SATELLITE
COMMUNICATION
LECTURE NOTES
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Lecture: 1
Principles of satellite comm.
Evolution and growth of comm. satellites
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Synchronous Satellite
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Lecture 2
Satellite frequency allocation and band spectra
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Lecture 3
Advantages of satellite communication
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Lecture4
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Lecture 5
MODEM
A modem (modulator-demodulator) is a device that modulates an analog carrier signal to
encode digital information, and also demodulates such a carrier signal to decode the transmitted
information. The goal is to produce a signal that can be transmitted easily and decoded to reproduce
the original digital data. Modems can be used over any means of transmitting analog signals, from
driven diodes to radio.
The most familiar example is a voice band modem that turns the digital data of a personal
computer into modulated electrical signals in the voice frequency range of a telephone channel. These
signals can be transmitted over telephone lines and demodulated by another modem at the receiver
side to recover the digital data. Modems are generally classified by the amount of data they can send
in a given time unit, normally measured in bits per second (bit/s, or bps). They can also be classified
by the symbol rate measured in baud, the number of times the modem changes its signal state per
second. For example, the ITU V.21 standard used audio frequency-shift keying, aka tones, to carry 300
bit/s using 300 baud, whereas the original ITU V.22 standard allowed 1,200 bit/s with 600 baud
using phase shift keying. A satellite modem or sat modem is a modem used to establish data
transfers using a communications satellite as a relay. There is a wide range of satellite modems from
cheap devices for home internet access to expensive multifunctional equipment for enterprise use.A
"modem" stands for "modulator-demodulator". A satellite modem's main function is to transform an
input bit stream to a radio signal and vice versa. There are some devices that include only a
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demodulator (and no modulator, thus only allowing data to be downloaded by satellite) that are also
referred to as "satellite modems". These devices are used in satellite Internet access (in this case
uploaded data is transferred through a conventional PSTN modem or an ADSL modem
CODEC
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Lecture 6:
Unit2: Communication satellite design link
General link design equation:
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Consider a transmitting source, in free space, radiating a total power Pt W
uniformly in all directions. At a distance R m from the source, the flux density
crossing the surface of a sphere with radius R is given by
F = Pt / 4 πR2
If the transmitting antenna has a gain Gt ,
Then the flux density becomes F = Pt Gt / 4 πR2
Where Pt Gt is called the effective isotropically radiated power.
If the receiving antenna has an effective aperture Ae , then power received by
the receiving antenna is
Pr = Pt Gt Ae / 4 πR2 W
Gain and area of an antenna are related by
G = 4 π Ae / λ 2
where λ is the wavelength at the frequency of operation.
Hence Pr becomes,
Pr = Pt Gt Gr/ ( 4 πR/ λ 2 ) W
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This expression is known as the link equation.
In decibel terms, Pr = EIRP + Gr - Lp
Where Lp = ( 4 πR/ λ 2 ) is the path loss which accounts for the
way energy spreads out as an EM wave travels away from a transmitting source
in three-dimensional space.
More generally, the link equation would be
Pr = EIRP + Gr - Lp – La - Lra - Lta dBW
Where La is the attenuation in atmosphere
Lta is losses associated with the transmitting antenna
Lra is losses associated with the receiving antenna
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Lecture 7 and 8
System noise temperature,C/N and G/t ratio
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/N=PrG/KTsBG
C/N= Pr/ KTsBG
Where Ts representsthe several sources of noise in the receiver in the form of
system noise temperature.
Let Rf amplifier ,downconverter,IF amplifier by noiseless units with their
respective equivalent noise temperature as Trf,Tm,Tif.Gr,Gm,Gif be the gains.
