By SSadhish Prabhu INTRODUCTION Cost to build and launch a GEO satellite is about 25000 dollars per kg Weight is the most critical factor in any design Dimension of the satellite dia ID: 555862
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Slide1
SATELLITE LINK DESIGN
By
S.Sadhish
Prabhu
Slide2
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 Slide3
Factors influencing system design
Weight of the satellite is driven by two factors
The number and the output power of he transponder on the satellite (requires large power from solar cells which in turn increases the weight )
Weight of the station keeping fuelSlide4
Factors influencing system design
The choice of frequency band
Atmospheric propagation effects
Multiple access techniques Slide5
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 Slide6
Basic transmission theory
Objective :
Calculation of the power received by an earth station from satellite transmitter
Two approaches for calculating :
Use of flux density
Link equation (
Friis
transmission equation
)Slide7
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
4
p
R
2
Flux density at distance R is given by
W/m
2
Equ
4.1Slide8
Isotropic Radiator
W/m
2
Pt
Watts
Distance R
Isotropic Source
Power Flux Density:
Surface Area of sphere =
4
p
R
2
encloses
Pt.Slide9
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(
) is variation of power with angle.
G(
) is gain at the direction .
P
0
is total power transmitted.
sphere =
4
p
solid radians
(
Eqn
4.2)Slide10
Antenna GainAntenna 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)Slide11
Flux density The flux density in the direction of the antenna
boresight
at distance R meter is
W/m
2Slide12
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. angleSlide13
EIRP
Receiver
Received
power P
t
Incident flux disunity, F
Pt
Watts
Isotropic Source
Receiving antenna area , A gain G
t
R
For an ideal receiving antenna with an aperture area of Am
2
,
P
r
= FASlide14
EIRPA antenna with physical aperture area of A
r
m
2
will not deliver the power
Thus the efficiency is reduced
It is descried by using effective aperture
A
e
A
e = ηAr
(4.5)
Where
η
– aperture efficiency of the antenna
Thus (4.6)
2
P
r
=
Slide15
Fundamental of antenna theory
2
(4.7)
Sub
A
e
in (4.6)
2
This expression is called as the
Friis
transmission equation Slide16
Contd..
In decibel term
Where,
EIRP = 10 log
10
(
P
t
G
t
)
dBW
Gr
= 10 log
10
dB
Lp
– path loss = 20 log
10
dB
2
(4.10)
(4.9)Slide17
In general Pr =
EIRP+G
r
-
L
p
-L
a
-
L
ta-Lra dBW (4.11)
Where
L
a
= attenuation in atmosphere
L
ta
= losses associated with transmitting antenna
L
ra
= losses associated with receiving antenna Slide18
Reference of dBSlide19
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 10m
2Slide20
Problem # 2A satellite operates at a frequency of 11 GHz. The receiving antenna has a gain of 52.3 dB, Find the received power. Slide21
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=P
rSlide22