1. Satellite Communication
Lecture 4:
Satellite Communication
2nd semester
References:

2. Introduction
In general terms, a satellite is a smaller object that revolves around a larger object in space. Ex. moon is a natural satellite of earth.
Communication refers to the exchange (sharing) of information between two or more entities, through any medium or channel.
If the communication takes place between any two earth stations through a satellite,  satellite communication
It is helpful in telecommunications, radio and television along with internet applications.
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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.

3. Need of Satellite Communication
In the satellite communication, satellites provide communication for long distances, which is well beyond the line of sight.
Ground wave and Sky wave propagation: The maximum hop or the station distance is limited to 1500KM
Satellite communication overcomes this limitation
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The following two kinds of propagation are used earlier for communication up to some
distance.
 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.
 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.
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.

4. Satellite Network
A satellite network is a combination of nodes, some of which are satellites, that provides communication from one point on the Earth to another.
A node in the network can be a satellite, an Earth station, or an end-user terminal or telephone
Although a natural satellite,
such as the Moon, can be used as a relaying node in the network, the use of artificial
satellites is preferred because we can install electronic equipment on the satellite to regenerate
the signal that has lost its energy during travel. Another restriction on using natural
satellites is their distances from the Earth, which create a long delay in communication.

5. Communication Satellite
Communications satellite is an artificial satellite stationed in space for the purpose of telecommunications.
It is nothing but a microwave repeater station in space.
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.
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.

6. Uplink and Downlink Channels and Frequency
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.

7. Uplink and Downlink Channels and Frequency
Uplink the link or channel from earth station up to a satellite.
Downlink the link or channel from a satellite down to one or more earth station.
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
The frequency with which, the signal is sent into the space is called as Uplink frequency.
The frequency with which, the signal is sent by the transponder is called as Downlink frequency
In similar way, second earth station can also communicate with the first one.

8. Frequency Bands for Satellite Communication
The frequencies reserved for satellite microwave communication are in the gigahertz (GHz) range.
The following table gives the band names and frequencies for each range
Each satellite sends and receives over two different bands.
Transmission
from the Earth to the satellite is called the uplink. Transmission from the satellite to the Earth
is called the downlink.

9. Satellite's Footprint
Satellite's footprint is the area which receives a signal of useful strength from the satellite.
The signal power at the center of the footprint is maximum.
The power decreases as we move out from the footprint center.
The boundary of the footprint is the location where the power level is at a predefined threshold.
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.
Satellites process microwaves with bidirectional antennas (line-of-sight). Therefore, the signal from a satellite is normally aimed at a specific area called the footprint.

10. Satellite's Footprint
There are two types of footprint: moving foot print and fixed footprint ( fixed only for a period of time).

11. Orbits
The path of satellite revolving around the earth is known as orbit
The orbit can be equatorial, inclined, or polar
Any satellite can achieve orbit at any distance from the earth if its velocity is sufficient to keep it from falling to earth and it is free of friction from earth's atmosphere, and gravity is strong enough to pull it back towards earth. Distance from the earth can become a problem. The farther the satellite is from the earth, the longer it takes for a radio or microwave frequency transmission to reach the satellite.
An artificial satellite needs to have an orbit~ the path in which it travels around the Earth
Satellite should be properly placed in the corresponding orbit after leaving it in the space.
It revolves in a particular way and serves its purpose for scientific, military or commercial.

12. Categories of Satellites
Based on the location of the orbit, satellites can be divided into three categories: geostationary Earth orbit (GEO), low-Earth- orbit (LEO), and middle-Earth-orbit (MEO)

13. Categories of Satellites
The following figure shows the satellite altitudes with respect to the surface
One reason for having different orbits is due to the existence of two Van Allen belts.
A Van Allen belt is a layer that contains charged particles.
A satellite orbiting in one of these two belts would be totally destroyed by the energetic charged particles.
The MEO orbits are located between these two belts.
We should choose an orbit properly for a satellite based on the requirement. For example, if the satellite is placed in lower orbit, then it takes less time to travel around the earth
and there will be better resolution in an onboard camera. Similarly, if the satellite is placed in higher orbit, then it takes more time to travel around the earth and it covers more earth’s surface at one time.
Below an altitude
of 2000 km

