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Basic Satellite Communication
(3)
Components of CommunicationsSatellite
Dr. Joseph N. Pelton
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Components of a Satellite
Main Mission Antennas & Communications System
Antennas are constantly pointed to the earth with larger
antenna serve the lower frequency and smaller antenna
service the higher frequency
The transponder in the communications system is
responsible for receiving the signal, amplifying it and re-
transmitting it in the lower frequency back to earth.
Most transponders in the C- and Ku- bands now use Solid
State Power Amplifiers (SSPAs) to amplify the signal
because of their weight, compactness and reliability.
Satellites can have 12 to 96 transponders plus spares,
depending on the size of the satellite.
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Components of a Satellite
Satellite Transponders & SSPA The transponder is the active
part of the satellite communications
system that provides the connection between the satellites
receive and
transmit antennas.
A transponder bandwidth can frequently be 36 MHz, 54 MHz, or
72MHz or it can be even wider.
A transponders function is to receive the signal filter out
noise, shift
the frequency to a downlink frequency and then amplify it
for
retransmission to the ground. The main amplifier may be a
Travelling
Wave Tube (TWT) or Klystron tube (now usually used for
higherfrequencies above 20 GHz and at very high power levels (i.e.
100 to
200 watts) or it may be a Solid State Power Amplifier
(SSPA)that
would be used at lower L, C or Ku bands frequencies. If the
transponder is a regenerative transponder then the signal will
be
converted to base band frequencies and processed there rather
than
handled at RF bands.
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Components of a Satellite
Antenna Gain & Path Loss
Gain measures the performance of an antenna. The larger
the size (or aperture) of an antenna the better it can point
to
or concentrate a signal towards the desired receiver
location on earth. Thus the more the antenna acts like a
spot light to illuminate the earth and not send signal
uselessly out into space, the higher the gain of the
antenna.
An Omni antenna that sends signals in all directions
equally has a gain of 1 and represents the lowest gain. Path
loss comes from the spreading out of a signal from
the antenna as it travels from the satellite to user antenna
and vice versa. Since the signal spreads in a circle
effective
power (or flux density) is reduced by the square of thedistance
traveled.
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Components of a Satellite
Satellite Transmit Power (EIRP)
EIRP = Effective Isotropic Radiate Power.
The product of final amplifier power, filtering andcoupling
losses and antenna gain.
Specified in watts or dB (relative to 1 watt) = dBw
EIRP density, typically specified in terms of dBw/Hz is a
key parameter in determining both uplink and downlink
performance.
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Components of a Satellite
Satellite Transponder & SSPAs
Transponder Traffic Loading is a Function of:
Transponder Bandwidth
Uplink and Downlink satellite and earth stationparameters
Number of Carriers per transponder. This
determines back-off which is the reduction inamplifier power
from the saturated or maximum
output power.
Modulation efficiency (i.e. bits/Hz)
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Methods to Cope with Rain
Attenuation Use lower frequencies where possible
Provide link margin + additional link
margin in beams where there is heavy rains
On-board processing
Site diversity
Ability to increase dwell time and/or reduce
information rate
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Components of a Satellite:Satellite Tracking, Telemetry, Command
& Monitoring (TTC&M)
Tracking is necessary to know exactly where a
satellite is and thus be able to send it commands,
receive telemetry and communication with it.
Omni antenna is used to be able to track and
command satellite even if orbit is disturbed.
The TTC&M system is highly automated with
computer alarms to sound in case of anomalies aredetected.
Analogy: School bus represents: spacecraft bus, and
passengers represents payload: telecommunications and TV
traffic. TTC&M is needed to keep bus running and check
on
status of passengers.
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Components of a Satellite:
Satellite Power System Main source of power is solar cell panels
that are
constantly oriented towards the sun.
New solar cells are increasingly efficient. The solar cell
system is backed up by battery
system that provides energy during solar eclipses
and other periods of outages.
Typical power levels of 2 to 5 KWs for FixedSatellite Systems
and 10 to 12 KWs for Mobile
and Broadcast Satellite Systems in GEO. Levels
are lower for LEO & MEO Systems.
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Components of a Satellite:
Satellite Bus & Stabilization System
The bus is the key parts of the satellite that allows its
operation in space.
The early generation of satellites use gravity-gradient or
spin stabilization but now 3-axis body stabilization using
momentum wheels is most common because of lifetime &
efficiency.
The bus includes the antenna mast, the thermal control,
the sun & earth sensors, the batteries, momentum
wheels,solar array controls & on-board computers.
Carbon-epoxy structures are used because of strength and
light weight.
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Components of a Satellite:
Satellite Bus & Stabilization System
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Assignment
Assignment 4:
Write short notes on the terms highlighted in
yellow color (Slides 4 and 9 of lecture 4)
