GPS

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GPS

• How it Works
• For a full tutorial on GPS and its applications
  visit the Trimble Website

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What is GPS?

• Global Positioning System is a worldwide
  radio-navigation system formed from a
  constellation of satellites
• Dependent on line of sight

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GPS Systems

• NAVSTAR (Navigation System Timing and Ranging)
• United States
• GLONASS
• Russian Global Navigation SS
• GALILEO
• European Union
• COMPASS (BeiDou)
• Chinese Navigation System
• IRNSS
• Indian Regional Navigational SS

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Components of GPS

• Space
• Satellites
• Control
• Ground control and antennas and monitoring
  stations
• User
• Military, civil, commercial, scientific

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Satellites (SVs)

• 32 satellites
• Orbit at 12,600 miles
• Six orbital planes
• Each SV has a ½ sidereal day orbital period
• Nine satellites are visible at any given time

Garmin
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Control Segment
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Users

• Civilian
• Cartography, Cellular networks, fleet tracking,
  navigationetc.
• Military
• Navigation, target tracking, munitions guidance,
  and reconnaissance
• Nuclear detonation detectors

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How do we determine our position?

• We have to determine our precise distance to at
  least 4 GPS satellites
• We need extremely accurate clocks
• In addition to distance we need to know exactly
  where the satellites are in space.
• Lastly, we correct for any delays the signal
  experiences as it travels through the atmosphere.

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How GPS works
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Information sent in the GPS signal

• GPS timing signal (PRC)
• Ephemeris
• Satellite orbit information
• Updated every 2 hours valid for 4 hours
• Almanac
• Orbit and status for all 32 satellites
• Error corrections

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Sync Satellite and Receiver

• We need to have an accurate measure of the radio
  signal travel time
• Atomic Clock
• Use oscillations of an atom for timing
• A fourth measurement is used to determine the
  position
• The receiver looks for a single correction
  factor that it can subtract from all its timing
  measurements that would cause them all to
  intersect at a single point

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Correcting Errors

• Charge particles in the ionoshpere and water
  vapor in troposhpere slows the GPS signal
• Multipath error
• Tiny errors that occur at the satellite
• Selective Availability

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Differential GPS

• Relies on the operation of two receivers
• One is stationary and its location is know
  precisely
• The second is roving and making measurements
  (this is YOU!)
• The reference station knows where it is located
• It uses that information to correct the timing
  from the GPS satellite
• This error correction is then sent to the rover
  to correct the measurements

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Wide Area Augmentation System

• WAAS
• Uses ground stations to measure the variation in
  the GPS system
• It corrects for this error and sends the
  correction to WAAS satellites
• Must have a specially equipped receiver to obtain
  the WAAS signal
• Typically accurate to within 1 meter

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Real Time Kinematic

• RTK
• Based on the use of carrier phase measurements
• RTK systems use a base station that transmits the
  measure carrier phase to mobile individuals
• Can result in cm accuracy