IIUC

EEE 3601
Wireless and Satellite Communication

2022

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Satellite Communication
A communication satellite is a spacecraft that carries abroad communication equipment, enabling a
communication link to be established between distant points. Long distance communication between the
continents that are difficult to reach by point to point communication systems is achieved through the use of
satellite stationed as if , in space above the earth.
The satellite is, infact, a radio relay station with one great advantage over the other communication system is
that it has the capability of a direct line of sight path to 98% of the earth surface.
From the operational point of view satellites are classified as-
(i) Communication satellite
(ii) Observatory satellite
Communication satellite: They are used for long distance communication purposes. They receive a signal
from earth, process it and retransmit down to earth. The signal carries the same intelligence in up-link and
down-link. They are infact synchronous satellite. Synchronous satellite also known as geostationary
satellites, go round the earth in 24 hours at a height of 3580km above the surface of the earth.
Observatory satellite: They are launched into a near elliptical earth orbit in order to gather information
about metrological factors like onset of rains, occurrence of cyclones, earthquakes etc.
Classification of communication satellites: Communication satellites are two types:
(i) Passive communication satellite
(ii) Active communication satellite
Passive communication satellite: It does not carry any equipment for receiving and transmitting the signal.
It is a mere reflector. The ground transmitting system beams power at the reflector. A part of the power is
intercepted by the reflector and is radiated towards the ground receiving system. Therefore in order to have a
suitable strength of signal at the receiver after reflection from satellite a large amount of power is required.
Active communication satellite: Satellites that carries equipments for receiving earth signals, processing
them and retransmitting them towards the earth is called an active satellite. The received power by satellite
is amplified by active electrical means. So such a satellite has to carry electrical power for operation of
electronic means. The power received by ground receiving system is determined by the power level of
satellite transmitter. Modern communication satellites are active satellites system.
Advantages: Communication through satellite has several advantages—
(i) It is a point to multipoint communication whereas all the terrestrial relays are point to point. Thus
they cover wide area.
(ii) The satellite circuits can be installed rapidly. Once the satellite is in the position, earth station can
be installed and communication may be established without
consuming much time.
(iii) Earth station can be shifted without much ado.
(iv) For search, rescue and navigational efforts, satellites offer the advantages which no other
systems can do.
(v) It has a unique degree of flexibility is interconnecting the mobile vehicles.
(vi) The satellite communication costs are independent of distance, so satellite communications are
economical.
(vii) Robustness against natural disaster.
(viii) Small fluctuation of delay-time.

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Disadvantages of satellite communication:

(i) Physically significant delay-time satellite communication.
(ii) It has some effects due to rain fall and airplanes.
Assignable satellite frequencies:
Satellite frequencies are usually classified as—
(i) Military only
(ii) Multimission for terrestrial and telecommunications and
(iii) Multiband for telecommunications only.
Frequency bands for satellite communications are given below:
Band Down-link Bands MHz Up-link Bands MHz
UHF Military 250-270(approx) 292-312(approx)
C-Band-commercial 3700-4200 5925-6425
X band-military 7250-7750 7900-8400
Ku Band-commercial 11700-12200 14000-14500
Ka Band-commercial 17700-21200 27500-30000
Ka Band- Military 20200-21200 4350-45500
Equipments on satellites:
Solar panel

Storage Power Command and

Batteries Conditioner telemetry
subsystem

Transponder

Frequency Position
Receiver transistor Transmitter control
subsystem

Antenna Subsystem

Antenna

(i) Antenna subsystem: It consists equipments for receiving and transmitting the signals. To keep
antennas pointing to earth, the antenna subsystem remain stationary while body of the satellite
spins.

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(ii) Transponders: The equipment which receives a signal, amplifies it, changes its frequency and
retransmits is called transponder. Most satellite has more than one transponder. The bandwidth
handled by a transponder differs from one satellite design to another, but most satellites have
transponder with a bandwidth of 36 MHz.
(iii) Power generation and conditioning subsystem: It generates the power for the operation of all
the units on the satellite, and converts the power generated into the form useful for operating
electronic equipments. Solar cells are used to convert sunlight into electricity.
(iv) Commands and telemetry subsystem: It transmits data about the satellite to earth and receives
command from earth.
(v) Thrust subsystem: It is used for keeping the satellite the antenna pointing in exactly the right
direction.

