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 S.Archana 10321A1004

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Evolution

Communication through Satellite wasconceived by auther C.Clarke

Introduced in the year 1945

Circular orbit radius of about 42,242 km

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Basic Satellite system

Line-of-sight microwave systems with asingle repeater.

"Messages and Transmission Channels,"

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Multiple Access points

Frequency division multiple access

Time division multiple access

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Basic optical communication

 Alexander Grahembell in1884 patented an"apparatus for signaling andcommunicating, called Photophone."

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Inter orbit link

Relaying data from the LEO space craft back to the ground in real time.

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SOUT

 ARTEMIS& EDRS- Programs of TheEuropeanSpaceAgency

SMALL OPTICAL USER

TERMINALS(SOUT) .

Hasthe features of low mass, small sizeandcompatibility with SILEX.

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Link Design

 Wavelength and design 

Link budgets for an asymmetric link

Pointing Acqusition

Tracking

Point Ahead

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 Wavelength and design:-

• The transmit and receive wavelengths aredetermined.

•

Circular polarization is used .

• Optical deplexing scheme can be used.

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Link budgets for an asymmetric link:-

 Requirement to transmit a much higherdata rate

Smaller telescope on LEO facilitates thedesign of a small user terminal.

 Adequate link margins is available

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Pointing:-

One satellite knows the location of theother

 It knows its own attitude

It can aim its beam knowing therequired direction.

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 Acqusation:-

Before communication can commence, ahigh power beam laser located on GEO endhas to scan over the region of uncertainty.

LEO and GEO terminals then track on thereceived communication beams,

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Tracking:-

The on-board disturbances which introducepointing fitter into the communication beamare alternated by means of a fine pointing

control loop (FPL) to enable acceptablecommunications to be obtained.

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Point Ahead:-

Point ahead angle=2Vt /c

 where

 Vt = transverse Velocity componentof the satellite.

C = Speed of light

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Block Diagram

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S UT Terminal

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Integrated transmitter

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Finite Pointing Loop

Fine pointing loop (FPL) is required toattenuate external pointing disturbances.

The SOUT FPL is used to compensate forfrequencies upto 80 HZ.

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Optical Bench

Diplexer, quarter wave plate and otherlens system required too acquisition andtracking are all placed in the optical

bench.

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Diplexer:-

Diplexer has a dietetric multilayer coating which provides efficient transmission ofone type polarised light at the transmit wavelength (848 nm) and rejects another

type poiarised light at the receive wavelength (800 nm).

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QWP:-

A quarter wave plate (QWP) converts thetransmit light to circular polarisation stateprior to the telescope.

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PAA, lasers, and redundancy switchingmechanisms are on one side while thediplexer, receive paths and calibration

path are on the other side of the optical bench.

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Structural Configuration

Main structural elements are a truss frameassembly.

Design drivers for the structure are theoptical bench pointing stability, soft mountconstrains and base-bending momentsassociated with the telescope CPA.

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The height of the terminal above the spacecraft depends upon the mounting interface

The head unit occupies an area of about 40 by 40cm depending upon the platforminterface.

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Mass And Power

The base-line SOUT has a total mass(including REM) of around 25 Kg and adynamic mass of 3.7kg due to the motion of

the CPA. The maximum power dissipationis around 65 W.

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 Advantages over Microwave

The antenna can be much smaller

Optical beam widths are less.

 Very high antenna gains on bothtransmit and receive.

Low mass, low power terminal.

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Conclusion

Future space infrastructure

 And considerable development effort iscurrently underway in Europe and

elsewhere.

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Quires

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  hank You