Pn =Gif K Tif B + Gif Gm K Tm B + Grf Gif Gm K B (Trf+Tin)
Pn= Grf Gif Gm[KTifB/GifGm+KTmB/Grf+kB((Trf+Tin)
Pn = Grf Gif Gm K Ts B
Final equation that we will get
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KTSB=KB[Trf+Tin+Tm/GRF+Tif/GmGrf
Ts=[Trf+Tin+Tin/GRF+Tif/GmGRf)
C/N=PTGTGR(λ/4πd)2/KTSBLA
(C/N0) = 10logPTGT-20log (4πd/λ)+ 10log(GR/TS)-10logLA-10logK
In the above equations GR/TS represents a ratio (dbk-1)that can be used to
specify the quality of an earth station.This GR/TS is called figure of merit.
Lecture 9:
Atmospheric & Ionospheric effects on link design
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Lecture 10:
Complete link design
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Lecture11:
Earth Station parameter
.
.
Here d is the range between transmitting and receiving anteena.The antenna
parameters such as its diameter D and antenna beam width Θ3DB affect the
C/N0 as these are linked to the gain by expressions given by
G=ŋ
.
G=ŋ
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Where ŋ is the antenna efficiency. The satellite antenna beam width
is
determined by the required earth coverage. for a given earth station diameter
DES,
ŋ ŋ
where ŋ and ŋ are the effeciencies of transmmiting and receiving
anteenas respectively.However ,if earth satation anteena beamwidth
is fixed,C/N0 is given by:
ŋ
!
()*+, ,-.(/0*1 23*42
ŋ
*
"#$
%
&'
01 5,(2,2 40/3 6*4 %,5-.,1 0,2.
In the case antenna diameter DSAT and DES are given ,the expression
for C/NO reduces to
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Lecture 12:
Unit 3: Analog satellite communication
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Baseband signal:
FDM techniques
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Lecture 13:
S/N and C/N ratio in frequency modulation in satellite link
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Lecture 14
S/N ratio in FM with multiplexed telephone signal In satellite link
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Single channel per carrier
Single channel per carrier (SCPC) refers to using a single signal at a
given frequency and bandwidth. Most often, this is used onbroadcast satellites to indicate
that radio stations are not multiplexed as subcarriers onto a single video carrier, but
instead independently share a transponder. It may also be used on other communications
satellites, or occasionally on non-satellite transmissions.
In an SCPC system, satellite bandwidth is dedicated to a single source. This makes sense if
it is being used for something like satellite radio, which broadcasts continuously. Another
very common application is voice, where a small amount of fixed bandwidth is required.
However, it does not make sense for burst transmissions like satellite internet access
or telemetry, since a customer would have to pay for the satellite bandwidth even when
they were not using it.Where multiple access is concerned, SCPC is essentially FDMA.
Some applications use SCPC instead of TDMA, because they require guaranteed,
unrestricted bandwidth. As satellite TDMA technology improves however, the applications
for SCPC are becoming more limited.
Advantages
•
simple and reliable technology
•
low-cost equipment
•
any bandwidth (up to a full transponder)
•
usually 64 kbit/s to 50 Mbit/s
•
easy to add additional receive sites
Disadvantages
•
inefficient use of satellite bandwidth for burst transmissions, typically encountered
with packet data transmission
•
usually requires on-site control
•
When used in remote locations, the transmitting dish must be protected.
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•
A dish which is moved out of alignment can result in fines as high as $1,100 per
minute (as of 2003) from the satellite operator
Lecture no 15:
Companded single sideband systems
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Lecture no16:
Analog FM/FDM TV satellite link:
For satellite Tv transmission the baseband video signal frequency modulates a video carrier and the
two audio signals frequency modulate two audio carriers. The details of the video modulation
depends on the transponder BW available. Typical values for network TV are a peak deviation ΔFp of
10.75 MHz and maximum video modulating frequency fv of 4.2 MHZ.
signal to noise ratio .For a sine wave, peak to peak power is ( √ )2 times the rms power. The peak to
The performance of FM-FDM television channel is expressed in terms of peak to peak luminance
peak value of luminance signal is 1/√ ) times the peak to peak value of the composite television
channel
S/N= ( √ )2 (1/√
= 6(C/N)(
8
;%<,(= 2
)(
)
%9(:
%9(:
8
;%<,(= 2
%9(: %9(:
(C/N)i.3/2.
