SATELLITE COMMUNICATION

PRESENTED BY-

ANAMIKA YADAV

NEHA SINGH

WHAT IS A SATELLITE?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 distance communication path.

TYPES OF SATELLITE

•Natural Satellite E.g. moon

•Artificial Satellite E.g. Aryabhata, INSAT..

•Active Satellite •Passive Satellite

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

WHAT IS SATTELITE COMMUNICATION?

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.

DownlinkUplink

WHY SATELLITE COMMUNICATION?Long distance communication beyond 10 – 20 MHz in three modes failed:-Ground wave due to conduction lossesSpace wave due to limited line of sightSky 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 revolutionized communication technology.

FREQUENCY BANDS USED IN SATELLITE COMMUNICATION

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

SATELLITE UPLINK AND DOWNLINK FREQUENCIES IN DIFFERENT BAND

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

ADVANTAGES AND DISADVANTAGES OF DIFFERENT BAND

C-BandAdv. : Broad Footprint, little rain fadeDisadv. : Weak signals, interference, large antenna sizes and amplifiers

Ku-BandAdv. : Focused Foot prints, no terrestrial interference small antenna and amplifierDisadv. : Interference to rain.

Ka-BandAdv. : Focused Foot prints, large unused bandwidthsDisadv. : Interference to rain.

COMPONENETS OF A SATELLITE SYSTEMSpace Segment 1. The Satellite 2. Tracking, Telemetry and Telecommand

The Ground Segment 1. Earth Stations

GROUND SEGMENTGround 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.

SATELLITE EARTH SATION UPLINK/DOWNLINK CHAIN

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 video signal.

ANTENNA•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...

TYPES OF ANTENNAPrime 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 converging on the Feed.

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 converging on the Feed.

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.

• High Antenna efficiency

OTHER TRANSMITTING AND RECEIVING DEVICES.. 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.

SPACE SEGMENTThe 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..

TYPES OF SATELLITE ORBIT ON THE BASIS OF ALTITUDE-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.

Three geosynchronous satellite covering entire earth

•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.

TYPES OF SATELLITE ORBIT ON THE BASIS OF ECCENTRICITY1. 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.

TYPES OF SATELLITE ORBIT ON THE BASIS OF INCLINATION1. 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 reference to the equatorial plane is not zero degrees.

FACTORS DECIDING THE SELECTION OF ORBIT

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.The performance of launcher.

TYPES OF SATELLITEGeo-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.

• Negligible Doppler shift

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 orbit (800 -900 km) or at 36000km apart.

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.

SATELLITE TRANSPONDERA 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 required for the transmitted signal.

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

LOW NOISEAMPLIFIER(L.N.A)

DOWNCONVERTER

POWERAMPLIFIER(P.A)

FILTER

6GHz4GHz

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 retransmitted at earth ground station.

LIFE AND COST OF SATELLITE

GEO 15 Yrs $2B

MEO 10 Yrs $2-3B

LEO 5 Yrs $1.5-3B

Lease 1 Yrs

Own 5-15 Yrs

WEIGHT OF SATELLITE

LARGE >1000Kg

MEDIUM 500-1000Kg

MINI 100-500Kg

MICRO 10-100Kg

NANO 1-10Kg

PICO <1 Kg

ADVANTAGES OF SATELLITE COMMUNICATION1. 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.

DIADVANTAGES OF SATELLITE COMMUNICATION1.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) 7.Congestion of frequencies and orbits

SATELLITE SERVICES

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.

MAJOR PROBLEMS FOR SATELLITE

1.Positioning in orbit

This can be achieved by several methodsOne 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 years

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 the earth’s magnetic field

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 recommended

5. Harsh Environment

Satellite components need to be specially “hardened”Circuits which work on the ground will fail very rapidly in spaceTemperature 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 beamsA 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 lostIn some countries sandstorms can also be a problem

A full size model of the Earth observation satellite ERS 2

TELSTAR

Picture from NASA

                                                                                                                                        

                                                                          

                                                                                                                            

                                                                                    

04/07/2023 60T.Shanmugaraju,ADE,STI[T],Delhi

MILESTONES

Following are the milestones in India’s Insat series programme.

1982 – Insat-1A launched in April using the Delta launch vehicle in the United States, but later deactivated in September and could not fulfil mission.

1983 – Insat-1B launched on board USA space shuttle.

1990 – Insat-1D launched using the American Delta launch vehicle.

1988 – Insat-1C launched by Ariane space from French Guyana in July.

1992 – Insat-2A, the first satellite of the indigenously built second generation Insat series launched by Ariane-4.

1993 – Insat-2B, the second satellite in the Insat-2 series launched on board Ariane-4.

1995 – Insat-2C, the third satellite in the Insat-2 series launched by Ariane-4.

1997 – Insat-2D, the fourth satellite in Insat series launched by Ariane-4.

1999 – Insat-2E,the last satellite in multi-purpose Insat-2E series, launched by Ariane from Korou, French Guyana.

2000 – Insat-3B, first satellite in the third generation Insat-3 series, launched by Ariane-5.

2002 – Insat-3C launched on board

Ariane-4.

2003 – Insat-3A launched by Ariane-5.

INSAT-3D

INSAT-4B

INSAT-4C

INSAT-4D

GSAT-4

GSAT-5

THANKYOU