Basic Satellite Communication (3) Components of Communications Satellite

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Basic Satellite Communication (3)Components of
Communications Satellite

• Dr. Joseph N. Pelton

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Components of a Satellite

• Payload Satellite Main Mission Antennas
  Communications System
• Satellite Bus Components
• Satellite Tracking, Telemetry, Command
  Monitoring (TTCM)
• Methods to Cope with Rain Attenuation
• Satellite Power System
• Satellite Bus, Orientation and Stabilization
  System

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Components of a Satellite Main Mission Antennas
Communications System

• Antennas are constantly pointed to the earth with
  larger antenna server 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 72 MHz 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 higher frequencies
  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 the
  distance 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
  and coupling 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 station
  parameters
• Number of Carriers per transponder. This
  determines back-off which is the reduction in
  amplifier 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 (TTCM)

• 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 TTCM system is highly automated with
  computer alarms to sound in case of anomalies are
  detected.
• Analogy School bus represents spacecraft bus,
  and passengers represents payload
  telecommunications and TV traffic. TTCM 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 Fixed
  Satellite 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)