the turns in helix structure.

SPACE CRAFT SUB-SYSTEMS
A communication satellite consists of two main functions, they are payload and bus.
Payload is required for communication whereas bus is required for mechanical and electrical support.
Bus supports altitude and orbit controls, propulsion, TT&C and electrical power where as payload supports the
band used for communication, the space links and the devices to remove interferences.
PAYLOAD
The payload comprises of a Repeater and Antenna sub-system and performs the primary function of
communication.
• REPEATER :
It is a device that receives a signal and retransmits it to a higher level and/or higher power onto the other side
so that the signal can cover longer distance.
A repeater in the satellite receives the uplink RF signal and converts it to an appropriate downlink frequency.
It does the work of processing the received signal.

Two types of repeater architectures are used.
 Transparent Repeater:
It only translates the uplink frequency to an appropriate downlink frequency. It does so without processing the
baseband signal. The main element of a typical transparent repeater is a single beam satellite. Signals from
antenna and the feed system are fed into the low-noise amplifier through a band-pass filter.
The band-pass filter attenuates all out of band signals such as transmission from the ground stations of
adjacent satellite systems. The low-noise amplifier provides amplification to the weak received signals.
 Regenerative Repeater:
A repeater, designed for digital transmission, in which digital signals are amplified, reshaped, retimed, and
retransmitted. Regenerative Repeater can also be called as a device which regenerates incoming digital signals
and then retransmits these signals on an outgoing circuit.
It not only translates and amplifies the signal, but is also does the task of baseband processing and
demodulation. This architecture of repeater is the best suited for digital systems and it offers several
advantages over transparent repeaters.

• Antennas:
The function of an antenna of a space craft is to receive signals and transmit signals to the ground stations
located within the coverage area of the satellite. The choice of the antenna system is therefore governed by
the size and shape of the coverage area. Consequently, there is also a limit to the minimum size of the antenna
footprint.
Antennas convert electromagnetic radiation into electrical current, or vice versa. Antennas generally deal in the
transmission and reception of radio waves, and are a necessary part of all radio equipment.
Antennas are used in systems such as radio and television broadcasting, point-to-point radio communication,
wireless LAN, cell phones, radar, and spacecraft communication.

BUS
The bus or payload platform consists of the subsystems that support the payload. These subsystems typically
include:
• Structures subsystem: the physical structure of the spacecraft, to which all electronics boxes, thrusters,
sensors, propellant tanks, and other components are mounted.
• Electric power/distribution subsystem (EPS or EPDS): the hard- and software used to generate and
distribute electrical power to the spacecraft, including solar arrays, batteries, solar-array controllers, power
converters, electrical harnesses, battery-charge-control electronics, and other components.
• Telemetry, tracking, and command subsystem (TT&C): The electronics used to track, monitor, and
communicate with the spacecraft from the ground. TT&C equipment generally includes receivers,
transmitters, antennas, tape recorders, and state-of-health sensors for parameters such as temperature,
electrical current, voltage, propellant tank pressure.

• Propulsion subsystem: Liquid and solid rockets or compressed-gas jets and associated hardware used for
changing satellite attitude, velocity, or spin rate. Solid rockets are usually used for placing a satellite in its
final orbit after separation from the launch vehicle. The liquid engines (along with associated plumbing lines,
valves, and tanks) may be used for attitude control and orbit adjustments as well as final orbit insertion after
launch.
• Power supply: The primary electrical power for operating electronic equipment is obtained from solar cells.
Individual cells can generate small amounts of power, and therefore array of cells in series-parallel
connection are required. Cylindrical solar arrays are used with spinning satellites, thus the array are only
partially in sunshine at any given time. Another type of solar panel is the rectangular array or solar sail. solar
sail must be folded during the launch phase and extended when in geo-stationary orbit.

• Attitude control: The attitude of a satellite refers to its Orientation in space. Much of equipment carried
abroad a satellite is there for the purpose of controlling its attitude. Attitude control is necessary, for
example, to ensure that directional antennas point in the proper directions. A number of forces, referred to
as disturbance forces can alter attitude, some examples being the gravitational forces of earth and moon,
solar radiation, and meteorite impacts.
• Station keeping: A satellite that is normally in geo-stationary will also drift in latitude, the main perturbing
forces being the gravitational pull of the sun and the moon. The force causes the inclination to change at the
rate of about 0.85 deg/year. To prevent the shift in inclination from exceeding specified limits, jets may be
pulled at the appropriate time to return the inclination to zero. Counteracting jets must be pulsed when the
inclination is at zero to halt that change in inclination.

