SATELLITE LINK DESIGN

By S.Sadhish Prabhu

INTRODUCTION

• Cost to build and launch a GEO satellite is about 25,000 dollars per kg

• Weight is the most critical factor in any design• Dimension of the satellite : dia must be less

than 3.5m• Antennas are the limiting factor

Factors influencing system design

Weight of the satellite is driven by two factorsI. The number and the output power of he

transponder on the satellite (requires large power from solar cells which in turn increases the weight )

II. Weight of the station keeping fuel

Factors influencing system design

• The choice of frequency band • Atmospheric propagation effects• Multiple access techniques

Performance objective• Bit error rate (BER) in a digital link• Signal-to-noise ratio (S/N) in an analog link

• BER or S/N is determined by Carrier - to- noise ration (C/N) at the input of the demodulator in the receiver

• C/N > 6 dB

Measured in base band channel

Basic transmission theory

Objective : Calculation of the power received by an earth station from satellite transmitter

Two approaches for calculating :i. Use of flux densityii. Link equation (Friis transmission equation )

Isotropic Radiator

• Consider an Isotropic Source radiating Pt Watts uniformly into free space.

• At distance R, the area of the spherical shell with center at the source is 4pR2

• Flux density at distance R is given by

24 RPF t

p W/m2 Equ 4.1

Isotropic Radiator

24 RPF t

p W/m2

Pt Watts

Distance R

Isotropic Source

Power Flux Density:

Surface Area of sphere = 4pR2encloses Pt.

Antenna Gain• We need directive antennas to get power to go in wanted

direction.• Defined as the ratio of power per unit solid angle radiated in a

direction to the average power radiated per unit solid angle

p4/

)()(0PPG

• P() is variation of power with angle.• G() is gain at the direction .• P0 is total power transmitted.

• sphere = 4p solid radians

(Eqn 4.2)

Antenna Gain

• Antenna has gain in every direction! • Usually “Gain” denotes the maximum gain of

the antenna.• The direction of maximum gain is called

“boresight”.• Gain is a ratio:• It is usually expressed in Decibels (dB)

G [dB] = 10 log10 (G ratio)

Flux density

The flux density in the direction of the antenna boresight at distance R meter is

24 RGP

Ftt

p W/m2

EIRP (Pt*Gt)• An isotropic radiator is an antenna which radiates in all

directions equally• Antenna gain is relative to this standard• Antennas are fundamentally passive

– No additional power is generated– Gain is realized by focusing power– Similar to the difference between a lantern and a flashlight

• Effective Isotropic Radiated Power (EIRP) is the amount of power the transmitter would have to produce if it was radiating to all directions equally

• Note that EIRP may vary as a function of direction because of changes in the antenna gain vs. angle

EIRP

Receiver

Received power Pt

Incident flux disunity, F

Pt Watts

Isotropic Source

Receiving antenna area , A gain Gt

RFor an ideal receiving antenna with an aperture area of Am2,

Pr= FA

EIRP

• A antenna with physical aperture area of Arm2 will not deliver the power

• Thus the efficiency is reduced • It is descried by using effective aperture Ae

Ae = ηAr (4.5)Where η – aperture efficiency of the antennaThus (4.6)R

AGP ett

p4 2Pr =

Fundamental of antenna theory

p e4 AGr

2(4.7)

Sub Ae in (4.6)

RGGP rtt

p

4Pr

2

This expression is called as the Friis transmission equation

Contd..

lossPathgainantennaceivingEIPRreceivedPower

Re

dBWLpGrEIRP )(Pr In decibel term

Where,EIRP = 10 log10 (PtGt)dBW

Gr = 10 log10 dB

Lp – path loss = 20 log10 dB

)4( e

pA

2

pR4

(4.10)

(4.9)

In general

Pr = EIRP+Gr-Lp-La-Lta-Lra dBW (4.11)

Where La = attenuation in atmosphere Lta = losses associated with transmitting antenna Lra = losses associated with receiving antenna

Reference of dB

Units Reference dBi isotropic gain antenna

dBW 1 watt dBm 1 milliwatt dBHz 1 Hertz dBK 1 Kelvin

dBi/K isotropic gain antenna/1 Kelvin dBW/m2 1 watt/m2

dB$ 1 dollar

Problem # 1

A satellite at a distance of 40,000km from a point on the earth’s surface radiates a power of 10W from an antenna with a gain of 17 dB in the direction of the observer, find the flux density at the receiving point, and the power received by an antenna at this point with an effective area 10m2

Problem # 2

• A satellite operates at a frequency of 11 GHz. The receiving antenna has a gain of 52.3 dB, Find the received power.

Answer

-126dbW for both questioNote:The received power is commonly called as carrier

power,C Because,Satellites use FM (Anlog transmission )or PM (digital

transmission)In both modulation the carrier is not changed So, C=Pr