(
) pq
pq
Where p is the pre emphasis factor, q is the weighting factor, fmax is the maximum video modulating
frequency. And modulation index m is the modulation index which is the ratio of
;%<,(=
%9(:
Eq can be written as:
(S/N)pp=6(C/N)I 3(1+m)2pq
Intermodulation products & their effects in FM/FDM system
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Lecture 17
Energy disposal in FM/FDM system
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Lecture18:
Advantages of digital communication
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Advantages of digital communication
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Lecture19:
Elements of Digital satellite communication system
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Lecture20
Digital baseband signals
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Digital modulation techniques:
Techniques are as follows:
1.BPSK
2.DPSK
3.QPSK
BPSK
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Lecture 21
Satellite digital link design :
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Time Division Multiplexing:
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Lecture22
Unit5: Multiple access technique
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‘
LECTURE 23
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Lecture 24
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TDMA compared to FDMA
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Lecture 25
TDMA Burst time plan
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Lecture 26
Multibeam TDMA satellite system
This is about the antennas that are used in satellite communication. There may
be a zone beam antenna or spot beam antenna at the satellite. Zone beam
antenna can receive from transmitters anywhere within the coverage zone
whereas the spot beam antennas have limited range. Thus, INTELSAT V there
are 4 beams at c band interconnected by 80MHz BW transponders in which the
beam interconnections are static and switched occasionally to accommodate
major adjustments in traffic patterns.
Thus narrowing the satellite beam antenna beam width increases
communications capacity .It provides a high gain for the coverage area and
hence permits power savings in both uplink and downlinks. The use of spot
beam has further advantage that the same frequency can be spatially reused
many times. Thus, if India is covered by N non overlapping spot beam, satellite
capacity will increase N fold over the achieved using one beam.
The main problem in the interconnection of UBS and DBS in the satellite
communication system such that of INTELSAT VI is their interconnectivity. This is
achieved by introducing an onboard switching technique called SATELLITE
SWITCH. The corresponding system would therefore called satellite switched
TDMA system. This satellite switch is actually the dynamic satellite switching
that uses a microwave switch matrix onboard the satellite. The satellite switch
can interconnect the six can interconnect the six up beams to six down beams in
accordance with the desired single point connectivity matrix pattern for dwell
interval of programmable location and duration. For rf case switching matrix
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mentioned above is utilized. The output signals from the matrix are amplified
eventually fed to the antennas .The switching matrix is steered by an inboard
Distribution control unit and acquisition and synchronization unit provides the
time refrences necessary for TDMA operation.
Beam hopping(Transponder Hopping)TDMA
In the beam hopping TDMA the switching is achieved by each station hopping
from one frequency to another according to the destination of the transmitted
burst.Each earth station selects its transmission time on the corresponding
frequency so as to transmit its burst within the correct time slot. It should be
noted that the beam hopping is also carried out along with the on-board signal
regeneration .This helps in avoiding the interburst interferences associated with
conventional TDMA .
CDMA and hybrid access technique
In CDMA access the whole BW of the transponder is used all the time and
signals from the users are encoded so that the information from an individual
transmitter can be detected and recovered only by a properly synchronized
receiving station that knows the code being used. Thus while the capacity of the
communication is increased by the multiple access, the secrecy of transmission
is maintained.This has the potential applications in military satellite
communication.
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Lecture 27
Unit6: Satellite Orbits
Introduction
The orbit of a satellite in use for communication purposes has special significance.Its
geostationary and has to be maintained geostationary at all costs.
Thus there are two main problems involved in communication satellites regarding
orbit, namely (i)Launching and putting the satellite into geostationary
orbit(ii)maintaining it
Once the satellite is placed in geostationary orbit,it has to be parked in desired slot till
its whole life. Communication satellite move around the earth as planets do around
the sun and therefore three keplers law apply on them
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1. Kepler’s first law states that, the path followed by a satellite around a primary is
elliptical with the center of masses at one of the foci.