TELEMETRIC TRACKING AND COMMAND SUBSYSTEM
Telemetry system
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.
The parameters monitored by the Telemetry system are:
• Voltage, current and temperature of all major sub-systems.
• Switch status of communication transponders.
• Pressure of the propulsion tanks
• Outputs from altitude sensors.
• Reaction wheel speed

Command systems
Command system receives instructions from ground system of satellite and decodes the instruction and sends
commends to other systems as per the instruction.
Example of commands are:
• Transponder switching
• Switch matrix configuration
• Antenna pointing control
• Controlling direction and speed of solar array drive
• Battery reconditioning
• Beacon switching
• Thruster firing
• Switching heaters of the various sub-systems

Tracking
• Tracking of the satellite is accomplished by having the satellite is accomplished by having the satellite
transmit beacon signals which are received at the TT&C earth stations. Tracking is obviously important
during the transmitter and drift orbital phases of the satellite launch.
• When on-station, a geo-stationary satellite will tend to shifted as a result of the various distributing forces,
as described previously. Therefore it is necessary to be able to track the satellites movements and send
correction signals as required. Satellite range is also required for time to time. This can be determined by
measurement of propagation delay of signals specially transmitted for ranging purposes.
The main functions of TT&C are:
• Monitor the performance of all the satellite sub-systems and transmit the monitored data to the satellite
control center.
• Support the determination of orbital parameters.
• Provide a source earth station for tracking.
• Receive commands from the control center for performing various functions of the satellite.

MULTIPLE ACCESS TECHNIQUES
Multiple accesses is defined as the technique where more than one pair of earth stations can simultaneously
use a satellite transponder.
FDD(Frequency Division Duplexing): refers to how the radio channel is shared between the uplink and
downlink.
FDM(Frequency Division Multiplexing): is a physical layer technique that combines and transmits low-
bandwidth channels through a high-bandwidth channel.
What is a Transponder?
A transponder is a wireless communications, monitoring, or control device that picks up and automatically
responds to an incoming signal. The term is a contraction of the words transmitter and responder.
simply, we can say that a satellite transponder is a series of interlinked devices that form a single
communication channel between transmitter and receiver.

The two major functions of a satellite transponder are as follows:
• Amplification of the received input signal.
• Frequency translation.
Types of Satellite Transponders:
It is to be noted here that there are basically two types of transponders, which are as follows:
• Bent pipe or Conventional transponders
• Regenerative or Processing transponders
Both of these transponders perform frequency conversion and amplification.
The regenerative transponder demodulates the radio frequency carrier signal to baseband signal along with
regeneration of signal and modulation.

Here, the 6 GHz signal is down-converted into 4 GHz while getting transmitted from an end to another.
Initially, a 6 GHz signal received from an antenna is provided to a low noise amplifier where amplification of the signal
is performed.
Further, the signal is down-converted to 4 GHz using a local oscillator with a frequency of 2.225 GHz.
Now, the intermediate frequency band pass filter takes the 4 GHz signal as output and removes the undesired
frequency signal.
Further, the filtered signal undergoes amplification by the use of a pre-amplifier like a traveling wave tube amplifier.
Then the output of the TWTA is fed to a high power amplifier that smooths out the amplitude and phase variation of
the received signal.

Transponder Assignment Modes
• Pre-assigned Multiple Access (PAMA):
The transponder is assigned to the individual user either permanently for satellite’s full life time or at least for
long duration. The pre-assignment may be that of a certain frequency band, time slot, or a code.
It is also known as Permanently Assigned Multiple Access (PAMA) or Fixed Assigned Multiple Access (FAMA).
• Demand assigned Multiple Access (DAMA):
DAMA allows multiple user to share a common link wherein each user is only required to put-up a request to
the control station or agency when it requires the link to be used.
It is very cost effective for small users who have to pay for the using the transponder capacity only for the time
it was actually used.
Note: - DAMA and PAMA are related only to channel/resource allocation and should not be confused with the
Multiple access/multiplexing methods intended to divide a single communication channel into multiple virtual
channels.

• Random Multiple Access (RMA):
In the case of RMA, access to the link or the transponder is by contention.
A user transmits the message without knowing the status of the message from other users.
Due to the random nature of transmission, data from the multiple users may collide. If a collision occurs, it is
detected and the data are re-transmitted.
Retransmission is carried out with random time delays and sometimes may have to be done several times.

Frequency Division Multiple Access (FDMA):
It allows several users to share the same time slot by dividing the frequency into different frequency channels.
Or, FDMA allows multiple users simultaneous access to a certain system.
Or we can say that the frequency band is divided into N non-overlapping channels. Guard band minimize the
interference between channels.
The maximum number of carriers that can access transponder is given by.
𝑛 =
𝐵𝑇𝑅
𝐵𝐶
Where 𝐵𝑇𝑅 is the total transponder bandwidth and 𝐵𝐶 is the carrier bandwidth.

