Sat Com Lecture 1

8/2/2019 Sat Com Lecture 1

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Satellite Adnan Qamar 1

SATELLITE


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What exactly is a satellite?

The word satellite originated from the Latinword Satellit- meaning an attendant, one who

is constantly covering around & attending to a

master or big man.

For our own purposes however a satellite is

simply any body that moves around another

(usually much larger) one in a mathematically

predictable path called an orbit.


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Types of Satellite

Weather Satellites

Communication Satellites

Navigation Satellite

Observing Satellite


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WEATHER SATELLITE

A weather satellite

called the

Geostationary

Operational

EnvironmentalSatellite observes

atmospheric

conditions over a

large area to help

scientists study and

forecast the weather


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COMMUNICATION SATLLITE

A communications

satellite, such as

the Tracking and Data

Relay Satellite(TDRS) shown here,

relays radio,

television, and other

signals between

different points in

space and on Earth


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A navigation

satellite, like this

Global Positioning

System (GPS)

satellite, sendssignals that

operators of

aircraft, ships, and

land vehicles and

people on foot can

use to determine

their location

NAVIGATION SATELLITE


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OBSERVING SATELLITE

An Earth observingsatellite surveys ourplanet's resources.This satellite, Aqua,

helps scientistsstudy oceanevaporation and otheraspects of themovement anddistribution ofEarth's water


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How do Satellites Work?

Two Stations on Earth want tocommunicate through radiobroadcast but are too far awayto use conventional means.

The two stations can use asatellite as a relay station

for their communication.

* One Earth Station transmitsthe signals to the satellite.Up linkfrequency is thefrequency at which GroundStation is communicating withSatellite.

* The satellite Transponderconverts the signal and sendsit down to the second earthstation. This frequency iscalled a Downlink.


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Transponders

The transponder is the brains of the satellite - provides the

connection between the satellites receive and transmit antennas.

Satellites can have 12 to 96 transponders plus spares, depending on

the size of the satellite.

A transponder bandwidth can frequently be 36 MHz, 54 MHz, or

72 MHz or it can be even wider.

A transponders function is to

Receive the signal, (Signal is one trillion times weaker then when transmitted) Filter out noise,

Shift the frequency to a down link frequency (to avoid interference w/uplink)

Amplify for retransmission to ground


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How Satellites Work1. A Earth Station sends

message in GHz range.(Uplink)

2. Satellite Receive andretransmit signals back.(Downlink)

3. Other Earth Stationsreceive message inuseful strength area.(Footprint)


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Launching the satellite

Space shuttles carry some

satellites into space, butmost satellites arelaunched by rockets thatfall into the ocean aftertheir fuel is spent. Manysatellites require minoradjustments of their orbitbefore they begin toperform their function.Built-in rockets calledthrusters make theseadjustments. Once asatellite is placed into a

stable orbit, it canremain there for a longtime without furtheradjustment.


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How does a satellite stay in its orbit?


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Satellite frequency band

BandDownlink,

GHz

Uplink,

GHz

Bandwidth,

MHz

L 1.5 1.6 15

S 1.9 2.2 70

C 4 6 500

Ku 11 14 500

Ka 20 30 3500


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Satellite Frequency Bands andAntennas (Dishes)

The size of Satellite Dishes (antennas) arerelated to the transmission frequency.

There is a inverse relationship betweenfrequency and wavelength.

As wavelength increases (and frequency

decreases), larger antennas (satellite dishes) arenecessary to gather the signal.


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Satellite Frequency Bands andAntennas (Dishes)

C-Band Ku-Band

Most commonly used bands: C-band (4 to 8 GHz) , Ku-band (11 to 17 GHz) , and Ka-band (20 to 30 GHz ).


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Classification of orbits:

* Circular orbits are simplest

* Inclined orbits are useful for coverage of

equatorial regions

* Elliptical orbits can be used to givestationary behavior viewed from earth using 3

or 4 satellites

* Orbit changes can be used to extend the life of

satellites


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Categories of satellites

Satellite orbits are also classified

based on their heights above the earth:

LEO(Low earth orbit satellite)

MEO(Medium earth orbit satellite)

GEO(Geostationary orbit)


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Satellite orbit Altitudes


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Low-Earth-Orbit (LEO) Altitude (375-1000 miles)

Revolution time: 90 min - 3 hours.

Advantages:

Reduces transmission delay

Eliminates need for bulkyreceiving equipment.

Disadvantages:

Smaller coverage area.

Shorter life span (5-8 yrs.)than GEOs (10 yrs).

Subdivisions: Little, Big, and Mega(Super) LEOs.


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Low Earth Orbits Satellite Arrays Satellite arrays

Launching a set of

satellites into lowearth orbits

Each point in groundhas at least onesatellite overhead

Satellites in an arraycommunicate withone another


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In LEO polarorbits are used

Altitude:500 to2000 km

Period:90 to 120min

Speed:20,000 to25,000 kmph

Footprint:8000km

diameter

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Little LEOs Applications

0.8 GHz range

Small, low-cost

Vehicle tracking,environmental

monitoring and two-waydata communication.Used for short,narrowbandcommunications.