2. Kepler’s second law states that, for equal time intervals, a satellite will sweep
equal areas in its orbital plane.
3. Kepler’s third law states that, the square of the periodic time of orbit is
proportional to the cube of the mean distance between the two bodies.
Synchronous orbit
A geostationary satellite is synchronous and has an equatorial circular and direct
orbit. At geostationary altitude and orbital velocity of satellite equals the velocity
of a point on earth’s equator. Let M and m be the mass of earth and satellite
respectively. similarly, let RE be the radius of synchronous circular orbit of satellite.
If w is the angular velocity of the circular motion of satellite in radians per second
then centripetal force acting on the satellite would be mRw2.
This centripetal force equal to the geostationary force b/w the satellite and earth
expressed by GmM/R2.
9>
9 4
At the earth’s surface the gravitational force in mg where g is the acceleration due
to the gravity.
From above two equations
?
R=[
]1/3
4
Where f=w/2π is the rotation rate n revolution per second. Putting f=1 revolution per
day and RE =6370Km,g=9.8,one obtains R=42,208Km.Subtracting RE form R one gets
35,838 km as the orbital height above the equator which is quite close to the precise
value of 35,860.
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Lecture 28
Orbital parameters:
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The satellite’s movement in an orbit follows the three kepler’s law.The first kepler’s
lkaw gives that the satellite moves along a conic on the orbit plane
r=p/(1+ecosv)
PHYSICAL
PHYSICALLAWS
LAWS
Kepler’s
Kepler’s1st
1stLaw:
Law: Law
LawofofEllipses
Ellipses
The
Theorbits
orbitsofofthe
theplanets
planetsare
areellipses
ellipseswith
with
the
thesun
sunatatone
onefocus
focus
Where p is a parameter,e is the eccentricity and v is the vertical angle.
Value of eccentricity e determines the type of conic
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*— 05 6,
e = @ < 1— DEEFGHD I
— <(5()*6(
but if e>1 then its hyperbola.
The elliptical orbit has been depicted.Here a is the semi-major axis and is linked to
the orbital parameter by the expression
T = 2πJ
(
K
Where µ is the quantity equal to GM,where G being the gravitional constant.eqaul
to 6.6728*10-11 m3kg-1s-2. And M be earth’s mass equals to the 5974*1024kg
Satellite velocity for the circular orbit is J where a is the orbit radius.
L2 = 2 K
5
K
(
K
(
So following should be remembered:
x=rcosv =a(cos.εε-e)
y=rsinv = a sinεε(1-e2)-1/2
r = p/(1+ecosv)
e= (ra - rp) /(ra + rp )
Area of the ellipse = πab.
Semi major axis = a = (ra+rp)/2
Apogee distance = ra =a (1+e)
Perigee distance = rp =a (1-e)
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Locus parameter = a (1-e2)
A synchronous satellite has T as sidereal period of rotation of its primary body TP.
T = TP = M1 N
O
PQRST
U* 24h = 86163,44 s = 23h 56 m 4 s
Satellite location wrt Earth:
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Lecture 29:
Look angles
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Earth coverage & slant range,
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ECLIPSE EFFECT
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Lecture30
Satellite placement in geostationary orbit:
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Lecture 31:
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station keeping
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Satellite stabilization:
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Lecture32:
Unit 7: Special purpose satellite
BDS:
Direct broadcast satellite (DBS) refers to satellite television (TV) systems in which
the subscribers, or end users, receive signals directly from geostationary satellites.
