Satellite Communications

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Satellite Communications

Adisak Jaisilp

Department of Electrical Engineering Faculty of Engineering Chiangmai University

Origin of Satellite Communications

• Arthur C. Clark

(1945)

‐

British Science fiction writer

propose the idea of

“a synchronous satellite communication”

3 satellites in an equatorial orbit,

radius of about 42,242 km.

spaced 120  apart from each other

could cover the whole world to transmit and

receive signals that relayed between the satellites

Brief History

• 1957

SPUTNIKS

USSR

• 1958

SCORE

USA

• 1965

Early Bird or

USA

INTELSAT 1

The first commercial satellite • Satellite Communications System is growing so rapidly • Difficult to summarize the list of satellites in space • Satellite positions are assigned by ITU (International Telecommunication Union) which is part of UN

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Components of Satellite Communications

Frequency Band Designations

Frequency range , GHz Band Designation 0.1‐0.3 VHF 0.3‐1.0 UHF 1.0‐2.0 L 2.0‐4.0 S 4.0‐8.0 C 8.0‐12.0 X 12.0‐18.0 Ku 18.0‐24.0 K 24.0‐40.0 Ka 40.0‐100.0 mm

Satellite Orbit

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Kepler’s First Law

radius of orbit r

0 p = semilatus rectum = h2_/  e = eccentricity = h2_C /  h = magnitude of angular momentum vector(constant)  = Kepler’s constant = 3.98x105_km3_/ s2 C = Boundary condition(constant) For e < 1 , the orbit is an ellipse e = 0 , the orbit is circle

Kepler’s Second Law

• The differential area swept out by vector r

0

dA = 0.5 h dt

• The radius vector to the Satellite sweeps

out equal areas in equal times

Kepler’s Third Law

• The orbital period T

• The square of the period of revolution is

proportional to the cube of the semimajor axis

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Types of Orbits

• Polar Orbit

– Circular movement from N to S, LEO or MEO – Navigation, Weather Forecasting, Resources Surveying

• Inclined Orbit

– Elliptical inclined orbit, LEO or MEO – Navigation, Weather Forecasting, Resources Surveying

• Equatorial Orbit

– Lies in equatorial plane and appear stationary relative to the earth – Geosynchronous orbit or geostationary orbit – Most widely used and more larger

Orbital Perturbation

• Effects of a nonspherical Earth

‐

The earth’s gravitational potential is not uniform

‐drift toward and circulate around the nearer of

longitudes of 105 W and 75 E

called “graveyards”

(they collect old satellites)

• Effects of the Sun and Moon

‐Gravitational attractions change the orbital

inclination, the Moon > the Sun

Orbital Effects in Communication

• Doppler Shift

‐The change in transmitting and receiving frequency

due to the different of their velocities

‐negligible for geosynchronous satellites

‐quite effected for low‐earth orbit satellites

‐compensated by tracking in narrow receiver

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Orbital Effects in Communication

• Eclipse

‐earth prevents sunlight from reaching satellite

‐

no power from

its solar cell

‐use batteries

‐

stop using some

transponders

Orbital Effects in Communication

• Sun Transit Outage

‐

satellite stay between the sun and the earth

‐

noise rise up

and interfere

normal operation

‐using other

satellites to help

‐prevent from losing

itself due to

no control signal

Satellite Launching Methods

• 1. ELV (Expendable Launch Vehicles)

• Delta (USA) • Ariane (Franc

e)

# put satellite on

transfer orbit

from 300 km to

their real orbit

 38,600 km

by its AKM

(apogee kick motor)

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Satellite Launching Methods

• 2. STS (Space Transportation System)

– The Space Shuttle

– by NASA

– 296 km circular orbit

– put satellite to transfer orbit

using PAM

(Payload Assist Module)

using AKM for geosynchronous

orbit

The Steps in Launching

Types of Satellites

• 1. Spinner

Cylindrical satellite

spin around the axis

that is parallel to

the N‐S axis

50‐100 rev/min

decrease disturbance

torques

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Types of Satellites

• 2. The three‐axis stabilization

‐

stabilizing for each

of the three axis

“body stabilization”

