Satellite Communication Ppt

A Satellite is a solid object which revolves

around some heavenly body due to the effect of gravitational forces which are mutual in nature.

COMMUNICATION SATELLITE-

A communication satellite is an artificial satellite that act as a radio relay station in orbit above the earth that receives, amplifies, and redirects

analog and digital signals carried on a specific radio frequency. It act as a repeater in long

•Natural Satellite  E.g. moon

•Artificial Satellite

 E.g. Aryabhata, INSAT.. •Active Satellite

ACTIVE SATELLITE

It is a functioning satellite that receives and transmits or retransmits radio-communication signals to or from a base station.

They have more complicated structures having a processing equipment called Transponder which is very vital for

functioning of the satellite. These transponders serve dual purpose i.e. provides amplification of the incoming and

performs the frequency translation of the incoming signal to avoid interference between the incoming and outgoing

signals.

PASSIVE SATELLITE

Passive satellites are relay stations in space. It simply reflects light or radio waves transmitted from one ground terminal to another without amplification or retransmission.

Satellite

communication is simply the

communication of the satellite in space with large number of earth stations on the ground. A Communication Satellite can be looked upon as a large microwave repeater.

Users are connected to the earth station via some telephone switch or some dedicated link. They

generate baseband signals, which is processed at the earth station and then transmitted to the

satellite through dish antennas. The satellite

receives the uplink frequency and the transponder present inside the satellite does the processing

function and frequency down conversion and then transmit the downlink signal at different

frequency. The earth station then receives the signal from the satellite through parabolic dish antenna and processes it to get back the

baseband signal. This baseband signal is then transmitted to the respective user via dedicated link or other terrestrial system.

Downlink Uplink

Long distance communication beyond 10 – 20 MHz in three modes failed:-

Ground wave due to conduction losses Space wave due to limited line of sight

Sky wave due to penetration of the ionosphere by the higher frequencies beyond critical

frequency.

And thus, there came the need of satellite communication.

We have seen that the waves of freq.> 30MHz can not propagate by conventional modes due to

penetration of frequencies beyond 30MHz through ionosphere.

A single satellite can provide coverage to over 30% of Earth’s surface and thus was adopted for long distance communication.

Communication links could be setup through out the entire world using satellites. This can’t be done with other modes of communication due to some severe limitations. It is often the only solution for some isolated areas.

And a new concept of communication, the communication through a Satellite

Frequency Band Range L-Band 1 to 1.5 GHz S-Band 1 to 3 GHz C-Band 3 to 8 GHz X-Band 8 to 12GHz Ku-Band 10 to 18 GHz Ka-Band 18 to 22 GHz

FREQUENCY BAND UPLINK DOWNLINK

C-Band 6.00GHz 4.00GHz

X-Band 8.00GHz 7.00GHz

Ku-Band 14.00GHz 11.00GHz Ka-Band 30.00GHz 20.00GHz

C-Band

Adv. : Broad Footprint, little rain fade

Disadv. : Weak signals, interference, large antenna sizes and amplifiers

Ku-Band

Adv. : Focused Foot prints, no terrestrial interference small antenna and amplifier

Disadv. : Interference to rain.

Ka-Band

Adv. : Focused Foot prints, large unused bandwidths Disadv. : Interference to rain.

Space Segment

1. The Satellite

2. Tracking, Telemetry and

Telecommand

The Ground Segment

Ground segment is basically consist of an earth station.

An earth station provides a complete uplink and downlink chain for the signal. It transmits and receives the signal to and from the

satellite. It is also consist of an antenna. Since the user baseband signal cannot be

transmitted directly, it is also consist of amplifiers, modulators and demodulators, frequency up- and down- converters.

The user generates the signal to be

transmitted known as baseband signal. This baseband signal is consist of video(5MHz),2 audio subcarriers(5.5MHz and 5.75MHz) and energy dispersal signal(25 MHz). After

modulation(70 MHz) and up conversion(6 GHz),the carrier is amplified and uplinked through solid parabolic dish antenna(PDA). Downlink signal can be received through same PDA using trans-receive filter (TRF) and low

noise amplifier(LNA). After down conversion to 70 MHz it is demodulated to get audio and

•Parabolic dish

antenna

•Diameter - gain (as a

function of frequency) •Noise - temperature (as a function of elevation) •Cross-polarisation isolation •Wind resistance •Temperature variations tolerance •Tracking...

Prime Focus Antenna

• Single Reflector Antenna. • Feed horn is placed at

the Focal point of the Reflector.

• Antenna Electronics are

placed on Feed.

• More susceptible to

Interference from Low elevation sources.

• More Blockage because

feed.

• Antenna Efficiency is in

the range of 60%.

• Low Cost Antenna. • Primarily Used for

Receive only applications.

Cassagrain Antenna

• Main reflector is

Parabolic

• Sub-Reflector is

hyperboloid and placed at Prime Focus

• Feed is Corrugated Horn

and is placed at Center of the Main Reflectors.

