COMMUNICATION SATELLITE SUB-SYSTEMS
WEEK 11
Topics:
Communication Satellite Sub-systems
Power Supply subsystems
Repeaters
Antenna Sub-system
Satellite Sub-systems can be divided into two groups
a.
Communication Subsystem
b.Other Sub-systems
Satellite Sub-systems can be divided into two groups
a.
Communication Subsystem
b.Other Sub-systems
a.
Repeaters
b.Antennas
Other sub-systems comprises of
a.
Power
b.
Stabilization
c.Propulsion
d.
Station keeping
e.Attitude Control
f.
Telemetry, Tracking and Command
Communication and other sub-system should least in
mass and power efficient
Other Sub-systems
Subsystem Functions Parameters of Importance
Repeaters Amplify Terminal Signals
and transmit back Noise Figure, Linearity and output RF power Antennas Receive and Transmit
Signals
Coverage, Gain
Electric Power Supply Provides Electric Energy
at various voltage Levels Power, Voltage Regulations Structure Support Equipment Stiffness
Attitude and Orbit
Control Attitude Stabilization, Orbit determination Accuracy
Thermal Control Temperature Regulations Heat Dissipation capability
Propulsion Provides Velocity
Increments and Torques Specific impulse, Mass of propellant Telemetry, Tracking
and Control Exchange of house keeping data with control center
Number of channels, security of
Electric Energy needed for
House Keeping
Communication System
Source of Electric Power is Sun, when Satellite is in the
Orbit
Electro-mechanical accumulators provides electric
energy during injections into transfer orbit and prior
to deployment of solar cells.
USSR uses thermonuclear energy for satellite.
Nuclear Cells are not used for communication
satellites.
Solar cells are photovoltaic that converts solar radiation
into useable voltage levels in power supply.
Normal conversion efficiency is about 12 to 15%.
Conversion efficiency decreases by 30 to 35% in seven
years.
Solar cells are covered with fused silica and cerium doped
micro sheets.
The ultrathin (50 um) low mass cells are not very
strongly affected by solar radiation.
For better conversion efficiency, solar cells with
aluminum or gallium arsenide are recommended.
This gives conversion efficiency around 18%.
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 1 to 2 KWs
For better conversion efficiency, solar cells with
aluminum or gallium arsenide are recommended.
This gives conversion efficiency around 18%.
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 1 to 2 KWs
Nickel- cadmium accumulators are used in case
Solar Eclipses
Solar cells are not aligned
DOD (depth of Discharge) must be maximum in battery
system
Nickel cadmium batteries are not allowed to discharge beyond 50%
Nickel hydrogen batteries can be used which can discharge up to 60%
Satellite life can be extended to one year due to these batteries
Battery voltages are 20 to 50 V and capacitors between 20 to 50 AH
When solar cells gets heated their efficiency gets down
In the spin stabilized satellites
The average temp. is low due to successive face to sun and cold
space (25C)
Efficiency is 14 % which is reduced to 22% at the end of satellite life
To compensate this 15% extra area is provided
In the three axis body stabilized satellite
Wings of solar panels are used
One revolution per day to keep the face toward sun
Can provide high power
But Higher weight and complexity of solar array drive motor
In Low power satellites Satellite spin type of stabilization is preffered
Conventional Transponders performs two function
Amplification of signals (-100dbW to 10dbW)
The frequency down conversion
Diplexer is used to transmit and receive from the same
path
Repeaters (Transponders)
Satellite
Antenna Diplex er
Front End
Carrier
Processin g
Power Amplifier
Regenerative Transponders performs
Receiving and detecting of uplink
Carrier processing for demodulation at low carrier
frequency
Power amplifier for demodulated carrier
There are certain advantages
Advantages in decoding
Generation of spot beams
Interference rejection
RF Filter(Microwave Filters):
Allow better packaging, lower weight and require less power
These filters are designed to achieve
Desired mask, noise rejection and equalization
Should not contribute significant circuit noise Chebyshev, Bessel and Legende filters are used Dual mode filters using cavity coupling are used
RF filters have 36 MHz bandwidth at C band
Center frequency to bandwidth ratio about 104 at C and 103
at k
Repeaters (Transponders)
RF Filter Equalizer Lowe Noise AmplifierBlock diagram of Front end Electronic System
Equalizer:
Are needed to cancel delay distortions of RF Filters
Low Noise Amplifier:
Solid state microwave amplifiers i.e. Tunnel Diode
Amplifier (TDA)
Have low noise contribution and high gains
Decreeing gain value at higher frequency
Now being replaced FET for k band
Repeaters (Transponders)
RF Filter
Equalizer Lowe Noise Amplifier
Block diagram of Front end Electronic System
Determines Transponder type
Bring the uplink frequency to downlink
Four different kind of frequency translation
RF-RF Translation
RF-IF- RF Translation
IF Remodulation
Demodulation – Remodulation
The Remodulation remove the uplink noise and
interference from the return modulation.
TWTAs are commonly used as power amplifiers
Power ranges from 8.5 to 20W
150 to 250W for Direct broadcast applications
Saturated gain about 55dB
TWTA is dominant device in transponder that
may affect the signal power significantly
TWTAs are being replaced by SSPA
SSPA uses GaAs FET devices and are lighter in
weight
SSPA offer significant improvement in reliability
In channelized satellite repeaters, tow additional units are
used
IMUX
OMUX
The IMUX separate the 500MHz into individual transponder bandwidth depend on the satellite mission
Power Amplifier
Input Circulator Group Delay Equalizer Input Filter Output Circulato r Amplitud e Equalizer Transponder Input Circulator Group Delay Equalizer InputFilter Output Circulato
r Amplitud e Equalize r Transponder Input Circulator Group Delay Equalizer Input
Filter Output Circulator Amplitud e Equalize r Transponder Input Signals 14.0
to 14.5 GHz
1
N-1
N
The output downlink signals from TWTAs of
different channels are combined by the output
multiplexer for transmission to earth
Redundancy is incorporated bot at the input
Antenna subsystem is also an essential
component of satellite system. Basically four
main type of antennas are used: these are
1. Monopoles and dipoles (wire antennas) which
are mainly used in VERY HIGH FREQUENCY AND
ULTRA HIGH FREQUENCY to provide
communication for TTC and M subsystem.
2. Horn antennas are mainly used at microwave
frequencies. Horns are actually used as feeds for
reflector.
3. Array antennas are actually phased array
antennas which are used on satellites to form
multiple beams from single aperture.
4. Reflector antennas are commonly used for
earth station antennas and the most widely
employed shape of it is the paraboloid with a feed
placed at its focus. The patterns for different
satellite antennas are shown as:
Antenna Subsystem
In the Satellite antenna it is essential that
• Mainlobe should be concentrated towards earth
• EIRP should be maximized
If mainlobe consideration given importance, the orbit utilization will not be
Efficient
Scanning beams has been introduced for efficient coverage of area.
Deployable antenna (large reflectors) are used of high directivity
