Satellite CommunicationFreeha Azmat

Configurations:1)star topology:

local telephone networkFailure at one point will cause a problemRedundancy is provided at central point

2)Mesh NetworkEach node is connected to every other nodeEncouraged for high capacity systems for security

purposesComplete connectivity with built in redundancyExpensive approachA hub must control the communication setup tear down

process, but need not be involved in carrying traffic.

Satellite Communication

Configurations

Components of Communication SatelliteEvery satellite involves the transmission of

information from An originating ground station to the

satellite(uplink)Retransmission of information from the satellite back

to ground(down link)Satellite must have

Receiver and receive antennaTransmitter and transmit antennaMethod for connecting the uplink to the down link

for retransmissionPrime electrical power to run all of the electronics

Basic ElementsSatellite or Space Segment: Besides satellite, the space segment includes the means

on Earth necessary for launching and station keeping2-way as in the case of long distance callGround stations' uplink is down linked over a wide

region’s in TV broadcast.

Ground Station ,or Earth Segment The earth segment includes earth stations containing

transmitters and receivers for transmission and reception of signals from satellites Transmitters Antennas

Basic Elements

Transmitters:The amount of power , which a satellite transmitter

needs to send out ,depends a great deal on whether it is in low earth orbit or in geosynchronous orbit. Geo synchronous satellite …..altitude of 22300 miles

while the low earth satellite is only a few hundred miles

Antennas :One of the biggest difference between a low earth

satellite and geosynchronous satellite is in their antennas.

Difference in power can be brought by changing the antenna design

The easiest way is to make antenna larger.

Power GenerationThe satellite must generate all of its own

power on the spacecraftPower is usually generated by large solar

panels covered with solar cells (sunlight electricity)

When there s no sunlight, the satellites requires the batteries on board which can supply the required power for the necessary time and then recharge by the time of the nest period of eclipse

Satellite Components

Basic Satellite systemsTransponder

A satellite that merely relays the uplink carrier as a downlink is referred to as a relay satellite , or repeater satellite.

The satellite transmits the downlink by responding to the uplink.

CrosslinkCommunication between two earth stations not visible

to the same satellite.By spacing multiple satellites in proper orbits around

the earth, worldwide communication between remote earth stations in different hemispheres can be performed.

Ground Relay:Satellite relay system involving earth stations,

near earth users( aircrafts, ships etc) and satellites

An earth station communicates to another earth station or to a user by transmitting to a relay satellite ,which relays the modulated carrier to the user.

Basic Element Systems

Satellite Repeater A satellite repeater is an electronic assembly

,which ensures the following functions:1)Amplification of signals: from an input

power in the order of -100dbW to an output power of about 10dbW.

2)Frequency Gain: which avoids interference between the powerful transmitted signal and the weak incoming signal.

Communication Repeaters1.Transparent Repeaters: Transparent repeaters in which the uplink

signals are translated in frequency ,but otherwise unchanged in form. The transparent repeaters which are widely used in all types of communications spacecraft, are relatively simple to design and use and are well suited to many system applications

2.Processing repeaters : are those in which the uplink signals are changed in form by de modulation ,correlation ,or some other complex process.

Processing repeaters are used to provide enhanced performance characteristics such as an increased immunity to interfering signals or increased communications capacity.

Transparent Repeater

Regenerative Repeaters

Up conversion and Down ConversionRF signal and IF signals: RF signal is coming from an antenna. So it

contains electromagnetic waves. But when the signal is processed using electronics circuits, the range of frequency may not be sufficient. So that RF has to be convert to some other frequency acceptable by the circuit. And IF is the intermediate frequency used for tuning & other purpose.

Up conversion and Down Conversion Up conversion is the translation of

intermediate frequency to RF frequency while Down conversion is translation of RF

frequency to intermediate frequency

(a)Up Conversion (b)Down Conversion

Low Noise AmplifierLow noise amplifier is a non-linear device as

its power gain characteristics are nonlinear. Two parameters used to describe the degree of non-linearity are1-db compression pointThird order intercept point

Low Noise Amplifier

Noise Figure LNA (Low noise Amplifier) is a special type

of amplifier used in communication systems to amplify very weak signals captured by an antenna.

LNA is often located very close to an antennaIt is a key component which is placed at the

front-end of a radio receiver circuit. Noise figure =(S/N)in –(S/R)out

High Power AmplifierOne of the key elements of any spacecraft transponder is the

output transmitter, consisting of a high power amplifier(HPA) TWT Amplifier characteristics: For maximum power efficiency ,it would be desirable to work at

the peak of the curve(saturation),but for linearity it is necessary to operate at a lower point.

When only a single input carrier is involved ,this saturation causes no carrier distortion, but only a limitation to its output power.When the TWTA input corresponds to multiple carriers ,however as in FDMA format, this non-linearity of the saturation effect cause inter-modulation products(beats) among these carriers.

These beat terms produce Additional frequency components at the non-linear output that can interfere with the desired carriers.

High Power Amplifier