Signals are broadcast in digital format at microwave frequencies. DBS is the
descendant of direct-to-home (DTH) satellite services. A DBS subscriber installation
consists of a dish antenna two to three feet (60 to 90 centimeters) in diameter, a
conventional TV set, a signal converter placed next to the TV set, and a length of
coaxial cable between the dish and the converter. The dish intercepts microwave
signals directly from the satellite. The converter produces output that can be viewed
on the TV receiver. A number of companies provide DBS and DTH service
throughout the world. In the United States, DirecTV and EchoStar are the main
vendors. DIGITAL DBS TV Digital TV signals are transmitted as a 20 Mbps QPSK
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signal occupying about 27 MHz of transponder bandwidth. The 40 Mbps signal has
a data rate between 23 and 27 Mbps with the remaining bits used for error control
and system operation. DBS-TV signals make extensive use of error correction and
error detection techniques in the form of a double layer of error control coding with
interleaving. Each DBS-TV satellite carries up to 32 high power transponders
covering part of the 12.2 to 12.7 GHz broadcast satellite band (BSS), and the
satellites at each orbit location transmit in opposite hands of circular polarization
(CP). Signals with opposite hands of circular polarization are orthogonal, and a
suitably designed earth station antenna can separate two signals with opposite
hands of circular polarization. In the simplest DBS- TV receiving terminal, an
electronically controlled polarizer is used immediately behind the antenna feed. The
polarizer can be set to receive LHCP (left-hand circular polarization) or RHCP by
changing the voltage supplied to the low noise block converter (LNB) unit at the
antenna. The polarizer converts the circularly received polarization signal to a
linearly polarized signal in a section of waveguide, and a linear probe in the
waveguide converts the signals to currents that drive the LNA input. Customers
wanting to receive signals from more than one orbital location need an antenna with
two feeds. DBS-TV receiving antennas are typically an offset parabolic reflector
design with the feed below the antenna aperture. The offset feed design eliminates
blockage of aperture by the feed which occurs in symmetrical reflector antenna
designs, and improves the aperture efficiency of the antenna, and therefore
increases its gain. Offset fed parabolic reflectors have a beam squint effect in the
plane of symmetry when operated in opposite hands of circular polarizations.
DBS TV RECEIVER: Fig below shows a block diagram of a DBS-TV receiver. The
entire front end of the receiver is located at the antenna feed in the form of an LNB
to minimize loss of signal and hence to maintain the lowest possible system noise
temperature. The electronic polarizer is switched by changing the voltage supplied
to the LNB via the cable that interconnects the antenna and set-top receiver. The
entire 12.2 to 12.7 GHz band is downconverted by the LNB to the 900-1400 MHz
band, where cable losses are much lower than at Ku band. The down-converter
consists of a dielectric resonator local oscillator and mixer, followed by an IF
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amplifier and band-pass
pass filter. The high gain LNB can drive 100 m of coaxial cable
without any reduction in signal quality. Where longer cable runs are needed,
amplifiers for the 900-1400
1400 MHz band can be used to boost the signal strength. The
set-top
top box accepts the entire 500 MHz band and separates the individual
transponder frequencies. Any one of these frequencies can be selected on demand by
the user.
The user enters a desired channel number into the set
set-top
top box using an IR remote
control, for example, channel 362, which is converted via a stored look-up
look
table in
the receiver to an RF-channel
channel frequency and polarization. The signal from the
required
equired transponder is then selected by the receiver by setting the correct
polarization at the antenna and tuning the set
set-up
up local oscillator to the appropriate
IF channel frequency. The QPSK signal is then demodulated. The result is a
multiplexed bit stream,
eam, typically at a rate up to 40 Mbps, which contains the bits for
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channel 362 and several other video signals. The bit stream is encrypted and
contains error control coding bits and data bits. The bit stream is processed to
correct and detect errors, de-interleaved, and decrypted. A digital demultiplexer
then extracts the bits for the wanted channel, 362 in this example, sends them to a
MPEG 2 decoder, and finally generates analog video and audio signals with D/A
converters to drive the TV set.