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Big LEOs Applications 2 GHz or above range

Can offer global services, whichcan be subject to regulatoryrequirements.

Used for technology devices suchas high-speed, high-bandwidthdata communications, and videoconferencing. They carry voice

and high-speed data services.The main uses are datacommunications and real-timevoice delivery to hand-helddevices.


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Mega (Super) LEOs Applications

20-30 GHz range

Mainly handles broadband

data. These systems areoptimized for packet-switched data rather thanvoice. They share the sameadvantages and drawbacks

of other LEOs and areintended to operate withinter-satellite links tominimize transmission timesand avoid dropped signals.


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Example of LEO:

Iridium sytem:A project started by

Motorola in the year 1990 with theobjective of providing worldwide Voiceand Data communication

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Middle-Earth-Orbiting (MEO)

MEOs orbits between the

altitudes of 5,600 and

9,500 miles.

These orbits are primarily

reserved for communications

satellites that cover theNorth and South Pole.

Unlike the circular orbit of the

geostationary satellites, MEOs are placed

in an elliptical (oval-shaped) orbit.Approximately a dozen medium Earth

orbiting satellites are necessary to

provide continuous global coverage 24

hours a day.


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Geosynchronous-Earth-Orbit (GEO)

Orbit is sychroneous withthe earths rotation.

From the ground the

satellite appears fixed.

Altitude is about 23,000miles.

Coverage to 40% ofplanet per satellite.


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Basics of GEOs

Geostationary satellites are commonly used forcommunications and weather-observation.

The typical service life expectancy of a geostationary

satellite is 10-15 years.

Because geostationary satellites circle the earth at theequator, they are not able to provide coverage at theNorthernmost and Southernmost latitudes.


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GEOs and Weather

The altitude is chosen sothat it takes the satellite24 hours to orbit theEarth once, which is alsothe rotation rate of theEarth.

This produces the cloudanimations you see on TV.

Can take imagesapproximately everyminute.


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GEOs

Satellites arepositioned every 4-8degrees.

Aproximately 300 GEOsatellites are in orbit.


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Apogee & Perigee

Apogee is the point at

which a satellite in an

elliptical orbit is

furthest from the Earth.

At its apogee, the

satellite travels slowerthan at any other point in

its orbit.

Perigee is the point at

which a satellite in an

elliptical orbit isclosest to the Earth.

At its perigee, the

satellite travels faster

than at any other point in

its orbit.

Definition of terms for earth


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Definition of terms for earth-orbiting satellite

ApogeeThe point farthest from earth.Apogee height is shown as hain Fig

PerigeeThe point of closest approach toearth. The perigee height is shown as hp

Line of apsidesThe line joining the perigeeand apogee through the center of the earth.

Ascending nodeThe point where the orbitcrosses the equatorial plane going from south

to north. Descending nodeThe point where the orbit

crosses the equatorial plane going from northto south.

Line of nodesThe line joining the ascendingand descending nodes through the center ofthe earth.

InclinationThe angle between the orbitalplane and the earths equatorial plane. It is

measured at the ascending node from theequator to the orbit, going from east to north.The inclination is shown as iin Fig.

Mean anomaly M gives an average value ofthe angular position of the satellite withreference to the perigee.

True anomaly is the angle from perigee tothe satellite position, measured at the earthscenter. This gives the true angular position of

the satellite in the orbit as a function of time.

D fi iti f t f


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Prograde orbitAn orbit in which the satellite moves inthe same direction as the earths rotation. Theinclination of a prograde orbit always lies between 0and 90.

Retrograde orbitAn orbit in which the satellite movesin a direction counter to the earths rotation. Theinclination of a retrograde orbit always lies between 90and 180.

Argument of perigeeThe angle from ascending nodeto perigee, measured in the orbital plane at the earthscenter, in the direction of satellite motion.

Definition of terms forearth-orbiting satellite


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Components of a satellite

There are 3 major

components in a

satellite, they are :

Transponder andantenna system-

Power Package-

Control and

information system &

rocket thrustersystem


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Basics: Advantages of Satellites

The advantages of satellitecommunication over terrestrial

communication are: The coverage area of a satellite greatly

exceeds that of a terrestrial system.

Transmission cost of a satellite is

independent of the distance from thecenter of the coverage area.

Satellite to Satellite communication is veryprecise.

Higher Bandwidths are available for use.

Disadvantage


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Disadvantage

The disadvantages of satellite

communication: Launching satellites into orbit is costly.

Satellite bandwidth is gradually becoming

used up. There is a larger propagation delay in

satellite communication than in terrestrialcommunication.

Security can be an issue


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Sat Com Lecture 1

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