Unit 6 Non Geostaitonary Orbit Satellite System

7/27/2019 Unit 6 Non Geostaitonary Orbit Satellite System

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NON GEOSTATIONARY ORBITSATELLITE SYSTEM

UNIT 6


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CONTENTS INTRODUCTION

REASONS

DESIGN CONSIDERATIONS

EXAMPLE OF SYSTEMS DTH

VSAT

GPS


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VSAT

Very Small Aperture Terminal

The first earth station antennas used incommercial satellite communications

systems were very large and expensive

with typical aperture diameters of 30 m.

These large antennas operated in C band

(6/4GHz).


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Most VSAT systems operate in Ku band with

earth station antenna diameters of 1 to 2 m and

transmitter powers of 1 or 2 W.

The earth stations are usually organized in a

STAR network, in which the earth stations

connect to a central hub station via a GEO

satellite.

Data rates on the links are from a few thousandbits per second up to 256 kbps,depending on

the traffic requirements.


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VSAT systems are used to link businesses and stores to a

central computer system so that sales transactions can be

completed more rapidly than by using a telephone line and

modem , and so that a central office can rapidly distributeand collect information from a large no. of locations in a

region or country.

VSAT systems are used for Voice Traffic, although thedata rates are well matched to digital voice bit rates.

For this reason , voice over IP (VOIP) has become a

growth segment in VSAT operations.


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The feed for the antenna can be

Cassegrain or Gregorian Designs.

These antennas require a sub reflector with

a minimum diameter of ~10 wavelengths.

Earth stations with antenna aperture

diameter less than 100 wavelength were

called Small Aperture Terminalsand as thesize reduced the term VSAT was coined

and then USAT (Ultra satellite aperture

terminal).


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These are typical sizes of VSAT antennas used in

International and domestic services.


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The standard VSAT antennas are not as small as the Ku-

band direct to home (DTH) antennas used for direct

broadcast satellite TV reception, which are typically

0.5 to 0.8 m in diameter.

DBSTV satellites use very powerful transponders,typically 160-240 W compared to 20 to 50 W of Ku- band

satellites used for VSAT service.

VSAT antennas are also much larger than USATahandheld satellite telephone as used in Iridium, Global

Star, New ICO, and other mobile satellite service (MSS)systems, which have an Omni-directional antenna.


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Geostationary satellites allied to

microwave cellular technologies have

been used to bypass completely thetraditional expansion of analog

telephony.

One such solution is wireless local loop

(WLL) coupled with VSAT distribution

architectures. The VSAT/WLL concept usually has an

optimum range of user densities where the

economics are most favorable.


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The geo stationary satellite is used to link

a large no. of VSATs with the main

switching center in a large city.

Each VSAT acts as the link to the local

switching center in the village or ruralcommunity with the final mile of the

telephony link being carried over a

wireless local loop(WLL).


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VSAT networks allow multimedia traffic to be brought

directly to the end user, but generally handle only

small traffic streams.

The traffic stream is also usually intermittent in nature; the

user accesses the satellite in a demand assigned

multiple access (DAMA) mode whenever a message is to

be sent and receives a short reply in due course.

This is typical in a point of sale (POS) VSAT system that

is used to transmit credit card information at a gas pump or

a store register.

Information about the sale and the customers credit is sent

to a central computer facility and an authorization or denial

is received in response.


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The interaction between the VSAT and the

main hub earth station in the POS (point of

Sale) transaction is completely automaticand transparent to the user, the customer in

this case.

Most VSAT networks do not generate

enough traffic to justify a dedicated satellite.


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NETWORK ARCHITECTURES There are three basic implementations of

any telecommunications service;

One way

Two way

And split two way(SplitIP, whenreferring to Internet Traffic, since the

outbound and inbound channels arerouted over different systems)


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Two way Implementation Classified into two basic network

architectures :

Star

Mesh


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One Way Implementation This is the mode of a satellite used in the

broadcast satellite service (BSS).