INMARSAT
INMARSAT is an acronym of International Maritime Satellite Organization with
headquarters at London. It is an international organization that controls satellite
systems for communication between ships and coasts so that emergency life saving
services may provide to the distressed people. It came into being at midnight on
15th/16th July 1979.INMARSAT has 85 member states. The USA maintains its lead
in INMARSAT by holding maximum share in it. Signatures from member countries
own INMARSAT, they contribute to the costs of organization & develop its policies.
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INTELSAT
VSAT:
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A VSAT consists of two parts, a transceiver that is placed outdoors in direct line of
sight to the satellite and a device that is placed indoors to interface the transceiver
with the end communications device, such as a PC. The transceiver receives or sends
a signal to a satellite transponder in the sky. The satellite sends and receives signals
from a ground station computer that acts as a hub for the system.
Each end user is interconnected with the hub station via the satellite, forming a star
topology. The hub controls the entire operation of the network. For one end user to
communicate with another, each transmission has to first go to the hub station
stati that
then retransmits it via the satellite to the other end user's VSAT.
The fig above shows a schematic of a VSAT/WLL network. The Geostationary
satellite is used to link a large number of VSATs with the main switching centre in a
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large city. Each VSAT acts as a link to the local switching centre in the village or
rural community, with the final mile of the telephone link being carried over a
wireless local loop.
Specifications of VSAT systems:
• They operate in Ku band.
• Earth station diameters range from 1-2 m.
• Transmitter power: 1-2 W.
• Data rate: Few thousand bps to 256 kbps.
VSAT systems do not transmit all the time. Hence a dedicated satellite need not be
used. Also DAMA (Demand Assigned Multiple access) is employed in some systems
in which channel capacity is assigned in response to fluctuating demands of the
VSATs in the network.
Network Architectures
VSAT systems can be implemented in 3 basic architectures.
• One-way implementation.
• Split-two-way implementation.
• Two way implementation.
One-way implementation
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This is the mode of a satellite used in the broadcast satellite service (BSS). The
introduction of digital telephony allows the provider and user much greater
flexibility in the operation of a broadcast network. Fig 1 shows the schematic of a
BSS.
Split-two-way
way implementation
This implementation is used when there is no normal return channel (E.g.: with Ku
band broadcast satellite service systems that carry internet traffic). The relatively
high capacity downlink stream is not complemented by an uplink capability from
the user terminal. If the BSS downlink is used as the download channel from an
internet service provider, the only option the user has for a return link is via
another telecommunications channel, such as a standard telephone line. The
Internet Protocol (IP) is therefore split between a satellite downlink (outbound)
channel and a terrestrial telephone (inbound) channel: hence the term split IP for
this implementation. The advantage of this approach is that the VSAT terminal
t
does
not require a transmit capability, which significantly reduces its cost and
complexity. The disadvantage is that the telephone line connection must usually be
through a modem, with a bit rate generally restricted to 56 kbps.
Two way implementation
The service includes a return link so as to establish two
two-way
way communication over
the same satellite, from the hub to the user and from the user back to the hub. Two
topologies are implemented: Star topology and Mesh topology.
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Star topology:Here, all off the traffic is routed via the master control station, or hub.
If a VSAT wishes to communicate with another VSAT, they have to go via the hub,
thus necessitating a “double hop” link via the satellite. Since all of the traffic
radiates at one time or another
another from the hub, this architecture is referred to as Star
network. Disadvantage: (1) Communication between two VSATs requires a double
hop thus increasing the time delay. (2) Since the low receive G/T of the VSATs ,
coupled with their limited transmit EIRP is compensated for by using a large hub
with high G/T and EIRP, the cost of the hub used is very high.
Mesh topology:Here,
Here, each of the VSATs has the capability to directly communicate
with any of the other VSATs. The network control and the duties of the hub is either
handled by one of the VSATs or the master control station functions are shared
among the VSATs. Since the traffic can go to or from any of the VSATs, this
architecture is referred to as Mesh network.
fig a
MSAT
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fig b.