15. GEO Earth Orbit Satellites
A satellite in a geostationary orbit appears to be in a fixed position to an earth-based observer.
A geostationary satellite revolves around the earth at the same angular velocity of the earth itself, 360 degrees every 24 hours in an equatorial orbit, and therefore it seems to be in a fixed position over the equator.
Line-of-sight propagation requires that the sending and receiving antennas be locked
onto each other's location at all times (one antenna must have the other in sight). For
this reason, a satellite that moves faster or slower than the Earth's rotation is useful
only for short periods. To ensure constant communication, the satellite must move at
the same speed as the Earth so that it seems to remain fixed above a certain spot. Such
satellites are called geostationary.
Because orbital speed is based on the distance from the planet, only one orbit can be
geostationary. This orbit occurs at the equatorial plane and is approximately 22,000 mi
from the surface of the Earth.
But one geostationary satellite cannot cover the whole Earth. One satellite in orbit
has line-of-sight contact with a vast number of stations, but the curvature of the Earth
still keeps much of the planet out of sight. It takes a minimum of three satellites equidistant
from each other in geostationary Earth orbit (OEO) to provide full global transmission.
Figure shows three satellites, each 120° from another in geosynchronous
orbit around the equator. The view is from the North Pole.
The satellites present in these orbits have the angular velocity same as that of earth.
Hence, these satellites are considered as stationary with respect to earth since, these are
in synchronous with the Earth’s rotation

16. GEO Earth Orbit Satellites
To provide full global transmission, a minimum of three satellites equidistant from each other is needed.
GEO Satellites are used for weather forecasting, satellite TV, satellite radio and other types of global communications.
But one geostationary satellite cannot cover the whole Earth. One satellite in orbit
has line-of-sight contact with a vast number of stations, but the curvature of the Earth
still keeps much of the planet out of sight. It takes a minimum of three satellites equidistant
from each other in geostationary Earth orbit (OEO) to provide full global transmission.
Figure shows three satellites, each 120° from another in geosynchronous
orbit around the equator. The view is from the North Pole

17. MEO Satellites
Medium-Earth-orbit (MEO) satellites are positioned between the two Van Allen belts.
A satellite at this orbit takes approximately 6-8 hours to circle the Earth.
Signals transmitted from a MEO satellite travel a shorter distance.
the signal strength is good  smaller and light weight receiving terminals.
Ten or more MEO satellites are required in order to cover entire earth.
One example of a MEO satellite system is the Global Positioning System (GPS)
The orbits and the locations of the satellites in each orbit are designed in such a way that, at any time, four satellites
are visible from any point on Earth. A GPS receiver has an almanac that tells the current
position of each satellite.
Signals transmitted from a MEO satellite travel a shorter distance. Due to
this, the signal strength at the receiving end gets improved. This shows that smaller and
light weight receiving terminals can be used at the receiving end.
Transmission delay can be defined as the time it takes for a signal to travel up to a
satellite and back down to a receiving station. In this case, there is less transmission delay.
Because, the signal travels for a shorter distance to and from the MEO satellite.
For real-time communications, the shorter the transmission delay, the better will be
the communication system. As an example, if a GEO satellite requires 0.25 seconds for a
round trip, then MEO satellite requires less than 0.1 seconds to complete the same trip.
MEOs operate in the frequency range of 2 GHz and above.
These satellites are used for High speed telephone signals. Ten or more MEO satellites are
required in order to cover entire earth.

18. MEO Satellites -- GPS
Constructed and operated by the US Department of Defense.
Orbiting at an altitude about 18,000 km (11,000 mi) above the Earth.
The system consists of 24 satellites in six orbits.
are designed in such a way that, at any time, four satellites are visible from any point on Earth
is used for land, sea, and air navigation to provide time and locations for vehicles and ships.

19. LEO Satellites Low-Earth-orbit (LEO) satellites have polar orbits.
The altitude is between 500 and 2000 km, with a rotation period of 90 to 120 min.
The satellite has a speed of 20,000 to 25,000 km/h.
The footprint normally has a diameter of 8000 km.
Because LEO satellites are close to Earth, the round-trip time propagation delay is normally less than 20 ms, which is acceptable for audio communication
. An LEO system usually has a cellular type of access, similar to the cellular
telephone system.

20. LEO Satellites
LEO satellites are mainly classified into three categories: little LEOs, big LEOs, and Mega-LEOs.
The little LEOs operate under 1 GHz. They are mostly used for low-data-rate messaging.
The big LEOs operate between 1 and 3 GHz.
Mega-LEOs operates in the GHz range. They are mostly used for of real-time, low delay video transmission
The higher frequencies associated with Mega-LEOs translates into more information
carrying capacity and yields to the capability of real-time, low delay video transmission
scheme.

21. Pros 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

22. Cons of Satellite Communication
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.

23. 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

24. Any Questions?