Applications of satellite communication:

Satellite may be used in the following applications-------

(i) Military purpose

(ii) Satellite internet access
(iii) Satellite television communication
(iv) Interconnectional long distance telephony
(v) Digital cinema transmission
(vi) Satellite radio communication

Altitude: In satellite communication altitude means the positional distance of satellite from the earth
surface.

Orbital velocity of the satellite:

Satellite can be placed in orbits around earth at different altitudes. Depending on the heights above the
earth’s surface, orbits are classified as low earth orbit (LEO), medium earth orbit (MEO) and geostationary
orbit (GEO).

A satellite remains in the sky in the circular orbit if its linear velocity is so adjusted that the resulting
centrifugal force caused by its rotation around the earth is equal and opposite to the earth’s gravitational
force. Equating the two forces,

Mm mv 2
G =
r2 r

We get orbital velocity of satellite

GM
v= m/sec.
r

Where M is mass of earth and r is the radius of the circular orbit.

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Orbital period of satellite: The orbital period of the satellite is the ratio of the periphery of the circular orbit
to the orbital velocity of the satellite.

2r r r3
T= = 2 = 2 = 1.66  10 −4  r minutes,
v GM GM
r

where r is in kilometers.

Height of the orbit:

The height of the orbit, h, is

h=(r-6370) km.

where 6370 km is the radius of the earth.

Calculation of the path loss of a satellite:

Let us assume that the signal propagating in the free space. Consider that average power, PT is to be radiated
equally in all directions (isotropically). At a distance d from the transmitting antenna, power per unit area
will be as follows—

PT
( PD ) iso = watt/m2.
4d 2

If the GT is the directive gain of the transmitting antenna, then the power density along the direction of
maximum radiation will be

PD = ( PD ) iso .GT

PT .GT
=
4d 2

Suppose directive gain of the receiving antenna is GR then the area over which antenna gathers the energy of
the incoming signal, called effective aperture is

G R 2 2
AR = m.
4

where λ is the wavelength of the signal.

Therefore power received by the receiving antenna is

PR = PD AR

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  
2

= GT GR   PT
 4d 

  
2
PR
= G R GT  
PT  4d 

2
 c 
= G R GT  
 4fd 

Where c is the velocity of the signal and f is the signal frequency. Expressing d in km and f in MHz, we can
write,
2
PR  3  108 
= G R GT  
6 
 4d .10 f .10 
3
PT

 0.57  10 −3 
= G R GT  2

 ( df ) 

Expressing the power ratio in decibels, we can write---

 PR 
  = (GT ) db + (G R ) db + 10 log(0.57  10 −3 ) − 20 log db d − 20 log db f
 PT  db

= (GT ) db + (GR ) db − (32.5 + 20 logdb d + 20 logdb f )

Where the third term in bracket at the right side of the above equation is known as path loss. Thus the path
loss in free space is

L = 32.5 + 20 logdb d + 20 logdb f

Now the above equation can be written as-

 PR 
  = (GT ) db + (G R ) db − ( L) db
 PT  db

Mathematical problem:

A geostationary satellite has orbit of 36000 km above the earth surface. The signal frequency is 4000MHz,
the transmitting and receiving antenna gains are 15db and 45db , if the transmitted power is 200 watt, then
determine

(i) Transmission path loss in free space

(ii) Received power.

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Working principle of a satellite:

Two Stations on Earth want to communicate through radio broadcast but are too far away to use
conventional means. The two stations can use a satellite as a relay station for their communication. One
Earth Station sends a transmission to the satellite. This is called a Uplink. The satellite Transponder
converts the signal and sends it down to the second earth station. This is called a Downlink.

Elevation Angle: The angle of the horizontal of the earth surface to the center line of the satellite
transmission beam.

This effects the satellites coverage area. Ideally, you want a elevation angle of 0 degrees, so the
transmission beam reaches the horizon visible to the satellite in all directions. However, because of
environmental factors like objects blocking the transmission, atmospheric attenuation, and the earth
electrical background noise, there is a minimum elevation angle of earth stations.

Coverage Angle: A measure of the portion of the earth surface visible to a satellite taking the minimum
elevation angle into account.