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Lecture 33:
SARSAT:
Low earth orbit satellite(LEO)
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Satellite communication wrt fibre optics:
Characteristics
Bandwidth
Fiber optic System
Bw of it is
1terahertz
Satellites
Bw of 36,54,72
MHz
Immunity to
interference
Immune to Em
interference
Durability
Storms can knock
down overhead
lines
Security
Difficult to tap
without detection
Multipoint
capabilities
Flexibility
Primarily a point
to point media
Difficult to
Transmission
subject to
interference from
various sources
including
microwaves.
Storms can disable
individual anteenas
but leave network
intact.
Signals are
encoded for
security.
Point to multipoint
Easy to reconfigure
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Connectivity to
customer size
reconfigure to meet if hardware has
changing demands. been apprx
designed.
Local loop
With antennas
required.
installed on
customer premises
,as with 14/12 GHz
band, local loops
not required.
Lecture 34:
LANDSAT:
LANDSAT-I was first satellite launched in 1972 by NASA followed by INTELSAT-2
,3,4,5,6 and 7.
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LANDSAT is one of the earth exploration satellite service .This system involves
observation of the earth for various purposes,e.g. meterological,geodesy,data
collection.
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Defense satellite :
Mission:The Defense Satellite Communications Systems (DSCS) is an important part of the
comprehensive plan to support globally distributed military users.
Features:Air Force Space Command operates ten Phase III DSCS satellites that orbit the earth
at an altitude of more than 22,000 miles. Each satellite uses six super high frequency
transponder channels capable of providing secure voice and high rate data communications.
DSCS III also carries a single-channel transponder for disseminating emergency action and
force direction messages to nuclear-capable forces.
The system is used for high priority command and control communication such as the
exchange of wartime information between defense officials and battlefield commanders. The
military also uses DSCS to transmit space operations and early warning data to various
systems and users.Background:The Air Force began launching the
DSCS IIIs in 1982. The
system is built with single, multiplebeam antennas that provide more flexible coverage than its
predecessor. The single steerable dish antenna provides an increased
power spot beam which can be tailored to suit the needs of different
size user terminals. DSCS III satellites can resist jamming and are
expected to operate twice as long as the previous generation.
DSCS users operate on the ground, at sea or in the air. Members of the 50th Space Wing's 3rd
Space Operations Squadron at Schriever Air Force Base, Colo., provide satellite bus command
and control for all DSCS satellites.
Air Force Materiel Command's Space and Missile Systems Center at Los Angeles AFB, Calif., is
responsible for development and acquisition of DSCS satellites and ground systems.
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Unit 8:Laser satellite communication:
Introduction:
LINK ANALYSIS:
(i)Atmospheric Effects: While analyzing the satellite cross-link,the effects of earth’s
atmosphere on the laser signals should b ecarefully studied.The earth’s atmosphere affects
the optical signals in a variety of ways such as attenuation due to the energy
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absorption,beam spreading due to the scattering of light waves,beam bending due to the
refocussing of the optical beams & beam breakup due to the loss of the field coherence over
the beamfront.
(ii) Complete link analysis: Satellite optical cross link analysis given below is suitable to the
case when the effects of atmosphere are considered to be the negligible in satellite cross
links. here it has been assumed that the RF link is used for the uplink and downlink
transmission to the satellite & the crosslink b/w two satellites is the optical link that uses
direct detection system.Thus,when the satellite receives uplink, it directly intensity
modulates the uplink carrier BW onto the optical carrier for the crosslink. For this it is
essential that the upper limit of the uplink frequency band is within the receiving
photodetector BW otherwise then the uplink BW is firstly down connected before Laser
modulation.
Thus the satellite RF uplink waveform may be represented
S(t) = u(t)+ nu(t)
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Where u(t) is the uplink carrier & nu(t) is the uplink noise and interference.Since the laser
power at the optical receiver is the intensity modulated so,
P(t) = Pr (1+βs(t))
Where the Pr is the average power and β is the intensity modulation index.
