Satellite Communication
Topics to be covered in unit-01:
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Introduction
Active and Passive satellites
Frequency Allocation
Types of orbits: LEO, GEO, MEO & HEO
Newtons and Kepler’s laws
Orbital elements
Orbit period and velocity
Look angles and slant range
Orbital perturbations
Satellite keeping
Rocket propulsion
Rocket equation
Launch and Multi Stage vehicles
Launch sequence
Testing and facilities
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01. Introduction:
In general terms, a satellite is a smaller object that revolves
around a larger object in space. For example, moon is a natural
satellite of earth.
We know that Communication refers to the exchange (sharing)
of information between two or more entities, through any
medium or channel. In other words, it is nothing but sending,
receiving and processing of information.
If the communication takes place between any two earth
stations through a satellite, then it is called as satellite
communication. In this communication, electromagnetic waves
are used as carrier signals. These signals carry the information
such as voice, audio, video or any other data between ground
and space and vice-versa.
Soviet Union had launched the world's first artificial satellite
named, Sputnik 1 in 1957. Nearly after 18 years, India also
launched the artificial satellite named, Aryabhata in 1975.
 Need of Satellite Communication:
The following two kinds of propagation are used earlier for
communication up to some distance.
i.
Ground wave propagation − Ground wave propagation is
suitable for frequencies up to 30MHz. This method of
communication makes use of the troposphere conditions
of the earth.
(This specific type of propagation of radio wave allows
getting the transmission of a signal with up to 30 MHz
frequency from one end to another. Using the layer of
the troposphere of the earth’s atmosphere, the artificial
satellites present in the space or orbit of the earth help
to propagate such wave transmission.)
ii.
Sky wave propagation − The suitable bandwidth for this
type of communication is broadly between 30–40 MHz
and it makes use of the ionosphere properties of the
earth.
(These specific types of radio waves are generally used in
order to transmit a specific signal that falls in the range of
up to 40MHz. The main difference that is shared with
ground wave is that such radio waves can be propagated
by getting refracted through the ionospheric layer (An et
al. 2018). In order to make this propagation stable, the
distance that needs to be maintained between the
ground and the space station is 1500 km otherwise the
activation of the satellite communication cannot be
activated successfully.)
(Satellite comm. System diagram above)
The maximum hop or the station distance is limited to 1500KM
only in both ground wave propagation and sky wave
propagation. Satellite communication overcomes this
limitation. In this method, satellites provide communication for
long distances, which is well beyond the line of sight.
Since the satellites locate at certain height above earth, the
communication takes place between any two earth stations
easily via satellite. So, it overcomes the limitation of
communication between two earth stations due to earth’s
curvature.
 How a Satellite Works
The communication satellites are similar to the space
mirrors that help us bounce signals such as radio, internet
data, and television from one side of the earth to another.
Three stages are involved, which explain the working of
satellite communications. These are:
- Uplink
- Transponders
- Downlink
A satellite is a body that moves around another body in a
particular path. A communication satellite is nothing but a
microwave repeater station in space. It is helpful in
telecommunications, radio and television along with internet
applications.
A repeater is a circuit, which increases the strength of the
received signal and then transmits it. But, this repeater works
as a transponder. That means, it changes the frequency band
of the transmitted signal from the received one.
The frequency with which, the signal is sent into the space is
called as Uplink frequency. Similarly, the frequency with which,
the signal is sent by the transponder is called as Downlink
frequency. The following figure illustrates this concept clearly.
The transmission of signal from first earth station to satellite
through a channel is called as uplink. Similarly, the
transmission of signal from satellite to second earth station
through a channel is called as downlink.
Uplink frequency is the frequency at which, the first earth
station is communicating with satellite. The satellite
transponder converts this signal into another frequency and
sends it down to the second earth station. This frequency is
called as Downlink frequency. In similar way, second earth
station can also communicate with the first one.
The process of satellite communication begins at an earth
station. Here, an installation is designed to transmit and
receive signals from a satellite in an orbit around the earth.
Earth stations send the information to satellites in the form of
high powered, high frequency (GHz range) signals.
The satellites receive and retransmit the signals back to earth
where they are received by other earth stations in the
coverage area of the satellite. Satellite's footprint is the area
which receives a signal of useful strength from the satellite.
