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.

Satellite Hardware

A satellite is comprised of a payload supported by a platform.

The platform takes up the largest proportion of the mass of the satellite - design objectives is therefore to support the largest possible payload with the smallest possible platform.

Platform comprises:

  • Physical support structure
  • Power
  • Propulsion
  • Thermal control
  • AOCS (altitude control and stationkeeping)
  • TTC&M (telemetry and telecommand)
  • control processor

Payload comprises:

  • Antenna sub-systems
  • Communications transponders (receivers, amplifiers etc)

Major Spacecraft Sub-Systems:

Payload Block Diagram:

Earth Illumination

One satellite in the GSO can "see" about one third of the Earth's surface so can provide telephone, data and TV links over the whole of that area. 

However, coverage is impossible to the polar regions (beyond about 80 latitudes). (An aircraft at 30,000 feet can still communicate at latitude 85)

MSS satellite antennas commonly have maximum gain (about 21 dBi) at the edge of coverage, and lower gain (18 dBi) at beam centre.

Three geostationary satellites at 120 spacing provide full Earth coverage (except for the polar regions).

 

Satellite Beam Options
Global Beam
  • Low gain (~18 dBi)
  • Shaped (~21 dBi) MSS - uplink budget critical

Shaped Beam

  • Hemispherical coverage (~22 dBi)
  • Zonal coverage (~30 dBi)

Continental Spot

  • CONUS
  • pan-European e.g. EUTELSAT SMS

Spot Beam

  • e.g. INTELSAT Ku-Band spots.

A choice of smaller beam means higher gain and EIRP but at the cost of a reduced coverage and possibly connectivity.

  • Ku-band BSS plan UK 1.84*0.72 elliptical beam (~44 dBi)
  • L-band super GSOs (~42 dBi) with 140 to 250 beams
  • Ka-band super GSO (~44 dBi) e.g. Spaceway
  • L-Band LEOs e.g. Iridium 37 beams per satellite and 2025 beams for earth coverage
  • Satellite on board processing needed for mesh networks

Next: Section 6 - Launch Vehicles.

 
 

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