The introduction of digital technologyallows the

provider and user much greater flexibility in the

operation of a broadcast network.

By means of proprietary software in the user

terminals, different parts of the downlink can beaccessed by different subscribers according to the

programs ordered from the supplier.


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One Way (broadcast)

Implementation This form of channel selection is called

Narrowcasting.

There can be many narrowcasting groups

within a larger broadcasting area.


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Split two way (Split IP) Implementation This implementation is used when there is no normal

return channel for eg. With Ku- Band broadcast satellite

service (BSS) systems that carryInternet Traffic.

The relatively high capacity downlink stream is not

complemented by an uplink capability from the user

terminal.

If the BSS downlink is used as the download channel from

an Internet service provider, the only option the user has

for a return link is via another telecommunications channel,such as a Standard telephone line.


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Split two way (Split IP) Implementation

TheInternet Protocol (IP) is therefore splitbetween a satellite downlink (outbound)

channel and a terrestrial telephone (inbound

or return) channel; hence the term Split IP

for this implementation.


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Advantages VSAT terminal does not require a transmit

capability, which significantly reduces itscost and complexity.

Disadvantage Telephone line connection must usually be

through a modem, with a bit rate generally

restricted to 56 kbps or less.


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TwoWay Implementation A return linkis designed into the service so that two-way

communications can be set up over the same satellite,

from the hub to the user and from the user back to thehub.

The VSAT/WLL implementation in fig. 9.2 is a two way

service between the hub in this case the satellite gatewayand any VSAT terminal.

The architecture selectedis the key to the economics of

twoway connections; it can be eitherMesh or Star.

These two architectures are illustrated in fig. 9.5a,b with the

topology as viewed by the satellite shown in fig. 9.6 a,b.


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Initially VSAT architectures were STAR networkssince the

very low receive G/T (gain to noise temperature ratio) of

the VSATs coupled with their limited transmit

EIRP(Effective Isotropic Radiated Power), wascompensated for by using a large hub with high G/T and

EIRP.

The cost of the hub was therefore quite high and at leastfor the smaller VSAT networks, somewhat prohibitive.

This led to the concept of a shared hub, where several

networks operate through one main hub.

A high speed terrestrial data link is required between the

host computers of the networks and the hub, which

increases the cost of the network.


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Rather than have one large hub for all of the

VSAT networks sharing the same satellite, the

over all network evolved to allow each sub networkto have its own hub as soon as the economics

made it attractive.

In this way, the host computer of each VSATnetwork can be co-located with its own hub,

thus eliminating the cost of the interconnection

between the hub earth station and the

computer controlling the service offered

through the VSAT network.


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Whether the hub is shared or dedicated on

the one hand or the VSAT is connected to a

single user or a local area network (LAN)with multiple users sharing access through

an Ethernet connection on the other, in

every case there will need to be an accesscontrol protocol.


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DTH Satellite Television The Direct to home Satellite Broadcast system (DBS/DTH)

enables viewers to receive many channels of high

quality TV programmes via high powered KU-BANDsatellites.

There are no distribution cables but the receiver

connects through MODEM to the customers telephoneline for communication with the subscription service

computer for billing purposes.

Thus the DTH system does not need a cable networkthat at present distributes TV signals to customers.

The data is compressed till significant digital artifacts

appear.


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Programmes containing frames with many fast moving

objects, like a game of basket ball , can be

compressed 3 or 4 to a transponder of 36 MHz channel

width.

Programmes containing mostly large still images can

be accommodated to about 5 to 6 to a transponder.

Movies can be compressed perhaps up to 7 or 8 to one

transponder by reducing frame rate and in some cases

even by not including interlacing lines which

otherwise is a standard practice to reduce flicker andimprove resolution.


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For DTH service, Ku-band frequencies are preferred

because these are not prone to interference from

ground point to point communicationand also need

much smaller diameter dish antenna.

