Global Posision System GPS

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Global Posision SystemGPS
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Litt historie

• 1960 nasA DoD are Interested in developing a
  satellite based position system with the
  following characteristics
• Global coverage
• Continuous/all weather operational
• Abillity to serve high dynamic platforms
• 1962 The satellite based positioning system
  Transit becomes operational for use on low
  dynamic platforms
• Polar orbits
• Circling the Earth evry 107 minutes
• Provided 2D position fixes for each orbital pass
• Time between position fixes, 35 to 100 minutes

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GPS FundamentalsGps Milestones

• 1969 The office of Secretary of Defense
  establish the Defense Navigation Satellite
  System to work for a single joint-use
  positioning system and from this NAVSTAR GPS was
  foemed
• 1978 The first 4 GPS satellites are launched
• 1993 The Standard Positioning Service Initial
  Operating Capability was attained (all 24
  satellites in orbit) (SPS)
• 1995 The Precise Positioning Sevice reached its
  Fully Operating Capability. (PPS)
• 2000 On May 1, the Selective Availability (which
  had degraded civil position accuracy to 100
  meter) was switched off.

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Principle of Operation

• 1.The fundamental technique for GPS is to use
  one-way ranging from the satellites.
• 2.The basis of the system is triangulation based
  on ranging from all visible satellites.
• 3. GPS measures distances using the travel time
  of a radio message.
• 4. To measures travel time, timing is crucial and
  the GPS satellites therefore need to have highly
  accurate clocks on board.
• 5. Every GPS satellite has up to 4 so-called
  atomic clocks and data transmission from each
  satellite is synchronized with GPS system time.

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• 6. Travel time from the signal left the satellite
  until it is received at the GPS receiver is
  measured.
• 7. By multiplying the travel time with the speed
  of light (300.000km/s)
• the distance can be computed.
• 8. Once you know the distance to each satellite,
  you need to know where these satellites are in
  space.
• 9. GPS satellites are launched into
  pre-determined orbits and their positions are
  transmitted to the user as part of the GPS
  signal.
• 10. Knowing the satellitesposition and distance
  to the user receiver, the geographical position
  of the user receiver can be computed.

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• 11. The clocks in the GPS satellites are
  extremely precise while the crystal
  clocks used in the GPS receivers are not.
• 12. The inaccurate time tagging of the received
  signal causes a timing offset (an extra unknown
  parameter in the position calculation).
• 13. Solving for this unknown parameter can be
  done by making an extra satellite range that
  makes up for the imperfect time sync on our part
  needed to compute a 3D position.
• 4 unknown
• Latitude
• Longitude
• Height and
• Time offset

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GPS Control Segment

• Consists of a Master Control Station in Colorad
  Spring as well as Uplink and Monitor Stations
  responsible for monitoring and maintenance of the
  satellite system.
• Monitor Stations.
• Hawaii
• Ascension
• Diego Garcia
• Kwajalein

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GPS Signal Characteristics

• The GPS satellites transmit a continuous signal
  on two frequencies, the
• L1 frequency 1575.42 MHz and
• L2 frequency 1227.60 MHZ
• On the L1 frequency the C/A code is avable and is
  used to identify the individual satellites and
  for determine the range between the satellite and
  the user receiver.
• The C/A code is a so-called PRN code whish is
  unique for each GPS satellite.
• The PRN code is composed of a series which have
  values of 1 or 0. When the received signal is
  compared with a copy of the signal available in
  the GPS receiver, instantaneous ranges for the
  tracked satellite can be computed.

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• The time (GPS Receiver Time) for when the
  received satellite signal (t1) arrives is
  compared with the time (GPS Satellite Time) for
  when the signal was transmitted (t0) and thus the
  distance to the satellite can be determined.

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Provided Positioning Services

• Precise Positioning Service (military users
  only)
• The information modulated onto both the L1 and
  L2 frequencies is used to compute position data.
• Posistion accuracy typicall 10 meter
• Standard Positioning Service (open to all users)
• Only the information modulated onto the L1
  frequency is used to compute position data
• Some receivers and software can lock onto the
  L2 frequency and since two different frequencies
  penetrates the ionosphere at different speed, the
  inospheric delay errors can be computed!
• Position accuracy using L1 receivers typically
  10 15 meters
• Position accuracy L1/L2 receivers typically 5
  10 meters

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Factors Affecting GPS Accuracy

• Satellite-to-use Range Estimation
• Due to the errors in the range measurements they
  will not be determined as if they were received
  correctly at the electrical center of the GPS
  antenna.
• When these incorrect range observations are
  used to compute positions, position data will be
  reported with an error estimate, e.g. (EPE 8.4
  meter)

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Ionospheric Disturbances

• Electrical loaded particles (electrons and
  protons) in the ionosphere causes radio signals
  travelling through the ionosphere to be delayed
  or lost.
• In periods with high ionospheric disturbances the
  GPS signal might be so delayed that calculated
  range are rejected from being used in the
  position calculation.
• In areas around equator the disturbances in the
  ionosphere are so rapidly changing that you may
  expect
• Loss of tracked satellites on the users system
• Loss of tracked satellites at the reference
  station
• Loss of transmitted corrections from Inmarsat or
  Spotbeam satellites

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The Sun Spot Cycles

• The sun spot activity goes in cycles of 11 years
  reaching its next peak year 2001/2002. This
  phenomena is expected to decrease in 2003/2004.
• From time to time ionospheric disturbances will
  influence on positioning but with less magnitude
  compared to what has been seen during the last
  few years.

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