• The paraboloid

converges towards the Sub Reflector ( prime focus), which is then reflected by

Sub-Reflector to form a Spherical Wave

Gregarion Antenna

• Main reflector is

Parabolic

• Sub-Reflector is

Parabolic and placed at Prime Focus

• Feed is Corrugated Horn

and is placed at Center of the Main Reflectors.

• The paraboloid

converges towards the Sub Reflector (prime focus), which is then reflected by

Sub-Reflector to form a Spherical Wave

Offset Fed Antenna

• Used for Smaller Earth

Stations.

• Main Reflector is a

section of Parabolic, cutoff above the axis.

• Feed is located below the

axis giving a completely unblocked Aperture.

 LNA - amplifies RF signal from the antenna

and feeds it into frequency converter (typically IF of 70/140 MHz)

 LNB - amplifies RF signal from the antenna

and converts it to an L-band signal (950-2100 MHz)

 LNA is more precise and stable but more

expensive than LNB (LO stability).

 Transmit power amplifiers provide

amplification of signals to be transmitted to the satellite

 Transceiver takes 70/140 MHz signal and

amplifies it to either C or Ku-band final frequency.

 Block Up-Converter takes L-band signal

and amplifies it to either C or Ku-band final frequency.

The space segment is consist of the satellite itself.

A satellite has various

transmitting and receiving antenna, transponders

and other control systems like temperature control, power supply control,

orbit and altitude control, tracking, telemetry and command equipment etc..

An orbit is the path that a satellite follows as it revolves around

Earth. In terms of commercial satellites, there are three main categories of orbits:

1. LEO( Low Earth Orbit)

• 500-2,000 km above the earth • These orbits are much closer

to the Earth, requiring

satellites to travel at a very high speed in order to avoid being pulled out of orbit by Earth's gravity

• At LEO, a satellite can circle

the Earth in approximately one and a half hours

2. MEO( Medium Earth Orbit)

• 8,000-20,000 km above the earth

• These orbits are primarily reserved for

communications satellites that cover the North and South Pole

• MEO's are placed in an elliptical

(oval-shaped) orbit

3. GEO ( Geosynchronous Orbit)

• 35,786 km above the earth

• Orbiting at the height of 22,282 miles above the equator

(35,786 km), the satellite travels in the same direction and at the same speed as the Earth's rotation on its axis,

taking 24 hours to complete a full trip around the globe. Thus, as long as a satellite is positioned over the equator in an assigned orbital location, it will appear to be

"stationary" with respect to a specific location on the Earth.

• A single geostationary satellite can view approximately

one third of the Earth's surface.

If three satellites are placed at the proper longitude, the

height of this orbit allows almost all of the Earth's surface to be covered by the satellites.

•R=6400 km T=84 minutes

• R=7100 km T=99 minutes (LEO)

• R=11400 km T=201 minutes (MEO)

• R=42350 km T=24 hrs (GEO)

So, an object placed at the orbit approx. 36 000 km above the equator will be seen at the same position in the sky from Earth.

1. Circular orbit: An orbit that has an eccentricity of 0 and whose path traces a circle.

2. Elliptic orbit: An orbit with an eccentricity greater than 0 and less than 1 whose orbit traces the path of an

ellipse.

3. Hyperbolic orbit: An orbit with the eccentricity greater than 1. Such an orbit also has a velocity in excess of the escape velocity and as such, will escape the

gravitational pull of the planet and continue to travel infinitely.

4. Parabolic orbit: An orbit with the eccentricity equal to 1. Such an orbit also has a velocity equal to the escape velocity and therefore will escape the gravitational pull of the planet and travel until its velocity relative to the planet is 0. If the speed of such an orbit is increased it will become a hyperbolic orbit.

1. Equatorial orbit: An orbit whose inclination in reference to the equatorial plane is zero degrees. 2. Polar orbit: An orbit that passes above or nearly

above both poles of the planet on each revolution. Therefore it has an inclination of (or very close to) 90 degrees

3. Inclined orbit: An orbit whose inclination in

The choice of orbit depends upon the nature of mission, the acceptable interference and the performance of the launcher:

The extent and latitude of the area covered. The elevation angle for earth station.

Transmission duration and delay. Interference.

Geo-Synchronous Satellite

• Orbit on the equatorial plane - appears stationary • Altitude of 36000 Kms.

• Circular orbit around earth with period of 24 hours. • Coverage of about 1/3 of Earth.

• 2 deg. apart. Identified by Longitudinal position with

ref. to Greenwich.

Advantage Of Geostationary Satellite

• Simple ground station tracking requirements. • Removes Satellite hand-over problems.

Polar Orbiting Satellite

• These satellites orbit the earth in such a

way as to cover the north and south polar regions.

• These satellites if in a low earth orbit

have to travel at a very high speed.

• These satellites can be kept in low earth

Inclined Orbit Satellite

• A disadvantage of Geostationary satellites

is that points on Earth beyond about 80 deg latitude are not visible.