Lecture 36:
Optical satellite link transmitter:
The transmitter part of an optical satellite link consists orf a laser source, modulator and
antenna along with some data handling electronics.
Laser source: A variety of LASER source may be used for optical satellite communication
systems. These may be gas lasers, solid stata lasers and semi conductor laser.Selection of
Laser source is dependent upon a no. of factors that include link range,propogation media,
data rate and platform limitation.
Modulators: In laser space communication the most preferable modulation is direct intensity
modulation. Various methods that can be used to modulate laser can be as:
Modulation type
Information Signal
Carrier parameter
Example
Analog
Time continuous
Continuous
Intensity Modulation
Pulse
Time continuous
Continuous
Pulse intensity
modulation
Digital
Time sampling
Quantized
Pulse code
modulation
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Modulation rates of 1Gbits/sec with the laser diodes have also been achieved.
Antennas : As with the RF communication system, the Laser satellite communication system
also utilize antennas to direct the transmitted energy. Here antennas are nothing more than
the conventional design telescopes where the size and geometry are dictated by wavelength
and requirement.
Optical satellite link Receiver: Optical receiver consists of an anteena, filter,
photo detector and the conventional receive electronic system. Receive
antennas are also the telescopes whose main purpose is to focus the optical
signal onto the photo detector and to reject as much as of background radiation
as is practical. The receive optical filters are employed to eliminate background
radiation that is not of the same wavelength as the optical signal. Infact the
range of wavelengths around the laser wavelength allowed by optical fibre is
called Optical bandwidth.
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Δλ/λ0 =BO/fo
Typical optical fiber BW at 1 micron generally ranges from 1-100 angstroms.
Optical detection are of two types:
1. Direct detection system :It responds to the signal intensity and are most
widely used in optical communication systems.
2. Hetrodyne detection systems: Here the optical signal is combined with a
local oscillator and then both signal are focused on to the same detector.
An optical receiver field of view is defined as the field arrival angles over
which the lens will focus the impinging field onto the photo detector
surface. This depends on the detector focal length fc.Thus the optical field
of view is given by:
Ώfv = Ad/fc2 =Ad/Ar = (Ad/Ar)(λ2/ Ar)
This equation is termed diffraction limited of view.
Photo detectors normally used in optical detection may be PIN diode or
avalanche photodiode. These phodetector are governed by their
characteristics properties such as detection efficiency,gain,responsitivity
and BW.Infact these are the parameters which a communication
engineer has to know while using photodetectors in the optical receiver
design. The detection efficiency indicates the fraction of the received
power which is actually detected.
Thus the detected count rate of the optical receiver given by:
ns=(V
5
3%*
The gain of photo detector is increased by cascading the photoemmisive
surface but this increases noise.
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The BW of photo detector determines the rate of power variation that
can be detected.
Shot noise
Pr(t)
Pb
R
Dark current
Thermal current
detector o/p
+
+
+
Here shot noise , dark current and thermal noise current are taken into
account. In addition to the some background noise power is also taken into
account. all these together form photo detector noise model.
Noise processes are as indicated in fig may be expressed as:
N0 = Nsn+Ndc+Nt
N0 represents the total receiver current processes in terms of 2 sided spectral
level.
In heterodyne detection, a strong local field relative to the received power is
added to the receive field prior photodetection.Thus here photo detector
responds to the intensity of combined field.
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Lecture 37:
Satellite beam acquisition ,tracking and pointing:
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World Institute Of Technology
8km milestone ,Sohna Palwal Road , NH-71 B ,Sohna , Gurgaon ,Haryana.
Website : www.wit.net.in
E-mail : info@wit.net.in
Lecture 38:
Deep space optical communication link:
This shows a typical; example of deep space optical communication link by
NASA.here main space craft communication instrument is the optical
transreceiver package called optranspac.The optranspac carries out various
functions of the optical transmitter and receiver .For example,receives beacon
signal and performs beam aquisition.Tracking and pointing functions along with
the plink optical commands and ranging signals.
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