R/(R+h) = sin(π/2 - β - θ)/sin(θ + π/2)

= cos(β + θ)/cos(θ)

Where,

R = 6370 km (earth’s radius)

h = satellite orbit height

β = coverage angle

θ = minimum elevation angle.

Types of Satellites:

⚫ Satellite Orbits

▪ GEO

▪ LEO

▪ MEO

▪ Molniya Orbit

Geostationary Earth Orbit (GEO)

• These satellites are in orbit 35,863 km above the earth’s surface along the equator.
• Objects in Geostationary orbit revolve around the earth at the same speed as the earth rotates. This
means GEO satellites remain in the same position relative to the surface of earth.

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Advantages:

• A GEO satellite’s distance from earth gives it a large coverage area, almost a fourth of the earth’s
surface.
• GEO satellites have a 24-hour view of a particular area.
• These factors make it ideal for satellite broadcast and other multipoint applications.

Disadvantages

• A GEO satellite’s distance also causes it to have both a comparatively weak signal and a time delay
in the signal, which is bad for point-to-point communication.
• GEO satellites, centered above the equator, have difficulty broadcasting signals to near polar regions.

Low Earth Orbit (LEO)

• LEO satellites are much closer to the earth than GEO satellites, ranging from 500 to 1,500 km above
the surface.
• LEO satellites don’t stay in fixed position relative to the surface, and are only visible for 15 to 20
minutes each pass.
• A network of LEO satellites is necessary for LEO satellites to be useful.

Advantages

• A LEO satellite’s proximity to earth compared to a GEO satellite gives it a better signal strength and
less of a time delay, which makes it better for point-to-point communication.
• A LEO satellite’s smaller area of coverage is less of a waste of bandwidth.

Disadvantages

• A network of LEO satellites is needed, which can be costly

• LEO satellites have to compensate for Doppler shifts cause by their relative movement.
• Atmospheric drag effects LEO satellites, causing gradual orbital deterioration.

Medium Earth Orbit (MEO)

• A MEO satellite is in orbit somewhere between 8,000 km and 18,000 km above the earth’s surface.
• MEO satellites are similar to LEO satellites in functionality.
• MEO satellites are visible for much longer periods of time than LEO satellites, usually between 2 to
8 hours.
• MEO satellites have a larger coverage area than LEO satellites.

Advantage

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• A MEO satellite’s longer duration of visibility and wider footprint means fewer satellites are needed
in a MEO network than a LEO network.
Disadvantage

• A MEO satellite’s distance gives it a longer time delay and weaker signal than a LEO satellite,
though not as bad as a GEO satellite.

Special Purpose Satellite:

There are some special purpose satellites which are listed below:

(i) Direct broadcast satellites (DBS)

(ii) International maritime satellite (INMARSAT)
(iii) International telecommunication satellite (INTELSAT)
(iv) Very small aperture terminals (VSAT)
(v) Search and rescue satellites (SARSAT)
(vi) LANDSAT
(vii) Mobile satellite communication systems (MSAT)

Direct broadcast satellite:

It is also called broadcast satellite system (BSS) and is operational in western countries since 1986 onwards.
Satellite broadcasting is intended to increase the number of TV programmes and is so becoming popular
worldwide. DBS utilizes high power satellite. It consist of

(i) A television repeater station in geostationary orbit

(ii) A ground station that transmits programme signals to the spacecrafts and
(iii) The consumer terminals that receive the signals from the satellite and convert them to a format
compatible with existing TV sets.

Most important features of DBS system is that it allows earth stations to be mounted in domestic area.
Emphasis in India is on satellite communication services rather than TV broadcasting services.

International maritime satellite (INMARSAT/INMSAT):

It came into being at 1979. It has now become quite essential to communications for global shipping.
International maritime satellite has headquarters in London and has about 53 member countries.

INMARSAT serves all areas where there is a need for maritime communications. It has provision for the
space segment for improving distress and safety of life at sea communications, efficiency and management
of ships, maritime public correspondence services and radio determination capabilities. Call to or from a
ship are routed via satellite to coast earth station where they are patched directly into the international
switched telecommunication networks.

Very small aperture terminals (VSAT):

Very small aperture terminals are low cost earth stations which use different modulation or multiple access
schemes especially for low speed packet data networks. These work mostly in C and Ku bands and use a

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very small antenna of diameter in the range of 1.2 to 2.5 meter. VSATs are often called microearth stations
and offer a low cost thin route data communication between a central host computer and a large numbers of
remote sites.

