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 factors
I. 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
Measured in
base band
channel
• 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
Basic transmission theory
Objective : Calculation of the power received by
an earth station from satellite transmitter
Two approaches for calculating :
i. Use of flux density
ii. 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
Pt
F
2
4pR
W/m2
Equ 4.1
Isotropic Radiator
Isotropic Source
Distance R
Pt Watts
Surface Area of sphere =
Power Flux Density:
4pR2encloses Pt.
Pt W/m2
F
2
4pR
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
P( )
G ( ) 
P / 4p
0
•
•
•
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
Pt Gt
W/m2
F
4pR
2
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
Isotropic Source
Incident flux
disunity, F
Receiver
Received power Pt
Pt Watts
Receiving antenna area , A gain Gt
R
For 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 antenna
PGA
Thus
(4.6)
Pr =
2
4pR
t
t
e
Fundamental of antenna theory
Gr 
4pAe

(4.7)
2
Sub Ae in (4.6)
  
Pr  PtGtGr 

 4pR 
2
This expression is called as the Friis
transmission equation
Contd..
Power received 
EIPR  Re ceiving antenna gain
Path loss
(4.9)
In decibel term
Pr  ( EIRP Gr  Lp)dBW
Where,
EIRP = 10 log10 (PtGt)dBW
Gr = 10 log10
(
4pAe

2
)dB
Lp – path loss = 20 log10
 4pR  dB


  
(4.10)
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
dBi
dBW
dBm
dBHz
dBK
dBi/K
dBW/m2
dB$
Reference
isotropic gain antenna
1 watt
1 milliwatt
1 Hertz
1 Kelvin
isotropic gain antenna/1 Kelvin
1 watt/m2
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 questio
Note:
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