FDMA is divided into two categories:
• Single Channel Per Carrier
• Multiple Channel Per Carrier
Single Channel Per Carrier (SCPC):
In this form each signal channel is transmitted over a single carrier frequency.
Multiple Channel Per Carrier (MCPC):
In this form multiple signals are grouped together and then it is transmitted over a single carrier frequency.

Time Division Multiple Access (TDMA)
It allows several users to share the same frequency channel by dividing the signal into different time slots.
This allows multiple stations to share the same transmission medium (e.g. radio frequency channel) while using
only a part of its channel capacity.

Code Division Multiple Access (CDMA)
CDMA is a system in which a number of users can occupy all of the transponder bandwidth all of the time.
CDMA signals are encoded such that information from an individual transmitter can be recovered by a
receiving station that knows the code being used, in the presence of all the other CDMA signals in the same
bandwidth.
Each transmitting station is allocated a CDMA code, any receiving station that wants to receive data from that
earth station must use the correct code.
CDMA codes are typically 16 bits to many thousands of bits in length, and the bits of a CDMA code are called
chips to distinguish them from the message bits of a data transmission.
CDMA was originally developed for military communication systems, where its purpose was to make detection
of the signal more difficult (called low probability of intercept).

Satellite Link Design
The designer of a satellite communication system must work to minimize the capital cost of the entire system
and must also ensure that sufficient revenue can be earned from the system to recover the large capital cost of
building and launching satellites.
All communication links are designed to meet certain performance objectives, usually a bit error rate (BER, the
probability that a received bit is in error) in a digital link or a signal to noise ratio (SNR) where the signal is
audio or video, measured in the baseband channel.
In a satellite link there are two signal paths: an uplink from the earth station to the satellite, and a downlink
from the satellite to the earth station. The overall CNR at the earth station receiver depends on both links, and
both must therefore achieve the required performance for a specified percentage of time.

Transmission Theory
In free space, radiating a total power Pt watts uniformly in all directions as shown in Figure 4.2. Such a source is
called isotropic.
At a distance R meters from the hypothetical isotropic source transmitting RF power Pt watts, the flux density
crossing the surface of a sphere with radius R m is given by
The flux density is the number of magnetic lines of
flux that pass through a certain point on a surface.
Pt
Figure 4.2

For a transmitter with output 𝑃𝑡 watts driving a lossless antenna with gain 𝐺𝑡, the flux density in the direction
of the antenna boresight at distance R meters is
The product 𝑃𝑡 𝐺𝑡 is often called the effective isotropic radiated power (EIRP), and describes the combination
of transmitter power and antenna gain in terms of an equivalent isotropic source with power 𝑃𝑡 𝐺𝑡 watts,
radiating uniformly in all directions.
𝐸𝐼𝑅𝑃 = 𝑃𝑡 𝐺𝑡
If we had an ideal receiving antenna with an aperture area of 𝐴𝑟𝑚2
we would collect power 𝑃𝑟 watts given by
Note: The aperture of the antenna is the area whose orientation is perpendicular to the direction from where
the electromagnetic wave is coming.
𝐴𝑟

A practical antenna with a physical aperture area of 𝐴𝑟 𝑚2
will not deliver the power 𝑃𝑟 Some of the energy
incident on the aperture is reflected away from the antenna, referred to as scattering, and some is absorbed by
lossy components. This reduction in efficiency is described by using an effective aperture 𝐴𝑒.
In practical relation between aperture area 𝐴𝑟 and effective aperture 𝐴𝑒 is given below.
η𝐴is the aperture efficiency of the antenna.
The aperture efficiency η𝐴 accounts for all the losses between the incident wave front and the antenna output
port.
power received by a real antenna with a physical receiving area 𝐴𝑟 and effective aperture area 𝐴𝑒 𝑚2
at a
distance R from the transmitter is

A fundamental relationship in antenna theory is that the gain and area of an antenna are related by
where λ is the wavelength (in meters for Ae in square meters) at the frequency of operation.
Substituting for Ae in Eq.
The above expression is known as the link equation, and it is essential in the calculation of power received in
any radio link. The term
4𝜋𝑅
λ
2
is known as the path loss, 𝐿𝑝.
In decibel terms, we have
Where,

Considering all factors we can write,

Example :
A satellite at a distance of 40 000 km from a point on the earth’s surface radiates a power of 10W from an
antenna with a gain of 17 dB in the direction of the observer. Find the flux density at the receiving point, and
the power received by an earth station antenna at this point with an effective area of 10 𝑚2
.