 Pros and Cons of Satellite Communication:
In this section, let us have a look at the advantages and
disadvantages of satellite communication.
Following are the advantages of using satellite communication:
- Area of coverage is more than that of terrestrial
systems
- Each and every corner of the earth can be covered
- Transmission cost is independent of coverage area
- More bandwidth and broadcasting possibilities
Following are the disadvantages of using satellite
communication:
- Launching of satellites into orbits is a costly process.
- Propagation delay of satellite systems is more than that
of conventional terrestrial systems.
- Difficult to provide repairing activities if any problem
occurs in a satellite system.
- Free space loss is more
- There can be congestion of frequencies.
 Applications of Satellite Communication
Satellite communication plays a vital role in our daily life.
Following are the applications of satellite communication −
- Radio broadcasting and voice communications
- TV broadcasting such as Direct To Home (DTH)
- Internet applications such as providing Internet
connection for data transfer, GPS applications, Internet
surfing, etc.
- Military applications and navigations
- Remote sensing applications
- Weather condition monitoring & Forecasting
 Satellites are specifically made for telecommunication
purpose. They are used for mobile applications such as
communication to ships, vehicles, planes, hand-held
terminals and for TV and radio broadcasting.
 They are responsible for providing these services to an
assigned region (area) on the earth. The power and
bandwidth of these satellites depend upon the preferred
size of the footprint, complexity of the traffic control
protocol schemes and the cost of ground stations.
 A satellite works most efficiently when the transmissions
are focused with a desired area. When the area is
focused, then the emissions don’t go outside that
designated area and thus minimizing the interference to
the other systems. This leads more efficient spectrum
usage.
 Satellite’s antenna patterns play an important role and
must be designed to best cover the designated
geographical area (which is generally irregular in shape).
Satellites should be designed by keeping in mind its
usability for short and long term effects throughout its
life time.
 The earth station should be in a position to control the
satellite if it drifts from its orbit it is subjected to any kind
of drag from the external forces
02. Active and Passive satellites:
Passive Satellite: Explanation
• The passive satellite is a reflector which receives the signal
from the transmitting earth station and scatters the signal in all
the directions.
• It works as large mirror and reflects the EM (ElectroMagnetic) radio waves without any modification and/or
amplification.
• It does not have its own power source or electronic
equipment unlike active satellite.
• Passive satellite can not generate power of its own and
simply reflects the incident signal energy.
• It is known as passive repeater due to its functionality.
Examples
• Passive remote sensing satellites which collect the energy
reflected or emitted from the surface.
• Starshine, PAGEOS
• Echo Satellites: Launched by NASA in the 1960s, these were
large metalized balloons used to reflect microwave signals.
• Moon: The moon can also be considered a natural passive
satellite as it reflects light from the Sun and radio signals from
Earth.
Advantages of passive satellite
Following are the benefits of passive satellite.
1. Passive satellites are cheaper to build and launch as they do
not require complex electronic systems.
2. They can last longer since there is no
onboard electronic system that can degrade over time.
3. The design and operation are simpler, reducing maintenance
costs.
Disadvantages of passive satellite
Following are the drawbacks of passive satellite.
1. Passive satellites cannot amplify signals, which can lead to
weak or degraded signals over long distances.
2. They only reflect signals and cannot provide other services
like broadcasting, navigation, or data transmission.
3. Due to lack of amplification, there can be higher signal loss
and attenuation.
Active Satellite: Explanation
• The active satellite has many benefits over passive satellite.
• It is equipped with transponders, amplifiers, transmitting and
receiving antennas.
• It amplifies the signal received from earth station or ground
station and retransmits the amplified signal back to earth.
• In addition to amplification, it performs frequency translation
of the received signal before retransmission. Active satellite
can generate power for its own operation.
• It is known as active repeater due to its functionality.
Examples
• Active Remote sensing satellites which transmit energy and
analyze received energy to form the image.
• ACRIMSAT, Courier 1B
• Communication Satellites (e.g., Intelsat, Inmarsat): These
satellites transmit telephone, television, and internet signals.
• Navigation Satellites (e.g., GPS Satellites): These satellites
provide navigation and positioning services.
• Weather Satellites (e.g., GOES): These are used for
meteorological observations.
Advantages of active satellite
Following are the benefits of active satellite.
1. Active satellites can amplify weak signals, making them
suitable for long-distance communication.