They have two major applications- the data broadcasting services and two way data services. The important
application of such a data broadcasting service is (i) news dissemination, including text and graphic
information, to small newspaper in remote locations, (ii) transmission of charts and documents including
handwritten information in various languages, and (iii) bulk data transfers between a company’s MIS centre
and corporate headquarters.

Earth station:

In satellite communication there are two stations (i) Earth station, (ii) Satellite station. The station which is
situated at the surface of the earth and from which the message or original signal is transmitted to satellite
station is known as earth station. An earth station is basically divided in to two parts:

(i) A RF terminal, which consists of an upconverter and a down converter, a high-power amplifier,
a low-noise amplifier and an antenna.
(ii) A baseband terminal which consists of baseband equipment, and encoder and decoder and a
modulator and a demodulator.

The basic requirements of an earth station antenna:

1. The antenna must have a highly directive gain, that is, it must focus its radiated energy into a narrow
beam to illuminate the satellite antenna in both the transmit and receive modes to provide the
required uplink and downlink carrier power.
2. The antenna must have low noise temperature so that the effective noise temperature of the receive
side of the earth station, which is proportional to the antenna temperature can kept low to reduce the
noise power within the downlink carrier bandwidth.
3. The antenna must be easily be easily steered so that a tracking (if it is required) system can be
employed to point the antenna beam accurately toward the satellite. This is essential for minimizing
antenna pointing loss.

Azimuth angle:

The azimuth angle is defined as the angle measured clockwise from the true north to the intersection of the
local horizontal plane TMP and the plane TSO (passing through the earth station, the satellite and the earth’s
centre). The azimuth angle A is between 0 to 360˚. Depending on the location of the earth station with
respect to subsatellite point the azimuth angle A is given by

1. Northern Hemisphere
Earth station west of satellite: A =180 − A
Earth station east of satellite: A =180 + A
2. Southern Hemisphere
Earth station west of satellite: A = A
Earth station east of satellite: A = 360 − A

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Elevation angle:

The elevation angle E is defined as the angle produced by the intersection of the local horizontal plane TMP
and the plane TSO with the line of sight between the earth station and the satellite.
Expression for azimuth and elevation angle:
To find out the expression for azimuth and elevation angle we must consider that the earth is a perfect
sphere with radius Re. Now from the following figure we have-
 MP 
A = tan −1   ………………..(1).
 MT 

Zenith North
North Pole
T Station
South Station Longitude θL
A
Satellite
South longitude θs
T

A΄ θl  o

E ß
ß δ
θs-θL

M Re
O
Equator S B

r
Figure: a
P

Figure: a

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Now from ΔMPO we have

MP
tan  s −  L =
MO
MP = MO tan  s −  L

Again for ΔMTO we have---

MT
tan  l =
TO
MT = TO tan  l
MT = Re tan  l
and
TO
cos l =
MO
Re
MO =
cos l

From equation (1) it can be written as

  Re  
  cos  tan  s −  L 
A = tan −1   l  
 Re tan  l 
 
 

To calculate the elevation angle E, consider the triangle TSO shown in the figure (a) and the redrawn figure
(b). We have

E = ß+δ-90˚
= (90˚-γ) + δ-90˚
= δ-γ
The angle γ can be calculated from the triangle TPO as follows:
R 
 = cos−1  e 
 OP 
Since OP=MO/cos|θs-θL|= Re/cosθlcos|θs-θL as seen from the triangle MPO and TMO, we have
 = cos−1 (cos l cos  s −  L )

To evaluate the δ in figure (b) we note that

 SB 
 = tan −1  
 TB 
 r − Re cos 
= tan −1  
 Re sin  

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 r − Re cos l cos s −  L 
= tan −1  
 e 
 R sin cos−1 (cos cos −  )
l s L  

Thus the elevation angle can be expressed by-
 r − Re cos l cos s −  L 
E = tan −1  
 e 
 R sin cos (cos cos −  ) 
−1
l s L 

− cos (cosl cos s −  L )
−1

Mathematical problem:
Consider an earth station located at longitude θL=80˚W and latitude θl=40˚N and a geostationary satellite at
longitude θS=120˚W. Calculate the azimuth and elevation angle.
Hints/Similar math:

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