• Inclined orbits, on the other hand can

provide visibility to the higher northern and southern latitudes, although they

require earth stations to continually track the satellite

Geosynchronous

Geosynchronous means that the satellite is synchronized with the earth in time and

direction. It means that is time taken by a satellite to complete its orbit around earth is equal to the time taken by to earth rotates around its own axis

Satellite Footprint

Coverage of entire surface of earth that is visible by the satellite.

A communications satellite’s transponder, is the series of interconnected units which form a communications channel between the receiving and the transmitting antennas .

A transponder is consist of:

An input band limiting device (a band pass filter). An Input low-noise amplifier (LNA) to amplify the

(normally very weak, because of the large distances involved) signals received from the earth station.

A frequency translator (normally composed of an

oscillator and a frequency mixer )used to convert the frequency of the received signal to the frequency

A output band pass filter.

A power amplifier (this can be a TWT or a solid

state amplifier).

Frequency band on the satellite is divided into several channels. Each channels are called transponder Each transponder have 40 MHz .

BLOCK DIAGRAM OF A SATELLITE

TRANSPONDER

The uplinked signal to satellite is 6GHz.it is received at the satellite and then amplified using a Low Noise Amplifier(L.N.A). This amplified signal is then down converted at

4GHz. It is sent through a filter and then power amplifier(TWT). The local oscillator frequency of the down converter is 2225MHz for C band and Ex-C band. This signal is then

GEO 15 Yrs $2B

MEO 10 Yrs $2-3B

LEO 5 Yrs $1.5-3B

Lease 1 Yrs

LARGE >1000Kg MEDIUM 500-1000Kg MINI 100-500Kg MICRO 10-100Kg NANO 1-10Kg PICO <1 Kg

1. Can reach over large geographical area. A single

satellite can provide coverage to over 30% of Earth’s surface. With just 3 geosynchronous satellite we can cover the entire earth.

2. Point to Multi point communication is possible. 3. Only solution for developing and isolated areas. 4. Ideal for broadcast applications.

5. No need for the local loop.

6. Wide bandwidths (155 Mbps) are available now. 7. Transmission cost and quality of signal is

independent of distances.

8. During critical condition earth stations can be removed and relocated easily.

1. Delay of 270+270 msec makes one feel annoying. 2. Delay reduces the of satellite in data transmission

during long file transfer.

3. Communication path between TX and RX is approximately 75000 km.

4. High atmospheric losses above 30 GHz limit carrier frequencies.

5. Large up front capital costs (space segment and launch)

6. Terrestrial break even distance expanding (now approx. size of Europe)

1.Communication Satellite Services

2.Broadcasting Satellite Services [BSS] 3.Mobile Satellite Services

4.Navigational Satellite Services 5.Metrological Satellite Services. 6.Military Satellite Services.

1. Positioning in orbit

This can be achieved by several methods One method is to use small rocket motors.

These use fuel - over half of the weight of most

satellites is made up of fuel.

Often it is the fuel availability which

determines the lifetime of a satellite.

Commercial life of a satellite typically 10-15

2. Stability

 It is vital that satellites are stabilised

•to ensure that solar panels are aligned

properly

•to ensure that communications antennae are

aligned properly

 Early satellites used spin stabilisation •Either this required an inefficient

omni-directional aerial

•Or antennae were precisely counter-rotated in

order to provide stable communications

 Modern satellites use reaction wheel

stabilisation - a form of gyroscopic stabilisation Other methods of stabilisation are also possible

 Including:

•eddy current stabilisation

•forces act on the satellite as it moves through

3. Reaction wheel stabilisation

Heavy wheels which rotate at

high speed - often in groups of 4.

3 are orthogonal, and the 4th (spare) is a

backup at an angle to the others.

Driven by electric motors - as they speed

up or slow down the satellite rotates.

If the speed of the wheels is inappropriate,

rocket motors must be used to stabilise the satellite - which uses fuel

4. Power

Modern satellites use a variety of power . Solar panels are now quite efficient, so

solar power is used to generate electricity.

Batteries are needed as sometimes the

satellites are behind the earth - this happens about half the time for a LEO satellite.

Nuclear power has been used - but not

5. Harsh Environment

Satellite components need to be specially

“hardened”

Circuits which work on the ground will fail

very rapidly in space

Temperature is also a problem - so

satellites use electric heaters to keep

circuits and other vital parts warmed up - they also need to control the temperature carefully

6. Alignment

There are a number of components

which need alignment

•Solar panels •Antennae

These have to point at different parts of

the sky at different times, so the problem is not trivial

7. Antennae alignment

A parabolic dish can be used which is

pointing in the correct general direction.

Different feeder “horns” can be used to direct outgoing beams more precisely.

Similarly for incoming beams

A modern satellite should be capable of at least 50 differently directed beams

8. Rain fade

Above 10 GHz rain and other disturbances

can have a severe effect on reception.

This can be countered by using larger

receiver dishes so moderate rain will have less effect.

In severe rainstorms reception can be lost

In some countries sandstorms can also be a problem



1990 – Insat-1D launched using the

American Delta launch vehicle.



1988 – Insat-1C launched by Ariane space