ECE 5233 Satellite Communications
Prepared by:
Dr. Ivica Kostanic
Lecture 6: Satellite sub-systems
(Section 3.1-3.4)
Spring 2014
Outline
Satellite subsystems
Communication subsystem
Satellite transponders
Examples
Important note: Slides present summary of the results. Detailed
derivations are given in notes.
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Satellite subsystems
 Major satellite subsystems
o Altitude and Orbit Control Systems (AOCS)
– maintain and stabilize satellite in the orbit
o Telemetry, Tracking, Command and
Monitoring (TTC&M) – take and process
measurements on satellite health and position
o Power subsystem – generate and distribute
power to various components of the satellite
o Communication subsystem – Receives,
processes and re-transmits the signals
o Satellite antenna – receive and transmit EM
waves.
o Superstructure – construction of the satellite
that is used as a mount for all other
components
o Thermal subsystem – maintains the
temperature of the satellite within prescribed
range
 Satellites have life expectancy 10-15 years
 Many components are deployed in redundant
configurations to minimize probability of satellite
failure
Major components of a Lockheed
Martin remote sensing satellite
BBC Documentary - How to build a satellite:
https://www.youtube.com/watch?v=_Rp53U4mzZA
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Altitude and Orbit Control
 Two principle tasks
o
Stabilize the orientation of the satellite
o
Maintain the position of the satellite in orbit
Example of spin
stabilized satellite
 Four ways of stabilizations
o
Spinning
o
Momentum wheels
o
Reaction wheels
o
Control moment gyro
 Orbit is maintained using control thrusters
 The amount of fuel available for thruster operation is a fundamental
limit on the satellite life span
Different methods for
satellite stabilization
Boeing 376 – one of the most popular GEO
Comm. Satellites
Operates in C, Ku bands
Usually 24 transponders
50 satellites over five continents, used by more
than 20 companies
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Telemetry, Tracking, Command and Monitoring
 TTC&M – distributed between satellite and Earth
station
 Satellite provides measurements
o Position sensors
o Environmental sensors
o Alarms
 Satellite may have few hundred of different sensors
 Measured data sent over TTC&M link to Earth station
 The TTC&M link is a narrowband link - allows for
high sensitivity reception
 At the Earth station measured data processed and
commands are issued to the satellite
 TTC&M may be operated by satellite owner or it may
be outsourced
 TTC&M systems are build with redundancy
Block diagram of
TTC&M system
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Power systems
 Used for generation and distribution of power
throughout a satellite
 Source of power generation – Solar panels
 Three types of power systems
o Solar – the most frequently used in commercial
satellites
o Chemical – used for backup to power satellite
during solar eclipses
o Nuclear – used for satellites leaving the Earth
orbit (deeper space exploration)
 Solar panels consist of many strings solar cells
connected in parallel
 Solar energy in Earth orbit has density of ~
1390W/m2
 Three axis stabilized satellites use flat solar panels
 Spinning satellites have solar panels on the
cylindrical surface of the satellite
 Efficiency of solar cells is about 20% (i.e. only 20%
of the sunlight might be converted to energy)
 The energy is used to charge satellite batteries and
to power rest of the satellite
 The power needed for a satellite may be in the
range 0.5-10KW
 Majority of the power is consumed by the
communication equipment - RF amplifiers on the
transponders
Block diagram of solar power generation system
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Power systems - examples
Example 1. Consider a case where a spin-stabilized
satellite has to generate 2000 watts of electrical power
from the solar panels. Assuming that the solar flux falling
normal to the solar cells in the worst case is 1250W/m2,
the area of each solar cell is 4 cm2 and the conversion
efficiency of the solar cells including the losses due to
cabling, etc., is 15 %, determine the number of solar cells
needed to generate the desired power. What would be
the number of cells required if the sun rays fell obliquely,
making an angle of 10◦ with the normal?
Example 2. It is desired that the battery system on board
the satellite is capable of meeting the full power
requirement of 3600 watts for the worst case eclipse
period of 72 minutes. If the satellite uses nickel–hydrogen
cells of 1.3 volts, 90 A h capacity each with an allowable
depth of discharge of 80 %, and discharge efficiency of 95
%, find
(a) the number of cells required
(b) (b) the total mass of the battery system. Given that the
specific energy specification for the battery technology
used is 60W h/kg.
Answers:
Required number of cells: 83777
For 10% angle, required number of cells is 85070
Answers:
a) Required number cells – 49
b) Mass of the battery system – 94.74 kg
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Communication systems
 Most important (i.e. revenue generating part
of the satellite)
 Satellite – repeater in the sky
 Bands for satellite operation: L(2GHz/1GHz),
S(4GHz/2GHz), C(6GHz/4GHz), X(7/8 GHz)
Ku(12-18GHz) and Ka(27-40GHz)
 Early communication satellites – power
limited, used narrowband transmission
 Contemporary satellite – bandwidth limited,
use wideband transmission and frequency
reuse
 Frequency allocation handled through ITU on
the global basis
 Management of the frequencies in the US
are conducted by Federal Communications
Committee (FCC)
Outline of satellite
communication system
 A unit of satellite communication capacity transponder
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Satellite transponders
 Two types of satellites
o “Bent pipes” (transparent)
o Regenerative (base band processing)
 Smallest assignable recourse
o Satellite transponder
o Satellite usually hosts many transponders
o Some of transponders may be spares
o Typical active transponder count is 24
o Satellite usually operates in single band
(although there are some multiband
satellites)
Basics of “bent pipe” architecture
o Bandwidth of the satellite transponder is a
compromise between power efficiency
(favors larger bandwidth) and limitations
on linearity of PA (favors smaller
bandwidth)
o Most common bandwidth of a transponder
in 36MHz (with 40MHz channelization)
o Some satellites adopt 54MHz or even
72MHz
Satellite with onboard processing
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Transponder arrangement – fixed frequency translation
 Basic design – each transponder
is individual chain with fixed
frequency translation
 Banks of transponders are
arranged to achieve higher
frequency separation (80MHz)
o Minimizes intermodulation
products
Example of transponder arrangement
for RCA’s SATCOM
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Simplified single conversion transponder
1. Input signal
3. Output signal- translated by frequency of LO
x1 t   A cos2f 6G t 
x3 t   GKAcos2  f 6G  f LO t 
2. After mixing stage
x2 t   K  A cos2f 6G t   X LO cos2f LO t 

1
1
KA cos2  f 6G  f LO t   KA cos2  f 6G  f LO t 
2
2
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