2. They can cover vast areas and multiple regions due to their
high altitude position.
3. Active satellites can manage multiple frequencies and
provide multiple services simultaneously.
4. Some active satellites have onboard processing capabilities,
allowing for better signal management and distribution.
Disadvantages of active satellite
Following are the drawbacks of active satellite.
1. They are more expensive to build, launch, and maintain due
to the onboard electronics and power systems.
2. The lifespan is limited by the onboard fuel and battery,
usually ranging from 10 to 15 years.
3. Requires complex technology and coordination, making it
challenging to design and operate.
Categories of Satellite Communication:
Satellite communication is classified into two distinct
categories as follows:
One-way satellite communication: In such communication, the
communication is generally established between two earth
stations using the artificial satellite present in the earth’s orbit
where the transmitted signal is unidirectional.
Two-way satellite communication: Such kind of communication
is established based on point-to-point connectivity, within two
particular earth stations and two uplinks and downlinks
happen between these two earth stations.
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03. Frequency Allocation:
Allocation of frequencies to satellite services s a complicated
process which requires international coordination and planning.
This is done as per the International Telecommunication Union
(ITU). To implement this frequency planning, the world is divided
into three regions:
Region1: Europe, Africa and Mongolia
Region 2: North and South America and Greenland
Region 3: Asia (excluding region 1 areas), Australia and southwest Pacific.
Within these regions, the frequency bands are allocated to various
satellite services. Some of them are listed below.
Fixed satellite service: Provides Links for existing Telephone
Networks Used for transmitting television signals to cable
companies
Broadcasting satellite service: Provides Direct Broadcast to
homes. E.g. Live Cricket matches etc
Mobile satellite services: This includes services for: Land Mobile
Maritime Mobile Aeronautical mobile
Navigational satellite services: Include Global Positioning
systems 6
Meteorological satellite services: They are often used to perform
Search and Rescue service Below are the frequencies allocated to
these satellites: Frequency Band (GHZ) Designations:
VHF: 01-0.3
UHF: 0.3-1.0
L-band: 1.0-2.0 (1.518-1.675 GHz, MSS (Mobile Satellite Service))
Civilian mobile communication services and global positioning
systems (GPS). For instance, Inmarsat use the frequency range to
facilitate communication across various mediums such as air,
water, and land, as well as for weather radar systems.
S-band: 2.0-4.0 (1.97 - 2.69 GHz, MSS (Mobile Satellite Service))
(Satellite TV, mobile broadband services, radio broadcasting, and
in-flight connectivity
For instance, communication between the International Space
Station (ISS) and the space shuttle.)
C-band: 4.0-8.0 (3.4GHz - 7.025 GHz, FSS (Fixed Satellite Service))
(Data services, unprocessed satellite feeds, and networks for
satellite TV
For Example: Telstar satellites use the frequency range for
facilitating transportation operations.)
X-band: 8.0-12.0 (7.25 - 8.44 GHz, FSS (Fixed Satellite Service))
(Military operations, pulsed radar systems, synthetic operational
and wave radars, weather monitoring, air traffic control, maritime
traffic regulation, defense surveillance, and the detection of
vehicle speeds)
Ku-band: 12.0-18.0 (Ku is Under K Band) (10.7 - 14.5 GHz, FSS
(Fixed Satellite Service), BSS (Broadband Satellite Service))
Fixed satellite television data services.
Ka-band: 18.0-27.0 (Ka is Above K Band) (17.3 - 30 GHz, FSS
(Fixed Satellite Service), BSS (Broadband Satellite Service))
Two-way broadband services for both mobile and fixed
applications, fixed satellite television services, and the deployment
of close-range targeting radars within military systems.
V-band: 40.0-75.0 (37.5 - 51.4 GHz, MSS (Mobile Satellite
Service), BSS (Broadband Satellite Service))
High-speed broadband services (fixed and mobile) and in-flight
connectivity.
W-band: 75-110
Mm-band: 110-300
μm-band: 300-3000.
