Satellite Network communication

SystemBY

ENG .. Mohamed Ahmed El Sadany

Main points History of satellite system . Usage of satellite . Types of Satellite Orbits. Coverage area of satellites. Bands of frequency. Look Angles . Transmitter system . Satellite system . Receiver system .

History The Moon is the 1st passive reflector and in the late of

1940 to 1950 , became the1st satellite transponder . In 1954 , U.S Navy successfully transmitted the 1st

message over this earth-to-moon-to-earth relay . In 1957 , Russia launched the first active satellite

(Sputnik1) , it transmitted telemetry information for 21 days .

Later in 1957 , US launched (Explorer 1) , it transmitted telemetry information for nearly 5 months .

In 1958 , NASA launched (Score) , it rebroadcasted President Eisenhower’s Christmas message .

In 1962 , American telephone and telegraph company (AT&T) launched (Telstar 1) ,the first simultaneously satellite .

Usage

Types of Satellite Orbits1. Based on the inclination , i, over the

equatorial plane.› Equatorial Orbits above Earth’s equator (i=0°)› Polar Orbits pass over both poles (i=90°)› Other orbits called inclined orbits (0°<i<90°)

Equatorial Polar Inclined

Types of Satellite Orbits

2. Based on Eccentricity :› Circular with centre at the earth’s centre.› Elliptical with one foci at earth’s centre .

Types of Satellite Orbits

3. Based on the Satellite Altitude: a- GEO-Geostationary Orbits b- MEO – Medium Earth Orbits c- LEO – Low Earth Orbits

Types of Satellite Orbits

Geo –Geostationary Orbit The most common type of communication satellite

today is Geostationary satellite(GEO). GEO satellites remain in the same position relative

to the surface of earth. If the satellite is in the circular orbit 35863 KM

above the surface of the Earth and rotates in the equatorial plane of the earth and rotates at the exactly same speed of earth then that satellite is referred as GEO satellite.

In GEO there is no problem with signal (freq) changes due to relative motion of satellite and antennas on the earth.

Geo –Geostationary Orbit Tracking of satellite by its earth station is very

simplified and easy. GEO satellites have a 24 hour view of a particular area.

Three satellites in Geostationary orbit separated by the angle of 120 degrees can cover the entire earth except the areas near south & north poles.

A GEO satellite’s distance from earth gives it a large coverage area.

The signal might be weak after travelling 35863 KM. Delay in the signal beneath and back of the satellite. The polar regions might poorly covered by the GEO

satellites.

MEO – Medium Earth Orbits A MEO satellite is in orbit somewhere between 8,000

km and 18,000 km above the earth’s surface. MEO satellites are similar to LEO satellites in

functionality. MEO satellites are visible for much longer periods of

time than LEO satellites, usually between 2 to 8 hours.

MEO satellites have a larger coverage area than LEO satellites.

Low Earth Orbit (LEO) LEO satellites are much closer to the earth than GEO

satellites, ranging from 500 to 1,500 km above the surface.

LEO satellites don’t stay in fixed position relative to the surface, and are only visible for 15 to 20 minutes each pass.

A LEO satellite’s proximity to earth compared to a GEO satellite gives it a better signal strength and less of a time delay, which makes it better for point to point communication.

A LEO satellite’s smaller area of coverage is less of a waste of bandwidth.

Low Earth Orbit (LEO) Large constellation of satellites is needed for continuous

communication (66 satellites needed to cover earth). LEO satellites have to compensate for Doppler shifts

cause by their relative movement. Atmospheric changes effect the LEO satellites,

causing gradual orbital distortion

Coverage area of satellites Every satellite have a footprint or

coverage area , it is differenced between each satellite and other .

it means the area that this satellite coverage it by it’s beam and in any difference position we can’t transmit or receive any signal to this satellite .

Coverage area

W2A :: 10° EAST

Frequency Bands ISSUES up-LINK(GHz) DOWN-LINK

(GHz)BAND

Interference with ground links

6 (5.925-6.425)

4 (3.7-4.2) C

Attenuation due to rain

14 (13.7-14.5) 11 (10.7-12.7)

Ku

High Equipment cost 30 (27.5-30.5) 20 (17.7-21.7)

Ka

Look Angles To orient an earth station antenna

toward a satellite , it is necessary to know the Elevation angle and Azimuth angle.

Angle of Elevation It is the angle formed between the

plane of a wave radiated from an earth station antenna and the horizon .

Angle of Azimuth It is defined as the horizontal pointing

angle of an antenna , it is measured in an clockwise direction in degrees from true north .

Uplink Model It is the model of the transmitter system towards the satellite

, it contains 4 steps :1. Encoding .2. Modulation .3. Up Converter .4. Power Amplification .

Encoding is an operation to convert audio and video to a digital stream of 0’s and 1’s.

Modulation is an operation to modulate the signal in 4MHz frequency range with a carrier signal in 70MHz frequency range by change in one only ch\s of the signal (Frequency , Amplitude or Phase ).

Up converter is an operation to convert the frequency from the IF range to the SHF range by multiplexing the freq, by Local oscillator freq Power Amplification is an operation to amplify the signal power to can reach to the satellite.

PSK,QPSK,QAM or

8PSK

Uplink Model

Modulator BPFIF

X

LO (mw generator)

BPF

Up converter

HPA

Free spacelosses

Otherlosses

Antenna

Satellite Transponderreceive section

RFEnc, signal I/P

Satellite Transponder It is an RF to RF repeater A typical satellite

transponder consists of :1. Band Pass Filter (BPF) .2. Low Noise Amplifier (LNA). 3. Frequency Translator .4. Low –Level Power Amplifier .

BPF is an electronic device that can make limiting to the signal and pass a determine band frequency.LNA is a tunnel diode.

Freq, Translator is a shift oscillator and BPF which converts the high band up link freq, to low band down link freq. ,

Low –Level Amplifier is a travelling wave tube that amplify the RF signal for transmission through antenna to earth

station .

Satellite Transponder

BPF

LNA

X

LO

Filter LPA

Frequency translator

Mux

Towards Earth Station

From Earth Station

Downlink Model

It is the model of the receiver system towards the earth station , it contains 5 steps :

1. Band pass filter(BPF).2. Low Noise Amplifier (LNA). 3. Down Converter .4. Demodulator .5. Decoding .

Down converter is an operation to convert the frequency from the RF range to IF range by multiplex the freq, by

Local oscillator freq. ,Demodulator is an operation to demodulate the signal in 70MHz frequency range to the original signal in 4MHz frequency range.

Decoding is an operation to have on the signal that was sent to sat, in the original form audio and video.

Downlink ModelSatellitetransmitter

Free spacelosses

Otherlosses

X

LO

BPF LNA

BPF DemodIF

Down converter

Earth stationAntenna

Dataout

Decoding

Q & A

Thanks