GIS 1710 The NAVSTAR GPS Segments

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GIS 1710The NAVSTAR GPS Segments

• Dr. Walter Goedecke
• Spring ? 2007

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Topics

• Description of the U.S. GPS system segments
• Space segment
• Spacecraft
• Orbit details
• Orbit anomalies
• Ground segment
• GPS Ground Control
• Ephemeris vs. Almanac

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The NAVSTAR GPS Segments GPS Description

• The Global Positioning System is divided into
  three segments space, control, and user.
• The space segment comprises the GPS satellite
  constellation.
• The control segment comprises ground stations
  around the world that are responsible for
  monitoring the flight paths of the GPS
  satellites, synchronizing the satellites' onboard
  atomic clocks, and uploading data for
  transmission by the satellites.
• The user segment consists of GPS receivers used
  for both military and civilian applications.

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The NAVSTAR GPS Segments GPS Description

• The system cost is about 400 million / year.
• There are two levels of service
• Precise Positioning Service (PPS) for exclusive
  military use
• Accuracy within 95 probability is 16 meters for
  horizontal component and 23 m for the vertical
• Standard Positioning Service (SPS) for public use
• Accuracy 100 meters for horizontal component and
  156 m for the vertical

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GPS Space SegmentSpacecraft

• Weight is 1,877 kg at launch
• The design lifetime is 10 years
• Some spares are in orbit, and some are ready to
  launch
• The older Block ? group are no longer operational
• As of July 2001 the constellation has
• 5 Block ?? satellites
• 18 Block ??A satellites
• 6 Block ??R satellites
• R designates replenishment, or replacement
• Block ??F will follow with enhanced capabilities.

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GPS Space SegmentGPS Orbital Details

• The system consists of 24 satellites in medium
  Earth orbit (MEO)
• This insures a ground station has access to at
  least four satellites at any given time, but as
  much as ten are possible.
• Orbital characteristics
• Period is half a sidereal day, 11 hours and 58
  minutes
• Speed is 3.87 km/s
• 20,200 km nominal altitude in circular orbit
• 550 inclination from equatorial plane
• 6 orbital planes with a constellation of 4
  satellites in each plane spaced 600 in orbital
  longitude

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GPS Space SegmentGPS Orbits

• After Dana

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GPS Space Segment Orbital Anomalies

• Orbits are never perfect, or nominal, as
  predicted
• Orbital perturbational forces in the space
  environment affect them, such as
• Non-central gravitational potentials
• The upper atmosphere and ionosphere
• The solar wind in relation to the geomagnetic
  field
• Plasmas, radiation, and high energy particles

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GPS Space Segment Drag, Solar Radiation, and
Solar Wind

• Low ballistic coefficient objects are more
  susceptible to radiation pressure, such as
  satellites with large solar panels.
• Above 800 km solar radiation pressure is a
  greater factor than atmospheric drag.
• Solar wind can perturb an orbit if the Earths
  magnetic field is sufficiently compressed.

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GPS Space Segment The Space Environment -
Plasmas

• Website http//www-istp.gsfc.nasa.gov/Education/I
  ntro.html

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GPS Ground Segment

• Spacecraft operated from the GPS master control
  station (MCS) by the US Air Force at Schriever
  Air Force Base (formerly Falcon AFB) near
  Colorado Springs, Colorado, and four other
  monitor stations, located in
• Hawaii
• Ascension Island in the Atlantic Ocean
• Diego Garcia in the Indian Ocean
• Kwajalein in the Pacific Ocean
• - Includes antenna for uploading to satellite

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GPS Ground SegmentGPS Up/Down Control

• After Dana

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GPS Ground Segment GPS Control Sites

• El-Rabbany

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GPS Ground SegmentGPS Ground Control

• MCS and the additional ground control stations
  monitor satellites determine orbits with
  precise cesium clocks every 1.5 seconds.
• The ground network calculates for each satellite
• Ephemeris
• Atomic clock error
• Other navigation message parameters

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GPS Space Segment Ephemeris vs. Almanac

• An almanac is a calendar of events, in this case
  GPS almanacs are computed for mathematical
  exercises to predict orbits.
• An ephemeris is closer to actual reality, taking
  into account new observations that deviate from
  nominal orbits due to perturbations.

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References

• Ahmed El-Rabbany, Introductions to GPS The
  Global Positioning System, Publisher Artech
  House.
• Wertz, James R., Wiley J. Larson, Space Mission
  Analysis and Design, Publisher Microcosm Press.
• Dana, Peter H., Department of Geography,
  University of Texas at Austin, 1994http//www.col
  orado.edu/geography/gcraft/notes/gps/gps_f.html