Low powered Ku- band satellite transmit in the 11.7 to 12.5

GHz range while the new high powered versions are

assigned 12.2 to 12.7 GHz band for efficient operation ofDTH transmissions.

Such satellites need receiving dish antennas of diameter

between 50 to 90 cm depending on the location ofreceiving site in the satellites foot print, area.


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Receiver Front Panel: The different operating controls and

buttons provided on it are:

A. The ON/Off switch: when set on OFF enables aflashing light close to the message to indicate that

necessary information has been sent to the customer

service centre.

B. TV/DTH or TV/DSB buttonswitches between the

satellite signal IN and the terrestrial antenna or cable

network IN as desired on turning ON the receiving.

C. Four cornered arrows to point to different items in the

menu system and program guide and also for channel

surfing while viewing video.


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A. On pressing the MENU button, a list of availableprogrammes appears on the TV screen.

B. The Select/Display provisionis for selecting items out

of the program guide and menu charts.

C. Access (SMART) card slotaccepts the smart card. It is

a credit card sized device which on plugging is the slot

allows the decoder to receive authorized programming.


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Receiver Back Panel It has both input and output connecting points.

Theinput connection include :

Jack for connecting phone line to DTH receiver.

Satellite IN and

N from antenna or cable

The receiver outputs areOut to TV with modulation on CH 3 or 4 ;

S videowhich connects Super Video to the compatibleTV set or VCR; Video signal to a monitor type TV or

VCR; audio output L/R for stereo sound and wide BankData designed for use in conjunction with future

technology such as High Definition TV(HDTV) and

Interactive TV etc.


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The 12.2 to 12.7 GHz bandwas set aside for exclusive

use byDBS-TV satellites in geostationary orbit so that

high power transponders could be used on specially

designed DBSTV satellites.

Typical transponder output levels are 100 to 240 Wwith

flux density at the earths surface up to-105 dB/W/m2.

The satellites can carry up to 32 transponders, giving a

total transmitted RF power up to 3.2 kW, higher than for

any other commercial.

DBS satellites are typically large and heavy, generally use

a three axis stabilizeddesign, and have large solar sails

to generate the DC power required by the transponders.


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The flux density at the earth surface produced by

medium and high power transponder used on DBS

satellites is in the range-105 to -115 dB W/m2,which

allows small receiving antennas dishes to be used for DBSTV reception, withdiameters in the range 0.450.9 m.

The small receiving antenna has a wide beam, typically4

to a 0.45m dish,which forces wide spacing of DBSTVsatellites to avoid interference at the receiving antenna by

the signals from adjacent DBS-TV satellites.

Each DBS TV satellites carries up to32 high powertransponders covering part of the 12.2 to 12.7 GHz

broadcast satellite band (BSS)and the satellites at each

orbit location transmit in opposite hands of circular

polarization (CP).


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Signals with opposite hands of circular polarization are orthogonal, and a

suitably designed earth station antenna can separate two signals with opposite

hands of circular polarization.

In simplest DBS TV terminal ,an electronically controlled polarizer is usedimmediately behind the antenna feed.

The polarizer can be set to receive LHCP (left hand circular polarization)

or RHCP( right hand circular polarization) by changing the voltage

supplied to the low noise block converter (LNB) unit at the antenna.

Typically,a supply voltage of 7 V will cause the antenna to receive one

polarization and reject the other. Increasing the voltage above 14 V

causes the antenna to switch polarizations.

The polarizer converts the circularly received polarization signal to a

linearly polarized signal in a section of waveguide, and a linear probe in

the waveguide converts the signals to currents that drive the LNA input.


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More complex receivers use an orthogonal mode

transducer(OMT) with two LNBs so that both hands of

circular polarization can be received at the same time by

using two LNBs and two receivers.

A dual channel DBSTV receiver with a dual- polarizedantenna allows two TV channels to be viewed by

different people in the same household at the same time.

