Lecture 2, Space Segment

Presentation / Lecture 2, Space Segment

Date Submitted: 06 June 2001

Written by RPC Telecommunications Ltd.. Website: http://www.rpctelecom.com

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This is the second in the series of general satcom tutorial lectures submitted by RPC Telecommunications.

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-Section 1
Orbit Theory: Kepler's Laws.
-Section 2
Orbit Theory: Formulae.
-Section 3
The Real World.
-Section 4
Types & Implications of Orbit.
-Section 5
Satellite Hardware.
-Section 6
Launch Vehicles.
-Section 7
Flight Plan & Maintaining Satellite In Orbit.
-Section 8
Stationkeeping & Stabilisation.

Launch Flight Plan

The launch flight plan is different for Expendable and Reusable Launch Vehicles.

The flight plan will be comprised of clear stages seperated by motor "burns"

  • boost phase
  • parking orbit
  • transfer orbit
  • final orbit

The objective is usually to achieve target orbital position with the minimum fuel used, but this takes the maximum time to achieve.

An operator can trade off fuel for speed. However, you cant refual a satellite in orbit (yet!).

Reusable Launch Vehicle

Elliptical transfer orbit - "Hohmann Transfer Ellipse" - is minimum energy path between two circular orbits.

Expendable Launch Vehicle

Maintaining The Satellite In Orbit
Stationkeeping:
  • Maintaining the nominal orbital position against perturbing forces
  • re-locating the satellite to a new orbital longitude position

Attitude Control and Stabilisation

  • Maintaining orientation of satellite's communications antennas in the direction of the Earth and with the correct rotational positioning
  • Maintaining orientation of solar panels in the direction of the sun to maximise electrical power generation
  • re-establishing orientation in the event of the satellite "tumbling" in space

Stationkeeping and Stabilisation require two elements:

  • Sensors to fix orientation in space
  • Reaction devices to provide stabilisation forces to move the satellite
Sources of Positional Data
Sun
  • bright and unambiguous
  • but, will not be visible during eclipse
  • accuracy is ~1 arc minute
  • direction must be known to align solar panels

Earth

  • always available, bright and unambiguous
  • but, large angle at most satellites may require a scanning motion, must be protected from sun
  • accuracy is ~0.1 because of horizon definition due to atmosphere
  • direction must be known to align antennas

Magnetic Field

  • economical, low power requirements, always available for low altitude satellites
  • but, poor resolution (~0.5), good only near the Earth, spacecraft must be magnetically clean

Stars

  • available anywhere in the sky with very high accuracy (~0.001)
  • but, sensors are heavy, complex and expensive, identifying targets is slow and complex, sensors need protection from the sun, multiple stars may cause problems

Inertial Space (gyroscopes and accelerometers)

  • requires no external sensors, highly accurate for limited time intervals
  • but, senses changes in orientation only, subject to drift, rapidly moving parts

Next: Section 8 - Stationkeeping & Stabilisation.

 
 

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