Based on the satellite service, following are the frequencies
allocated to the satellites: Frequency Band (GHZ) Designations:
VHF: 01-0.3 ---Mobile & Navigational Satellite Services
L-band: 1.0-2.0 --- Mobile & Navigational Satellite Services
C-band: 4.0-8.0 --- Fixed Satellite Service
Ku-band: 12.0-18.0 --- Direct Broadcast Satellite
04. Types of Orbits: GEO, LEO, MEO and HEO-
1.5.1) Geostationary or geosynchronous earth orbit (GEO)
GEO satellites are synchronous with respect to earth. Looking
from a fixed point from Earth, these satellites appear to be
stationary. These satellites are placed in the space in such a
way that only three satellites are sufficient to provide connection
throughout the surface of the Earth (that is; their footprint is
covering almost 1/3rd of the Earth). The orbit of these satellites
is circular.
There are three conditions which lead to geostationary
satellites. Lifetime expectancy of these satellites is 15 years.
1) The satellite should be placed 37,786 kms (approximated to
36,000 kms) above the surface of the earth.
2) These satellites must travel in the rotational speed of earth,
and in the direction of motion of earth, that is eastward.
3) The inclination of satellite with respect to earth must be 00
.
Geostationary satellite in practical is termed as geosynchronous
as there are multiple factors which make these satellites shift
from the ideal geostationary condition.
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1) 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.)
2) These satellites experience the centrifugal force due to the
rotation of Earth, making them deviate from their orbit.
3) 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 of GEO: Northern or southern regions of the
Earth (poles) have more problems receiving these satellites due
to the low elevation above a latitude of 60°, i.e., larger antennas
are needed in this case. Shading of the signals is seen in cities
due to high buildings and the low elevation further away from
the equator limit transmission quality. The transmit power
needed is relatively high which causes problems for battery
powered devices. These satellites cannot be used for small
mobile phones. The biggest problem for voice and also data
communication is the high latency as without having any
handovers, the signal has to at least travel 72,000 kms. Due to
the large footprint, either frequencies cannot be reused or the
GEO satellite needs special antennas focusing on a smaller
footprint. Transferring a GEO into orbit is very expensive.
1.5.2) Low Earth Orbit (LEO) satellites:
These satellites are placed 500-1500 kms above the surface of
the earth. As LEOs circulate on a lower orbit, hence they exhibit
a much shorter period that is 95 to 120 minutes. LEO systems
try to ensure a high elevation for every spot on earth to provide
a high quality communication link. Each LEO satellite will only
be visible from the earth for around ten minutes.
Using advanced compression schemes, transmission rates of
about 2,400 bit/s can be enough for voice communication. LEOs
even provide this bandwidth for mobile terminals with
Omnidirectional antennas using low transmit power in the range
of
1W. The delay for packets delivered via a LEO is relatively low
(approx 10 ms). The delay is comparable to long-distance wired
connections (about 5–10 ms). Smaller footprints of LEOs allow
for better frequency reuse, similar to the concepts used for
cellular networks. LEOs can provide a much higher elevation in
Polar Regions and so better global coverage.
8
These satellites are mainly used in remote sensing an providing
mobile communication services (due to lower latency).
Disadvantages: The biggest problem of the LEO concept is the
need for many satellites if global coverage is to be reached.
Several concepts involve 50–200 or even more satellites in
orbit. The short time of visibility with a high elevation requires
additional mechanisms for connection handover between
different satellites. The high number of satellites combined with
the fast movements resulting in a high complexity of the whole
satellite system. One general problem of LEOs is the short
lifetime of about five to eight years due to atmospheric drag and
radiation from the inner Van Allen belt1. Assuming 48 satellites
and a lifetime of eight years, a new satellite would be needed
every two months. The low latency via a single LEO is only half
of the story. Other factors are the need for routing of data
packets from satellite to if a user wants to communicate around
the world. Due to the large footprint, a GEO typically does not
need this type of routing, as senders and receivers are most
likely in the same footprint.
1.5.3) Medium Earth Orbit (MEO) satellites:
MEOs can be positioned somewhere between LEOs and GEOs,
both in terms of their orbit and due to their advantages and
disadvantages. Using orbits around 10,000 km, the system only
requires a dozen satellites which is more than a GEO system,
but much less than a LEO system. These satellites move more
slowly relative to the earth‟s rotation allowing a simpler system
design (satellite periods are about six hours). Depending on the
inclination, a MEO can cover larger populations, so requiring
fewer handovers.
Disadvantages: Again, due to the larger distance to the earth,
delay increases to about 70–80 ms. the satellites need higher
transmit power and special antennas for smaller footprints.
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