‐3 flywheels react

disturbance torques

from each axis

Controlling the orbit and attitude

• Telemetry Tracking and Command (TT&C)

• Telemetry

Collect data from many sensors and send to earth station

• Tracking

observe and determine position of satellite

• Command

message from earth station to control attitude,

position (using gas jet to correct them)

and control all communication systems

Antennas for Earth Station

• 1. Offset Paraboloidal Refector Antenna

‐feeder is on the

focal point of the dish

‐suitable for small

diameter antenna

‐often used in direct

satellite TV

such as UBC

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Antennas for Earth Station

• 2. Cassegrain

Antenna

‐very large

‐dia. up to 30 m.

‐high gain

‐suitable for

control station

Signal Impairments

• Propagation Impairment

‐

Rain Attenuation

‐

Atmospheric Losses ‐Refraction ‐Signal Scintillations ‐Reflection Multipath ‐Propagation Delays ‐Intersymbol Interference ‐Signal Depolarization ‐Interference • Physical Cause

‐

rain and cloud

‐

O₂, H₂O ‐Atmospheric gases ‐Refractivity Fluctuations ‐Objects on the earth surfaces ‐Distance ‐Ducting, Scatter, Diffraction ‐Ice Crystals ‐Signals from other systems

Types of Interference

Types

• A

• B

• C

• E

• F

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Satellite Access

• 1. Frequency Division Multiple Access (FDMA)

assign specific frequency for each earth

station

Satellite Access

• 2. Time Division Multiple Access (TDMA)

assign time slot for each earth station

Satellite Access

• 3. Code Division Multiple Access (CDMA)

each earth station use its own code

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VSAT

• Very Small Aperture Antenna

• Diameter of antenna  2 meters

• Such as UBC

MSAT

• Mobile Satellite

‐Transportation

‐Logistic

‐TV station

GPS

• Global Positioning Satellite System

‐Navigation

‐Mapping

‐Surveying

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Satellite for Earth Observation

• Natural Resource Surveying

• 2 Technologies

– Remote Sensing

• Using electromagnetic spectrum • Need data interpretation

– Camera

• Real picture • Such as THEOS – Thailand Earth Observation System

Meteorological Satellite

• Weather Forecasting

– Cloud, Rain, Wind, Temperature and Turbidity

– Various effects in atmosphere

– Ozone

• For

– Disaster prevention

– Aviation

– Education

Video Conference via Satellite

• Distance meeting

• Distance learning

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More about Orbits

• GEO

Geostationary orbit

– Satellite appears stationary related to the earth – 35,800 km., equatorial plane

• MEO

Medium‐earth orbit

– 10,000 km., Inclined orbit or polar orbit

• LEO

Low‐earth orbit

– 1,000 km., Inclined orbit or polar orbit

• Sun‐synchronous orbit

– Remains fixed relative to the sun – Orbit plane incline with constant angle to the sun

Satellite Systems in Thailand

• THAICOM

– THAICOM 1A : Spinner, 120E, for TV, Telephone,Cable TV

– THAICOM 2: Spinner, 78.5E, for TV, Telephone, Cable TV

– THAICOM 3: 3‐axis stabilization, damage => released

– THAICOM 4: IPSTAR, 3‐axis, 119.5E, more efficiency due

to frequency reused, for high speed Internet

– THAICOM 5: 3‐axis, 78.5E, instead of THAICOM 3, most

users are cable TV

Satellite Systems in Thailand

• THEOS

‐Thailand Earth Observation Satellite

– Owner = GISTDA: Geo‐Informatics and Space Technology Development Agency – LEO ‐Low Earth Orbit and Sun‐synchronous Orbit – 5 years lifetime and 822 km. from surface – Time period in 1 round = 101.46 minutes – Use for ‐natural resource management ‐agriculture surveying – 2 cameras • Panchromatic Telescope(B&W) resolution 2 meters • Multispectral Camera(color) resolution 15 meters