System Noise Temperature and G/T Ratio
Noise temperature is a useful concept in communications receivers, since it provides a way of determining how
much thermal noise is generated by active and passive devices in the receiving system.
The noise power is given by
In satellite communication systems we are always working with very weak signals (because of the large
distances involved) and must make the noise level as low as possible to meet the CNR requirements.
This is done by making the bandwidth in the receiver. Usually set by the IF amplifier stages, to be just large
enough to allow the signal (carrier and sidebands) to pass unrestricted, while keeping the noise power to the
lowest value possible.
Intermediate-frequency (IF) amplifiers are amplifier stages used to raise signal levels in radio and television
receivers.

To determine the performance of a receiving system we need to be able to find the total thermal noise power
against which the signal must be demodulated. We do this by determining the system noise temperature Ts.
If the overall end-to-end gain of the receiver is 𝐺𝑟𝑥 (𝐺𝑟𝑥 is a ratio, not in decibels) and its narrowest bandwidth
is 𝐵𝑛 Hz, the noise power at the demodulator input is
The noise power referred to the input of the receiver is 𝑃𝑛 where
Let the antenna deliver a signal power 𝑃𝑟 watts to the receiver RF input. The signal power at the demodulator
input is 𝐶 = 𝑃𝑟 𝐺𝑟𝑥 watts, Hence, the carrier to noise ratio (CNR) at the demodulator is given by

By substituting the 𝑃𝑟 equation we have,
𝐶
𝑁
=
𝑃𝑡𝐺𝑡𝐺𝑟
𝐾𝑇𝑠𝐵
λ
4𝜋𝑅
2
The above equation is called carrier to noise power ratio.
This measures amount of noise received in a satellite system.
Nosie power spectral density is given by, 𝑁0 =
𝑃𝑛
𝐵
Substitute it in above equation.
𝐶
𝑁0
=
𝐾𝑇𝑠
λ
4𝜋𝑅
2
The above equation is called carrier to noise power spectral density ratio.
This is used for analysis of GPS receiver performance or quality.

G/T ratio is also called figure of merit. Figure of merit specify the quality of earth station.
𝐶
𝑁
=
𝐾𝑇𝑠𝐵
λ
4𝜋𝑅
2
Rearrange the above equation so we have,
𝐶
𝑁
=
𝑃𝑡𝐺𝑡
𝐾𝐵
λ
4𝜋𝑅
2
𝐺𝑟
𝑇𝑠
Therefore, (
𝐶
𝑁
) ∝ (
𝐺𝑟
𝑇𝑠
)

Example: An earth station antenna has a diameter of 30m with an aperture efficiency of 68% and is used to
receive a signal at 4150MHz. At this frequency, the system noise temperature is 60K when the antenna points
at the satellite at an elevation angle of 28°.What is the earth station G/T ratio under these conditions? If heavy
rain causes the sky temperature to increase so that the system noise temperature rises to 88K,what is the new
G/T value?
We have, λ=c/f.

Complete link design
A complete link is consist of two earth station and a satellite, therefore a complete link is made up of uplink
and downlink.
The quality of information received on earth station depends on uplink, satellite transponder and downlink.
Uplink design:
Uplink of a satellite is the one which earth station is transmitting the signal to satellite.
Cost of transmitter is high in comparison to receiver because generation of high power microwave carrier is
very expensive.
Design of uplink is easier compare to downlink.

Input backoff:
• When the TWTA operates on multiple number of carriers, intermodulation distortion is generated.
• This distortion can be minimized by operating TWTA in it’s linear position of transfer characteristic.
• Backoff is required for operation of TWTA.
The shifting of operating point is known as input backoff.
Or, we can say that difference in dB between carrier input power at the operating point and saturation point.
Saturation: to a very full extent, especially beyond the point
regarded as necessary or desirable.

The earth station EIRP will have to be reduced by the Specified BO, resulting new EIPR value.
𝐸𝐼𝑅𝑃 𝑈 = 𝐸𝐼𝑅𝑃𝑆 𝑈 + 𝐵𝑂𝑖
The input backoff varies from 3 to 7 dB
When non-linear effects in uplink are included by considering backoff term, so equation of carrier to noise
spectral density will be.

Downlink Design:
Downlink of a satellite is the one which Satellite is transmitting the signal to earth station.
Output backoff:
• When the input backoff is employed, a corresponding output backoff must be allowed.
• Due to non-linear characteristic of TWTA the input and output backoff are not same.
Therefore above the saturation, the linear characteristics are extrapolated and backoff is taken 5 dB below the
extrapolated linear characteristic.
We can say that output backoff is drop in the output power.
𝐵𝑂𝑜 − 5𝑑𝐵 = 𝐵𝑂𝑖 ≫ 𝐵𝑂𝑜 = 𝐵𝑂𝑖 + 5𝑑𝐵

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