Reception from two satellites spaced 9 apart in GEOcan be achieved with a larger antenna, 0.45 X 0.6 m

that produces two beams separated by the appropriateangle.


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DBS TV i


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DBS- TV receiver The entire front end of the receiver is located at the

antenna feed in the form of an LNB :

to minimize loss of signal and

hence to maintain the lowest possible system noise

temperature.

the electronic polarizer is switched by changing the

voltage supplied to the LNB via the cable that

interconnects the antenna and set-top receiver.

The entire 12.212.7 GHz band is down converted bythe LNB to the 900-1400 MHz,where cable losses are

much lower than at Ku band.


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The down converter consists of a dielectric resonator

local oscillator and mixer, followed by an IF amplifier

and band pass filter.

The high gain LNB can drive 100m of coaxial cable

without any reduction in signal quality.

Where longer cable runs are needed, amplifiers for the900-1400 MHz band can be used to boost the signal

strength.

The set top box accepts the entire 500-MHz band andseparates out the individual transponder frequencies.

Any one of these frequencies can be selected on demand

by the user.


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The user enters the desired channel no. into the set top

box using as IR remote control, for eg. 363, which is

converted via a stored look up table in the receiver to

an RF channel frequency and polarization.

The signal from the required transponder is then

selected by the receiver by setting the correct

polarization at the antenna and tuning the set top localoscillator to the appropriate IF channel frequency.

The QPSK signal is then demodulated. The result is a

multiplexed bit stream, typically at a bit rate up 40Mbps, which contains the bits for channel 363 and

several other video signals.


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The bit stream is encrypted and contains error

control coding bits and data bits. The bit

stream is processed to correct and detect

errors, deinterleaved , and decrypted.

A digital de-multiplexer then extracts the bits

for the wanted channel, 362 in this eg, sendsthem to a MPEG 2 decoder, and finally

generates analog video and audio signals with

D/A converters to drive the TV set.


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The look up table in the receiver that relates channel numbers to

frequencies, polarizations, and instructions for the TDM de-

multiplexer is downloaded from the satellite on a regular

schedule.

This allows the service provider to change the transponder which

carries a particular signal, and to alter the mix of signals on a

given transponder as required, without the customer being aware

of the changes.

The satellite is used to address individual receivers and to load

another look up table that specifies which channels the user is

authorized to receive.

If the user fails to pay his or her bills to the service provider, the

receiver will eventually be instructed to show only a message that

it has been disconnected for failure to make timely payment for

the service.


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This process involves a smart card, which identifies each

receiving system and enables decryption of the satellite signals.

The high level of protection applied to the DBS-TV signals isintended to prevent unauthorized reception by users who have

not paid monthly fees.

Hackers have reportedly broken the encryption system of Direct

TV from time to time by reprogramming smart cards, but DirectTV retaliated in early 2001, disabling the pirated cards and

shutting down nonpaying viewers.

Pay per view channels are handles differently from broadcast

channels. A customer wishing to buy a movie or a sporting eventselects the desired channel and authorizes the system to make a

charge.


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The Direct TV receive terminals have no uplink capability, and

must therefore use terrestrial telephone circuits to send charging

information to the central office of the TV service provider.

The cost of the pay per view event is then added to the customersmonthly bill. This requires a connection between the DBS-TV

receiver and the PSTN at the customers premises.

The receiver dials a toll free no. , and down loads a record of thecharges for that customer, and any other information that the

receiver is programmed to deliver to the service provider.

Such information might include the pattern of channels that the

customer selects and watches , which is valuable data foradvertisers.

Using the satellite to convey instructions to the receivers brings

some notable advantages to the DBS-TV customer.


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A customer who wishes to change the

level of service he or she receives need

make only a single phone call to obtain aservice upgrade.

The customer owns the DBS-TVreceiving equipment and is responsible

for its maintenance, so there are no

service calls by satellite TV providers.

Unit 6 Non Geostaitonary Orbit Satellite System

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